Mike Howell
Especialista de Soporte Técnico de EASA

Las dos razones principales para realizar la prueba de núcleo del estator en el centro de servicios son (1) verificar que el núcleo del estator es apto para uso continuo y en caso de rebobinado y (2) verificar que el proceso de rebobinado no ha alterado de forma adversa la condición del núcleo del estator.

El propósito de este artículo es discutir como determinar, evaluar y comparar los resultados de la prueba de núcleo. Es muy importante comprender que la variación de los procedimientos de prueba puede invalidar la comparación.

David Sattler
L&S Electric, Inc.

A no ser que se tenga mucho cuidado, tirar del alambre magneto al desmantelar el estator de un motor a menudo deforma o dobla los dientes de las laminaciones. Estos dientes deformados comprometerán la calidad de la reparación y hay estudios que demuestran que este problema puede reducir la eficiencia del motor. Sin embargo, aunque esta reducción puede ser pequeña, genera altos costos y desperdicio de energía.

Aunque los clientes rara vez notan la merma del rendimiento, nuestro objetivo durante la reparación de los motores es siempre llevar a cabo rebobinados de la más alta calidad posible. Por lo tanto, hemos diseñado e implementado el uso de discos (platos) retenedores para mantener los dientes del estator en su lugar mientras se saca el alambre magneto de las ranuras. Los discos que se ven en las fotografías nos han ayudado a evitar y garantizar dañar los dientes del estator al sacar el alambre del estator.

Tom Bishop, P.E.
Especialista Sénior de Soporte Técnico de EASA

La norma para realizar las pruebas de resistencia de aislamiento en los devanados de motores y generadores del Instituto de Ingenieros Eléctricos y Electrónicos (IEEE), publicada en el 2002 ha sido revisada. La edición del 2013 fue publicada en Marzo del 2014. 

El primer cambio en el nuevo documento, consiste en una pequeña modificación del título, el cual pasó de ser “Práctica Recomendada IEEE para Probar la Resistencia de Aislamiento de las Máquinas Rotativas” a “Práctica Recomendada para Probar la Resistencia de Aislamiento de las Máquinas Eléctricas”. La justificación para este cambio fue emplear los términos más frecuentemente utilizados por la IEEE en motores y generadores. Este artículo describe los cambios más importantes realizados en los apartados de la norma que afectan las reparaciones y las pruebas en los centros de servicio.

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Índice de Polarización
Un cambio importante realizado en el apartado 5.4, titulado “ Valores de índice de polarización” afecta a las pruebas de los bobinados en alambre redondo. El texto en concreto ahora establece: “Esta prueba podría no aplicar a pequeñas máquinas con bobinados en alambre redondo ya que la corriente de absorción IA se vuelve insignificante en cuestión de segundos (vea un debate adicional en el Anexo A).” En el Anexo A, la norma acepta que para los devanados de alambre redondo, “el valor de la corriente de absorción puede decaer aproximándose a cero en 2 ó 3 minutos”, este tiempo dista mucho de los 10 minutos de duración prescritos en la prueba de índice de polarización (IP). En la edición previa de la norma, éste apartado se centraba en los bobinados de pletina y no trataba específicamente el tema de los bobinados de alambre redondo. La importancia de este cambio radica en que se clarifica que en muchos, si no la mayoría de los casos, la prueba de IP no es aplicable a bobinados de alambre redondo. Por consiguiente no aportará información útil y podrá crear confusión entre el usuario final y los que realizan la prueba. Por lo que hacerla sería básicamente una pérdida de tiempo.

Con relación al IP de los devanados de armadura de las máquinas de C.C., un texto del apartado 12.2.1 establece lo siguiente: “La prueba de índice de polarización no aplica a armaduras de C.C. con colectores de cobre expuestos, esto significa obligatoriamente con aislamiento no encapsulado”. Por consiguiente, la prueba de IP no aplica a las armaduras convencionales.

Nota: Para los bobinados con sistemas de aislamiento clase B (130° C) o superiores, el valor mínimo del IP sigue siendo 2.0. De igual forma, la regla de los 5000 megohmios no cambia. Esto significa que no es necesario realizar pruebas de IP a bobinados con resistencias de aislamiento de 5000 megohmios o superiores.

Corrección por Temperatura
Durante más de medio siglo, las características de la resistencia de aislamiento (IR) versus la temperatura establecidas en la IEEE 43, han seguido la regla simple que el valor de la IR se dobla cada que la temperatura del bobinado baja 10° C, y a la inversa, que el valor de la IR se reduce a la mitad cuando la temperatura del bobinado aumenta 10° C. No obstante, el apartado 6.3 de esta nueva edición, proporciona dos factores de corrección por temperatura, uno de los cuales utiliza dos fórmulas distintas de corrección. Ahora, los bobinados se diferencian entre “termoplásticos” o “termoestables”. Los devanados con aislamientos termoplásticos son aquellos fabricados con sistemas asfálticos y otros sistemas de aislamiento que fueron usados antes de principios de 1960. Los bobinados con aislamientos termoestables aparecieron a finales de 1960 e incluyen sistemas basados en polyester y materiales epóxicos.

Desafortunadamente, la regla previa de los “10 grados” aplica a bobinados termoplásticos, que son devanados relativamente raros ya que se remontan a más de 5 décadas. La “regla” para los sistemas de aislamiento termoestables, los cuales son mucho más comunes, se expresa mediante dos fórmulas ligeramente complicadas. Una fórmula cubre las temperaturas del aislamiento que van desde los 10° C hasta  menos de 40° C, y la otra cubre las temperaturas del aislamiento que van desde los 40° C hasta menos de 85° C. Las fórmulas se muestran a continuación.

Fórmula para temperaturas que van desde los 10° C hasta menos de 40° C:
K_t=  exp [-1245 {(1/(T+273) - (1/313)}]
(Ecuación 1)

Fórmula para temperaturas que van desde los 40° C hasta menos de 85° C:
K_t=  exp [-4230 {(1/(T+273) - (1/313)}]
(Ecuación 2)

Donde:
T = Es la temperatura (en grados C) a la que fue medida la resistencia de aislamiento.
Kt = Es el factor por el que se debe multiplicar T para poder corregir la resistencia de aislamiento a 40° C.

La Tabla 1 muestra la variación del factor Kt para un rango de temperaturas. Determinar el valor de Kt utilizando la tabla en lugar de calcularlo con fórmulas, es más rápido y facilita el proceso.

Note que la Tabla 1 tiene un rango de temperaturas comprendidas entre los 10° C y los 60° C, mientras que el rango especificado por la fórmula va desde los 10° C hasta temperaturas inferiores a los 85° C. La IEEE 43 explica esta aparente inconsistencia mediante una nota que se lee de la siguiente forma: “Las dos ecuaciones 1 y 2 anteriores, son aproximaciones y podrían llevar a cometer errores significativos si se utilizan para calcular la resistencia de aislamiento a temperaturas que se encuentren fuera del rango comprendido entre los 10º C y los 60º C.”

Para ilustrar el efecto del factor de corrección por temperatura utilizando la nueva norma versus la versión previa, tenemos el siguiente ejemplo: La resistencia de aislamiento de un bobinado es de 160 megohmios a 20° C (68° F) y la temperatura de referencia para la resistencia de aislamiento es de 40° C (104° F). Utilizando el método antiguo, tendríamos que rebajar a la mitad el valor de la IR para obtener su valor a una temperatura que se encuentre 10° C por arriba. En nuestro ejemplo, tendríamos que hacer esto dos veces, rebajando a la mitad el valor medido a los 20° C y rebajando a la mitad el valor obtenido a los 30° C y así calcular la resistencia de aislamiento corregida a la temperatura de referencia de 40° C.

Matemáticamente estamos multiplicando por ½ y por ½, o lo que es lo mismo, multiplicando el valor de IR medido a 20° C por ¼. Lo anterior permite corregir el valor de la resistencia de aislamiento a 40° C. Por tanto, la resistencia de aislamiento de 160 megohmios a 20° C corregida a 40° es de 40 megohmios (160/4).

A continuación, convertiremos la medida utilizando la nueva norma. De la Tabla 1, tenemos que para una temperatura de 20° C, el factor de conversión es 0.76. Al multiplicar la resistencia de aislamiento de 160 megohmios por 0.76, obtenemos un valor de 122 megohmios. Por consiguiente la resistencia de aislamiento a 40° C es de 122 megohmios. Note que este valor es mucho más alto que el calculado con el método antiguo. La Tabla 2 muestra la diferencia entre los dos métodos, tomando como base una resistencia de aislamiento de 100 megohmios a 40° C.

Para obtener mayores detalles sobre la corrección por temperatura, consulte el artículo publicado en julio de 2013 en la revista Currents de EASA, titulado “Revisiting insulation resistance temperature correction.”

Resistencia de aislamiento mínima
El apartado 12.3 incluye una tabla titulada “Valores mínimos recomendados para la resistencia de aislamiento a 40° C (todos los valores en MΩ).” El cambio más importante realizado en esta tabla es, que el valor mínimo de la resistencia de aislamiento para las armaduras pasó de 100 megohmios a 5 megohmios. La razón para realizar este cambio fue la de reconocer que independientemente del tipo de bobinado, las barras de cobre expuestas de los colectores tienen un efecto limitador sobre la resistencia de aislamiento. En la Tabla 3 se puede apreciar una comparación de los valores de los mínimos valores de resistencia de aislamiento establecidos por la IEEE 43-2013 y la IEEE 43-2000 para distintos bobinados. Note que los niveles de resistencia de aislamiento mínimos listados en la primera columna son los mismos para ambas ediciones de la norma. Además, los cambios relacionados con las armaduras se resaltan en color azul.

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The topics covered included in this webinar recording:

• One circuit wye connection — Best, no parallel paths, turns per coil may prevent this
• Delta or multiple parallel circuits—Produces closed circuits, Circulating currents
• Open delta (4 wire connection)
• Permissible connections—Skip pole, adjacent pole
• Determined by speed combination

T​arget audience: This webinar recording will benefit service center technicians and supervisors. 

When a customer requests a motor be rewound for a new set of conditions, that is typically what we in the service center industry provide them. However, there are occasions where the customer request may be fulfilled by reconnection; in some cases, this is done simply by revising the motor nameplate data. The purpose of this article is to identify and explain some of these scenarios.

Reconnections covered include:

• Part winding start (PWS)
• Single voltage 12 leads
• 2 wye and 1 delta
• 230/460-575 volts 380 volts 50 Hz and 460 volts 60 Hz
• 2300 and 4000 volts

For an additional reference, see "Variables to consider when making motor frequency changes between 50, 60 Hz" published November 2008.

Mike Howell
EASA Technical Support Specialist

The two primary reasons for performing stator core testing in the service center are (1) to verify that the stator core is acceptable for continued use and in the event of a rewind, (2) to verify that the repair process has not adversely changed the stator core condition. This testing can be done using a commercial core loss tester or a manual loop test using an appropriate AC source, cables and meters. Some typical reasons a manual loop test may be performed are: 

• Customer or service center preference / specs 
• Commercial core loss tester not available 
• Stator size is inappropriate for available commercial core loss tester 

Additionally, some service centers have forgone core loss testing on small stators for various reasons including difficulties with test configuration, calculations, cost or even appearance. The purpose of this article is to explore a low-cost test setup for loop testing small stators.

Matthew Conville, P.E.
EASA Technical Support Specialist

When we consider putting a machine into service, we must consider the duty rating of the machine. If we do not, there is a good chance that the machine being placed into service will have thermal degradation of the windings. Not every application is created equal. For example, a crane motor doesn’t need the same duty rating as a punch press motor that runs continuously, even though they may have the same horsepower ratings. Likewise, a chop saw motor wouldn’t need to have the same duty rating as a pump motor where the pump is operated continuously.

A special discounted collection of 6 webinar recordings focusing on AC electric motor design.

Once purchased, all 6 recordings will be available on your "Downloadable products purchased" page in your online account.

Downloadable recordings in this bundle include:

The Basics: AC Motor Design
Presented July 2016

This webinar recording covers: 

• Various types of AC motors and bases for operation
• Squirrel cage induction motor rotor design / construction
• Squirrel cage induction motor stator design / construction

How Winding Changes Affect Motor Performance
Presented January 2019

This webinar recording focuses on the effect of three-phase stator winding changes on efficiency and reliability.

Specific changes addressed will include:

• Connection
• Circuits
• Turns
• Span/pitch
• Grouping sequence
• Concentric to lap, and vice versa
• Wire area per turn and per slot

Target audience: Service center technicians and supervisors.

Motor Starting Capabilities and Considerations
Presented March 2014

This webinar addresses the topic of a three phase squirrel cage motor’s ability to successfully accelerate a driven load. Although a motor can drive a running load, that is not assurance that it has the capability to accelerate the load up to rated speed. The difference between success and failure is determined by some complex conditions. For example, the motor torque during starting is not constant, and unless the load is a pure inertia load (very rare), it does not have a constant speed-torque relationship. Key considerations addressed include acceleration time, acceleration torque, motor heating, stator and rotor limits, and torque variables.

Target audience: This presentation will be most useful for service center sales personnel, engineers, supervisors and managers. The content will be beneficial for moderate through highly experienced persons.

AC Motor Redesign: Speed Changes
Presented January 2015

This presentation focuses on AC motor redesigns involving speed changes. Service centers encounter scenarios such as the procurement of a single-speed motor that must be redesigned for two speeds or redesign of an existing two-speed motor for use on an adjustable-speed drive.

Topics covered include:

• Single-speed, one-winding to two-speed, one-winding
• Single-speed, one-winding to two-speed, two-winding
• Two-speed, two-winding to single-speed, one-winding
• Two-speed, one-winding to single-speed, one-winding

The redesign examples are performed using EASA’s AC Motor Verification & Redesign program, including use of the integrated motor winding database for locating comparative data. Examples will include other changes such as voltage, frequency and horsepower.

Magnetic Wedges
Presented January 2019

An increasing number of manufacturers are using magnetic wedges in their form-wound machines. When a winder fails to replace magnetic wedges in kind, the winding temperature rise can increase by 20°C, and the magnetizing current can increase by 20% or more.

This recording explains why they are used, provides a balanced review of the benefits and negative issues associated with their use, and explains how to avoid the problems.

• Why some manufacturers use magnetic wedges
• Benefits of magnetic wedges
• Downside of magnetic wedges
• Fitting and installation to prevent them from falling out in service

Target audience: This will benefit service center technicians and supervisors.

Speed/Torque Curves
Presented March 2017

This recording covers:

• Starting torque
• Breakdown torque
• Full load torque
• Speed current curve
• Load torque curve
• Impact of reduced voltage start (autotransformer, PWS, wye-delta)
• Slot combination problems (noise, torque cusp, cogging)

It is very important to understand speed/torque curves and how they impact motor operation.

Target audience: Engineers, mechanics, winders and sales persons with fundamental knowledge of motor operation. 

A special discounted collection of 11 webinar recordings focusing on AC motor electrical procedures.

Once purchased, all 11 recordings will be available on your "Downloadable products purchased" page in your online account.

Downloadable recordings in this bundle include:

The Basics: Motor Repair Burnout Procedures
Presented October 2016

• Interlaminar insulation materials / properties of AC stators
• Core testing before and after
• Processing equipment, controls and records

The Basics: The Why and How of Core Testing
Presented October 2016

• The reasons for performing core testing and why they are important
• An explanation of the two core testing methods:
• Loop testing
• Use of a core tester
• How to properly perform a core test
• How to assess the results
• Stator Core Testing: Know What You Have Before You Wind It

Stator Core Testing: Know What You Have Before You Wind It
Presented April 2017

This presentation covers:

• The importance of the stator core test 
• Simple theory to share with technicians and customers 
• Practical approach for testing small stators demonstrated 
• Eliminating pen + paper; loop test calculations for any device 
• Assessing the results

High Potential Testing of AC Windings
Presented December 2019

High-potential testing is routinely used to assess the ground insulation of AC stator windings in-process, after completion of a rewind and post-delivery. This webinar covers:

• Differences between AC and DC high-potential tests
• Sizing AC test sets when testing large windings
• What relevant standards address (and what they don’t)
• Communicating test requirements to all stakeholders
• When to test and at what levels
• How to evaluate results

Target audience: Beneficial for service center managers, supervisors and technicians responsible for high-potential testing.

Squirrel Cage Rotor Testing
Presented October 2014

Determining whether or not a squirrel cage rotor is defective is an issue that is a challenge to every service center as there is often no simple way to determine the integrity of a rotor. The primary focus of this session is to describe many of the available tests that can be utilized in the service center or at the motor installation site. In addition to conventional squirrel cage rotor testing methods such as the growler test, techniques that will also be covered are the use of a core loss tester, high current excitation, and spectrum analysis of vibration.

Target audience: This presentation will be most useful for service center and field technicians with at least 2 years experience, service center supervisors and managers, engineers, or anyone with previous experience dealing with suspected open rotor issues.

Evaluating High No-Load Amps of Three-Phase Motors
Presented December 2011

This presentation focuses on the steps to take before rewinding to avoid the undesirable situation of high no-load motor amps after the rewind.

The presentation covers the following steps that should be performed on every AC stator rewind:

• Inspect the stator bore and rotor outside diameter for evidence of machining or damage
• Record the original winding data exactly as found
• Verify the winding data
• Test the stator core before and after rewinding removal

Target audience: This is most useful for service center mechanics and winders with any level of experience, and service center supervisors and managers.

Insulation Technology Improvements and the Repair Market
Presented July 2019

Most modern rotating electric machines operate on the same principles their predecessors have for 100+ years. However, improvements in materials technology over that time have allowed for increasingly greater power density in machine design.

There is a natural time lag between OEM technology improvement and repair of equipment containing that technology. This session will explore some of these improvements and their implications for service centers attempting to provide a quality repair.

Target audience: This webinar will be appropriate for service center managers and technicians responsible for rewind activities.

Motor Temperature Rise and Methods to Increase Winding Life
Presented December 2018

This webinar discusses:

• Temperature rise: Method of detection, Insulation class, Enclosure, and Service Factor
• Increasing winding life: Insulation class, Cooling system, and Winding redesign

Target audience: This will be most useful for service center engineers, supervisors, managers and owners. The content will also be beneficial for mechanics and winders.

Air Gap: What It Is, What Does It Do, and Why Is It Important?
Presented October 2019

The physical air gap between the rotor or armature and the stator or field frame is complex and plays a critical role in the performance of AC and DC machines. Most repairers do not realize how little they understand about this subject.

This webinar explains the role air gap plays in AC motor performance, how to recognize the symptoms of an uneven air gap, and share corrective measures. For DC machines, this webinar will cover the distinctly different role of the field air gap as opposed to the air gap of the interpoles.

• Air gap tolerance of AC machines
• Air gap tolerance of DC fields and interpoles
• Allowable runout of rotor / armature
• Recognizing the signs of air gap anomalies
• Corrective actions

Target audience: This webinar recording is of benefit to managers, supervisors, winders, mechanics and field service personnel.

Troubleshooting AC Generators & Alternators
Presented May 2015

This recording covers theory of operation, inspection, operation and troubleshooting tips for AC generators and alternators. For the supervisor, field service technician or service center personnel, generators can present unique challenges. Topics covered include:

• Theory of operation
• Testing tips
• Stator winding cautions
• How to interpret the exciter motor connection
• In-shop and on-site testing methods
• How to test the voltage regulator
• How to test a generator without a regulator

Core Repair and Restack Techniques
Presented April 2014

This webinar teaches:

• How to repair damaged stator cores and how to know when a restack is necessary.
• There are often cases when repairs can be accomplished without a labor intensive restack.
• When a restack is required, there are pitfalls to watch out for to avoid problems with geometry, vibration and core losses.

Target audience: This presentation is useful to the supervisor, winder and sales personnel who interact with the end user.

EASA’s AC Motor Redesign manual explains how to make all possible changes in the ratings of AC electric motors, within design limitations. Besides mathematical formulas, it provides guidelines on the limitations for each type of redesign. These are useful in determining whether a desired new rating is possible before the motor is stripped. Terms are expressed in both English and metric units. Each chapter contains at least one example to guide you through your own redesigns.

This book is available as a FREE download (see link below) or printed copies can be purchased.                                                                                    

Chapters in this book include:

• Wire size change
• Voltage change
• Horsepower or kilowatt change
• Frequency change
• Phase change
• Circuit change
• Span or chord factor change
• Winding connection change
• The master formula
• Converting concentric windings to lap windings
• Converting lap windings to concentric windings
• Notes on pole changing
• Decreasing speed by increasing poles
• Increasing speed by decreasing poles
• Single-speed to two-speed, one winding
• Single-speed to two-speed, two winding
• Developing a winding for a bare core
• Strengthening or weakening a motor - short method
• Determining the proper connection
• Single-phase redesign
• Calculation of secondary voltage
• Determining three-phase coil grouping 

This presentation focuses on AC motor redesigns involving speed changes. Service centers encounter scenarios such as the procurement of a single-speed motor that must be redesigned for two speeds or redesign of an existing two-speed motor for use on an adjustable-speed drive.

Topics covered include:

• Single-speed, one-winding to two-speed, one-winding
• Single-speed, one-winding to two-speed, two-winding
• Two-speed, two-winding to single-speed, one-winding
• Two-speed, one-winding to single-speed, one-winding

The redesign examples are performed using EASA’s AC Motor Verification & Redesign program, including use of the integrated motor winding database for locating comparative data. Examples will include other changes such as voltage, frequency and horsepower.

Mike Howell
EASA Technical Support Specialist

This technical paper from the 2014 EASA Convention focuses on AC motor redesigns involving speed changes. Service centers encounter scenarios such as the procurement of a single-speed motor that must be redesigned for two speeds or redesign of an existing two-speed motor for use on an adjustable-speed drive. Topics covered include:

• Single-speed, one-winding to two-speed, one-winding
• Single-speed, one-winding to two-speed, two-winding
• Two-speed, two-winding to single-speed, one-winding
• Two-speed, one-winding to single-speed, one winding

The redesign examples use EASA’s AC Motor Verification & Redesign program, including use of the integrated Motor Winding Database for locating comparative data. Examples include other changes such as voltage, frequency and horsepower.

EASA's AC Motor Verification & Redesign - Ver. 4 software has been further refined and now contains and is fully integrated with EASA's Motor Rewind Database. This makes it the perfect program to lookup motor data, to verify existing winding data, and to perform motor winding redesigns.

This valuable resource is available only to EASA Members.

The AC Motor Verification and Redesign software provides easy verification of either concentric or lap windings, as well as redesigns with changes in poles/speed, horsepower, frequency or voltage. The redesign report with original and new winding data is output as an Adobe Reader (PDF) file and can be printed or saved. The program also allows you to search EASA’s extensive motor winding database. Choose to use the included database containing more than 250,000 windings or connect to the live, ever-expanding online database. Once found, motors from the database can be automatically imported as a starting point for further redesign.

Key software features include:

• Improved redesign accuracy and database search options.
• Includes the EASA Motor Rewind Database with more than 250,000 reported AC and DC windings. Use static built-in rewind database, or choose to use the constantly-updated, online database.
• Allows multiple simultaneous input cases for comparison of different motors.
• Users can opt to exclude half-wire sizes from automatic calculations.
• Automatic conversion from AWG to metric wire and square/rectangular wire to round magnet wire.
• The user can limit redesigns to only those matching in-stock wire sizes.
• Standard "one line formula calculations" are available from the Reference menu.
• Help files provide context-sensitive help. Includes the full EASA AC Motor Redesign book. Spanish translation of Help reference materials is provided.
• Built-in reference tables for chord factor, coil grouping, distribution factors, flux densities, and more.

System requirements

• Windows® XP, Windows® Vista, Windows® 7, 8 or 10 (Note: To run on a Mac, you must run a supported Windows OS using virtual machine software such as Parallels or Fusion.) 
• CD-ROM or DVD drive
• Approximately 1.25 GB free space on hard drive
• Screen resolution of at least 1280x768 (with text size set at 100%)
• Java™ Virtual Machine 1.8 or higher (Version 1.8 included on CD-ROM)
• Adobe® Reader (for report output/printing; free download from https://get.adobe.com/reader/)
• Internet access for retrieving future software updates and optional online motor rewind database

This handy form provides fields to record all of the details needed to document and/or order replacement coils for an AC form-wound machine. The form also includes detailed descriptions/definitions of the data to be recorded in each field.

This 4-page motor service order form provides fields to record:

• Customer contact information
• Nameplate data
• Reason for service
• Inspection activity checklist for the stator and rotor
• Incoming inspection notes
• Winding tests
• Rotor single-phase test
• No-load test
• Mechanical inspection before and after repair
• Winding tests after rewind but prior to treatment
• Winding tests prior to assembly
• No-load run test
• Shipping details

Presented by Chuck Yung
EASA Senior Technical Support Specialist

The physical air gap between the rotor or armature and the stator or field frame is complex and plays a critical role in the performance of AC and DC machines. Most repairers do not realize how little they understand about this subject.

This webinar will explain the role air gap plays in AC motor performance, how to recognize the symptoms of an uneven air gap, and share corrective measures. For DC machines, this webinar will cover the distinctly different role of the field air gap as opposed to the air gap of the interpoles.

• Air gap tolerance of AC machines
• Air gap tolerance of DC fields and interpoles
• Allowable runout of rotor / armature
• Recognizing the signs of air gap anomalies
• Corrective actions

Target audience
This webinar recording is of benefit to managers, supervisors, winders, mechanics and field service personnel.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

Although aluminum magnet wire theoretically can be converted to copper magnet wire of about 5/8 of the original wire area, in some cases this is not advisable. In others, it may result in a change in the magnetic strength of a coil or winding. In this article we will address the most common aluminum-to-copper magnet wire conversions as well as how to deal with whether the wire area should be reduced.

Chuck Yung 
EASA Technical Support Specialist 

While shaft currents are not a new problem (papers on the subject date back prior to 1930), what is “new” is our understanding of how to solve the problem. Shaft currents have been described as shaft voltages, circulat­ing currents, bearing currents and circulating voltages. This article will refer to the phenomenon as “shaft currents” because it is the current that causes the damage.

When a conductor is passed through a magnetic field, voltage is induced into the conductor. 

It is not the voltage that damages a bearing, but rather the current. (Fuses fail because the current is too high, not the voltage.) We don’t have a practical way to measure the current through the shaft, so we measure the magnitude of the voltage instead. 

Chase Fell
Precision Coil and Rotor

Un aislante ideal no permite el flujo de la corriente de fuga. El factor de potencia de un aislante se define como el coseno del ángulo de fase entre el voltaje y la corriente. En un aislante ideal, la corriente adelanta al voltaje exactamente 90 grados y en este sistema ideal el factor de potencia sería cero. Los sistemas de aislamiento de las bobinas de los motores y generadores eléctricos tienen pérdidas inherentes que causan el flujo de corrientes capacitivas y resistivas (Ver Figura 1). Para estos aislamientos, el factor de potencia no puede ser cero. 

El factor de potencia tip-up (FP) se utiliza comúnmente para medir la calidad de las bobinas nuevas y devanados fabricados para motores y generadores de C.A. de 6 kV o tensiones superiores. En los sistemas de aislamiento modernos de los devanados estatóricos, el factor de potencia y el factor de disipación dieléctrica son casi los mismos (Ver Figura 2). La prueba de FP tip-up puede ser útil para verificar la calidad del proceso de fabricación del bobinado, el comportamiento del material aislante, la consolidación de los conductores, la uniformidad del encintado del muro aislante y la condición del curado de la resina. Una vez el sistema de aislamiento alcance el voltaje de inicio de efecto corona (CIV), la descarga parcial (DP) cortocircuitará efectivamente algo de la capacitancia del aislamiento y el factor de potencia aumentará. La prueba FP aplica a bobinas individuales tratadas con impregnación por presión y vacío (VPI) y bobinas resin-rich, así como también a bobinados completamente curados. La prueba de factor de potencia tip-up no aplica a pruebas en banco de bobinas VPI sin curar (verdes) o para evaluar bobinados completos pre-procesados VPI. 

En los centros de servicio, la prueba de factor de potencia tip-up puede ser útil para verificar la calidad de un sistema de bobinas recién instalado, incluyendo la eficacia del proceso VPI. La prueba FP de los bobinados en servicio puede establecer una medida de referencia para el mantenimiento por análisis de tendencias. Esta prueba FP en servicio puede identificar potencialmente el envejecimiento del muro aislante, ya que la capacitancia entre el conductor de cobre y el núcleo del estator, generalmente se reduce a medida que se presenta separación de las cintas (delaminación) y/o burbujas de aire en el aislamiento (voids), entre las bobinas y el núcleo. La separación de las cintas de aislamiento normalmente aparece o se acelera por el envejecimiento térmico o mecánico del bobinado. La separación de las cintas y/o las burbujas de aire en el aislamiento pueden ocasionar descargas parciales y el fallo prematuro del sistema de aislamiento.

By Mike Howell
EASA Technical Support Specialist

Manufacturers deploy various external connection schemes to produce three-phase induction motors for multiple voltages and/or starting methods. Be sure to follow the relevant connection diagram, which is usually affixed to the motor or contained in its manual. If the diagram is lost, damaged, or ignored, you could find yourself dealing with a costly rewind.

The tips in this article apply to connections commonly encountered on machines with one speed at power frequency. If the external connection information isn’t available, ask your local service center for assistance, especially if several lead tags are missing or there are multiple nameplate speed ratings at power frequency. The service center can also help with unconventional numbering or cross-referencing IEC and NEMA numbering.

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Tom Bishop, P.E.
EASA Senior Technical Support Specialist

When the number of coils per group is the same throughout a three-phase lap winding, the grouping sequence is simply that number of coils repeated three (since it is three-phase) times the number of poles. For example, a 48-slot 4-pole winding has 12 groups of 4 coils.

The formula used to determine the average number of coils per group is:
Coils per group = slots divided by groups.

We don’t advocate using full slot coils with a lap winding; thus, the total number of coils is equal to the number of slots. The number of groups in an alternating pole winding is equal to the number of phases times the number of poles. In many cases, there are windings that have unequal coils per group, such as a 36-slot 8-pole winding, which has 24 groups with an average of 1.5 (36/24) coils per group.

Mike Howell
EASA Technical Support Specialist

The October 2012 Currents article titled "How to properly test AC stator and wound rotor windings" provides a thorough explanation on the proper application of insulation resistance, winding resistance, surge testing and high potential testing for stators and wound rotors. The article emphasizes that NEMA MG 1-2011 specifies AC and DC high potential (hi-pot) test levels for new windings and does not recommend repeated application of the high potential test. This is reinforced in EASA's Recommended Practice for the Repair of Rotating Electrical Apparatus (ANSI/EASA AR100-2010) which calls for reduced voltage levels for repeated tests should they be required. This article is intended to provide additional information on the high potential test performed on new windings. Specifically, it addresses the advantages of AC high potential testing for new, form coil stator windings. Topics discussed include: A real-world example Destructive test? Sizing the AC test set Example calculation Bibliographic references to additional reference materials

Presented by Chuck Yung
EASA Senior Technical Support Specialist

This webinar recording shares some of the “best practice” rewind methods used by (and learned from) EASA service centers around the world: connection recognition, best insulating materials, wire choices and tips to save time and effort. Topics covered include:

• Slot liner, separators and phase insulation
• Managing voltage stresses
• Making the connection: solder, crimp fittings or silphos
• Lacing tips
• Testing the completed winding

This webinar is intended for experienced and prospective winders, and those who supervise winders.

Mike Howell
EASA Technical Support Specialist

The March 2013 Currents article titled “Stator I²R loss: considerations for rewinds and redesigns” describes the stator I²R loss, its calculation and how to control it during rewinds. This follow up will provide a brief review and then explore the additional stator copper losses mentioned in that article.

Cyndi Nyberg
Former EASA Technical Support Specialist

It is very important to take accurate data when you rewind a form-wound motor or generator, especially if the coils will be made by an outside coil manufacturer. There are a couple of variations to the standard coil design that is not common, but that you may come across from time to time.

Brick-type design
A brick-type winding uses two different sizes of wire arranged as shown in Figure 1. Rather than use one large rectangular wire for each turn, this example uses four smaller wires. When more than one conductor is used, each individual conductor must be insulated. However, it is not necessary to insulate every wire to achieve the required separation.

Carlos Ramirez
Especialista de Soporte Técnico de EASA

En este webinario explicaremos como los diferentes cambios efectuados en el bobinado impactan en el desempeño del motor. Si no es realizado de forma correcta, cualquier cambio en el bobinado podría llegar a tener consecuencias negativas. El torque de arranque, la potencia nominal y la eficiencia en general podrían verse afectadas. 

El webinario incluye:  

• Efectos de realizar conexiones Delta o Estrella incorrectas 
• Consecuencias de un error al contar las vueltas (espiras)  
• La fórmula maestra  
• Efectos al usar los dientes abarcados (Span) envés del paso (Pitch) 
• Limitaciones en la secuencia de agrupamiento  
• Conversión de bobinados concéntricos a excéntricos (imbricados)  

 Este webinario está dirigido a bobinadores, supervisores y técnicos de pruebas.   

Mike Howell, PE
EASA Technical Support Specialist 

The insulation system chosen for any rewind should be suitable for the application, the voltage class, and the winding process capability of the service center. In most cases, adherence to “equal to or better than” selection is a good practice.

By Chuck Yung
EASA Senior Technical Support Specialist

Existen beneficios e inconvenientes al usar circuitos en paralelo en un bobinado trifásico. Sea que estemos hablando de un bobinado de alambre redondo o de pletina (solera), algunas de las consideraciones se comparten. Comencemos con lo básico: Entre más alta la potencia y/o más bajo el voltaje nominal, menos vueltas por bobina se utilizan. Debido a que un devanado trifásico tiene grupos por fase y por polo que alternan ABC, ABC, ABC, etc., los puentes entre grupos podrían ser 1-4, 1-7, 1-10, 1-13, etc., o cualquier combinación de ellos, siempre y cuando se conserve la polaridad alternada de los grupos y que las fases no se crucen entre sí.

Presented by Chuck Yung
EASA Senior Technical Support Specialist

This webinar recording discusses the equalized connections found in an increasing number of factory windings, explains why they are used, and addresses whether or not they are needed when converting a concentric winding to a lap winding. Alternatives, such as changing the number of circuits, or the special extra-long jumpers, are also compared.

The webinar recording covers

• Explanation of why machine-wound concentric windings use equalizers
• Effect of unbalanced voltage
• Role of air gap in causing circulating currents
• Labor involved and risk of failures due to increased complexity
• How to properly locate the equalizers

This webinar is useful for engineers, service center managers, mechanics and sales representatives.

Chuck Yung
EASA Senior Technical Support Specialist 

My purpose in writing this article is to explain in layman’s terms what electromechanical professionals refer to as circulating currents, why they exist in three-phase electric motors and to offer practical solutions.

Chuck Yung 
EASA Technical Support Specialist 

There are times when a winding cannot be processed through the burn­out oven, so it must be removed “cold.” The bond strength of most resins is approximately 8-10 psi (55-70 kPa), which means that a fairly large coil might have nearly 3,000 pounds (1350 kg) of bonding force with the slot. 

In those cases, there are some use­ful tips that can be used to reduce the difficulty in removing the coils. Many of the techniques in this article can be adapted for open slot wound rotors and armatures. 

Mike Howell
EASA Technical Support Specialist

Before rewinding a stator, EASA strongly recommends winding data verification. This is a required criterion for rewinds covered under the EASA Accreditation Program audit checklist. With tools like EASA’s AC Motor Verification & Redesign program, this can be done easily within minutes. Additionally, the verification is an EASA member benefit provided at no additional charge by submitting an inquiry to EASA’s technical support staff. We see many cases where failure to invest a few minutes up front costs a service center an additional rewind. There are probably as many cases where service centers identify issues with the as-found winding data before investing time and materials. 

One of the most common winding data changes made by service centers is a wire size change. This is inherent to most redesigns where the number of turns per slot is changed. But it is also routinely done to accommodate a service center’s available inventory. Although this type of change is easily done in EASA’s verification & redesign program, there are various situations that restrict winders to pen and paper changes. And, as processes become more manual, they typically have a higher risk for error. Minimizing the manual calculations associated with this change can increase process efficiency while reducing errors.

Chuck Yung
EASA Senior Technical Support Specialist

When an electric motor is expected to be stored for an appreciable time before it is placed into service, certain steps should be taken to ensure that it will be suitable for operation when it is needed. The practical limitation we need to recognize is that much of what we do when putting a motor into long-term storage has to be undone when the same motor is moved into operation. This article addresses common recommendations for stored motors.

Carlos Ramirez
EASA Technical Support Specialist

¿El motor consume mucha corriente en vacío, aunque los datos del bobinado son correctos? ¿El motor se calienta con carga de forma inusual? Estas son preguntas comunes que pueden ser resueltas verificando la condición del núcleo del estator. En este webinario discutiremos cómo realizar la prueba de lazo en el núcleo de un estator y cómo analizar los resultados obtenidos, proporcionando información sobre los equipos utilizados, consejos para reparar el núcleo del estator y otras pruebas alternativas.

El seminario incluye:

• Teoría de la prueba de lazo (“toroide”)
• Procedimiento de prueba
• Límites aceptables para las pérdidas y las temperaturas en el núcleo
• Equipo asociado
• Consejos para la reparación de núcleos dañados
• Pruebas alternativas

Este webinario es útil para supervisores, bobinadores y personal encargado de realizar las pruebas.

Chuck Yung
EASA Technical Support Specialist

One of the most misunderstood winding connections is the part-winding start. Many customers (and some members) tend to blur the differences between the part-winding-start (PWS) connection and wye-start, delta-run connections.

Let’s review the Wye-Delta first before looking more closely at the part-winding-start connection. The wye-start, delta-run connection is designed to reduce starting current, heating of the windings and rotor, and starting torque. It does this by temporarily connecting the motor for a voltage higher than line voltage.

Two-pole motors present special rewind issues, especially when converting them from concentric to lap windings. The pitch is especially important as certain coil pitches will cause harmonics that have a negative impact on performance. Optimum pitches are often very difficult to wind and shorter pitches result in sacrificed conductor area.

This presentation explores sample redesigns and present some guidelines to assist in deciding between the concentric and lap winding.

Target audience: This webinar will be most useful for service center winders, engineers, supervisors and managers. The content will be beneficial for beginners through highly experienced persons.

Tom Bishop, PE
EASA Senior Technical Support Specialist

One of the most frequent member requests to our technical support group is for conversion of a 3 phase winding from concentric to lap. An excellent alternative to requesting the conversion is to use the EASA AC Motor Verification and Redesign (ACR) program to calculate the changes. In fact, many members have purchased the redesign program and have called us to confirm their conversions as they develop their proficiency and “comfort level” with the program. However, our emphasis here is not to convince you to purchase the ACR program but to cover the important details for a proper concentric to lap winding conversion.

Chuck Yung
EASA Senior Technical Support Specialist

While manufacturers use concentric windings due to their ability to wind the coils directly into a core, many repairers convert them to lap windings to take advantage of the superior MMF (magneto-motive force) curve.

Although the former Tech Note 12 (see page 2-187 of the EASA Technical Manual), and the AC Motor Verification and Redesign Program, Version 4 allow us to convert a concentric winding to a comparable lap winding, there are still some winders using “shortcuts” they have learned over the years.

Chuck Yung
Especialista Sénior de Soporte Técnico de EASA

Mientras los fabricantes usan devanados concéntricos por su capacidad para bobinarlos directamente dentro del núcleo, muchos reparadores los convierten en bobinados excéntricos para aprovechar su curva FMM (fuerza magnetomotriz) superior.

Aunque la antigua Tech Note 12 (vea la página 2-187 del Manual Técnico de EASA), y la versión 4 del AC Motor Verification and Redesign Program, permiten convertir un bobinado concéntrico en un bobinado excéntrico comparable, aún existen algunos bobinadores empleando “atajos” que han aprendido con el tiempo.

Vicki Warren — Iris Power - Toronto, Ontario
Brian F. Moore – Georgia Power - Atlanta, Georgia
Jim Williams – Bradley’s Motors - Corpus Christi, Texas 
Special thanks to Gary Castle at Bradley’s Motors

Traditional tests of insulation resistance, polarization index (IEEE 43) and the controlled DC high voltage test (IEEE 95) have been effective in evaluating certain aspects of global vacuum pressure impregnation (GVPI) stator windings; however, they have not proven adequate for determining whether or not the insulation system is well-consolidated. Recently there has been the development of an IEC standard (IEC 60034-27) that defines the test procedures for performing off-line partial discharge testing as part of quality assurance testing. In addition, globally there has been a move towards using a dielectrics characteristic test, either power factor or dissipation factor, as part of the QA testing for GVPI systems. Partial discharge tests have proven to be effective in locating isolated problems that could lead to failure; whereas, the dielectrics characteristic tests provide a more general condition assessment. Based on experience to date, both are needed to fully evaluate how well the winding is consolidated. 

This paper, presented at the 2014 EASA Convention, describes research done by EASA service shops on the effectiveness and practicality of using offline partial discharge combined with a dielectrics characteristic test to evaluate the consolidation of stator windings in medium voltage machines manufactured by GVPI. Advantages and disadvantages of each test and industrial standards will be described as appropriate.

Presentado por Carlos Ramirez, EASA Technical Support Specialist

La conexión incorrecta de los motores eléctricos es una causa frecuente de fallo y es más común de lo que parece. La falta de información y la mala interpretación de los datos de placa son algunas de sus causas. En este webinario se explican los diferentes tipos de conexiones para los motores eléctricos trifásicos de una o varias velocidades con al menos 6 cables de salida y se comparan las equivalencias NEMA e IEC para el marcado de cables. La información proporcionada también será de gran utilidad para evitar el conexionado incorrecto en los diferentes voltajes. También incluye las conexiones por devanado partido (Part Winding) y como interpretar la información de la conexión de la placa de datos.

El webinar incluye:

• Conexiones Estrella y Delta (“Triángulo”)
• Conexiones para motores de una sola velocidad con al menos 6 cables de salida
• Conexiones para motores de dos velocidades con al menos 6 cables de salida
• Conexiones para Devanado Partido (Part winding)
• Equivalencias NEMA e IEC para el marcado de cables  
• Interpretación de la información de la conexión de la placa de datos

Este webinario es útil para supervisores, personal encargado de realizar pruebas y responsables del centro de servicio.

Anthony Sieracki
Spina Electric Co.

In all aspects of electric motor repair specifications and instructions, we should refer to industry standards, government standards, electrical codes, manufacturing recommendations and of course EASA's Recommended Practice for the Repair of Rotating Electrical Apparatus (ANSI/EASA AR100-2010). All are written to help us perform and accomplish the best repairs possible. This article covers a topic that is often times taken for granted, yet it is key to making sure your repaired motor does not fail prematurely. We will cover the proper methods for making connections on AC electric motors under 600 volts. The most common methods are crimped terminals, multiple bolt connector points and split bolts connectors.

Chuck Yung
Especialista Sénior de Soporte Técnico de EASA

Uno de los temas más debatidos en nuestra industria es la comparación- y los procedimientos- de impregnación por presión y vacío (VPI) versus la inmersión y secado en horno. En este artículo, he ampliado la discusión para incluir bobinas semicuradas (B-stage) y el método de goteo (trickle). Los centros de servicio que cuentan con un tanque de VPI resaltarán rápidamente los muchos beneficios del VPI, como un mejor sellado de los devanados y una mejor transferencia de calor entre los conductores de los bobinados y la carcasa para mejorar la disipación de calor.

Los bobinados de pletina (solera/bobinas formadas) y de alambre redondo tienen dos problemas claramente diferentes. Para las máquinas con bobinas de pletina, la penetración de la resina es la mayor preocupación, lo que le brinda una clara ventaja al proceso VPI. En los bobinados de alambre redondo, la inquietud es la retención de la resina.

Jacob Snyder
Evans Enterprises, Inc.

When a modern temperature controlled (i.e., controlled pyrolysis) burnout oven is not available, the method described here can be used to process aluminum frame motors.

Mike Howell
EASA Technical Support Specialist

Most AC stator windings installed by EASA service centers are balanced, three-phase, two-layer, lap windings. But, what does it mean for such a winding to be balanced? If balanced, the voltages generated in each phase are equal in magnitude and displaced from each other by the same angle (See Figure 1 Balanced). If there is any difference in magnitude or angle displacement, the winding is unbalanced (See Figure 1 Unbalanced). It is well established that unbalanced windings can cause undesirable vibration, electromagnetic noise, and additional conductor heating due to circulating currents.

Chuck Yung
Especialista Sénior de Soporte Técnico de EASA

Con el aumento continuo de los tamaños de los motores CA y la obvia superioridad de los devanados con bobinas preformadas (pletina o solera), un área en la que podemos ayudar a mejorar la confiabilidad de los motores de nuestros clientes es rediseñando estos motores grandes de alambre redondo para que acepten bobinas preformadas. La mayoría de los reparadores estarían de acuerdo en que las máquinas de alambre redondo por arriba de 600 hp (450 kW) deberían rediseñarse con bobinas preformadas. Así mismo, aquellas con tensiones nominales superiores a 2 kV serían más confiables con bobinas de pletina.

Nadie quiere rebobinar un motor con 60 #14 AWG (62- 1.6 mm). Con la abundancia de proveedores especializados en laminaciones de estatores, el costo y la practicidad para convertir motores de alambre redondo a pletina está al alcance de casi todos los centros de servicio. Las laminaciones para reemplazar el núcleo pueden ser troqueladas o cortadas con láser o agua y entregadas en tiempos muy razonables.

Mike Howell
EASA Technical Support Specialist

Una de las actividades a realizar menos populares relacionadas con el tratamiento de los bobinados, es la preparación y la limpieza de los ajustes, agujeros roscados y superficies mecanizadas. Muchos centros de servicio invierten tiempo adicional durante la etapa de preparación para minimizar la etapa de limpieza. El enfoque más común para proteger estas superficies durante el tratamiento del bobinado consiste en utilizar compuestos para enmascarar o aerosoles de liberación de película seca.

Durante el último año, el departamento de soporte técnico de EASA ha recibido una serie de consultas por parte de los miembros buscando recomendaciones para reemplazar el producto “Special Masking Compound” de Famous Lubricants’ (ver Figura 1) que actualmente no se encuentra disponible. Se cree en estos momentos que el fabricante tiene la intención de continuar con la producción en el futuro, aunque el plazo se desconoce. Este problema específico conlleva a una pregunta más general: ¿Cuál es una buena práctica para escoger un producto para enmascarar estas superficies?

Tom Bishop, P.E.
Especialista Sénior de Soporte Técnico de EASA

Cuando se energiza inicialmente un motor de CA a su voltaje nominal (y frecuencia), el motor toma varias veces la corriente nominal hasta que alcanza toda la velocidad de operación. Las consecuencias de la corriente de arranque incluyen: Breve sobrecalentamiento de los bobinados del estator y del rotor, disparos no deseados de los dispositivos de protección y caídas de voltaje en el suministro eléctrico.

De acuerdo con las normas NEMA MG 1 e IEC 60034-1, la corriente a rotor bloqueado es la corriente de estado estable a rotor bloqueado. Sin embargo, según la norma NEMA MG 1 clausula 12.36, al momento de la energización, existe un valor pico de medio ciclo que varía entre 1.8 y 2.8 veces la corriente de estado estable, en función del diseño del motor y del ángulo de conmutación. Un factor relevante en la amplitud de este valor pico instantáneo es el magnetismo residual en los núcleos del rotor y del estator y los ángulos instantáneos de los voltajes de fase aplicados. Aunque la medición de la corriente no sea lo suficientemente rápida para capturar el primer pico de medio ciclo, la Figura 1 ilustra un breve intervalo de tiempo cuando la corriente de arranque es transitoria, antes de alcanzar su estado estable. Esto se puede ver claramente a los 95ms, donde las amplitudes de las corrientes son diferentes en cada fase ya que se encuentran desfasadas entre sí.

Un término que debería evitarse o definirse es “Corriente de Inrush”. Dicha corriente no está definida en las normas NEMA o IEC. Por tanto, en algunos casos, podría significar la corriente a rotor bloqueado en estado estable y en otros podría ser el valor pico instantáneo de la corriente de arranque u otra cosa diferente. Con relación a la corriente a rotor bloqueado en estado estable, la norma NEMA MG 1 clausula 10.37.2 asigna letras para los kVA a rotor bloqueado por caballo de potencia medidos a voltaje y frecuencia nominal, tal como lo muestra la Tabla 1.

Diferencias en la Corriente de Arranque
Para apreciar mejor las diferencias en los dos tipos de corriente de arranque, evaluaremos un motor de 25 hp a 460 voltios con kVA a rotor bloqueado letra G. Incidentalmente, las letras más comunes para motores con potencias mayores o iguales a 10 hp (7.5 kW) son F y G. La Figura 2 muestra la fórmula para calcular la corriente a rotor bloqueado en un motor. Note que la c o rriente nominal del motor no se usa en la fórmula.

Por tal motivo, utilizar guías como: “La corriente a rotor bloqueado es 5 a 8 veces la corriente nominal” en el mejor de los casos es emplear valores estimados y de acuerdo con la fórmula de la Figura 2 no se puede confiar en su exactitud. Cuando la letra kVA (CODE) no se indique en la placa de datos del motor, un mejor enfoque para calcular la corriente a rotor bloqueado, siempre que sea posible, consiste en usar la fórmula de la Figura 2 y un rango guía. Suponga que el motor se encuentra en el centro de servicio. En ese caso, la corriente a rotor bloqueado se puede calcular siguiendo el procedimiento descrito en la sección “Need Locked-Rotor Current Only?” del artículo “Working with Motor Locked-Rotor Test Data” publicado en la revista Currents en febrero del 2016.

La corriente a rotor bloqueado calculada para el motor de 25 hp se encuentra entre 176 y 198 Amperios. Para determinar el valor pico instantáneo de la corriente de arranque potencial, multiplicamos los amperios a rotor bloqueado por 1.8 y por 2.8. El resultado es un rango entre 317 (1.8 x 176) y 554 (2.8 x 198) Amperios.

Dispositivos de protección
Los dispositivos de protección pueden ser fusibles o interruptores, siendo los fusibles sin retardo y los interruptores de disparo instantáneo o de tiempo inverso. La forma en que funcionan estos dispositivos puede explicar porque la protección interrumpe el circuito del motor durante el arranque.

Los fusibles sin retardo tienen una velocidad de respuesta rápida bajo condiciones de sobre corriente, proporcionando a los componentes del circuito una protección muy efectiva contra corto circuitos. Sin embargo, las dañinas sobre cargas temporales o sobre corrientes transitorias pueden causar interrupciones molestas a menos que los fusibles estén sobre dimensionados. Los fusibles son más adecuados para circuitos con cargas inductivas como motores o transformadores, que no estén sometidos a grandes sobre corrientes transitorias y a fuertes sobrecargas temporales. Para las cargas con motores CA, puede ser necesario sobredimensionar un fusible sin retardo al 300 por ciento de la corriente nominal del motor para poder soportar la corriente de arranque. Una mejor alternativa para aplicaciones con motores es la de un fusible con retardo de tiempo.

Los fusibles con retardo de tiempo se pueden dimensionar cerca de la corriente nominal del equipo para proporcionar una protección muy efectiva contra corto circuito y una protección confiable contra sobre cargas en circuitos sujetos a sobre cargas temporales y sobre corrientes transitorias. Dependiendo del tipo de fusible, en cargas con motores CA, un fusible con retardo se puede dimensionar al 125-175 por ciento de la corriente nominal para soportar la corriente de arranque. Comparado con los fusibles instantáneos, los valores de corriente más bajos pueden proporcionar una mejor protección contra corto circuitos con menos corriente máxima instantánea y una reducción potencial en el riesgo de arco eléctrico.

Tipos de Interruptores
Los interruptores de disparo instantáneo operan inmediatamente cuando la corriente del circuito alcanza el valor de disparo ajustado en el dispositivo. Estos interruptores son solo de disparo magnético y también se denominan Interruptores de protección para motores (MCPs por sus siglas en inglés). El circuito magnético del interruptor consiste en un núcleo de hierro con una bobina arrollada en el mismo, lo que crea un electroimán. La corriente de carga pasa a través de la bobina, por lo que el interruptor dispara cuando se presenta un corto circuito. Para soportar la corriente de arranque en cargas con motores CA, puede llegar a ser necesario dimensionar un interruptor de disparo instantáneo al 800 por ciento del valor nominal de corriente alterna (hasta 1700 por ciento en motores de alta eficiencia con Diseño B).

Los interruptores con retardo tienen un mecanismo que permite demorar la función de disparo del dispositivo. El tiempo de retardo necesario disminuye a medida que la corriente aumenta. Los interruptores de tiempo inverso son termo magnéticos. Este tipo de interruptores tienen dos mecanismos de conmutación: Un interruptor bimetálico y un electroimán. La corriente que excede el valor de sobrecarga del interruptor calienta el bimetálico lo suficiente para doblar una barra de disparo. El tiempo requerido por el bimetálico para doblar la barra y disparar el circuito es inversamente proporcional a la corriente. La parte magnética del interruptor funciona igual que en un interruptor de disparo instantáneo. Para soportar la corriente de arranque en cargas con motores CA, puede ser necesario ajustar un interruptor de tiempo inverso al 300 por ciento del valor nominal de corriente alterna.

Otro tipo de dispositivo de protección es un interruptor con unidad de disparo electrónica. El disparo de estos interruptores se pueden configurar con un tiempo de retraso largo, corto, instantáneo o disparo por aterrizamiento (L,S,I,G por sus siglas en inglés). Estas características permiten al usuario diseñar un disparo a medida para la aplicación. Si los interruptores se ajustan correctamente, los beneficios adicionales incluyen la reducción de la energía térmica en eventos potenciales con arco eléctrico y una mejor protección del motor.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

When an AC motor is initially energized at rated voltage (and frequency), the current drawn is many times rated current until the motor attains full operating speed. Consequences of starting current include short time overheating of stator windings and rotors, undesirable operation of overload protective devices and a sag in the supply voltage.

Chuck Yung
EASA Senior Technical Support Specialist

With a steady increase in random wound AC motor sizes and the obvious superiority of the form coil winding, one area where we can help improve customers' motor reliability is by redesigning those large random wound motors to accept form coils. Most repairers would agree that machines rated larger than 600 hp (450 kW) should be designed as form coil machines. Likewise, those rated over 2 kV will be much more reliable as form coil machines.

No one wants to rewind a motor using 60 #14 AWG (62- 1.6 mm) wires in hand. With an abundance of niche suppliers of stator laminations, the cost and practicality of converting a random wound motor to form coil are available to nearly all service centers. Replacement laminations can be punched, laser-cut or water-cut, and supplied with very reasonable delivery times.

Mike Howell
EASA Technical Support Specialist

One of the least popular tasks to perform related to winding treatment processes is preparation and cleanup of fits, threaded holes and machined surfaces. Many service centers invest additional time in the preparation stage so as to minimize the cleanup stage. The most common approach to protecting these surfaces during winding treatment is to utilize masking compounds or dry release sprays.

In the last year, EASA’s technical support staff has received a number of inquiries from members seeking replacement recommendations for Famous Lubricants' “Special Masking Compound” which is currently unavailable. It is believed at this time that the manufacturer intends on continuing production at some point in the future though the time frame is not known. This specific problem leads to a more general question: What is a good practice for choosing a product to mask these surfaces?

Mike Howell
Especialista de Soporte Técnico de EASA

La mayoría de los bobinados de los estatores de CA instalados por los centros de servicio de EASA son imbricados de doble capa, trifásicos y están balanceados. Pero ¿qué significa que estén balanceados? Si los devanados están balanceados, los voltajes generados en cada fase tienen la misma magnitud y el mismo ángulo de desfase (Ver Figura 1 ‒ Balanceado). Si hay alguna diferencia en la magnitud o en el ángulo de desfase, se trata de un devanado desequilibrado (Ver Figura 1 ‒ Desequilibrado). Está bien establecido que los devanados desequilibrados pueden causar vibraciones indeseables, ruido electromagnético y calentamiento adicional de los conductores debido a las corrientes circulantes.

Jacob Snyder
Evans Enterprises, Inc.

El método aquí descrito se puede utilizar para procesar motores con carcasa de aluminio cuando no se tenga un horno moderno de quemado con temperatura controlada (es decir de pirolisis controlada).

Mike Howell
Especialista de Soporte Técnico de EASA

El artículo publicado en marzo del 2013 en la revista Currents de EASA titulado “Stator I²R loss: considerations for rewinds and redesigns” describe las pérdidas I²R del estator, su cálculo y cómo controlarlas durante el rebobinado. Esta continuación, proporcionará una breve revisión y luego explorará las pérdidas adicionales en el cobre del estator mencionadas en ese artículo.

Presented by Mike Howell
EASA Technical Support Specialist

EASA’s Energy Policy states that members will strive to ensure that the methods, techniques and materials they use to service and rebuild rotating electrical machines will maintain or improve their energy efficiency, whenever possible. Controlling stator copper losses during rewinds is a significant part of that effort. This webinar recording looks at several aspects of winding design to prevent increased temperature rise and decreased efficiency.

• I²R losses and conductor area / length
• Eddy current losses and laminated conductors
• Circulating current losses and transposed conductors

This webinar recording will benefit service center managers, supervisors and technicians responsible for rewinds.

Carlos Ramirez
Especialista de Soporte Técnico de EASA

¿Recibió un motor antiguo bobinado con alambre de aluminio? Este webinario explicará como realizar la conversión adecuada de alambre de aluminio a alambre de cobre en máquinas de CA y CC, incluyendo ejemplos para el rebobinado de estatores y campos shunt.  Los temas cubiertos son:

• Sección de los alambre de cobre y aluminio 
• Alambres AWG y métricos 
• Devanados de motores de CA 
• Bobinas de campos shunt
• Ejemplos 

Este webinario está enfocado a bobinadores, supervisores e ingenieros.

Disclaimer: All video captions and translations are AI-generated.
EASA is not responsible for any inaccuracies that may occur.

Want to test your knowledge after watching the webinar?

TAKE THE QUIZ

Tom Bishop, PE
Especialista Sénior de Soporte Técnico de EASA

Una de las solicitudes más frecuentes a nuestro grupo de soporte técnico es la conversión de un devanado trifásico de concéntrico a imbricado (excéntrico). Una excelente alternativa para dicha conversión es utilizar el programa EASA AC Motor Verification and Redesign (ACR). De hecho, muchos miembros compraron el programa de rediseño y nos han llamado para confirmar sus rediseños a medida que desarrollan su competencia y su "nivel de comodidad" con el programa. Sin embargo, nuestro énfasis aquí no es convencerlo de que compre el programa ACR, sino cubrir los detalles importantes para rediseñar el devanado concéntrico a imbricado.

This webinar teaches:

• How to repair damaged stator cores and how to know when a restack is necessary.
• There are often cases when repairs can be accomplished without a labor intensive restack.
• When a restack is required, there are pitfalls to watch out for to avoid problems with geometry, vibration and core losses.

Target audience: This presentation is useful to the supervisor, winder and sales personnel who interact with the end user.

Chuck Yung
EASA Senior Technical Support Specialist

When a core loss test reveals localized hot spots, or visual inspection identifies physical damage, the ability to repair the damage in a cost-effective manner means the difference between repair or replacement.

Topics covered in this recording include:   

• What core loss flux level is correct?
• Clearing small localized hot spots
• What is the best way to clear surface shorting?
• Grinding versus spreading the laminations
• “Watt knocking” or physically manipulating the core
• Debunking the rusting myth — Coreplate, class F red insulator, waterglass
• Restack: complete or partial — Does lamination grade really matter? 

This presentation is intended for owner-managers, shop supervisors, machinists, service center technicians, and safety directors.

Chuck Yung
Especiaslista Sénior de Soporte Técnico de EASA 

Mi propósito al escribir este artículo es explicar en términos sencillos a qué se refieren los profesionales de la electromecánica como corrientes circulantes, por qué existen en los motores eléctricos trifásicos y ofrecer soluciones prácticas.

David Sattler
L&S Electric, Inc.

Unless great care is taken, pulling magnet wire from a motor stator often bends or splays the lamination’s end teeth. Bent teeth, or teeth that have been splayed outward at the ends of the core stack, will likely compromise the quality of the repair job. Studies¹ show that motor efficiency may be reduced by splaying end teeth. Even if that reduction in efficiency is slight, any reduction in efficiency results in higher costs and wasted energy.

Even though these performance reductions are seldom noticed by customers, our goal in motor repair is always to produce the highest quality rewind possible. Therefore, we have designed and implemented the use of disc clamps to hold the stator tooth tips in place while pulling magnet wire from the slots. The clamping fixtures described in the photos have helped ensure that we avoid damaging the stator teeth during the stripping process.

Chuck Yung
EASA Senior Technical Support Specialist 

Historically, one emergency repair used for large, low-speed machines (motors as well as generators) was to remove the damaged/failed coils from the circuit. Cutting out a single damaged coil permitted the machine to be quickly returned to service with minimal disruption.

As long as some basic principles are followed, this method can be safely used. It is still popular with operators of large, low-speed synchronous ma­chines. One common application for such machines is hydro power stations. There are many old hydro generators operating with a dozen or more coils bypassed. The underlying goal when cutting out coils is to minimize the negative side effects and keep coil groups intact for future removal.

WARNING: This procedure is not recommended for 2-pole machines. The odds of success are slim.

Chuck Yung
EASA Senior Technical Support Specialist

Flooding in the aftermath of tropical storms (hurricanes, monsoons and cyclones) with heavy rainfall will often shut down hundreds of plants along the Gulf Coast from Florida to Texas and other places around the world.

To get them up and running again, maintenance departments and motor repairers face the daunting task of cleaning muck and moisture from many thousands of electric motors and generators. See Figure 1. The process in such situations can take weeks, if not months, and requires special clean-up procedures for motors contaminated by saltwater.

Although the problems are huge, affected plants can get back in production more quickly by working closely with service center professionals and following a few tips that will make the cleanup more manageable. These include prioritizing motors and generators for repair or replacement, storing contaminated machines properly, and using proven methods to flush away saltwater contamination. Constructing temporary ovens on site or at the service center can also add capacity for drying the insulation systems of flooded motors.

Chuck Yung
EASA Technical Support Specialist

Sometimes when redesigning a motor, the desired speed requires more poles than are possible for the number of stator slots. Or, a motor arrives in the service center with a nameplate speed that does not seem to be compatible with the number of stator slots (e.g.,18 poles with 36 slots). In both cases, the answer may be a part-salient, part-consequent winding.

To understand how this winding works, let’s compare it to ‘normal’ winding designs. One winder’s trick for verifying the integrity of a connection diagram is to trace through each phase and “arrow-diagram” the groups. For a salient- pole winding, the polarities alternate with each physical group (Figure 1). With a consequent- pole connection, all the arrows point the same direction (Figure 2).

Tom Bishop, PE
Especialista Sénior de Soporte Técnico de EASA 

Con la llegada de los variadores de frecuencia electrónicos (VFD) de estado sólido a fines de la década de 1980, se descubrió que los bobinados de los motores que funcionaban con VFDs fallaban con más frecuencia que al estar alimentados con la energía convencional (onda sinusoidal). A principios de siglo, los fabricantes de motores habían comprendido mejor cómo los VFD afectaban los devanados del motor, y al igual que los proveedores de materiales electro aislantes habían desarrollado materiales y métodos para mejorar la confiabilidad de los devanados de los motores alimentados con VFDs. El término general para los devanados es "inverter duty (a prueba de inversor)". En este artículo, describiremos los materiales y métodos asociados con los devanados inverter duty. 

Alambre magneto
Antes de que se desarrollara el alambre “spike-resistant (resistente a picos)” a finales de la década de los 90s, una práctica común para bobinar los motores que funcionaban con VFDs consistía en utilizar alambre con un aislamiento más grueso a base de poliéster y algunos de ellos utilizaban películas de aislamiento triples o cuádruples. Estos alambres son muy efectivos cuando se les somete a voltajes de onda sinusoidal o voltajes transitorios intermitentes. Los alambres con aislamiento para trabajo pesado (heavy duty) son efectivos contra el efecto corona (Figura 1) porque la distancia entre los conductores reales es mayor con el aislamiento agregado. Esta mayor separación entre los conductores individuales obliga a que cualquier voltaje que se presente entre los conductores sea menor. Sin embargo, cuando la forma de onda del VFD somete a esfuerzos los alambres, la rigidez dieléctrica de los alambres con aislamiento para trabajo pesado, no es tan efectiva. Los alambres magneto modernos utilizados para motores con inversores tienen mayor capacidad dieléctrica con una vida útil más significativa (Figura 2). También pueden soportar picos de voltaje mejor que el alambre con aislamiento para trabajo pesado, pero con la misma estructura que el alambre magneto estándar. La Figura 3 ilustra el impacto en la vida útil del alambre magneto a medida que aumenta la frecuencia de conmutación de un variador. La vida del alambre con aislamiento de trabajo pesado se ve muy afectada, mientras que la del alambre inverter duty no se acorta por la frecuencia de conmutación. 

Usar alambres con mayor diámetro aumentará el voltaje donde comienza a producirse el efecto corona. Por eso, al rebobinar motores inverter duty puede ser importante utilizar la menor cantidad de alambres más gruesos disponibles. Al contrario, los alambres más delgados tienen menos pérdidas por efecto superficial a frecuencias más altas, como la frecuencia portadora de un variador. El efecto superficial hace que la corriente en un conductor redondo esté cerca de la superficie, y la frecuencia portadora es la velocidad a la que el voltaje de CC se "corta" en segmentos para simular la potencia de una onda sinusoidal. Si la frecuencia portadora es alta, por ejemplo, 12 kHz o más, utilice alambres con diámetros más pequeños si es posible; de lo contrario, considere utilizar alambres más gruesos. 

Llenado de ranura y sistema de aislamiento
Incluso el mejor sistema de aislamiento eventualmente comenzará a fallar, especialmente con el uso de un VFD. Para mayor resistencia eléctrica y mecánica, un diseño típico inverter duty maximizará el llenado de la ranura. Esto aumenta la eficiencia y permite que el motor funcione más frío, y también ayuda a evitar el movimiento de las bobinas que puede romper el aislamiento. Es una buena práctica utilizar amarres en al menos cada 3.ª o 4.ª cabezas de bobina, en el lado conexión y lado opuesto conexión para sujetar aún más el devanado.

Como lo ilustra la Figura 4, el fallo más común de los devanados que funcionan con VFDs ocurre en las primera(s) vuelta(s) conectada(s) al cable de salida, por lo que como protección eléctrica adicional algunos bobinados la primera vuelta de las bobina(s) conectada(s) al cable de salida pueden estar aislada(spaguetti). El aislamiento entre fases está diseñado para separar las bobinas de las diferentes fases. La mayor parte de la magnitud de los picos de voltaje vistos por el devanado se concentra en las bobinas conectadas a los cables de salida. Las vueltas inicial y final de una bobina de alambre redondo pueden estar en contacto y se puede presentar un pico de voltaje entre esos dos alambres adyacentes, así como a través de las bobinas. Debido a que los picos de voltaje pueden alcanzar los 2000 voltios o más, también se debe usar aislamiento de ranura adicional para el voltaje más alto, siempre que no sea necesario reducir la sección del alambre para poder insertar el bobinado en las ranuras. Maximice el aislamiento y utilice separadores en las ranuras y vueltas finales. Un motor que funciona con un VFD normalmente se calienta más que el mismo motor alimentado con una onda sinusoidal. Si la temperatura del devanado es 10°C más alta, la vida térmica útil del aislamiento se reduce a la mitad. El aislamiento Clase H (180 °C) tiene una clasificación de temperatura más alta que los devanados Clase B o F (130 °C o 155 °C), por lo que se puede extender la vida útil del devanado. Cuando el motor funciona a una velocidad inferior a la nominal o base, la disminución del flujo de aire hará que el devanado del motor se caliente más. Por esta razón, es ventajoso un sistema de aislamiento Clase H (180°C). 

Impregnación y barniz
Se debe utilizar un proceso de doble inmersión y horneado. Si está disponible, una mejor alternativa sería sumergir y hornear(dip & bake)seguido de impregnación por presión y vacío (VPI) y luego secar. Asegúrese de seguir las instrucciones del fabricante del barniz/resina en cuanto a la temperatura de precalentamiento del bobinado como támbién la temperatura y el tiempo de curado. Tenga en cuenta que el tiempo de curado no comienza hasta que el devanado se haya calentado a la temperatura mínima de curado recomendada para el barniz/resina. Precaución: La mayoría de los alambres magneto tienen una capa lubricante que se utiliza para facilitar su fabricación. El proceso de precalentamiento del devanado tiene dos propósitos: Primero, evaporar el lubricante del alambre, lo que luego permite que el barniz/resina se adhiera al conductor y el segundo es que ayude a aliviar las tensiones residuales en la película aislante del alambre para que este no se agriete (crazing). 

Técnica de bobinado inverter
Cuando se fabrica o rebobina un motor que funciona con un VFD, se debe tener mucho cuidado al insertar las bobinas en las ranuras para evitar que la película aislante del alambre no se raye ni se pele. Es una buena práctica utilizar papel mylar en las ranuras para ayudar a la inserción de las bobinas y protegerlas de daños. Algunos fabricantes utilizan una técnica de bobinado que hace que quede menos "aleatorio" al alinear el alambre en las ranuras empleando un espaciado más ordenado de las vueltas. La idea es mantener el principio y el final de las bobinas lo más alejados posible entre sí para reducir la magnitud del voltaje entre los conductores adyacentes. Las máquinas bobinadoras semiautomáticas utilizadas en los centros de servicio se acercan a este nivel de espaciado ordenado de las vueltas. 

Especificaciones para bobinados inverter duty
La siguiente es una especificación guía para un sistema inverter duty. 

General 

• Aislamiento Clase H o superior 

Alambre magneto 

• Inverter duty 

Sección del conductor 

• Conserve o aumente la sección transversal 
• Conserve o aumente el número de alambres (reduce las pérdidas por corrientes parásitas 

Aislamiento 

• Separadores entre fases 
• Como mínimo aislamiento a tierra de 0.015” (0.38 mm) 
• Arriba de 80 voltios por bobina instale separadores en la mitad de cada grupo 

Atado y soporte 

• Amarre al menos cada tercera o cuarta bobina 
• Encinte las cabezas con un mínimo de 3 medias capas de de vidrio virgen [1 pulgada (25 mm)] a partir de 1 pulgada (25 mm) del núcleo hasta 1 pulgada de las puntas 

Impregnación 

• Pre caliente el barníz de acuerdo con las instrucciones del fabricante 
• Sumerja y cure(dip & bake) dos veces 
• Cure durante el mayor tiempo recomendado por el fabricante 
• Note que el tiempo de curado no comienza hasta que el devanado se haya calentado a la temperatura de curado

Mike Howell, PE
Especialista de Soporte Técnico de EASA

Siempre que sea posible, EASA recomienda usar el cable de salida especificado por el fabricante original del equipo. Si no está disponible, la sección 6 del Manual técnico de EASA proporciona orientación al respecto y hay una calculadora en línea disponible en go.easa.com/calculators para determinar el tamaño mínimo recomendado según la clasificación de temperatura, la corriente esperada, la cantidad de cables y el tipo de conexión. Este artículo describirá la función de la calculadora. Es importante tener en cuenta que no existe una respuesta correcta en este proceso cuando se desconoce la información original. Al seleccionar un cable conductor, se deben considerar los siguientes aspectos

Chuck Yung
Especialista Senior de Soporte Técnico de EASA

Algunas veces cuando rediseñamos un motor, la velocidad requerida, necesita  más polos de los que son posibles de obtener con el número de ranuras del estator. O el motor ingresa en el centro de servicios con una placa que indica una velocidad que no parece compatible con el número de ranuras del estator (ej. 18 polos para 36 ranuras). En ambos casos, la respuesta puede ser un bobinado híbrido (o que funciona parcialmente con polos alternos y parcialmente con polos consecuentes).

Richard Huber, P. Eng.
Richard Huber Engineering. Ltd
North Vancouver, BC
Canadá
Miembro del Comité Técnico de Servicios

Introducción
Recientemente trabajé en una máquina nueva de 13.8 kV de tensión nominal, enfriada por aire,   que producía grandes cantidades de ozono y tenía grandes niveles de descargas parciales. El problema básico con  los bobinados de esta máquina era  el espacio incorrecto entre las cabezas de bobina y los cables  de salida principales. También había poco espacio entre las series y las conexiones entre los  grupos del bobinado.

Chuck Yung 
EASA Technical Support Specialist 

In the world of three-phase electric motors, one area which seems to cause great confusion is the use of electric motors which are rated for more than one voltage. Especially today, with so much international commerce, it is understandable that different meanings might be assumed for this simple term. 

Those readers in the U.S. are ac­customed to “dual-voltage” 230/460v ratings. The 1:2 ratio lends itself to 9-lead windings, with connection combinations such as 1- and 2-circuit wye, 2 and 4-delta, 3 and 6-wye, etc. The common factor is that the circuits and the possible operating voltages have the same 1:2 ratio.

Gene Vogel
EASA Pump & Vibration Specialist

The EASA AC Motor Verification & Redesign - Version 4 software (ACR-MotorDb) is a powerful tool for service centers providing the capability to meet their customer’s needs for AC stator and wound rotor redesigns. In most cases, the data from the existing winding is recorded when that winding is removed from the core. But occasions arise where that original data is not available; it may have been recorded incorrectly or a different service center may have stripped the core but not completed the repair. In those cases, the service center must come up with a new “bare core” winding design. ACR-MotorDb has some specific features to address this need.

LEARN MORE ABOUT THE SOFTWARE

HOW TO CALCULATE A WINDING FROM A BARE CORE

The MotorDb segment of the program is the EASA Winding Database compiled over decades from winding data submitted by EASA members. With over 300,000 winding records, it is likely that windings similar to the original winding are available in MotorDb. By simply searching the database for the core dimension criteria, a list of prospective matching windings is returned. A winding from the database does not have to match the original motor nameplate exactly to be used as basis for the bare core design. When a matching winding is selected, that data can be automatically transferred to the Redesign program where modifications needed to match the desired criteria can be adjusted. The process is smooth, effortless and accommodates most 3 phase induction motor windings.

The first step is to display a list of prospective windings that closely match the bare core dimension criteria. Enter the core length, bore diameter, number of slots and poles into the MotorDb search dialog box. As an example, we will search for a 12” core length, 14” bore diameter and 72 slots for a 125 HP, 6 pole Marathon motor. Initially enter only the core dimensions, number of slots and number of poles (Figure 1); the Get Count feature will quickly return the number of matching records. If the result is about 50 or fewer motors, click OK to retrieve those records into a spreadsheet format where the records can be sorted by columns and reviewed. If the Record Count is too large, enter additional criteria to narrow the search. For our example, 44 records were found, and the resulting spreadsheet is illustrated in Figure 2.

The spreadsheet can be sorted by columns to easily review the data. It is useful to sort by the AirDensity (AGD) and Power (Pwr) columns to assess if the bare core is a good candidate for the desired resulting winding. If there are several windings with the desired power rating and the AGD is within acceptable limits, there is assurance the redesign will be successful. For our example, there are 16 windings rated at 125 HP and 10 of them are Marathon motors. So, in this example, it is likely original factory data is available. Of course, that is not always the case. Suppose our bare core is a Siemens motor, which is not listed. We can still select a different manufacturer winding as the basis for our bare cored design. Select one of the windings from the spreadsheet that matches the desired nameplate data as closely as possible. The full winding data will be displayed in an editor (Figure 3).

This original data record was in the database so no redesign was necessary; the bare core can be wound directly from the database record. Such is not always the case, and the EASA software has a function in MotorDb to transfer data from a MotorDb winding record to ACR for redesign. The Send to ACR function in MotorDb creates a new record in ACR where all the ACR redesign functions are available. Taking our example motor, suppose the desired winding is 575 Volts. MotorDb records are only 230 or 460 Volts.

Figure 4 illustrates a MotorDb record sent to ACR and the Volts redesigned from 460 Volts to 575 Volts. The winding is redesigned for 575 Volts and the connection was changed from 6Y to 3Y to keep the Volts per Coil within acceptable limits (Figure 5).

The combination of the EASA Winding Database and the Verification and Redesign program is an easy-to-use solution when presented with bare core winding challenges. For complete step-by-step instructions on bare core redesign, view the tutorial video How to calculate a winding from a bare core available at go.easa.com/wbc.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

Determining the source of noise in an electric motor is often more challenging than correcting it. A methodical investigative approach, however, can narrow the possibilities and make it easier to resolve the issue—with one caveat. If the noise is due to something in the motor design (e.g., a manufacturing defect or anomaly), a solution may be impossible or impractical. With that in mind, let’s review the primary sources of noise in electric motors—magnetic, mechanical, and windage—as well as their causes and ways to reduce or eliminate them.

Areas examined in this article include:

• Magnetic noise
  • Slip noise
  • Skewing
  • Unequal air gap
• Mechanical noise
  • Loose stator core
  • Bearings
  • Airborne noise
• Windage noise

READ THE FULL ARTICLE

Cyndi Nyberg Esau
Former EASA Technical Support Specialist

Most rotor, stator and armature laminations used in industrial electric motors and generators are made from non-oriented, cold-rolled electrical steels. However, there is a lot of variation in the specific properties in these electrical steels. This article will discuss properties of electrical steels, as well as the methods used to optimize the magnetic characteristics of the laminations, specifically through the annealing process.

Richard Huber, P. Eng. 
Richard Huber Engineering, Ltd. 
North Vancouver, BC
Canada 
Technical Services Committee Member

Introduction
Recently I worked on new air-cooled machines rated at 13.8 kV that generated large quantities of ozone and had very high partial discharge levels. The basic problem with the windings in these machines was incorrect spacing of the end turns and the main leads. There was also a lack of space between the series and group connections in the windings.

Spacing requirements
The spacing suggested here is a guide only and original equipment manufacturers (OEMs) and service centers may have their own guide for such values. It should be noted, how­ever, that if spacing is much reduced from that suggested here, there is a real risk that partial discharge activity will develop in the winding.

Global vacuum pressure impregnation is the most common insulation system processing method utilized for form wound stators today. A successful VPI depends on several variables including materials, methods and maintenance. This recording will provide information to assist the service center with ensuring success with form wound VPI projects.

Target audience: This recording will be most useful for service center winders, engineers, supervisors and managers. The content will be beneficial for beginners through highly-experienced persons.

Kent Henry 
EASA Technical Support Specialist 

When taking winding data, equal­izer connections can be mistaken for wye points. You may wonder what purpose equalizer connections serve and whether they can just be elimi­nated to simplify the repair process. Before discussing equalizers, we will explore the factors that lead to a need for equalizers. 

A magnetic unbalance within a motor or generator can be a very seri­ous problem. The magnetic balance Stator relies on a marriage of electrical and mechanical elements. When either of these electro-mechani­cal elements changes, it may create a magnetic unbalance. 

Mike Howell, PE
Especialista de Soporte Técnico de EASA 

El sistema de aislamiento escogido para cualquier rebobinado debe ser el adecuado para la aplicación, el voltaje y la capacidad del proceso de rebobinado del centro de servicio. En la mayoría de los casos, seleccionar una opción "igual o mejor" es una buena práctica.

This presentation focuses on the steps to take before rewinding to avoid the undesirable situation of high no-load motor amps after the rewind.

The presentation covers the following steps that should be performed on every AC stator rewind:

• Inspect the stator bore and rotor outside diameter for evidence of machining or damage
• Record the original winding data exactly as found
• Verify the winding data
• Test the stator core before and after rewinding removal

This presentation is most useful for service center mechanics and winders with any level of experience, and service center supervisors and managers.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

Determining the source of noise in a motor is often much more challenging than correcting it. However, a methodical approach to investigating the noise can narrow down the possible causes and therefore make it easier to resolve the noise issue. In this session we will address the causes and characteristics of the primary sources of noise in AC motors. Specific topics to be addressed:  

• Magnetic noise (aka “electromagnetic noise” or “electrical noise”) 
• Mechanical noise 
• Windage noise 
• Guidance for reducing or eliminating the intensity of these noise sources

This webinar recording is intended for mechanics, supervisors and testing technicians.

Tom Bishop, P.E.
Especialista Sénior de Soporte Técnico de EASA

Cuando el número de bobinas por grupo es el mismo a lo largo de un devanado trifásico excéntrico (imbricado), la secuencia de agrupación es simplemente ese número de bobinas repetido tres veces multiplicado por el número de polos (ya que es trifásico). Por ejemplo, un devanado de 4 polos y 48 ranuras tiene 12 grupos de 4 bobinas.

La fórmula utilizada para determinar el número promedio de bobinas por grupo es: Bobinas por grupo = Ranuras divididas por grupos. Ya que no recomendamos el uso de bobinas a ranura llena en bobinados imbricados, el número de bobinas es igual al número de ranuras. El número de grupos en un devanado de polos alternos es igual al número de fases multiplicado por el número de polos. En muchos casos, existen devanados que tienen bobinas por grupo desiguales, como un bobinado de 8 polos de 36 ranuras, que tiene 24 grupos con un promedio de 1,5 (36/24) bobinas por grupo.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

The most frequent concern about a motor with high current is high no-load current with a 3-phase motor. That topic has been addressed in three prior Currents articles beginning with “Test Run Tips: Common Causes For High No-Load Current On Rewound Motors” (June 2002); “Avoiding High No-Load Amps On Rewound Motors” (February 2004); and “Taming Those Misbehaving Motors” (December 2009). This article will cover the broad topic of high no-load current with 3-phase motors and the issue of high current with load with 3-phase motors. Also considered will be cases of lower than expected no-load current.

Presented by Tom Bishop, PE
EASA Senior Technical Support Specialist

This recording covers the broad topic of high no-load current with 3-phase motors and the issue of high current with load with 3- phase motors. Also covered are the cases of lower than expected no-load current. 

Primary topics are:

• High no-load current – motor not rewound
• High no-load current – rewound motor
• High current with load

This presentation is intended for mechanical technicians, winders, supervisors, engineers and managers. 

Mike Howell
EASA Technical Support Specialist

Los bobinados fraccionarios concentrados, en inglés Fractional-Slot Concentrated Windings (FSCW), han sido empleados durante décadas, principalmente en máquinas pequeñas. Sin embargo, el avance continuo en la electrónica de potencia junto con la necesidad de tener máquinas más eficientes y con mayor densidad de potencia está aumentando el uso de este tipo de bobinados en máquinas de diferentes tipos y tamaños.

Gene Vogel
Especialista de Bombas y Vibraciones de EASA

El programa de Verificación & Rediseño de Motores C.A. de EASA- Versión 4, lanzado en mayo, es una actualización fácil de usar de la Versión 3 y de la Versión 2 más antigua del programa. El diseño y las funciones del nuevo programa están adaptados de la Versión 2 más antigua, por lo que los usuarios se podrán mover a través de él en una curva de aprendizaje fácil. En la Versión 4 del programa también existen funciones que ayudarán al reconocimiento de aspectos del proceso de rediseño, que pueden haber sido pasados por alto. Si su programa está operativo, continúe leyendo para aprender acerca de estas funciones.

Al ingresar un nuevo diseño, la Versión 4 del programa proporciona un campo para identificar el cliente por su nombre o por un número. Un “Nuevo” botón permite al usuario introducir los datos del cliente sobre la marcha, pero se considera práctico que haya ingresado antes los datos de sus clientes habituales. Los elementos del menú Database -> Customer, le permiten ganar tiempo al gestionar su lista de Clientes. Por tanto los clientes se pueden ingresar fácilmente por nombre o ser seccionados desde una lista de nombres desplegable.

Utilice el teclado o el ratón
Al ingresar los Datos del Bobinado Original, algunos usuarios prefieren el teclado mientras otros se sienten más cómodos moviéndose por los campos con el ratón. El programa soporta ambos métodos. Con el tabulador se accede a los campos de los datos (shift-Tab para devolverse). Cualquier campo que cuente con flechas para ajuste de valores se puede configurar con las teclas situadas Arriba-Abajo del cursor o se pueden escribir los valores. El botón de opción y las casillas de verificación se pueden ajustar con la Barra Espaciadora. Con el ratón, solo manipule las flechas para ajustar los valores o haga clic en cualquier campo y escriba el valor. También puede situar el cursor encima de un ícono para ver su descripción (ver Figura 1).

Una vez ingresados los datos, al oprimir el botón Calculate (Calcular), se visualiza una tabla de selección con las posibles opciones de rediseño. Dicha Tabla es flexible y se puede ampliar para incluir más columnas (clic derecho). Estas columnas se pueden ordenar haciendo clic sobre sus encabezados. Hay varias opciones disponibles para ver los diferentes números de circuitos (ver Figura 2).  En la ayuda (HELP) del programa o en la página web de EASA: http: //www.easa.com/resources/software/ac-motor-verification-redesign, existe un buen video tutorial sobre cómo utilizar esta Tabla de selección. A menudo, la elección de los parámetros de rediseño implica comparar diferentes opciones. El programa permite al usuario escoger y ver múltiples opciones desde la tabla de selección – solo sostenga la tecla CTRL y haga clic en las filas deseadas. Las diferentes opciones de rediseño se pueden seleccionar con las pestañas situadas en la parte inferior de la pantalla. Para comparar los parámetros críticos en forma de columnas, seleccione en el menú Rediseño la opción Side-by-Side.

Además, justo encima del botón Calculate situado en la página del motor Original, existen los botones denominados Round Turns (Redondear Espiras) que sirven para ajustar el número de espiras: Integer (entero) o Half-Turn (promedio). Al escoger la opción Half (promedio) se incluyen esas opciones en la tabla de selección. Por ejemplo, si una máquina de 4 polos, 48 ranuras, 12 grupos, 4 bobinas por grupo tiene 7.5 espiras (half turns), esta podría ser rebobinada con 7-7-8-8 espiras y paso 1-11 ó utilizando 7-8-7-8 espiras y paso 1-12.

Datos de rebobinado integrados
Algunas veces, cuando los datos no están disponibles o son sospechosos, se requieren los datos de rebobinado para un “núcleo sin alambre”, Los Datos de Rebobinado de EASA- Versión 4, integrados al programa de rediseño, pueden ayudar en estos casos. Al encontrar en la base de datos un motor con parámetros similares, estos datos se pueden usar como base para diseñar el bobinado del “núcleo sin alambre”. Al localizar en la base de datos un motor adecuado, la opción del menú MotorDb -> Send to ACR, convierte los datos de ese motor en un nuevo caso de rediseño y no es necesario volver a escribir los datos. La página web de EASA cuenta con un video tutorial de este sencillo proceso.

Es común experimentar con diferentes parámetros del bobinado como el número de espiras, el paso y las conexiones. La mejor alternativa consiste en utilizar el botón Calculate para desplegar la tabla de selección y escoger la opción deseada; el resultado del rediseño se abrirá en una nueva pestaña como Rewind option # (ver Figura 3). La opción de Rebobinado Manual proporciona un cuadro de diálogo para ajustar esos valores. Sea precavido a la hora de rediseñar bobinados manualmente, ya que es posible ingresar valores “imposibles” como 4 circuitos con 6 polos, lo que el programa no permitiría normalmente.

Ajustes del tamaño del alambre
Uno de los ajustes más comunes es el del tamaño del alambre. La ranura de un bobinado puede estar muy floja o muy apretada, o el tamaño del alambre escogido podría no estar disponible en la cantidad suficiente de hilos en paralelo. El programa proporciona una calculadora poderosa para escoger la combinación entre dos tamaños (calibres) de alambre incluyendo el cálculo del porcentaje del cambio. Los valores de los nuevos tamaños de alambre calculados se ingresan automáticamente en el rediseño con los valores nuevos de CM/A (A/mm2) (ver Figura 4).

En el programa de rediseño existe un buen número de opciones convenientes. Las dimensiones del núcleo se pueden escribir en fracciones como 3 11/16, o emplear matemática simple para convertir los valores; 3.5 x 25.4 (conversión de pulgadas a mm). En el menú de Tools (Herramientas) existe una opción disponible para Definir los Tamaños de Alambre Disponibles de tal forma que solo aparecerán los alambres que se encuentren en inventario. Este listado trabaja en conjunto con la casilla de verificación de Solo Tamaños de Alambre Disponibles (Wires Sizes: Only Available), localizada en la página del Motor Original justo arriba del botón Calculate. Existen Valores Pre-establecidos para los datos de los Motores Originales. En el menú Tools seleccione Motor Defaults e ingrese cualquier dato que pueda ser usado con frecuencia. Los datos de entrada comunes son hp – kW y los calibres de los alambres AWG o Métricos. Guarde y cierre el Motor Pre-establecido  y estos valores serán fijados por defecto para nuevos rediseños. El menú Reference tiene un conjunto de calculadoras sencillas y varias tablas comúnmente utilizadas, como los datos del Alambre Magneto Redondo y las Corrientes Trifásicas a Plena Carga. También, en el menú Reference existe una versión en PDF del manual de Rediseño de Motores C.A. que tiene muchas fórmulas básicas usadas para el cálculo de rediseños. Las tablas de conversión de Concéntrico a Imbricado también están ahí, estas son usadas por algunos miembros para revertir una conversión de Imbricado a Concéntrico. Para obtener más información a cerca de la conversión a bobinados concéntricos, contacte al Soporte Técnico de EASA.

Una vez se haya calculado el rediseño de un Motor Original, los datos del Motor Original se aseguran y no pueden ser editados sin borrar los rediseños. Esto es para prevenir que esté presente un rediseño que no coincida con el Motor Original. El menú Editor -> Allow Edits permite la opción de Borrar todos los Rediseños (Delete all Redesigns) o de Clonar el Motor (Clone Motor). Al seleccionar Clone Motor se creará un nuevo Motor Original en los que los datos se pueden editar. Esto es útil en escenarios de “simulación” y para motores con varios bobinados en los cuales la mayoría de los datos físicos de cada bobinado son los mismos.

A lo largo de la búsqueda de datos de motores similares en la base de datos de bobinados y durante el rediseño de múltiples motores, puede haber un número de motores abiertos en cualquier momento. Las pestañas arriba del editor muestran la identificación del motor con un ícono para el tipo de dato. Cuando existen varias pestañas, al hacer clic derecho en cualquiera de ellas, se visualizarán las opciones para cerrar las pestañas que no son necesarias: Cerrar las otras (Close Others), Cerrar las Pestañas a la Izquierda (Close Tabs to the Left), Cerrarlas Todas (Close All).

La ayuda del programa cuenta con información útil para el rediseño de bobinados en las secciones de Concepts y Task. Los vídeos de los tutoriales están disponibles en la web de EASA y se puede acceder a ellos a través de la pantalla de Bienvenida del programa, en el menú Ayuda. Los tutoriales y la Ayuda son recursos útiles cuando surgen preguntas. Por supuesto, el soporte técnico de EASA también puede responder inquietudes relacionadas con las características y funciones del programa o ayudar con cuestiones de rediseño.

Nota del Editor: Los que habían comprado la Versión 3 recibieron automáticamente en mayo la Versión 4. De lo contrario, el programa se puede adquirir utilizando el formulario adjunto. 

Chase Fell
Precision Coil and Rotor

An ideal insulator allows no leakage current to flow. The power factor of an insulator is defined as the cosine of the phase angle between voltage and current. For an ideal insulator, the current leads the voltage by exactly 90 degrees and the power factor for this ideal system would be zero. Coil systems in electric motors and generators have inherent losses causing capacitive and resistive current flow. For this insulation, the power factor cannot be zero. 

The power factor (PF) tip-up test is commonly used as a quality measurement for new coils and windings manufactured for AC motors and generators rated 6 kV and higher. For modern stator winding insulation systems, the power factor and the dielectric dissipation factor are very nearly the same. PF tip-up testing can be useful to verify the quality of the winding manufacturing process, insulation material performance, consolidation of conductors, uniformity of groundwall taping and state of resin curing. Once an insulation system reaches corona inception voltage (CIV), partial discharge (PD) will effectively short out some of the capacitance of the insulation and the power factor will increase. PF testing is applicable to individual vacuum pressure impregnation (VPI) coils and resin-rich coils as well as cured complete windings. Power factor tip-up testing is not applicable for bench testing of green VPI coils or evaluating pre-processed complete VPI windings. 

The power factor tip-up test can be useful in the rewind shop to verify the quality of a newly-installed coil system including the effectiveness of VPI processing. PF testing of in-service windings can set a baseline measurement for maintenance trending. The in-service PF tip-up test can potentially identify groundwall insulation aging since the capacitance between the copper conductor and the core is generally reduced as delamination and/or air pockets become present in the insulation between the coils and the core.

Winding RTDs are resistance-based temperature monitoring devices. Aside from just reporting winding temperature, here are some tips for maximizing the benefit of RTDs. Place six RTDs, spacing them uniformly around the core so there are two per phase. Provide a location map, numbering the RTDs, starting with the number 1 RTD in the 12:00 position. Number the RTDs clockwise facing the connection end.

Knowing where each RTD is located (which phase, as well as the physical location in the stator) provides some powerful diagnostic ability. Possible causes for deviation in temperature are:

• Two RTDs reading high, and both in the same phase: Check for voltage / current unbalance; higher current in one phase causes higher temperature in that phase.
• If the number of circuits is half the number of poles, circulating currents can occur. This situation can be exacerbated by uneven airgap which cause a further temperature increase. The corrective action, in this case, is to use the appropriate extra-long jumpers when connecting the winding.
• Higher temperature indicated in adjacent RTDs may indicate obstructed ventilation. Some possible causes are clogged filters, missing soundproofing, displaced weather-stripping, poorly positioned air baffles, or a missing J-box cover.
• Some manufacturers place all six RTDs across the 10:00 to 2:00 portion of the winding, to report more uniform temperatures. By distributing the RTDs symmetrically around the stator -- instead of just on the top -- the reported apparent temperatures often look alarming. Before returning the motor, let the end-user know where they were originally, and explain that the symmetrical placement will yield more realistic results.

Mike Howell
EASA Technical Support Specialist

Fractional-slot concentrated windings (FSCW) have been used for decades, primarily in small machines. But continued technological advancement in power electronics along with the need for more efficient and power-dense machines is increasing use of FSCW in a variety of machine types and sizes.

The purpose of this guide is to provide repair/rewind practices and tips that will help service center technicians and motor winders maintain or increase the efficiency, reliability and quality of the motors they repair.

Some of the included procedures derive directly from the 2019 and 2003 rewind studies by EASA and AEMT of the impact of repair/rewinding on motor efficiency. Others are based on the findings of an earlier AEMT study [1998] of small/ medium size three-phase induction motors and well-established industry good practices . 

The procedures in this guide cover all three-phase, random-wound induction motors. Much of the guide also applies to form-wound stators of similar sizes. 

(Note: This guide provides many specific procedures and recommendations. Alternative practices may accomplish the same results but must be verified.)

Table of Contents Overview

• Terminology
• Energy losses in induction motors
• Motor repair processes
  • Preliminary inspection
  • Dismantling the motor
  • Removing the old winding and cleaning the core
  • Rewinding the motor
  • Reassembling the motor
  • Confirming the integrity of the repair

El propósito de esta guía es suministrar prácticas y consejos de reparación/rebo­binado que ayudarán a los técnicos y a los bobinadores del centro de servicios a conservar o aumentar la eficiencia, confiabilidad y calidad de los motores que reparan.

Algunos de los procedimientos incluidos derivan directamente de los estudios sobre el impacto de la reparación/ rebobinado en la eficiencia del motor realizados por EASA y AEMT en los años 2003 y 2019. Otros se basan en los hallazgos del estudio previo efectuado por AEMT [1998] en motores trifásicos pequeños/medianos y en las buenas prácticas industriales bien establecidas.

Los procedimientos de esta guía cubren todos los motores trifásicos de inducción de alambre redondo. Mucha información también aplica a motores con bobinas preformadas (pletina o solera) de tamaños similares.

(Nota: Nota: Esta guía proporciona muchas recomendacio­nes y procedimientos específicos. Se pueden lograr los mismos resultados con otras prácticas, pero deberán ser verificadas.)

Tabla de Contenido

• Terminología
• Pérdidas de energía en los motores de inducción
• Procesos de reparación del motor
  • Inspección inicial
  • Desmontaje del motor
  • Remoción del antiguo bobinado y limpieza del núcleo
  • Rebobinado del motor
  • Montaje del motor
  • Confirmando la integridad de la reparación

The challenge for every motor repair firm is twofold: to repair the equipment properly; and to demonstrate to their customers by means of adequate testing and documenta­tion that rewound motors retain their operating efficiency. Following the guidelines in the “DOs” and “DON’Ts” below will help you accomplish both.

Numerous studies have been done to determine the effect rewinding has on motor efficiency. These studies identified several variables that can impact the efficiency of a rewound motor, including core burnout temperature, winding design, bearing type, air gap and winding resistance. The following guidelines were developed as a result of those studies, which found that the efficiency of both standard and energy efficient electric motors can be maintained during rebuilding and rewinding. 

To ensure that motors retain their efficiencies when rewound, EASA also strongly recommends that electric motor repair centers comply with ANSI/EASA Standard AR100: Recommended Practice For The Repair Of Rotating Electrical Apparatus and strictly adhere to the “DOs” and “DON’Ts” that follow. These guidelines, which contain safe values (based on available data) and correct procedures, apply to both energy efficient and standard motors. Further study of the matter continues, and these guidelines will be revised if additional information warrants.

This presentation covers:

• An explanation of partial discharge
• Description of the damage mechanism
• PWM drive and partial discharge
• How to evaluate partial discharge
• Repair tips for dealing with partial discharge

When installing winding Resistance Temperature Detectors (RTDs), divide the number of stator slots by the number of RTDs to install (usually six) and mark the slots accordingly. For example, a 72-slot stator with six RTDs would position an RTD in every 12th slot. That results in two RTDs per phase. Be sure to number the RTDs and provide a map of their locations to aid the customer in interpreting temperature differences. For example, unbalanced voltage might result in higher temperature in two RTDs in the same phase, while obstructed ventilation is likely to cause higher temperature in two or three adjacent RTDs.

One anomaly is WPI or WPII (weather protected) enclosures, where the top hood is integral to airflow. Some manufacturers place all six RTDs across the top of the windings (from the 10:00 - 2:00 positions) so that all RTDs are within the area receiving better cooling. This is not deceptive; it’s just meant to avoid a customer asking questions about temperature differences. For repairers, it’s a talking point with your customer when rewinding such a motor. Do they want the RTDs evenly spaced, recognizing that they will see the differences in actual operating temperature? Or do they want them placed as the manufacturer did? Better to have that conversation first, rather than raise doubts after the motor returns to service.

Note that, depending on the coils/ group and pitch, an RTD might be between top and bottom coils of the same phase, or of different phases.

Chase Fell
Technical Education Committee Chair
Jay Industrial Repair

High potential (hipot) testing procedures for motor and generator windings are usually performed with 50/60 Hz AC or DC as the power source. Hipot testing is a critical step in validating the quality of new windings. AC and DC hipot tests are also useful to understand the condition of aged insulation for machines in service. DC hipot testing is widely used in motor repair because the equipment is portable, and the steady state test current comes mostly from leakage through the insulation.

When a breakdown occurs, DC causes less damage to material adjacent to the fault when compared to AC tests. A disadvantage of DC testing is the voltage is not distributed in the same way as what the winding sees with AC. Specifically, the DC test stresses the end turns much higher.

AC hipot testing is much more consistent with the voltage stress of the machine in service. Studies have shown that the AC hipot test can reveal insulation defects that are left undetected with DC tests. An AC test can better detect voids and delamination inside the insulation system. The disadvantage of the AC test at power frequency is when the size of the test set and/or complexity of the setup becomes problematic in motor repair and in the field.

High-potential testing is routinely used to assess the ground insulation of AC stator windings in-process, after completion of a rewind and post-delivery. This webinar covers:

• Differences between AC and DC high-potential tests
• Sizing AC test sets when testing large windings
• What relevant standards address (and what they don’t)
• Communicating test requirements to all stakeholders
• When to test and at what levels
• How to evaluate results

Beneficial for service center managers, supervisors and technicians responsible for high-potential testing.

Cyndi Nyberg 
Former EASA Technical Support Specialist 

“I have rewound a two-speed, two-winding motor. The high speed runs fine — the no-load current seems right. But when I test the low speed, the amps are far too high at rated voltage. It draws significantly above the rated current, at no-load! I know that the winding data is correct. What could be wrong?” 

This is one of the most common problems we encounter at the EASA office.  The solution is almost always the same. There are three questions we ask in return.

1. What are the two speeds?
2. What are the number of circuits in each winding?
3. What jumpers are used to connect each winding? 

Jim McKee (deceased)
Alabama Electric Motor Service
Sheffield, Alabama 
Technical Education Committee Member 

There seems to be a long and never ending list of equipment and facilities needed by most EASA service centers. But often we are constrained by cost, available space, and more urgent priorities that keep us from fulfilling some of these needs. One such (almost mandatory) need is a vacuum pressure impregnation (VPI) system. 

Normally there are two ways to provide this service. One way is to have another EASA service center do the VPI process for you. Another is to buy your own system at a consider­able capital investment. 

There is also another option. We decided that the way to provide VPI processing was to build our own system. Most EASA service centers have a talented collection of people with numerous skills. Ours is no exception. We felt that we had the skills in house to do the job. All we needed was some hardware and to do a bit of research into how the process works. 

This video shows one step in collecting motor winding data: how to measure magnet wire. A service center could use this data to:

• Duplicate an original winding
• Verify that a previous rewind was done correctly
• Serve as a basis for redesigning a winding
• Store recorded data for future reference

Helpful tools

• EASA's magnet wire table can be found in Section 6 of EASA's Technical Manual and can be downloaded for free by EASA members.
  
  DOWNLOAD FREE PDF BUY LAMINATED WIRE TABLE
   
• EASA's polyphase and single phase winding data cards are available for purchase in EASA's online store.
  
  BUY WINDING DATA CARDS

There is much discussion in the industry about how to properly electrically test AC stator and wound rotor windings. Topics include test voltage, procedure, frequency and when to perform the various tests. This article describes how the following standards address these questions:

• NEMA MG 1-2011
• (MG1) IEEE 43-2000
• (IEEE 43) IEEE 62.2-2004
• (IEEE 62.2) IEEE 522-2004
• (IEEE 522) IEEE 1068-2009
• (IEEE 1068) ANSI/EASA AR100-2010
• (AR100) CSA C392-2011 (C392)

These standards are regularly reviewed and coordinated, so some of the information may not match the old yellowed reference taped to your toolbox lid. These updated references should replace anything dated previous to the dates indicated on the standard. AR100 Section 4.3.1 lists the recommended tests for stator and wound rotor windings. They are insulation resistance (IR), winding resistance, growler, phase balance, surge comparison, polarity and ball rotation tests. This article covers the IR, winding resistance and surge tests. Noticeably absent from this list is the ever popular high potential (hi-pot) test. Topics covered also include:

• IR (or megohm) test
• Polarization index test
• Winding resistance test
• Surge comparison test
• Hi-pot test.

Presented by Carlos Ramirez
EASA Technical Support Specialist

Is the motor drawing high no-load amps and winding data are correct? Are you experiencing unusual heating of the stator under load? Those common questions can be answered by checking the stator core condition. This presentation will discuss how to perform a stator core test using a loop test. It also will explain how to analyze the results, providing information about the associated equipment, tips for repairing core damage and explain other alternatives for stator core testing.

The presentation covers:

• Loop test theory
• Testing procedure
• Acceptable limits for losses and core temperatures
• Associated equipment
• Tips for repairing core damage
• Alternative stator core test

This presentation will be useful for supervisors, winders and test technicians.

This EASA software is a valuable interactive training tool ideal for training your novice(s). Even experienced winders will learn from it. The CD teaches how to wind in a richly detailed, step-by-step approach. It includes narrative, animations and video clips, with tests to assess student comprehension. The training, which is divided into 13 lessons, covers data taking, core testing, coil cutoff, burnout, stripping, core preparation, coil making, stator insulation, coil insertion, internal connections, lacing and bracing, inspection and test of untreated and treated windings, and winding treatment. Features include "Pro Tips" and "Drill Downs" that enhance the learning experience and assure that even the most experienced technician will learn from this product. The course is delivered as an interactive Adobe PDF file containing text, audio, video, supporting documents and quizzes.

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Presented by Tom Bishop, P.E.
EASA Senior Technical Support Specialist

This webinar recording focuses on the effect of three-phase stator winding changes on efficiency and reliability.

Specific changes addressed will include:

• Connection
• Circuits
• Turns
• Span/pitch
• Grouping sequence
• Concentric to lap, and vice versa
• Wire area per turn and per slot

Target audience: Service center technicians and supervisors.

Por Tom Bishop P.E.
Especialista Sénior de Soporte Técnico de EASA
 
Algunas veces, los cables de salida de los motores no se encuentran  identificados o sus marcas no  son legibles, por lo que se hace necesario  marcarlos para poder  conectar el motor adecuadamente  a la línea de alimentación. Este artículo se ocupará de cómo identificar  los cables de los motores eléctricos con 9 cables de salida y se  basa en la premisa de que ninguno de los  cables se encuentra marcado. Si alguno de ellos estuviera identificado, el procedimiento es el mismo, pero se requerirán menos pasos. Nota: Vea el artículo publicado en  mayo de 2008, en la revista Currents de EASA, titulado “Identifying Unmarked Leads Of 6-Lead Motors With 1 Or 2 Windings”.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

The markings on the external leads of a motor sometimes become defaced or are removed, which makes it neces­sary to identify and mark them before the motor can be properly connected to the line. This article will address lead identification of three-phase motors with 9 leads, based on the premise that none of the leads are marked. If some of the leads are marked, the process is the same, but may require fewer steps. Note: See the May 2008 issue of Currents for the article “Identifying Unmarked Leads Of 6-Lead Motors With 1 Or 2 Windings.” 

Anthony Sieracki 
Spina Electric Co.

Often during the rebuild and overhaul of a piece of electrical apparatus, we are faced with the challenge to identify the type of temperature detection devices that need to be replaced. The identification process should start at the beginning of the rebuild when the apparatus starts its way through the service center. Most major manufacturers identify the temperature detection device on a connection plate in the motor terminal box and many have a connection diagram indicating the type of device used. You can also refer to the manufacturer's catalog where they identify the type of temperature detector that has been used. Needless to say, it is usually too late to start identifying the temperature sensor after the winding is reclaimed and the device is now dust. Because that does happen, let's look at the styles and types of winding temperature detectors. Types of temperature detectors discussed include:

• Resistance Temperature Detectors (RTDs)
• Thermocouples
• Thermistors
• Thermostats

Chuck Yung 
EASA Technical Support Specialist

One frequent request of EASA’s technical support staff is for help in identifying unmarked motor leads. This article introduces a set of proce­dures for identifying unmarked leads of 6-lead motors with 1 or 2 wind­ings. For most connections, the only tools required for these procedures are an ohmmeter and surge tester. 

An additional benefit is that these procedures can be used to identify the type of connection (Table 1); for example, when a motor is received without a nameplate. With 6 leads, the motor connection could be part-winding start, wye-delta, or a 2-speed design. 

Mike Howell, PE
EASA Technical Support Specialist

The markings on the external leads of a motor sometimes become defaced or are removed, which makes it necessary to identify and mark them before the motor can be properly connected to the line. This presentation reviews procedures that explain how to identify unmarked leads of three-phase motors with one or two windings.

Topics include:

• IEC / NEMA numbering systems 
• Three-lead machines 
• Six-lead machines 
• Nine-lead machines
• Twelve-lead machines

This presentation is intended for all personnel who troubleshoot machines with unmarked leads.

Mike Howell
EASA Technical Support Specialist

As service organizations, we should examine every phase of our projects and the related decisions we make in terms of SQDC (safety, quality, delivery and cost) every time and in that order.

• Safety – keep people safe
• Quality – fulfill requirements
• Delivery – meet time commitments
• Cost – achieve strong business results

When operational excellence principles have been adopted, organizations typically find that if they properly and intentionally attend to safety, quality and delivery, then cost can more easily be controlled leading to predictable and satisfactory business results.

Often times, the projects that are most difficult to manage properly are the ones that divert from the norm, removing us from our standard business activities. Approximately 48% of EASA service centers report providing at least one type of fieldservice as a standard business activity. These fieldservice activities are primarily vibration analysis, alignment, balancing and thermography.

A less common activity is the on-site stator rewind. Certainly, there are classes of machines (e.g., large generators) that are almost always wound on-site and there are service organizations that specialize in these activities. However, most service centers perform on-site stator rewinds infrequently and even then, many of the personnel required for the scope of work are not the personnel usually involved with fieldservice work. When working on-site and out of our element, how do we ensure the safety of all people affected by our work while delivering a quality product on-time and on-budget?

One way to mitigate risks associated with these types of projects is quality planning.

Chuck Yung 
EASA Technical Support Specialist 

When a customer calls and orders a motor, he usually specifies only the Hp/kW rating, rpm, frame, enclosure and voltage rating. That leaves at least one critical area where the elec­trician can go wrong: The starting method and number of leads. 

Bill Finley and Tyler Gaerke
Siemens Industry, Inc., Norwood, OH

There is always a need to push to higher and higher efficiencies. This can be seen in the revisions to IEEE 841 which pushed efficiencies up to NEMA premium levels. DOE has continued to pass legislation increasing efficiencies to higher levels up to 500 HP. There has also been action recently to establish higher minimum efficiency levels on machines as large as 2500 HP. Motor manufacturers have been motivated to find creative ways to increase efficiency levels through optimization of manufacturing processes, designs, active material increase and better more efficient materials such as magnetic sticks.

To better understand the steps required, it is helpful to understand, how losses are generated and to identify the levers that reduce these significantly, all at an acceptable cost for the investment of the motor. Life cycle costs should also be investigated. 

This paper investigates the impact on the motor performance during starting and normal operation by employing magnetic wedges versus non-magnetic wedges and other potential design changes. The type of induction motor, open (ODP, WPII) or enclosed (TEFC), along with the number of poles, influences the effect on the motor these design changes may have.

Magnetic forces (stresses) acting on the wedges are also investigated in this paper. This paper also discusses qualification processes that are necessary in order to avoid failures and ensure reliable magnetic wedge systems.

This paper covers:

• Designing and testing for NEMA and IEC premium efficiency levels
• History of high efficiency standards activities
• Industrial facility opportunities
• Magnetic wedges (purpose)
• Impact of magnetic wedges on motor performance
• Experimental data for different magnetic wedges
• Qualification of magnetic slot wedges
• Designing with magnetic wedges

Chuck Yung
EASA Senior Technical Support Specialist

One of the pleasures of helping EASA members is in discovering challenges or specific areas where we can improve on the original design of the motor manufacturer. The most recent of these, for me, has been a noticeable cluster of calls about motors with multiple windings. The call usually starts with something like this: "We wound this motor, and one speed was terribly burned." Another one I often hear is: "We rewound both speeds, and the surge test pattern for one speed indicates a winding problem." These are but a couple of examples of a design issue we are seeing with motors having more than one winding. While the use of variable-frequency drives (VFDs) is increasingly common, there are still applications using 2-speed, 2-winding motors. Cranes are a good example of one such application. When a core has more than one winding, the two windings behave as a transformer. Applying voltage to either winding induces voltage in the other winding because the two windings are inductively coupled. As long as both windings are connected 1 wye, and the leads of the second winding are left open, no magnetizing current is drawn by the second winding. When both windings are conventional and symmetrical, this arrangement works just fine. The problems start when either winding deviates from the symmetry that is so important to 3-phase motor performance.

Topics covered include:

• Transformer effect
• Visualizing the current flow
• Parallel circuits

This presentation covers the following topics:

• Induction motor basics for operation
• Squirrel-cage
  • Conductor material
  • Deep-bar effect
  • Multiple-cage windings
  • Phase resistance
  • IEC/NEMA design letters
  • Speed-torque characteristics
• Wound-rotor
  • Winding construction
  • Wave-wound connections
  • Distribution factor and chord factor
  • Rotor phase voltage
  • Speed-torque characteristics

Target audience: This webinar will benefit service center technicians and supervisors. 

Jim Bryan
EASA Technical Support Specialist (retired)

En la edición de Febrero de nuestra revista Currents, Mike Parsons proporcionó excelentes consejos para contactar y formular preguntas al Departamento de Soporte Técnico de EASA. Mike hace parte de Hupp Electric Motors Co. en Marion, Iowa y es miembro del Comité de Educación Técnica y me gustaría resaltar una declaración que hizo: “¡Ustedes no son ninguna molestia!” De hecho, son nuestro sustento. 

Durante años, sus Juntas Directivas y Gerentes han asignado recursos para aumentar el número de especialistas de soporte técnico y en encuestas anteriores de evaluación de necesidades, los miembros han calificado al soporte técnico/ingeniería de EASA como el beneficio número uno de la membresía. Así que aprovechad esto por todos los medios. 

Para ayudarle a obtener el mayor beneficio, este artículo explicará la información requerida para llenar la Solicitud de Verificación de Datos & Rediseño. Puede completar y enviar su solicitud en línea en www. easa.com/resources/tech_support/ redesign_inquiry o descargarla en este link y enviarla por fax o correo electrónico. Ver Figura 1. En el caso que llene la solicitud a mano, asegúrese de escribir claramente. Por ejemplo, los números “1” o “7”, o “5” o “6”, se parecen cuando se escriben muy rápido.

Figura 1

Cuando esté llenando la solicitud impresa o en línea, verá que ciertos campos están marcados con un asterisco (*), esto implica que son obligatorios para poder completar la solicitud. Algunas veces toda esta información no está disponible por lo que se debe informar de esto en el campo apropiado. Entonces nosotros haremos lo posible para determinar lo que se debe hacer. 

Comenzando con la información de la empresa, los datos importantes son su número de identificación de EASA, su nombre y la información de contacto por si nos surgen algunas preguntas. Indique porque medio prefiere recibir su respuesta, proporcionando un número de fax, un correo electrónico o su número telefónico. Nosotros utilizamos los datos de placa y la información del fabricante para ingresarla en la base de datos de EASA. Aunque esta información será de ayuda para futuras consultas, en caso que falte, no impedirá que la solicitud sea procesada. 

Datos de placa 

*Hp o kW es la potencia de la placa de datos que determina la capacidad de carga de la máquina. Cuando existen opciones como esta, se debe encerrar dentro de un círculo la unidad correcta. Las rpm o el número de polos y la frecuencia determinan la velocidad de la máquina. La frecuencia, el voltaje y los amperios también son tomados de la placa de datos. Si existe más de un valor todos deberán ser reportados.

Datos y dimensiones del núcleo 

Uno de los puntos clave para evaluar o rediseñar un bobinado es verificar si las densidades de flujo magnético en el entre hierro y en el núcleo del estator son razonables. Estas se pueden expresar en miles de líneas de flujo por pulgada cuadrada (klíneas/ pul²) o Teslas (T) y se comparan con los valores máximos establecidos en el entre hierro (65 klíneas/pul² o 1 T), yugo (130 klíneas/pul² o 2 T) y diente (130 klíneas/pul² o 2 T). También son comparadas con motores de la base de datos de EASA con dimensiones y potencias similares. Podemos calcular el número de líneas de flujo por pulgada cuadrada utilizando el voltaje, la frecuencia, los datos del bobinado y las dimensiones del núcleo. Los cálculos requieren mediciones precisas de cada uno de los componentes así como también el número de ranuras del estator. La Figura 2 proporciona las directrices para tomar estas medidas.

Figura 2

El número de ranuras del rotor es opcional a no ser que se requiera un cambio de velocidad. Dependiendo del número de polos, ciertas combinaciones de ranuras rotor-estator producirán ruido, variación del torque a muy baja velocidad (cogging) o una reducción del torque cuando el motor comienza acelerar (cusp). Con el número de barras del rotor podemos verificar si esta combinación ocasionará problemas en su motor. Tenga en cuenta que si las ranuras del estator o del rotor son inclinadas, dicha combinación no deberá causar estos problemas. 

El largo del núcleo deberá ser la distancia total entre los dos extremos del núcleo. Si existen ductos de aire, el número y ancho de los mismos se deberán anotar en los campos provistos. Estos datos serán incluidos posteriormente en la evaluación. 

En los bobinados preformados (pletina) son necesarias las dimensiones de la ranura del estator. Las medidas exactas facilitarán el cálculo de los calibres o tamaños de alambre y del aislamiento, para que se ajusten adecuadamente en la ranura cuando se construyan las bobinas. 

Información del bobinado 

El número de grupos y bobinas se requiere para evaluar el bobinado. Aquí tenga cuidado con la matemática. Un ejemplo es un motor de 48 ranuras con 3 bobinas por grupo, en el cual el llenado de todas las ranuras es el mismo. Muchas veces este diseño se reporta como un bobinado concéntrico de 12 grupos y 4 bobinas por grupo con un total de 48 ranuras y tres pasos de bobina. Realmente existen 36 ranuras y la tercera bobina de cada grupo tiene el doble o casi el doble del número de espiras de las otras dos bobinas del grupo. En cada grupo de bobina, dos bobinas compartirán una ranura y la otra bobina llenará por completo una ranura. Esto puede causar confusión y muchas veces requiere de una llamada telefónica para aclarar lo que realmente hay ahí. 

La sección de los datos del alambre indica el número de alambres en paralelo y los calibres de cada bobina. Recuerde que el alambre redondo puede ser métrico o AWG. Si no está seguro, proporcione las medidas con micrómetro y nosotros tomaremos la decisión. Tenga en cuenta que si una máquina no está fabricada en Norte América, bien podría tener alambre métrico. Se debe evitar el uso de galgas ya que generalmente no cuentan con la suficiente precisión para poder determinar la diferencia entre los calibres medios o AWG versus los métricos. Incluso un alambre medio puede marcar la diferencia. Una mejor práctica consiste en medir el alambre con un micrómetro y utilizar la tabla de Alambres Redondos de EASA para identificar el calibre del alambre. 

Sin duda, la parte de este rompe cabezas que se reporta erróneamente con más frecuencia, es la conexión. Una buena regla a tener en cuenta es que si el motor tiene más de tres cables, existe más de una conexión. Una discusión acerca de esto y de cómo determinar la conexión, se encuentra en el artículo publicado en octubre de 2011 en la revista Currents titulado “Understanding three-phase motor connections.” 

Sera necesario contar las espiras en varios grupos de bobina ya que no es raro que exista un número de espiras diferentes en las bobinas de un mismo grupo o en grupos distintos. Algunas de estas pueden parecer impares, por lo que es bueno contar varios grupos de bobinas hasta que se identifique el patrón. El número de espiras será el número total de alambres dentro de la ranura dividido por los alambres en paralelo y el número de lados de bobina. Por ejemplo, en la ranura compartida de un bobinado excéntrico (imbricado) con 90 alambres dentro de la ranura y con 3#16, 2#17, el número de espiras es: 

90/2 = 9 espiras
( 3 + 2 ) 

El paso indica la ranura en la cual se aloja el primer lado de bobina y la ranura en la cual se inserta el lado opuesto. Ver Figura 3. Por lo que en un paso 1-8, el primer lado de bobina está insertado en la ranura 1 y el otro lado cae en la ranura 8. Un paso de bobina 1-8 abarca 7 dientes, por lo que la extensión de la bobina es igual a 7 (span). Todas las bobinas de ese grupo tendrán el mismo espacio entre ellas y a continuación comenzará la siguiente fase.

Figura 3

Los bobinados concéntricos siempre tendrán más de un paso, como 1-8, 10, 12. Éste es un grupo de tres bobinas que están concéntricamente anidadas como se ve en la Figura 4 a la derecha. No todas las potenciales combinaciones de paso permitirán una distribución equitativa de las bobinas y por consiguiente no se pueden utilizar. Por ejemplo, un motor de 4 polos con 48 ranuras y 36 bobinas no puede utilizar un paso 1-7, 9, 11 ya que la bobina a ranura llena caerá en la parte superior de la ranura de una bobina compartida y habrá ranuras vacías. 

Datos nuevos

Esta sección contiene las instrucciones de lo que usted desea obtener. Esto puede incluir cambios de potencia, velocidad, frecuencia o de voltaje. El motivo de la solicitud es muy importante ya que no necesitamos adivinar o suponer nada. Entre más detalles proporcione, especialmente en solicitudes complicadas, mejor serán los resultados. Si alguna información no está disponible, incluya una nota al respecto para que podamos hacer nuestro mejor esfuerzo para llenar los vacíos. 

Conclusión 

Entre más completa y exacta sea la información, recibirá una respuesta más rápida y precisa. Agradecemos mucho cuando nos envían al Departamento de Soporte Técnico de EASA sus solicitudes en línea o mediante los formatos que pueden descargar en la WEB. Al parecer cada centro de servicio tiene su propia forma de registrar esta información y esto es bueno. No obstante toma más tiempo encontrar la información cuando no estamos familiarizados con dichos formatos. Enviar la información en los formatos estandarizados por EASA acelera el proceso.

Jim Bryan
EASA Technical Support Specialist (retired)

In the February 2017 edition of Currents, Mike Parsons provided excellent advice on contacting EASA Technical Support with questions. Mike is with Hupp Electric Motors Co. in Marion, Iowa, and is a member of the Technical Education Committee. I would like to underline one statement he made: “You are not a bother!” In fact, you are our livelihood. 

Over the years, your Board of Directors has allocated resources to increase the number of technical support specialists. And in past Member Needs Assessment Surveys, members have consistently rated technical/engineering support as the number one benefit of membership. By all means, take advantage of it. 

To help you get the most from the benefit, this article will explain the information required to complete the Data Verification & Redesign Request form. You can complete and submit the request online or you may complete and fax or email the form that is available to download. See Figure 1. If the forms are filled out manually, be sure to write clearly. For example, numbers such as “1” or “7”, or “5” or “6”, can look the same if written too quickly.

Figure 1: EASA's Data Verification & Redesign Form

When completing both the printed and online forms, certain fields are marked with an asterisk (*) implying that they are required to complete your request. Sometimes all of this information is not available and should be noted so in the appropriate spot. We will then make every attempt to determine what should be done. 

Starting with the company information block, the important data are your company EASA identification number, a name and contact information in case we have questions. Let us know which medium you prefer to receive your response by filling in either the fax, email, or phone area. We use information in the manufacturer block to enter into the EASA database. While helpful for future reference, it will not impede the request if it is missing. 

Nameplate data 

*Hp or kW is the rating from the nameplate for the machine’s load capacity. When there are choices such as this, the correct unit should be circled. Entering rpm or poles and frequency determines the speed of the machine. Frequency, volts and amperes are also from the nameplate. If there is more than one of any of these values, all should be reported. 

Core data & dimensions 

One of the keys to evaluating or changing a design is to determine if the magnetic flux densities in the air gap and core iron are reasonable. This can be expressed in thousands of lines of flux per square inch (klines/in²) or Tesla (T). This is compared to the maximum values established for the air gap (65 klines/in² or 1T), core (130 klines/ in² or 2T) and tooth (130 klines/in² or 2T). They are also compared to motors with similar cores and ratings found in the EASA motor database. We can calculate the number of lines of flux per square inch using the voltage, frequency, winding data and core dimensions. The calculations require accurate measurements for each of the components as well as the number of stator slots. Figure 2 provides guidelines for these measurements.

Figure 2: Important information for taking measurements

The number of rotor bars is optional unless a speed change is requested. Depending on the number of poles, certain combinations of numbers of rotor bars compared to stator slots will produce noise, cogging or torque cusps. With the number of bars, we can check to see if the combination in your motor will be a problem. Note that if the rotor bars or stator slots are skewed, the combination should not cause these problems. 

The core length should be given as the overall distance from one end of the core to the other. If there are air ducts, the number and width of these can be reported in the spaces provided. They will then be included in the evaluation. 

For form coil windings, the stator slot dimensions are needed. Accurate measurements will facilitate designing the wire size and insulation to fit properly in the slot when the coil is made. 

Winding information 

The number of groups and coils is required for the winding evaluation. Be careful with the math here. An example is a motor with 48 slots with 12 groups of 3 coils, but all the slots are equally full. Many times this will be reported as 12 groups of 4 for 48 total coils with 3 pitches in the concentric winding. Actually, there are 36 total coils and the third coil in each group has twice or nearly twice the number of turns of the other two coils in the group. In each coil group, two coils will share a slot and one of the coils will fill the slot alone. It can be confusing and often require a phone call to clarify what is really there. 

The wire data section tells the number and size of the wires in hand for each turn. Remember the wire could be AWG or metric. If you are not sure, provide the micrometer readings for the wire sizes and we will make the determination. Note that if the machine is not made in North America, it well could be metric wire. Wire gauges should be avoided; they are generally not sufficiently accurate to determine the difference between half sizes or AWG versus metric sizes. Even a half wire size can make a difference. It is best practice to measure the wire with a micrometer and use the EASA Round Magnet Wire Data chart to identify the wire size. 

By far the most often misreported piece in this puzzle is the connection. A good rule to remember is that if the motor has more than three leads, there is more than one connection. A discussion of this and how to determine the connection is found in the October 2011 Currents article titled “Understanding three-phase motor connections.” 

Several examples should be taken to determine the number of turns in each coil; it is not uncommon for there to be different turns in the coils in the same group or for different groups. Some of these may seem odd, so it is good to count multiple coil groups until you recognize a pattern. The number of turns will equal the total number of strands (wires) in the slot divided by the number of wires in hand and the number of coil sides. For instance, in a shared slot lap winding with 90 total wires in the slot and 3#16, 2#17 the number of turns is: 

90/2 = 9 turns
(3+2) 

The pitch is the slot the first coil side falls in and the slot for the opposite side reached. See Figure 3. Such as in a pitch of 1-8, the first coil side is in slot 1 and the other side is in slot 8. A coil pitch of 1-8 spans 7 teeth, so the span = 7. All of the coils in that group will have the same space between them and then the next phase begins.

Figure 3: Coil pitch

Concentric windings will always have more than one pitch listed such as 1-8, 10, 12. This is a group of three coils that are concentrically nested as seen in Figure 4 at the right. Not all potential pitch combinations will allow the coils to be distributed evenly and therefore cannot be used. For instance, a 4-pole motor with 48 slots and 36 coils cannot use a pitch of 1-7, 9, 11; the full slot coil will fall on top of a shared slot coil and there will be empty slots. 

New rating 

This section contains the instructions for what you would like to accomplish. This may include changes in the horsepower, speed, frequency or voltage. The reason for the inquiry is very important so we do not need to guess. The more detail provided, especially for complicated requests, the better the results. If any of the information is not available, include a note to that effect so we can do our best to fill in the gaps. 

Conclusion 

The more complete and accurate the information provided, the more quickly and accurately the answer will be received. We very much appreciate when you submit your requests to EASA Technical Support online or using one of the downloadable forms. It seems that every facility has its own way to record this information and that is good. It does take extra time to find the information if you are not familiar with the format; submitting on standardized EASA forms expedites the process.

Chuck Yung
EASA Technical Support Specialist

Member Question: We recently received an 800 hp, 2-pole 460- volt motor for repair. It had a 4-Delta connection, and the windings show severe thermal stress. The customer confirmed that the motor was recently installed, drew high current, and failed quickly.

Published: August 2006 & Revised: September 2024

EASA members can download a PDF of Internal Connection Diagrams for FREE. Use the link below for the free PDF.

This edition of EASA Internal Connection Diagrams contains significantly more connections and winding diagrams than the previous version (1982), as well as improved templates for drawing connection diagrams. It provides internal connection diagrams for three-phase windings. It can be used with either concentric or lap windings. It also covers all possible parallels; wye and delta, 2 - 48 poles; part windings; two-speed windings; wye-delta and consequent-pole connections, 2 - 48 poles. It includes PAM connections, as well as triple- and quadruple-rated connections.

In terms of the number of connections, this edition covers more poles than before. It also now contains some less-common connections. These include the European pole amplitude modulation (PAM) design; multi-torque ratings; and part-salient, part-consequent pole connections that permit pole/slot combinations that otherwise would be unattainable.

Although the “by-the-numbers” method of drawing connections remains basically unchanged in this edition, the winding connection templates have been greatly improved. New templates for skip-pole and adjacent-pole diagrams also have been added to simplify drawing these connections. The jumpers are shown in gray with different line patterns for each phase. 

The book also includes templates for 2-pole through 30-pole, adjacent (1-4 jumpers) and skip pole (1-7) connections.

A printed version of this book is available for purchase in the online store.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

This webinar recording reviews the failures associated with 3-phase motors on Variable Frequency Drives (VFDs) and how to rewind to limit future failures. The transient over-voltages produced by the VFD can cause the winding insulation to break down. Motor manufacturers and service centers have recognized that the winding insulation system must be enhanced to help withstand the effects of being used on a VFD. Topics include:

• Brief overview of the transient voltage phenomena
• Materials for an inverter-duty winding system
• Processes for an inverter-duty winding system
• Other considerations: cables, VFDs

This webinar is intended for winders, shop supervisors and engineering staff.

Tom Bishop, PE
EAS A Senior Technical Support Specialist 

With the advent of solid-state electronic variable frequency drives (VFDs) in the late 1980s, it was found that the windings of motors used on VFDs failed more frequently than when powered by a utility (sine wave) supply. By the turn of the century, motor manufacturers had gained a better understanding of how VFDs affected motor windings, and motor manufacturers and suppliers of winding materials had developed materials and methods to improve the reliability of motor windings supplied from VFDs. The general term for the windings is “inverter duty.” In this article, we will describe the materials and methods associated with inverter duty windings.

Much has been written in EASA publications and elsewhere about the consequences of excessive temperature on a motor’s performance. We know that excessive temperature and moisture are the largest contributors to bearing and winding failures. Understanding the source of the increased temperature will help us to correct the problem and improve the machine’s life expectancy.

A chart included in this article illustrates the theoretical impact of increased temperature on the life of the motor insulation system. This chart only addresses the impact of thermal aging and not various other conditions that will affectthe motor’s life. In other words, it says that for every 10ºC increase in operating tem-perature, the expected life is reduced by one-half. Conversely, if we can re-duce the temperature of the motor by 10ºC, we can expect the life to double. Note that this is true at any point on the curve. However, there is the rule of diminishing returns: at some point the cost of designing and operating a motor to run cooler out-weighs the benefts of doing so.  Here we will explore some of the factors that con-tribute to increased temperature.

Topics covered include:

• Overload
• Ventilation
• Voltage
• Electrical steel (core iron)
• Current density
• Circulating currents
• Harmonics

Presented by Mike Howell
EASA Technical Support Specialist

Choosing an appropriate lead wire for a new stator winding is an important task. The manufacturer’s information is not always available, or the number of circuits or external connection may have been changed, requiring a redesign of the lead wire.  This webinar reviews: 

• Commonly available materials 
• Lead wire insulation classes 
• Lead wire voltage classes 
• General sizing procedures 

This webinar is intended for repair technicians and anyone who needs to select lead wire.  

Mike Howell, PE
EASA Technical Support Specialist

EASA recommends using the lead wire specified by the original equipment manufacturer (OEM) whenever possible. If not available, some guidance is provided in section 6 of the EASA Technical Manual and an online calculator is available at easa.com/calculators to determine a minimum recommended size based on temperature rating, expected current, number of leads and type of connection. This article will describe the calculator’s function. It’s important to note that there is no one right answer in this process when the original information is unknown. When selecting a lead wire, the following topics should be considered.

Chuck Yung
Especialista Sénior de Soporte Técnico de EASA

A menudo, las inundaciones producidas por las intensas lluvias de las tormentas tropicales (huracanes, monzones y ciclones) colapsan cientos de plantas industriales a todo lo largo de la costa del golfo de México, desde la Florida hasta Texas y en otros lugares del mundo.

Para retomar las actividades productivas, los departamentos de mantenimiento y los reparadores enfrentan la difícil tarea de limpiar la suciedad y desalojar la humedad en miles de motores y generadores eléctricos. Ver Figura 1. El proceso en tales situaciones puede tomar semanas o meses y requiere procedimientos especiales para limpiar los motores contaminados con agua salada.

Aunque el problema es enorme, las fábricas pueden volver a producir más rápidamente aunando esfuerzos con centros de servicio profesionales y siguiendo algunos consejos que facilitan las tareas de limpieza. Estos incluyen, priorizar los motores que requieren ser reparados o reemplazados, almacenar adecuadamente las máquinas contaminadas y utilizar métodos contrastados para eliminar la contaminación con agua salada. Fabricar hornos provisionales in situ o en el centro de servicio también puede aumentar la capacidad de secado de los sistemas de aislamiento de los motores inundados.

John Allen
Sheppard Engineering

Clachan Hydro Power Station (HPS) went into service in 1956. Clachan HPS is located on Scotland’s west coast about 40 miles north of Glasgow. The underground power station is at the head of Loch Fyne sea loch. See Figure 1. The tailrace discharges into the Fyne River, a salmon fishing river. Loch Fyne has a renowned fishery and seafood restaurant within a mile.

The 900 ft (275 m) head vertical shaft Francis turbine driven 50 MVA, 40 MW 428.6 rpm 11 kV generator was designed by English Electric. The generator stator was recored and rewound in 1984 by Peebles Field Services (acquired by Dowding & Mills in 1998).

During the 1984 rewind, the original split core stator was rebuilt as a complete annulus. And the Class B winding was replaced by a resin rich Class F epoxy winding. The turns were insulated with Samicaflex insulation tape and the slot cell would typically have been an S5 mica tape; this is a 180g/m2 epoxy mica tape on a glass fabric. The dielectric stress for the slot cell (wall) insulation was very conservative at 41.9 v/mil (1,650 v/mm).

The rewind used 5 mm (0.197”) epoxy glass wedges with Nomex 410 packing and 4 mm (0.157”) phenolic glass coil separators. The punched slot width was 22.15 mm (0.872”) which would typically have resulted in a built slot width of 21.8 mm (0.858”) and the specified slot cell width was 21.1 mm (0.831”).

As part of Scottish & Southern Electric (SSE) program of power station refurbishment, Clachan HPS was refurbished in 2000. The program included refurbishment of the generator stator with an option to rewind if the partial discharge levels could not be improved. 

Dowding & Mills refurbished the generator stator, re-insulated the rotor and replaced the DC exciter with a brushless excitation system. They also replaced the complete station control systems together with all the low voltage (LV) and high voltage (HV) electrical installations.

Gene Vogel
Especialista de Bombas y Vibraciones de EASA

Las nuevas funciones del programa AC Motor Verification and Redesign – Version 4 (ACRewind) de EASA mejoran la capacidad de los miembros de EASA para enviar datos originales de forma electrónica e incluirlos en la base de datos Motor Rewind Data – Version 4 (MotorDB). Pase directamente a la sección “mejoras” si ya está familiarizado con los programas y cómo funcionan.

El software de la Version 4 es un conjunto de herramientas poderosas que sirve para que los miembros de EASA validen los datos de un bobinado, rediseñen devanados concéntricos a imbricados y efectúen cambios en los parámetros de un motor. Una característica clave de esta última versión del programa es la integración de la base de datos MotorDB con el programa ACRewind. La disponibilidad de ambas funciones del programa dentro de una interfaz de usuario común no es por simple conveniencia. La capacidad de los programas para compartir las fuentes de datos crea nuevas capacidades que la versión de cada programa independiente no podría.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

The wires that are associated with most motor and generator windings are copper magnet wires. For some special application machines, there are other wire types that have been used, such as Litz wire (very fine woven strands) or lead wire. In this article, we will address some issues relating to magnet wire type conversions and combinations.

The term magnet wire brings to mind the thought that the wire is somehow “magnetic,” which is not the case. The reason for the name is that it is wire used in magnetic coils. Thus, they are coils that make use of electro-magnetism. The two physical types are round and rectangular magnet wire. Strictly speaking, square wire is a form of rectangular wire. Having mentioned round and rectangular wire, we will move on to our first topic: the conver­sion of rectangular to round wire.  

An increasing number of manufacturers are using magnetic wedges in their form-wound machines. When a winder fails to replace magnetic wedges in kind, the winding temperature rise can increase by 20°C, and the magnetizing current can increase by 20% or more.

This recording explains why they are used, provides a balanced review of the benefits and negative issues associated with their use, and explains how to avoid the problems.

• Why some manufacturers use magnetic wedges
• Benefits of magnetic wedges
• Downside of magnetic wedges
• Fitting and installation to prevent them from falling out in service

Target audience: This will benefit service center technicians and supervisors.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

One of the main objectives in rewinding is to maintain the original performance characteristics of the machine. Our focus with this article will be on three-phase motor stator windings. If it is known that the winding in the motor was original and there is certainty that the as-found data is correct, the original winding can be duplicated with confidence by using the as-found data. However, in some cases there is uncertainty that the as-found winding had original factory data. In those cases, extra steps need to be taken as described here.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

Uno de los principales objetivos del rebobinado es poder conservar las características de funcionamiento originales de la máquina. Nuestro artículo estará enfocado en los bobinados de los estatores de motores trifásicos. Si se sabe que el devanado del motor era original y estamos seguros que los datos que hemos tomado son correctos, el bobinado original se puede duplicar con confianza utilizando estos datos. Sin embargo, en ciertas ocasiones, no existe la certeza que los datos que hemos tomado correspondan al bobinado original de fábrica. En estos casos, es necesario tomar medidas adicionales como las aquí descritas.

This webinar presents a product quality planning process for industrial motor stator windings rated above 4 kV. Emphasis is placed on analyzing gaps between these projects and lower voltage rewinds as they relate to:

• Stator winding design
• Insulation system validation
• Process control

Target audience: This presentation is most useful for service center winders, engineers, supervisors and managers. The content targets beginners through highly experienced persons.

Mike Howell
Especialista de Soporte Técnico de EASA

Diferentes normas relevantes incluyendo la IEC 60085 y la IEEE 1 definen de forma similar los materiales electro aislantes (EIM) y los sistemas de aislamiento eléctrico (EIS). Resumiendo, los EIM son materiales idóneos para separar las partes conductoras a diferentes voltajes y los EIS son estructuras aislantes que contienen uno o más de estos materiales.

Como en cualquier sistema, existe una interacción entre los materiales usados y los diseñadores de los sistemas de aislamiento cuidan todos los detalles para evitar que esta interacción produzca resultados indeseados. Por ejemplo, es posible que dos materiales (EIM) clasificados individualmente como clase H (180ºC) tengan vida térmica en un sistema (EIS) limitado a una clase térmica F (155ºC).

Mike Howell
EASA Technical Support Specialist

Relevant standards including IEC 60085 and IEEE 1 have similar definitions for electrical insulating materials (EIM) and electrical insulation systems (EIS). To summarize, EIM are materials suitable for separating conducting parts at different voltages, and EIS are insulating structures containing one or more of these materials.

As with any system, there is an interaction between the materials used, and the insulation system developers take great care to ensure that this interaction does not lead to undesirable outcomes. For example, it is possible for two materials (EIM) classified individually at thermal class H (180°C) to have thermal endurance in a system (EIS) limited to thermal class F (155°C). Far worse outcomes could exist if material compatibility is an issue. At the service center level, our resources are generally insufficient for these types of insulation system development activities. For this reason, two approaches often seen are (1) relying on a third party (e.g., resin manufacturer) to provide a qualified insulation system bill of materials, or (2) applying commonly used materials based on their individual ratings. The first approach is strongly recommended, and the second approach can lead to disaster.

Chuck Yung
EASA Senior Technical Support Specialist

You've just dismantled a special motor for a customer, and the core test indicates the watts loss/pound is excessive.  The high core losses are caused by shorts between the laminations.  This may be the result of a ground failure.  Or excessive temperatures may have caused the deterioration of inter-laminar insulation (called coreplate.)

Whatever the cause, a replacement is 16 weeks away, and your customer wants his motor repaired.  This motor sounds like a prime candidate for a restack, but you are hesitant.  Your company has a reputation for quality, and the finished product has to meet your usual high standards. 

You want to know the best procedure for repairing this core.  Here are some guidelines to help you do the best possible repair. 

By Mike Howell
EASA Technical Support Specialist

Manufacturers deploy various external connection schemes to produce three-phase induction motors for multiple voltages and/or starting methods, so successful installation depends on using the relevant connection diagram. If this information is lost, damaged, or ignored, a connection mistake could lead to a costly rewind.

The following tips apply to common connections on machines with one speed at power frequency. If the manufacturer's external connection diagram isn't available, ask a service center for assistance, especially if there are several missing lead tags, multiple speed ratings at power frequency, unconventional numbering, or NEMA-IEC cross-references.

READ THE FULL ARTICLE

This valuable resource is available only to EASA Members.

Active and Allied members can download this software for FREE!

This version of the EASA Motor Rewind Database software takes a large leap forward with the data that it provides members. Most notably, it now has the ability to connect to a live, ever-expanding online database of more than 250,000 windings. This live database will be continuously monitored, updated and corrected as needed by EASA’s Technical Support Staff. Using the online database guarantees you’ll have the most up-to-date information available at all times. If your computer does not have an Internet connection, the software will automatically switch to the static, local database that was included and loaded during installation. (Note: The local database does not receive updates.)

The database includes:

• Three-phase, single-speed AC motors
• Three-phase, multi-speed AC motors
• Single-phase AC motors
• DC motors & generators

This webinar discusses:

• Temperature rise
  • Method of detection
  • Insulation class
  • Enclosure
  • Service Factor
• Increasing winding life
  • Insulation class
  • Cooling system
  • Winding redesign

Target audience: This webinar will be most useful for service center engineers, supervisors, managers and owners. The content will also be beneficial for mechanics and winders.

This presentation begins with an overview of ambient, winding temperature rise, and winding temperature. It also covers factors for motor temperature rise limits such as motor size (medium or large), insulation class rating, service factor and the enclosure. The final part of the presentation addresses detectors for measuring winding temperature, namely thermostats, resistance temperature detectors (RTDs), thermocouples and thermistors.

Target audience: This presentation is most useful for service center winders, engineers, supervisors and managers. The content will be beneficial for beginners through highly-experienced persons.

Mike Howell
Especialista de Soporte Técnico de EASA 

La pregunta sobre los niveles de tensión utilizados en la prueba de impulso para comprobar el aislamiento entre espiras de los estatores de pletina surge muy a menudo. ¿Y por qué nos la hacemos? Muchos centros de servicio tienen buenos fabricantes de bobinas en sus listados de proveedores calificados y varios clientes que poseen motores de pletina. No es improbable que cada fabricante de bobinas y cada cliente puedan especificar niveles de prueba diferentes. Aún más, la mayoría de las normas o guías disponibles proporcionan un rango de niveles de prueba. Esto es debido a que no todos los sistemas de aislamiento están diseñados con la misma capacidad para soportar impulsos. Además, las propiedades del aislamiento entre espiras de un sistema en particular pueden variar ampliamente, dependiendo del grado de procesamiento alcanzado al momento de la prueba.

Chuck Yung
EASA Senior Technical Support Specialist

How much no-load current should I expect when testing a motor? We would like to have a ratio of no-load amps / full-load amps, for quality control purposes. Many of us expect a motor to draw approximately one-third of rated current, when operating from rated voltage on our test panel. That is a good rule of thumb - most of the time. While there are lots of exceptions, most of them are predictable.

The intent of this article is to explain why those statements are valid and, in the process, to offer practical guidelines for assessing no-load current. Many of us apply these principles daily. 
Knowledge is power. We should, whenever possible, improve our knowledge by gathering facts: 

• Use the AC Motor Verification & Redesign Program to check densities before rewinding the motor. 
• Keep records of tests for comparison of identical machines. 
• Get information from the manufacturer to supplement your records.

Topics covered in this article include:

• Practical guidelines
• Effects of applied voltage
• Different designs affect rule
• Flux and air gap
• Number of poles
• Considering scale, manufacturing tolerances
• Exceptions to every rule

Mike Howell
EASA Technical Support Specialist

The open stator impedance test (a.k.a, ball test or dummy rotor test) is used by many service centers as a quality control check before winding treatment and/or a troubleshooting test. This webinar recording reviews test procedures, expected outcomes and incorporating thermal camera imaging. Topics covered include:

• Understanding the principles of the test
• Performing the test safely and consistently
• Expanding the value of the test with thermography 

This webinar recording is intended for personnel responsible for testing stator assemblies.
 

Chuck Yung
EASA Senior Technical Support Specialist

There are benefits and drawbacks to the use of multiple circuits in a 3-phase winding. Whether discussing a random winding or form coil winding, some of the considerations are shared. Let’s start with the basics:  The higher the power rating, and/or the lower the voltage rating, the fewer turns/coil used. Because a 3-phase winding has pole-phase groups alternating ABC, ABC, ABC, etc., the intra-phase jumpers could be 1-4, 1-7, 1-10, 1-13, etc., or any combination of these so long as the alternating polarity of the groups is maintained and the phases are not cross-connected.

Chuck Yung
EASA Senior Technical Support Specialist

Concerns about partial discharge (PD) used to be limited to repairers and users of machines rated over 7 kV. PD is a common consideration for machines rated 11 kV and higher. This article will describe PD, explain how to detect it, and offer repair solutions.

What is partial discharge?
Air, like Nomex^®, Mylar^®, mica and Dacron^®, is an insulator. Like any insulation, it will break down electrically if subject to too high an im­pressed voltage. Air is capable of with­standing approximately 75 volts/mil (3000 volts/mm), as compared to mod­ern insulating materials rated many times higher. (See Table 1.)

Gene Vogel
Especialista de Bombas & Vibraciones de EASA 

El artículo de la revista Currents de noviembre del 2023: “Pautas Para el Fluido de Sellado de las Bombas Sumergibles” cubre las opciones para el aceite de sellado de las bombas sumergibles. Además del aceite de sellado, algunas bombas sumergibles tienen una cavidad del estator llena de aceite. Este aceite transfiere de manera muy eficiente el calor desde el devanado y el núcleo del estator, y especialmente desde el rotor, hasta la carcasa de la bomba, donde la convección lo lleva al líquido de bombeo en el que está sumergida. Si bien las especificaciones para el aceite de sellado son bastante flexibles, el aceite de la cavidad del estator tiene más restricciones.

This video explains how to check the ground insulation of an AC motor winding using the insulation resistance (IR) test. The IR test is usually the first electrical test because it indicates if the motor winding can withstand further testing, or the machine can return to service. This video shows:

• How to select the megohmmeter and IR test voltage
• How to connect the megohmmeter to the winding and ground the leads
• How to perform the IR test and for how long
• How to safely discharge the winding
• How to correct the IR test result to the standard temperature of 40°C and determine if it is acceptable

David Sattler
L&S Electric

El objetivo de la impregnación por vacío y presión (VPI) es saturar completamente un devanado con resina aislante. A medida que la resina penetra en los materiales aislantes, los oscurece y al drenar la resina VPI del devanado, todo el aislamiento de las cabezas queda oscuro en un tono uniforme. El aislamiento de la conexión también debe quedar oscuro de forma pareja. Si algún aislamiento muestra un tono más claro, la bobina o el puente no han quedado completamente saturados y el devanado no está debidamente protegido. Si no se soluciona el problema, es probable que esto provoque un fallo prematuro en el equipo. Esto podría generar una garantía costosa o, como mínimo, la reparación no brindará a sus clientes la calidad que esperan y merecen.

Jim Bryan (retired)
EASA Technical Support Specialist

Voltage surges come in many forms, all of which can be devastating to an electric motor. Transient conditions, rapid bus transfer, ungrounded systems, reclosure, improperly located power factor correction (PFC) capacitors, accidental connection of a new dual-voltage motor for the wrong voltage, and lightning are all sources of damaging surges. Here, we would like to discuss two of these sources: rapid bus transfer and reclosure.  Rapid bus transfer occurs when the voltage source is changed from the primary to a secondary source or back. This might occur when an automatic back-up source is brought on line during a power outage. Reclosure is similar but may include the automatic reclosing of an overcurrent device or “chatter” in a switch or circuit breaker.

Las versiones impresas y en forma de descarga del valioso manual didáctico / recurso de EASA, “Principios de Motores C.A. Medianos y Grandes”, se encuentran ahora disponibles en inglés y en español. El manual incluye gráficos e ilustraciones, fotografías y mucha información técnica sobre máquinas C.A., incluyendo como funcionan, información específica sobre los tipos de encerramientos, fabricación de componentes y aplicaciones.  Muchos de los principios incluidos en el libro aplican a todos los motores C.A., especialmente a aquellos accesorios que fueron asociados en el pasado con las máquinas más grandes (como encoders, RTDs, termostatos, calentadores de espacio, sensores de vibración, etc.).

Las versiones  forma de descarga ofrecen funciones prácticas ya que contienen toda la información que contiene el manual impreso, pero en formato PDF, fácil de usa, ya que contiene marcadores que permiten a los lectores navegar rápidamente por el documento y “saltar” a la página deseada.

Las secciones del manual incluyen:

• Terminología y Definiciones del Motor
• Tipos de Encerramientos de Motores
• Aplicaciones Típicas para Motores
• Consideraciones de Manejo y Seguridad
• Teoría Básica del Motor
• Normas para Motores
• Estatores
• Rotores de Jaula de Ardilla
• Ejes
• Lubricación y Rodamientos
• Accesorios del motor & Cajas de Conexiones
• Procedimientos de Inspección y Prueba
• Alineamiento del Motor, Vibración y Ruido
• Procedimientos de Almacenamiento
• Máquinas Sincrónicas

COMPRAR

This version of Principles of Medium & Large AC Motors manual is now available to address applicable IEC standards and practices. This 360-page manual was developed by industry experts in Europe along with EASA's engineering team. (The "original" version of this book based on NEMA standards remains available as a separate document.)

This manual includes drawings, photos and extensive text and documentation on AC motors, including how they work, information on enclosures, construction on components and applications. Many of the principles included apply to all AC motors, especially those with accessories that are associated with larger machines in the past (such as encoders, RTDs, thermostats, space heaters and vibration sensors).

While the manual covers horizontal and vertical squirrel-cage induction motors in the 37 to 3,700 kW (300 to 5,000 hp) range, low- and medium-voltage, most of the principles covered apply to other sizes as well. 

This valuable instructional/resource manual is available in printed and downloadable versions, and focuses primarily on IEC motors.

Sections in the manual include:
(Download the PDF below for the complete Tables of Contents)

• Motor nomenclature & definitions
• Motor enclosures
• Typical motor applications
• Safety & handling considerations
• Basic motor theory
• Motor standards
• Stators
• Squirrel cage rotors
• Shafts
• Bearings & lubrication
• Motor accessories & terminal boxes
• Test & inspection procedures
• Motor alignment, vibration & noise
• Storage procedures
• Synchronous machines

BUY A COPY FOR YOUR OFFICE

PRINTED BOOK DOWNLOADABLE PDF

This book is also available focusing on NEMA Standards — in both English and Español.

NEMA - English NEMA - Español

This valuable instructional/resource manual is available in printed, downloadable and CD-ROM versions.

For this second edition, this 320-page manual has been reorganized, updated with new information, including revised standards and published articles, and edited extensively. The manual includes drawings, photos and extensive text and documentation on AC motors, including how they work, specific information on enclosures, construction of components and applications. Many of the principles included apply to all AC motors, especially those with accessories that were associated with larger machines in the past (such as encoders, RTDs, thermostats, space heaters, vibration sensors, etc.).

While the manual covers horizontal and vertical squirrel-cage induction motors in the 300 to 5,000 horsepower range, low- and medium-voltage, most of the principles covered apply to other sizes as well.

This manual focuses primarily on NEMA motors.

Sections in the manual include:

• Motor nomenclature & definitions
• Motor enclosures
• Typical motor applications
• Safety & handling considerations
• Basic motor theory
• Motor standards
• Stators
• Squirrel cage rotors
• Shafts
• Bearings & lubrication
• Motor accessories & terminal boxes
• Test & inspection procedures
• Motor alignment, vibration & noise
• Storage procedures
• Synchronous machines

BUY NOW

BOOK DOWNLOAD CD-ROM BOOK & CD-ROM

This book is also available focusing on IEC Standards ... IEC VERSION

Chuck Yung 
EASA Technical Support Specialist 

We've all faced those rush jobs, where a cus­tomer is desperate to get his motor back faster than humanly possible. It is our nature to try to do the impossible that's why we are in this business.

We enjoy a challenge and the opportunity to help people. Turning a motor around quickly, when a customer is in need, is rewarding. One of the most time-consuming steps drying the windings after they have been cleaned Ð can be frustratingly slow when an anxious customer is calling. How long does that motor need to bake to be safe? 

Most service centers have built in a safety factor, based on some long-forgotten problem we had (or heard about) with a winding that was not properly dried. "Joe" opened the oven door right before lunch and left it open, then someone pulled a stator out a couple of hours later and it megged low. Maybe it failed on the test panel. Of course, "Joe" didn't fess up to what happened.  So you told everyone: "From now on, all windings bake 2 hours longer than we used to."

John Allen
Sheppard Engineering

La Central Hidroeléctrica de Clachan, comenzó a funcionar en 1956. Esta central subterránea se encuentra localizada en la costa oeste escocesa, a unas 40 millas del norte de Glasgow y en la cabecera del lago salado Loch Fyne. Ver Figura 1. El ducto de descarga vierte el agua dentro del rio Fyne, en el cual se pesca salmón. Loch Fyne cuenta en una milla con pesca reconocida y marisquerías de renombre.

La turbina Francis de eje vertical con salto de agua de 900 pies (275 m) que acciona un generador de 50 MVA, 40 MW 428.6 rpm 11 kV fue diseñada por English Electric. En 1984, el núcleo del estator fue reconstruido y el estator fue rebobinado por Peebles Field Services (adquirida en 1998 por Dowding & Mills).

Durante el rebobinado de 1984, el núcleo original fabricado en secciones, fue reconstruido totalmente en forma de anillo y el bobinado Clase B fue reemplazado por un bobinado epóxico resin-rich Clase F. Las espiras fueron aisladas con cinta Samicaflex y el aislamiento a tierra generalmente estaba fabricado con cinta de mica S5; esta es una cinta de mica de 180g/m2 sobre un tejido de fibra de vidrio. La rigidez dieléctrica del aislamiento a tierra (Pared de aislamiento) de 41.9 v/mil (1,650 v/mm) era muy conservadora.

Durante el rebobinado se utilizaron cuñas epóxicas de vidrio de 5 mm (0.197”) con un relleno de Nomex 410 y separadores de bobinas de vidrio fenólico de 4 mm (0.157”). El ancho de la ranura perforada era de 22.15 mm (0.872”), lo que normalmente habría dado lugar a un ancho de ranura construida de 21.8 mm (0.858”) y el ancho de la pared de aislamiento especificada era de 21.1 mm (0.831”).

Como parte del programa para el reacondicionamiento de centrales eléctricas de la Scottish & Southern Electric (SSE), La Central hidroeléctrica de Clachan fue reacondicionada en el año 2000. El programa incluía el reacondicionamiento del estator del generador con la opción de rebobinarlo, si no se podían mejorar los niveles de descargas parciales. 

Dowding & Mills reacondicionó el estator del generador, re-aislando el rotor y reemplazando la excitatriz de CC por un sistema de excitación sin escobillas. Ellos también cambiaron completamente los sistemas de control de la estación, conjuntamente con todas las instalaciones eléctricas de bajo y alto voltaje.

Reclosure occurs when power to a motor is briefly interrupted and restored before the magnetic field has fully collapsed in the motor’s winding. If this occurs while the applied power is out of phase with the collapsing field, significant damage can result. This webinar will address how this can happen and what measures can mitigate damage potential.

Topics covered include:

• Reclosing a switch or breaker before the magnetic field collapses
• Utilities’ automatic reclosure
• Contactor “chatter”
• Determining the time constraint
• Remedies: Time delay; Zero crossing

Jim Bryan
EASA Technical Support Specialist (retired)

Reclosure occurs when power to a motor is briefly interrupted and restored before the magnetic field has fully collapsed in the motor’s winding. If this occurs while the applied power is out of phase with the collapsing field, significant damage can result. This paper, presented at the 2014 EASA Convention, addresses how this can happen and what measures can mitigate damage potential. Topics covered include:

• Reclosing a switch or breaker before the magnetic field collapses
• Utilities’ automatic reclosure
• Contactor “chatter”
• Determining the time constant
• Remedies
• Time delay
• Zero crossing

Durante los últimos años, el rediseño de los motores eléctricos es un servicio que ha aumentado de popularidad en las compañías que reparan motores eléctricos. Al variar  uno o más datos del diseño, los centros de servicio muchas veces pueden adaptar motores para que cumplan con nuevos requisitos de forma más rápida y económica  que al comprar motores nuevos.
El manual de Rediseño de Motores de CA de EASA explica  la forma de realizar todos los cambios posibles en los valores nominales de los motores eléctricos de CA dentro de sus limitaciones de diseño. Además de fórmulas matemáticas, este manual proporciona directrices relacionadas con las limitaciones propias de cada tipo de rediseño, que  le ayudarán a determinar  si es posible obtener un nuevo valor nominal antes de retirar el bobinado. Los términos se expresan en Español y en unidades métricas y cada capítulo contiene al menos un ejemplo para guiarlo durante sus propios rediseños.

Los miembros de EASA pueden descargar GRATIS este  libro  o pueden comprar copias impresas del mismo.                                                                         

Los capítulos del libro incluyen:

• Cambio del  tamaño del conductor
• Cambio de Voltaje 
• Cambio de Potencia
• Cambio de Frecuencia
• Cambio del Número de  Fases
• Cambio de los Circuitos en Paralelo
• Cambio de  Paso o del Factor de Cuerda
• Cambio en la conexión del bobinado
• La fórmula maestra
• Conversión de bobinados concéntricos en imbricados
• Conversión de bobinados imbricados en concéntricos
• Notas para el cambio del número de polos
• Disminución de la velocidad aumentando el número de polos
• Aumento de la velocidad disminuyendo el número de polos
• Una velocidad a dos velocidades con un solo devanado
• Una velocidad a dos velocidades con dos bobinados
• Cálculo de un bobinado para un núcleo sin datos
• Fortaleciendo o debilitando el motor- método corto
• Determinando la conexión apropiada
• Rediseño monofásico
• Cálculo del voltaje secundario
• Determinando el agrupamiento trifásico de las bobinas 

Chuck Yung
EASA Senior Technical Support Specialist

Have you ever had a curing issue with your DAP monomer (diallyl-phthalate ) solventless resin (hereafter referred to as resin for simplicity)? If you haven’t, read on for guidance on preventing issues in the future. If you have, this article provides guidance on correcting the issues as well. 

As expensive as resin is, all service centers should be diligent about the care of their resin dip tank and VPI (vacuum pressure impregnation) systems.

Chuck Yung
EASA Senior Technical Support Specialist

One of the most briskly debated issues in our industry is the comparison – and procedures for – vacuum pressure impregnation (VPI) versus dip & bake. For this article, I have expanded the discussion to include trickle epoxy and B-stage coils. Service centers that have a VPI tank will quickly point out the many benefits of VPI, such as better sealing of the windings and improved heat transfer from the winding conductors to the frame for enhanced heat dissipation.

Form and random windings have two distinctly different issues. For the form-wound machine, resin penetration is the biggest concern – giving a clear advantage to a VPI process. For random windings, the concern is retention of the resin.

Mike Howell
Especialista de Soporte Técnico de EASA
 
La prueba de resistencia de aislamiento (IR), es realizada en las máquinas eléctricas rotativas por varias razones, incluyendo la evaluación de la condición del aislamiento, verificar la aptitud para su puesta en servicio y determinar si es conveniente someter los bobinados a pruebas adicionales. La norma IEEE Std 43-2000 proporciona la práctica recomendada para la industria.

La resistencia de aislamiento se calcula  de la siguiente forma:

R = E / I_T  

donde

R es la IR en MΩ
E es la tensión CC de prueba  en V
I_T es la corriente resultante total en µA

Mike Howell
EASA Technical Support Specialist
 
Insulation resistance (IR) testing is performed on rotating machines for several reasons including evaluation of condition, suitability for service and suitability for additional testing. IEEE Std 43-2000 provides the industry recommended practice. The insulation resistance is defined as follows:

R = E / I T  

where

R is the IR in MΩ
E is the applied direct voltage in V
I_T is the total resultant current in μA

Revisit EASA's 2021 Convention & Solutions Expo by buying access to recordings of the general sessions and education events streamed from EASA's website!

These recordings provide just over 22 hours of training. Downloadable PDFs of slides and technical papers are included!

Technical presentations include:

• Overview, Operation, Troubleshooting, Testing & Repair of Synchronous Motors - Javier Portos, Integrated Power Services, LLC, La Porte, TX
• Understanding Corrosion in Pumps - Gene Vogel, EASA Pump & Vibration Specialist
• Tips & Tricks for Submersible Pump Repair - Gene Vogel, EASA Pump & Vibration Specialist
• Proper Field Installation of Vertical Turbine Pump Motors - Gene Vogel, EASA Pump & Vibration Specialist
• Carbon Brush Applications, Tip & Tricks - Matthew Conville, EASA Technical Support Specialist
• Generator Repair Tips - Wayne Hall, Jenkins Electric, Charlotte, NC
• Repair Best Practices to Maintain Motor Efficiency & Reliability - Tom Bishop, P.E., EASA Senior Technical Support Specialist
• Failure Modes, Troubleshooting & Maintenance Best Practices - Calvin Earp & Ron Widup, Shermco Industries, Irving, TX
• Estimating Performance of Small Induction Motors Without a Dyno - Mike Howell, EASA Technical Support Specialist
• Use the Latest Tools of the Trade for Field Testing of Electric Motors - Calvin Earp & Ron Widup, Shermco Industries, Irving, TX

En Español

• Construcción del Estator (de Principios de Motores C.A. Medianos y Grandes) - Carlos Ramirez, EASA Especialista de Soporte Técnico
• Lo Qué Necesita Saber para Comprar, Instalar, Operar & Reparar Motores Eléctricos - Carlos Ramriez, Especialista de Soporte Técnico de EASA

Sales presentations include:

• 5 Fundamentals for a Successful Sales Attack! - Mike Weinberg Speaker, Consultant, Best-Selling Author
• Getting The Meeting - Mike Weinberg Speaker, Consultant, Best-Selling Author
• Every Sales YES Begins with a KNOW - Sam Richter, SBR Worldwide, LLC, Minnetonka, MN

Management presentations include:

• EASA Research on Growing Your Business: Key Info from the Manufacturer/Supplier Community - Jerry Peerbolte, J. Peerbolte & Associates, Fort Smith, AR
• EASA 2021 and Beyond - Brian Beaulieu, ITR Economics, Manchester, NH
• Global Update: Marketplace Trends on Motor-Driven Systems (and IIoT) - Ivan Campos, Analyst, Manufacturing Technology, OMDIA (formerly IHS Markit)
• Engaged Leadership - Clint Swindall, Verbalocity, Inc., San Antonio, TX
• Connecting Generations - Clint Swindall, Verbalocity, Inc., San Antonio, TX
• Optimizing Service Center & Management Efficiency - Chuck Yung, EASA Senior Technical Support Specialist

How do I access this content?
After purchasing, you can access the streaming content by going to the Convention or Training menus at easa.com and looking for "Past Convention Presentations" ... or you may go to https://easa.com/convention/past-convention-presentations/easa-rewind-2021.

NOTE: Access to the streaming content will be granted only to the person making the purchase.

Revisit EASA's 2024 Convention & Solutions Expo by buying access to recordings of the general sessions and education events streamed from EASA's website! These recordings provide just over 32 hours of training. Downloadable PDFs of slides and technical papers are included!
 

General sessions

• Keynote - World of Opportunity: Unlocking Passion, Performance and Transformation - Sebastian Terry
• EASA Industry Research - Industry Insights: Executive Perspectives on Market Trends and Conditions - Jerry Peerbolte, Prof. Emeritus, University of Arkansas - Fort Smith, AR
• Global Business Economy Trends/Forecasts - Christopher Kuehl, Ph.D., Armada Corporate Intelligence
• Presentation of the EASA Award

Technical presentations

• DC Machines 101: How They Work and How to Repair Them - Chuck Yung, EASA Senior Technical Support Specialist
• Best Practices in Medium and High Voltage Rewinds - Javier Portos, Integrated Power Services, LLC
• Motor Testing Fundamentals, Myths and Meaning - Preston Thompson, Megger
• Importance of Spring Force on Carbon Brush Function - Jeff Koenitzer and Nitin Kulkarni, Helwig Carbon Products, Inc
• Diagnosing Failures: Methodology & Case Studies - Mike Howell, PE, EASA Technical Support Specialist
• Very Low Frequency AC High-Voltage Testing - Michael Peschel, High Voltage, Inc
• Diagnosing Induction Motor Rotor Cage Faults - Gene Vogel, EASA Pump & Vibration Specialist; and Noah Bethel, PDMA Corp.
• No Load Run Bearing Temperature Criteria - Blake Bailey, designmotors
• Electric Motor Bearing Lubrication Frequency and Quantity - Tom Bishop, PE, EASA Senior Technical Support Specialist
• Dynamic Balancing Machine Setup and Operation - Gene Vogel, EASA Pump & Vibration Specialist (Note: no recording; handout and technical paper only)

En Español

• Construcción del Estator - Carlos Ramirez, Especialista de Soporte Técnico de EASA
• Las Mejores Prácticas de Reparación - Carlos Ramirez, Especialista de Soporte Técnico de EASA
• Corrientes por el Eje/Rodamientos - Carlos Ramirez, Especialista de Soporte Técnico de EASA
• Criterios de Temperatura del Rodamiento/Cojinete en Prueba sin Carga - Mario Lanaro, Flopower

Sales/marketing presentations

• 7 Fundamentals of Sales Success - Jeff Bajorek
• Tapping into Your Sales Superpower - Jeff Bajorek
• You Don’t Have a Sales Closing Problem - Jeff Bajorek
• The Four Types of Sales Managers - Jeff Bajorek

Management presentations

• Transitioning Family Wealth - Thomas Deans, Ph.D., Détente Financial Group
• Business Models and Case Studies for Remote Condition Monitoring Services - Geoffrey Brewer, HECO, Inc.; and Mike Huber, American MTS
• US Members: Workforce Development Assistance and Resources in Your State - Nathan Ginty, NIST Manufacturing Extension Partnership; Crystal Bristow, Jenkins Electric Co.; and Jan Schmidlkofer, K&N an Impel Company
• How EASA Accreditation Benefits Your Shop - Matthew Conville, MBA, PE, EEMSCO, Inc.
• Changing the Way You Recruit Forever - Chris Czarnik, Author of Winning the War for Talent
• Global Trends in Electric Motor Regulations - Benjamin Hinds, ABB
• State of the Low Voltage AC Motor Market - Blake Griffin, Interact Analysis
• Retaining and Developing Great Employees - Chris Czarnik, Author of Winning the War for Talent
• Sustainability: Where Is the Profitability? - Bjorn Mjatveit, EMR Consulting AS

Chuck Yung
EASA Technical Support Specialist

Note: This article was originally published October 2001 and was updated September 2021.

When rewinding a motor, the service center is restricted by the original design. Sometimes, we encounter a motor design we wish had never been developed. The random-wound, 2300-volt motor design falls into that category. Most of us would prefer to see medium voltage (2300-4160 volt) machines built exclusively using form coils. The form coil winding assures uniform volts/turn stresses, and reliably seals the windings against hostile environments.

From the manufacturer’s perspective, a random-wound, 2300-volt motor represents a substantial reduction in manufacturing cost. And competitive pricing is important if they want to sell motors. The great challenge to the service center is in successfully rewinding this design while maintaining profit.

Chuck Yung 
EASA Technical Support Specialist 

One of the more common sources of exaspera­tion for a winder is rewinding a motor that has odd turns. How would you like to find a better way to deal with odd turns? Here are some tips to make life easier for the winder: 

This webinar recording covers: 

• Importance of core loss testing
• Methods to reduce core losses
• Slot fill improvement without reducing copper

This book was developed to help electric motor technicians and engineers prevent repeated failures because the root cause of failure was never determined. There are numerous reasons for not pursuing the actual cause of failure including:

• A lack of time.
• Failure to understand the total cost.
• A lack of experience.
• A lack of useful facts needed to determine the root cause.

The purpose of this book is to address the lack of experience in identifying the root cause of motor failures. By using a proven methodology combined with extensive lists of known causes of failures, one can identify the actual cause of failure without being an “industry expert.” In fact, when properly used,  this material will polish one’s diagnostic skills that would qualify one as an industry expert.

The book is divided into the various components of an electric motor. In addition to a brief explanation of the function of each component and the stresses that act upon them, numerous examples of the most common causes of failure are also presented.

For this second edition, the manual has been reorganized and updated with new information including a new approach to methodology, new case studies and a new section covering synchronous machine failures. This could not have been done without many contributions from EASA members and the Technical Education Committee. 

The all new “Root Cause Methodology” section goes into great detail explaining that effective root cause failure analysis must take place within the context of a practical problem-solving methodology or framework. It covers a modified Plan-Do-Check-Act process that emphasizes the importance of planning and the related problem-solving methodology components. This section also explains A3, a high-level reporting tool that is very effective for problem solving.

In addition, besides a systematic approach to problem solving, root cause failure analysis of motors and motor systems requires familiarity with contributing factors attributable to various kinds of applications, environments and industries. This includes how various stresses can affect motor components and the reciprocal impact the motor system may have on the motor. This section includes a table with a detailed summary of motor stresses. 

There also is a new section on “Synchronous Machine Failures” and an expanded “Case Studies” section. Readers are guided through eight case studies.

With 328 pages, the book provides extensive information, including a wide range of failures, the likely causes listed, and the methodology for confirming the probable cause of each failure. 

Members may purchase a printed manual and/or a PDF download. The printed manual is in black and white, while the download shows most of the failure photos in color.  

Sections in the manual include:

• Root Cause Methodology (all new)
• Bearing Failures
• Stator Failures
• Shaft Failures
• Rotor Failures
• Mechanical Failures
• DC Motor Failures
• Synchronous Machine Failures (all new)
• Accessory Failures
• Case Studies (expanded)
• References

This book is available as part of EASA's Root Cause Failure Analysis seminar.

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DISCOUNTED BLACK & WHITE BOOKS!
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B&W BOOK B&W BOOK & DOWNLOAD

Jim Bryan
EASA Technical Support Specialist (retired)

The most stressful time for electric motors is during starting. The speed-current curve in Figure 1 illustrates why. At starting, the motor current is the highest it will ever be. This is referred to as starting or locked rotor current. These different terms describe that when the shaft speed is zero, the current is maximum. Note also the impact of applied voltage to the current characteristics. This will be discussed later.

Many performance parameters of the motor are directly proportional to the current. The parameter of most concern in this article is the heat produced which is proportional to the square of the current as represented by P = I²R. Where P is the power lost in heat (kilowatt-hours [kW•h]) due to the square of the current flow (I²) through a resistance (R). Once the motor has been successfully started, the load current level is reached and the cooling circuit of the motor is able to dissipate the additional heat produced by the starting current. Restarting the motor before this additional heat has been dissipated means more heat in the form of kW•h will be added on top of that which is there. Each subsequent start before the additional heat has been dissipated will add more heat — raising the temperature until some component in the motor reaches its failure point.

The limiting factor as determined by the design is the temperature increase resulting in component failure in a squirrel cage induction motor of one of three components: the winding, the rotor bars or the rotor shorting end rings. Depending on the design, the thermal “weak link” could be any of these.

Carlos Ramirez
Especialista de Soporte Técnico de EASA

Para monitorear la temperatura de los bobinados y en los rodamientos se pueden usar diferentes tipos de dispositivos. La correcta identificación de los mismos es importante para determinar el tipo de  sensor en casos en los que el dispositivo es desconocido o para escoger el dispositivo correcto para una determinada aplicación.

El webinario incluye:

• RTDs
• Termopares, termostatos y termistores
• Determinar el tipo de sensor desconocido y pruebas
• Controladores

Este webinario es útil para bobinadores, mecánicos, supervisores y técnicos de pruebas.

Chuck Yung
EASA Senior Technical Support Specialist

One of the recurring questions asked of EASA technical support specialists is:  “Should I use 1-4 or 1-7 jumpers?” This article is a tutorial on jumper selection to help the reader recognize when it does – or does not – matter. 

Let’s start with a short review for the experienced winders and good fundamentals (Table 1) for the newer winders. First, three-phase windings are symmetrical. The connection is log­ical if we apply some basic rules. The groups are positioned symmetrically, in sequence of A-B-C-A-B-C, with an equal number of coil groups required in each phase.

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

The most common method of start­ing squirrel cage three-phase motors is across the line (direct-on-line). Some applications require limiting the mo­tor starting current and/or torque to reduce the stress on the electrical and mechanical systems. 

Although there are other meth­ods such as autotransformer, reactor and using a variable frequency drive (VFD), the focus in this article will be on the reasons behind the selection of the three most common methods of achieving these objectives. Specifically, these methods are part winding, wye-delta, and electronic soft-starting. The windings in all three of these methods usually have 6 leads.

Cyndi Nyberg Esau
Former EASA Tenchincal Support Specialist

As we move into January, it's time to put into practice those New Year's resolutions we made. Many of us have the same ones every year. You know: eat less, exercise more, spend more time with the family, etc. However, rather than personal challenges, presented here are the"resolutions" for some of the more common calls we receive in the Technical Support Department at EASA Headquarters. A review of the basics is always a good idea to start the new year fresh!

EASA’s Stator Core Test Form provides a step-by-step procedure for calculating the number of turns and cable size required for a loop test. The form also has provision for recording the meter and temperature readings obtained during the test. Core sketches that show the location of measured dimensions and a wiring diagram of instrument connections are also included.

For more details on stator core testing, see Section 7 of the EASA Technical Manual.

This presentation covers:

• The importance of the stator core test 
• Simple theory to share with technicians and customers 
• Practical approach for testing small stators demonstrated 
• Eliminating pen + paper; loop test calculations for any device 
• Assessing the results

Mike Howell
EASA Technical Support Specialist

Aside from managing his family’s brewery in England, J.P. Joule did some pretty amazing work in physics dur­ing the mid-nineteenth century. Joule discovered that the rate at which heat is produced by a steady current in any part of an electric circuit is proportional to the resistance and the square of the current. So, the I²R loss of a conductor is not creatively named.

How does this apply to motor stator windings? The stator I²R loss is typically the largest contributing factor to the stator winding tempera­ture rise and the largest detractor of efficiency.

When preparing to rewind random or form wound stators, sometimes there just doesn’t seem to be enough room in the stator slot for the desired conductor area and insulation quantities. Common scenarios encountered are redesigns from concentric to lap, changes to higher voltages or aggressive designs from the OEM.

This webinar will look at balancing stator copper losses against insulation reliability.

Presented by Mike Howell, PE
EASA Technical Support Specialist

Note: This presentation is an update to the webinar originally presented June 2015.

When preparing to rewind random or form-wound status, sometimes there just doesn’t seem to be enough room in the stator slot for the desired conductor area and insulation quantities. Common scenarios encountered are: 

• Redesigns from concentric to lap
• Changes to higher voltages
• Newer designs from the OEM

This presentation looks at balancing stator copper losses against insulation reliability and is intended for technicians and engineers working with stator rewinds. 

Gene Vogel
EASA Pump & Vibration Specialist 

“Submersible Pump Seal Fluid Guidelines,” the November 2023 Currents article, addressed options for seal oil in submersible pumps. In addition to seal oil, some submersibles have an oil filled stator cavity. The oil in the stator cavity very efficiently transfers heat from the stator winding and core, and especially from the rotor, to the pump casing where convection carries it to the pumpage in which the pump is submerged. While the specifications for seal oil are fairly flexible, stator cavity oil has more constraints.

Mike Howell
EASA Technical Support Specialist

The question of surge test voltage levels used for testing the turns insulation of form wound stators comes up often. And, why wouldn’t it? Many service centers have several good coil suppliers in their qualified supplier lists and quite a few customers who own form wound motors. It’s not unlikely that each of the coil suppliers and each of the customers could be specifying different test levels. What’s more, most authoritative standards or guides available provide a range of test levels. This is because not all insulation systems are designed to have the same surge withstand capability. Also, the turn insulation properties of one particular system can vary widely depending on the extent of processing complete at the time of test.

Chuck Yung
EASA Senior Technical Support Specialist
 
When taking winding data, the area most prone to error is in identifying the connection. This article includes a reference page (see Figure 1) that I encourage you to print and laminate for the winders to use.

Blake Parker
Technical Education Committee Member
High-Speed Industrial Service

When looking at a stator core that requires repair, it can be easy to jump to conclusions. There are many factors to consider when re-stacking a stator. Those include the materials, core compression, length of the core, vents, spacers, vent construction and more. This article focuses on taking proper measurements when re-stacking a stator core and how to go about stacking the stator to ensure those dimensions are met.

This presentation stresses the importance of taking accurate winding data and explains and emphasizes the consequences of inaccurate data. Details are provided on how to take accurate electrical and mechanical data as well as how to verify the data is correct. It gives you and improved ability to "get it right the first time" so as to avoid the added cost and time of another rewind to correct errors.

Tom Bishop, P.E. 
EASA Technical Support Specialist 

When we mention temperature detectors for motors we usually think of winding temperature detectors. However, temperature detectors can also be used to monitor bearings and airflow. In this article we will describe the more common types of tempera­ture detectors and how they can be applied to windings, bearings and to check ventilation (airflow). 

We will begin by describing the different types of temperature detectors most commonly used in motors and generators. For simplicity we will use the term “motors” to mean both motors and generators. Although our focus is on temperature detectors for motors and generators, the detectors may also be found in transformers and other equipment. These are resistance temperature detectors (RTDs), thermocouples, thermostats, and thermistors. 

Editor's Note: This "encore" technical article first appeared in the September 2003 issue of Currents. It was written by former Technical Support Specialist Cyndi Nyberg Esau.

To make more efficient use of time and materials, winders may want to increase the number of parallel circuits when winding an AC stator (or wound rotor). However, there are limits to the number of parallel circuits that can be used in an AC stator (or wound rotor) design. In this article, some of the potential problems associated with increasing the number of parallel circuits will be analyzed.

If the original design of a mo­tor has few turns with large wires, or many wires in hand, it may be easier to rewind if the number of parallel circuits can be increased (see Figure 1). Doubling the circuits, for example, doubles the turns per coil and cuts in half the wire size or the number of wires in hand. Of course, doubling the circuits also doubles the volts per coil.

This webinar recording covers: 

• Various types of AC motors and bases for operation
• Squirrel cage induction motor rotor design / construction
• Squirrel cage induction motor stator design / construction

This webinar covers:

• Internal connections
• Connections in the outlet box
• Connections in the MCC Ladder diagrams 

This webinar will cover burnout procedures for AC stators: 

• Interlaminar insulation materials / properties
• Core testing before and after
• Processing equipment, controls and records

This webinar covers:

• Photo documentation
• Paper documentation
• Measurements
• Winding data: turns, wire size, connection, core dimensions
• Keeping cause of failure questions in mind 

Chuck Yung
EASA Senior Technical Support Specialist

Anyone who has spent much time reading the EASA AC Motor Redesign book, or motor design textbooks, will recall that there are certain stator-rotor slot combinations that can detract from the performance of an electric motor.

Steve Skenzick
HPS Electrical Apparatus Sales & Service

By this time we should all know that stator core loss testing is a required part of a quality rewind.  A core loss test before and after burn-off is speci­fied in the EASA Recommended Practice for the Repair of Rotating Electrical Ap­paratus (ANSI/EASA AR100-2010) and The Effect of Repair/Rewinding on Motor Efficiency; EASA/AEMT Rewind Study and Good Practice Guide to Maintain Mo­tor Efficiency. I would like to share some core loss testing experiences we have had over the years in our service center.

Jim Bryan
EASA Technical Support Specialist

When designing or redesigning an electric motor, there are many trade-offs. For instance, opening up the air gap to improve power factor might diminish the efficiency. For every action taken in adjusting the motor’s design, there is a reaction at some other point that affects the motor’s performance.

One of the critical considerations involves the interaction of the turns per coil and pitch. As you shorten the pitch, it requires more turns to generate the same torque. But, as you increase the number of turns, the wire takes up more space in the slot. The way to overcome this problem is to decrease the wire size. But decreasing the wire size decreases the current density, in­creasing the I²R losses and operating temperature. 

Cyndi Nyberg 
Former EASA Technical Support Specialist 

To make more efficient use of time and materi­als, winders may want to increase the number of parallel circuits when winding an AC stator (or wound rotor). However, there are limits to the num­ber of parallel circuits that can be used in an AC stator (or wound rotor) design. In this article, some of the potential problems associated with increas­ing the number of parallel circuits will be analyzed. 

If the original design of a motor has few turns with large wires, or many wires in hand, it may be easier to rewind if the number of parallel cir­cuits can be increased. Doubling the circuits, for example, doubles the turns per coil and cuts in half the wire size or the number of wires in hand. Of course, doubling the circuits also doubles the volts per coil. 

Tom Bishop, P.E.
EASA Senior Technical Support Specialist

What are thermistors?

Thermistors, derived from the term thermally sensitive resistors, are a very accurate and cost effective method for measuring temperature. Thermistors are usually two-terminal semiconductor devices made from semi-conductor materials that have an electrical resistance that varies non-linearly with temperature (see Figure 1). Some materials provide better stability while others have higher resistance ranges and are fabricated into smaller thermistors.  Each specific thermistor has its own unique resistance versus temperature characteristic. 

By Gene Vogel
EASA Pump & Vibration Specialist

EASA’s AC Motor Verification & Redesign – Version 4 program, released in May, is an easy-to-use update to the prior Version 3, and older Version 2 program. The layout and function of the new program is mimicked from the older Version 2 program, so users can move up to the new program with an easy learning curve. Also there are features in the Version 4 program that will aid in recognizing aspects of the redesign process that can be overlooked. If you have the program running, follow along to learn about these features.

When entering a new design, the Version 4 program provides a field to identify the Customer by name and/or number. A “New” button lets the user enter customer data on the fly, but it is helpful to have the data already entered for your regular customers. The Database -> Customer menu items let you manage your Customer list ahead of time. Then customers can be easily entered by number or selected from the Customer Name dropdown list.

Use keyboard or mouse
When entering the Original Winding Data, some users prefer using the keyboard while others are more comfortable moving through the fields with the mouse. The program supports both methods. The Tab key steps through the data fields (shift-Tab for reverse). Any field with scroll arrows can be set with the Up – Down cursor key, or values can be typed in. Radio button and check boxes can be set with the Space Bar. With the mouse, just operate the scroll arrows or click in any field and type the value. Also, hover the cursor on an icon to see the icon description (see Figure 1).

When the data has been entered, the Calculate button displays a selection grid of possible redesign options. The selection grid is flexible and can be expanded to include additional columns (right click), and columns can be sorted by clicking the column heading. Several options are available to display various numbers of circuits (see Figure 2).  There is a good video tutorial on using the selection grid available from the program Help or though the EASA website. Often, the choice of redesign parameters involves comparing various options. The program allows the user to select and display multiple selections from the selection grid – just hold down the CTRL key and click the desired rows. The various redesign options can then be selected with the Tabs at the bottom of the display. For a columnar comparison of critical parameters, select the Side-by-Side option from the Redesign menu.

Also, there are Round Turns buttons for Integer or Half-Turn settings just above the Calculate button on the Original Motor page. Selecting Half Turns includes those options in the selection grid. An example of half turns is 7.5 turns for a 48-slot, 4-pole machine with 12 groups of 4 which might be wound as 7-7-8-8 with a pitch of 1-11 or 7-8-7-8 with a pitch of 1-12.
Integrated rewind data

In some cases, a “bare core” design is needed when winding data may be unavailable or suspect. The EASA Motor Rewind Data – Version 4, integrated with the redesign program, can help in these cases. By finding a motor in the database with similar parameters, that winding data can be used as the basis for the bare core design. When a suitable motor is found from the database, the MotorDb -> Send to ACR menu item converts the data from that motor into a new redesign case. There is no need to re-enter the data. A video tutorial on this simple process is also available on the EASA website.

It is common to experiment with various winding parameters such as turns, pitch and connections. The best approach is to use the Calculate button to display the selection grid and choose the desired option; the resulting redesign will be opened on a new Rewind option # tab (see Figure 3). The Manual Rewind option provides an input dialog box to adjust these values. Use caution when manually redesigning windings. It is possible to enter “impossible” values such as 4 circuits with 6 poles, which the program would not ordinarily allow. 

Wire size adjustments
One of the most common adjustments is wire size. A winding slot may be too full or too loose, or the chosen wire size may not be available in enough in-hand quantity. The program provides a powerful wire-size calculator to choose combinations of two wire sizes with the calculated percent change. The new wire size values from the calculator are automatically entered into the redesign with new CM/A (A/mm2) values (Figure 4).

There are a number of convenience options available in the redesign program. Core dimensions can be entered as fractions such as 3 11/16, or use simple math to convert values; 3.5 x 25.4 (convert inches to mm). On the Tools menu is an option to Set Available Wire Sizes so that only the wire sizes in your inventory appear. This list works in conjunction with the Wire Sizes Only Available checkbox on the Original Motor page just above the Calculate button. There are Default Value settings for Original Motor data. Select Motor Defaults from the Tools menu and enter whatever data you might commonly use. Common entries are hp – kW and AWG or Metric wire. Save and close the Default Motor and those values will be pre-set for new redesigns. The Reference menu has an array of simple calculators and various commonly used tables, such as Round Magnet Wire data and Three-Phase Full Load Currents. Also on the Reference menu is a PDF version of the AC Motor Redesign manual that has many of the base formulas used to calculate redesigns. The Concentric-to-Lap conversion charts are there also, which some members use for reverse conversions of Lap-to-Concentric. Contact EASA Technical Support for more information on converting to concentric windings.

Once a redesign has been calculated for an Original Motor, the Original Motor data is locked and can’t be edited without deleting the redesigns. This is to prevent a redesign being present that does not match the Original Motor. The Editor -> Allow Edits menu item has options to Delete all Redesigns or Clone Motor. Selecting Clone Motor will create a new Original Motor where the data can be edited. This is helpful for “what if” scenarios, and for multi-winding motors where most of the physical data is the same for each winding. 

In the course of looking up similar motor data in the winding database and redesigning multiple motors, there may be a number of motors open at any time. The tabs at the top of the editor show the motor ID with an Icon for the type of data. When several tabs are present, right clicking any tab will display options for closing unneeded tabs: Close Others, Close Tabs to the Left, Close All.
The program Help has useful information on winding redesign in the Concepts and Tasks sections. The video tutorials available on the EASA website may also be accessed from the program Welcome screen on the Help menu. The tutorial and Help are handy resources when questions arise. Of course EASA Technical Support can also answer questions about features and functions of the program or help with redesign questions.

LEARN MORE ABOUT THIS SOFTWARE

Editor’s Note:  Those who had purchased Version 3 automatically received Version 4 in May 2016. Otherwise, the program may be purchased. 

This webinar covers:

• Procedural tips for coil insertion
• Creating slot room where there is none
• Faster, easier separators
• Lacing technique to prevent phase paper pull-out
• Interspersed coil winding made simple
• Better braze joints

Blake Parker
Miembro del Comité de Educación Técnica
High-Speed Industrial Service

Al revisar el núcleo de un estator que requiere reparación, puede ser fácil sacar conclusiones precipitadas. Hay muchos factores que se deben considerar al re apilar el núcleo del estator. Estos incluyen los materiales, la compresión y largo del núcleo, los orificios de ventilación, los espaciadores, la construcción de los orificios de ventilación y más. Este artículo se centra en tomar las medidas adecuadas para volver a apilar el núcleo de un estator y cómo garantizar que se cumpla con esas dimensiones.

Mike Howell
Especialista de Soporte Técnico de EASA

El núcleo del estator de un motor de inducción se puede fabricar utilizando laminaciones de una sola pieza (vea la Figura 1 a la izquierda) de hasta un diámetro exterior de 48 pulgadas (1200 mm) aproximadamente. Para estatores más grandes, o cuando se minimiza el material de desecho, las láminaciones del estator son segmentadas (vea la Figura 1 a la derecha). El espacio circunferencial típico entre las láminaciones segmentadas es de solo unas 0,012 pulgadas (0,3 mm), por lo que se exagera en las cifras incluidas. El número de segmentos elegidos por un fabricante para un diseño determinado puede depender de varios factores, algunos técnicos y otros económicos. Para la mayoría de las actividades de reparación del centro de servicio, las máquinas con estatores con laminaciones segmentadas se procesan de la misma forma que aquellas con laminaciones de una sola pieza. Sin embargo, hay algunas áreas que vale la pena explorar que podrían ser útiles cuando se trabaja con estatores con laminaciones segmentadas.

Teaches how to determine type of connection, number of parallel circuits, turns per coil, wire size, span and groups. Shows step-by-step way to properly record all information.

This training film is archived here solely for historical purposes. The film was produced many years ago and does not meet EASA's current presentation standards. Some procedures may have also changed.

Step-by-step disassembly of a capacitor motor, both ball bearing and sleeve bearing. Includes bearing removal (both types), plus operation and replacement of various types of  starting switches and governors. Stripping not included.

This training film is archived here solely for historical purposes. The film was produced many years ago and does not meet EASA's current presentation standards. Some procedures may have also changed.

Shows method of winding concentric coils, which includes determining the coil width for the various size coils within each group, how to set the concentric winding head and select the proper grooves for winding the coils. Also shows how to wind coils using the Holden Head.

This training film is archived here solely for historical purposes. The film was produced many years ago and does not meet EASA's current presentation standards. Some procedures may have also changed.

Shows proper procedures for inserting concentric coils into a single-phase stator. Shows tools and insulations and points out relationship between running and starting winding coils.

This training film is archived here solely for historical purposes. The film was produced many years ago and does not meet EASA's current presentation standards. Some procedures may have also changed.

Step-by-step assembly of a capacitor motor, including installation of both ball and sleeve bearings. Shows tools needed and how to align parts marked during disassembly.

This training film is archived here solely for historical purposes. The film was produced many years ago and does not meet EASA's current presentation standards. Some procedures may have also changed.

Outlines the basic steps and procedures commonly used for testing three-phase, AC motors rated 600 volts or less, including preliminary insulation resistance, continuity and no-load tests, as well as the single-phase test for rotors. Also explains how to test components once the motor has been disassembled; these procedures include surge-comparison, polarization index, growler and DC high-potential tests, as well as the “loop” test for stator cores. Additionally, examines performance of final insulation resistance, noload, vibration and dynamometer tests of repaired or rewound motors.

This training film is archived here solely for historical purposes. The film was produced many years ago and does not meet EASA's current presentation standards. Some procedures may have also changed.

Teaches procedures for winding diamond and round-nosed coils. Includes determining coil dimensions from a bare core and winding multiple groups at one time.

This training film is archived here solely for historical purposes. The film was produced many years ago and does not meet EASA's current presentation standards. Some procedures may have also changed.