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.

Mike Howell
EASA Technical Support Specialist

Inspection and testing are two of the most important activities performed by service centers. These activities involve the collection of two types of data: attribute data (go/no-go information) and variable data (measurement information). When collecting variable data or measurement information, service center technicians obtain data consisting of two components – the actual value of the measured dimension and the error associated with the measurement. The service center should be focused on minimizing measurement error such that the measurement values are as close to actual values as needed to properly evaluate the measured item.

This article covers:

• Accuracy and precision
• Calibration
• Subcontracting calibration
• In-house calibration
• Calibration checks
• What should be calibrated?

Tom Bishop, P.E. 
EASA Technical Support Specialist 

A frequent question that arises during repair or maintenance is how much voltage to apply when performing a high potential test. The test voltage for a new winding will be higher than that of a winding that has been in service, but what is the appropriate test voltage? Seeking answers to this question can sometimes lead to confusion. Our goal in this article is to clarify high potential test­ing and test voltages. 

What is meant by high potential (hipot) testing is not always clear. The term “high potential test­ing” is defined by NEMA (MG1-1.57) and IEEE (IEEE Standard 858) as a test that “consists of the application of a voltage higher than the rated volt­age for a specified time for the purpose of determining the adequacy against breakdown of insulating materials and spacings under normal conditions.” For example, a 5000-volt hipot test on a motor rated 4000 volts would be considered a high potential test. 

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.

This webinar recording focuses on:

• Motor assembly issues
• Electrical and mechanical inspection
• Static and run testing
• AC motors with ball, roller and sleeve bearings

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

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.

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

This presentation shares tips that are not covered in “Fundamentals of DC: Operation and Repair Tips,” such as:

• Tips for interpreting armature and interpole tests
• Finding that ground in the newly rewound armature
• Interpreting questionable drop test results

It also covers final assembly tests including how to determine whether the cause of sparking is the interpoles or the armature.

This presentation is aimed at the experienced technician and supervisor.

Chuck Yung
EASA Senior Technical Support Specialist

This paper covers:

• Interpreting AC and DC drop test results (Is that coil really shorted?)
• Differentiating between interpole and armature problems
• Locating an armature short/ground
• Locating shorted/open equalizers in an armature
• Working neutral: Did that motor arc when it left the factory? Let’s cure that problem!

Scott Clark, Ph.D., P.E.
Brandon & Clark, Inc.

Traditional rotor bar defect detection systems are often insufficient in sensitivity, record-ability and numerical analysis of test results. Many means of rotor defect detection require the motor to be fully assembled with a functional stator. These limitations have laid the groundwork for the development of a new rotor defect detection technique.

A new method termed Rotor Magnetic Field Analysis (RMFA) extends traditional induction motor rotor testing techniques with precision magnetic field measurements and data processing tools. This is the first new technology in rotor testing in more than three decades.

Carick “Joe” Howard
Red Stick Armature Works, Inc.

Es bien conocido por aquellos que trabajan con el almacenamiento de motores eléctricos que las filosofías y procedimientos de mantenimiento varían. La revisión minuciosa de la información de EASA y siete fabricantes de motores diferentes sobre el almacenamiento de motores reveló algunas diferencias y similitudes interesantes en la información disponible en cada una de las fuentes consultadas.

Varias fuentes comparten elementos comunes como el ambiente, la protección contra humedad, el mantenimiento de los rodamientos y la resistencia de aislamiento. Aquí, la finalidad de nuestro debate es describir algunas de las diferencias y ojalá dar a conocer algunos puntos a tener en cuenta cuando se crea un procedimiento de almacenamiento a largo plazo para los usuarios finales.

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.

Jim Bryan
EASA Technical Support Specialist

The squirrel cage induction motor (SCIM) functions by applying a voltage to the stator winding and inducing a voltage across the air gap in the rotor circuit. The squirrel cage rotor consists of a lamination stack with slots to ac­commodate some number of rotor bars and shorting (end) rings that tie all the bars together. The squirrel cage consists of bars and end rings that are typi­cally made from alloys of aluminum or copper. This squirrel cage is illustrated in Figure 1 with the lami­nation stack removed.

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

Chase Fell
Precision Coil and Rotor

Los fallos a tierra, cortocircuitos y malas conexiones en las bobinas de los interpolos, campos serie y devanados de compensación de las máquinas de CC pueden causar problemas de funcionamiento que incluyen: Chisporroteo, flameo (flashover), frenado y fallos catastróficos. Algunas bobinas de campo shunt están bobinadas con muchas espiras y un alambre relativamente delgado y generalmente son excitadas con una fuente de CC independiente a la de la armadura. Por lo general, los campos serie, interpolos y devanados de compensación del circuito de armadura están bobinados con pocas espiras y alambre grueso, ya que por ellos circula la corriente de armadura. Para obtener resultados de prueba precisos asegúrese que los bobinados están limpios y secos y verifique visualmente las conexiones de los campos de baja resistencia. Para detectar problemas de calentamiento irregular o conexiones flojas o corroídas aplique voltaje CC a las bobinas de un estator de CC y realice una inspección termográfica. Compruebe que las marcas de los cables de salida sean las correctas. Estas deben coincidir con los datos de placa del fabricante original (OEM) o con las normas NEMA MG1 o IEC 60034-8, lo que aplique.

Welcome by 2019-2020 EASA Chairman Brian Larry
Presentation by Tom Bishop, P.E., EASA Senior Technical Support Specialist

In 2003, EASA and AEMT (Association of Electrical and Mechanical Trades in the UK) issued a report that proved rewinding motors in accordance with prescribed good practices would maintain efficiency and reliability. But in recent years, claims abounded that premium efficient motors could not be rewound without degrading efficiency.

This recording discusses the latest EASA/AEMT research that included independent, third-party testing.

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

In this article, we will discuss testing of capacitors for electric motors in general and tests associated with specific uses of capacitors such as for power factor correction, and for electric motor starting (see Figures 1 and 2). For information on sizing power factor correction capacitors see Subsection 2.10 of the EASA Technical Manual, and for determining the correct size capacitor for a motor, see Subsection 2.11 of the EASA Technical Manual.

Gene Vogel
Especialista de Bombas & Vibraciones de EASA 

Existen muchas causas comunes de vibración alta en la maquinaria rotativa; Demasiadas para enumerar aquí. Pero a menudo, lo que de otro modo sería un nivel aceptable de vibración se ve amplificado por la resonancia. Todas las máquinas son susceptibles a la resonancia. La resonancia ocurre cuando la frecuencia natural de algún componente de una máquina coincide con una fuerza excitadora. Cuando se produce resonancia, es la combinación de una fuerza excitadora y una frecuencia natural lo que da como resultado una alta vibración; ambos deben estar presentes en la misma frecuencia para que se produzca la resonancia. Cuando la resonancia causa una vibración excesiva, es importante identificar la frecuencia natural y la forma modal de la vibración. Una simple prueba de impacto (bump test), realizada a máquina parada, es un buen primer paso para identificar la frecuencia natural (Figura 1). 

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.

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.

Mike Howell
EASA Technical Support Specialist

When possible, it is good practice to perform an uncoupled, no-load run on an induction motor as an incoming diagnostic test. A no-load run should also be performed after assembly, and ANSI/EASA AR100-2020 states that “no-load running tests should be made at rated voltage and rated frequency.” In this article, we’ll discuss some of the reasons why this is important and some things to consider when you cannot meet both requirements.

Mike Howell
EASA Technical Support Specialist

La precisión y exactitud de los equipos de prueba & medida (M&TE) han sido tratadas en artículos previos de Currents (noviembre y diciembre de 2014). Un tema relacionado que no fue cubierto en dichos artículos es la resolución. El documento JCGM 200:2012 del Joint Committee for Guides in Metrology define resolución como: “El cambio más pequeño en una cantidad medida que causa un cambio perceptible en la indicación correspondiente”. Simplificado, es la diferencia más pequeña que puede ser medida por el equipo en cuestión. La exactitud de la M&TE debe ser mayor (menos exacto) o igual a la resolución. Es decir, durante la calibración, el M&TE debe ser capaz de indicar el valor comparado con el estándar.

Precisión y exactitud
Repasemos brevemente la importancia de la precisión y exactitud. Al recoger la información de las medidas, los técnicos del centro de servicio obtienen datos con dos componentes: El valor auténtico de la medida (valor real) y el error asociado a la medida (componentes de precisión y exactitud). Así mismo, entre más pequeño sea el error de medida, más se acerca la indicación o valor medido a la medida real. Como lo muestra la Figura 1, a menudo los términos precisión y exactitud se demuestran y diferencian gráficamente utilizando el ejemplo de la diana.

La precisión se refiere al grado de repetibilidad y reproducibilidad en el sistema de medida, Repetibilidad es la capacidad que tiene un técnico para obtener la misma medida varias veces midiendo el mismo elemento con el mismo M&TE. Reproducibilidad es la capacidad de varios técnicos para obtener la misma medida midiendo el mismo elemento con el mismo M&TE. Normalmente, la precisión del M&TE es evaluada con estudios de repetibilidad & reproducibilidad (R&R).

La exactitud es el grado en el que la medida concuerda con el valor real. La exactitud de un M&TE es evaluada por calibración.

Resolución
De nuevo, podemos simplificar la resolución como la diferencia más pequeña que puede ser medida con nuestro M&TE. Aunque para cualquier medida la exactitud de nuestro M&TE se debe comparar con nuestro rango de tolerancia aceptable.  Tendemos a ver rápidamente la resolución de un indicador o medidor solo por observación. Por esta razón, la resolución es un buen “primer paso” cuando se selecciona un M&TE para una tarea específica. Es decir, si usted tiene una herramienta con una resolución de 1 cm y necesita medir algo con un diámetro nominal de 1 mm+/-0.1mm, ya debería saber que tiene la herramienta incorrecta para el trabajo. 

Existen algunos ejemplos obvios de malas elecciones que podemos identificar en un típico centro de servicio. Nunca pensaríamos utilizar una balanza industrial para pesar los pesos de balanceo o una regla para medir el diámetro de un alambre magneto. En estos dos casos, sabemos que la resolución de un M&TE probablemente es más grande que el valor medido; si la resolución no está ahí, seguramente la exactitud deseada no estará ahí. La selección del M&TE apropiado depende del propósito de la medición. Para balancear, muchos pueden considerar apropiada una balanza con una exactitud de 0.1 gramos que pese hasta 100 gr. Pero, los centros de servicio que balancean rotores de husillos o conjuntos extremadamente largos pueden necesitar algo diferente. 

Para el alambre magneto, la precisión y exactitud requeridas para identificar simplemente un calibre durante la toma de datos pueden ser muy diferentes a las requeridas para determinar si las dimensiones de una muestra de alambre magneto están dentro de la tolerancia de fabricación de las normas NEMA o IEC. Además, una galga para alambres nunca es una buena opción para medir alambres magneto.

Los M&TE escogidos por cada centro de servicio variarán de acuerdo con los requisitos de diferentes fuentes como clientes y entes reguladores o de certificación. Siempre deben evaluarse primero los requisitos de los clientes antes de tomar cualquier decisión sobre el proceso de negocios. Un centro de servicio cuyo cliente más importante es un lavadero de vehículos puede tener requisitos muy diferentes a uno que repara motores relacionados con la seguridad de una central nuclear. Sin embargo, todos los centros de servicio deben escoger los M&TE adecuados para darles una seguridad razonable en las actividades de seguimiento del proceso e inspección y pruebas que realizan.

Cuando se trata del seguimiento de procesos, para la mayoría de parámetros existen muchos medidores y sensores que varían ampliamente por rango, resolución y exactitud. Por ejemplo, si se usa un manómetro en un sistema VPI donde el proceso está calibrado a 80±5 psi (5.5±0.3 bar) y el manómetro tiene un rango de 0-150 psi (0-10.3 bar), es razonable tener una calibración limitada, tal vez de 70-90 psi (4.8-6.2 bar). La Figura 2 muestra un manómetro que puede usarse de esa forma.

Ahora, veamos un parámetro diferente que debe ser controlado durante el ciclo de vacío-VPI. Durante un proceso de impregnación global-VPI, existe una fase de vacío seco y algunas veces también una fase de vacío húmedo. Normalmente, los niveles de vacío seco deben estar por debajo de los 5 Torr (0.007 bar) y es deseable alcanzar un nivel menor o igual a 1 Torr (0.001 bar), especialmente en estatores con bobinas de pletina. El manómetro de la Figura 2 sirve para algún proceso industrial simple pero no es adecuado para las mediciones de vacío en el proceso VPI de un centro de servicio. Examinemos la resolución de la porción de vacío de la escala, desde 0 hasta 30 pul-Hg. La Tabla 1 muestra las unidades para convertir pul-Hg en Torr. Si estamos interesados en niveles de vacío seco menores o iguales a 5 Torr, resulta evidente por que el manómetro de la Figura 2 es inadecuado. No se puede diferenciar un vacío de 0.5 Torr de un vacío de 10 Torr.

Esto no significa que si su centro de servicio tiene un manómetro de vacío inadecuado, no esté logrando niveles de vacío aceptables- esto solo significa que usted no tiene un control de proceso adecuado y no sabe el nivel de vacío que está obteniendo. Una opción más razonable para medir el vacío en un sistema VPI se muestra en la Figura 3. Un manómetro similar a este puede tener un rango de 0.2 a 20 Torr y una exactitud del 20%.

Los centros de servicio deben evaluar cada medida que afecte la calidad del servicio o producto suministrado. Para cada uno, considere el rango de valores posible, así como también la precisión y exactitud de los M&TE necesarios para realizar el trabajo. incluso para los técnicos más calificados y experimentados, contar con los M&TE es crítico para la disposición adecuada de cualquier máquina o componente.

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
EASA Senior Technical Support Specialist

Cuando se considera la construcción de un gran growler para probar rotores y armaduras, la decisión inicial típica es seleccionar la potencia en kVA. La razón principal para esto es que el growler necesitará ser conectado a una fuente de alimentación que tenga suficiente amperaje. Para ayudar a simplificar el complejo proceso de diseño, en este artículo hemos seleccionado cinco potencias expresadas en kVA. Uno de los valores de potencia seleccionados cumplirá con las necesidades de la mayoría de los centros de servicios.

Este artículo abarca:

• Un ejemplo de diseño
• Determinación de las vueltas y el tamaño del cable
• Construyendo el núcleo
• Determinación de las dimensiones de la bobina

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.

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.

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.

Mike Howell
Especialista de Soporte Técnico de EASA

Como forma de diagnóstico y cuando sea posible, una buena práctica consiste en probar en vacío un motor de inducción que entra al centro de servicios. Esta misma prueba también se debe efectuar después del ensamblaje y la norma ANSI/ EASA AR100-2020 establece que "las pruebas en vacío se deben realizar a la tensión y frecuencia nominales". En este artículo, discutiremos algunas de las razones por las que esto es importante y algunos factores a considerar cuando no se puede cumplir con ambos requisitos.

Prueba Dinámica/en Vacío
Cuando un motor de inducción funciona desacoplado a tensión y frecuencia nominales, la velocidad del rotor debería estar muy próxima a la velocidad síncrona, que es proporcional a la frecuencia. Por lo general, al medir la velocidad del rotor esta estará dentro de 1 rpm de la velocidad síncrona. Además, el campo magnético en el motor es proporcional al voltaje aplicado e inversamente proporcional a la frecuencia de red y a menudo esta relación se denomina simplemente voltios por hercio (V/Hz). La corriente en vacío es predominantemente la corriente de magnetización y se deben evaluar los voltios por hercio nominales. La Figura 1 muestra los valores típicos de la corriente en vacío (NLA) en porcentaje de la corriente nominal (FLA) vs. la potencia nominal en motores de 2, 4 y 6 polos. En general, la corriente en vacío en porcentaje de la corriente nominal aumentará a medida que aumente el número de polos y disminuirá a medida que aumente la potencia de salida.

Si no se dispone de un variador adecuado, los motores diseñados para trabajar con inversores se deben operar a los voltios por hercio nominales con las siguientes excepciones: No se debe exceder el voltaje nominal y la velocidad máxima indicada en la placa de datos. Por ejemplo, si se usa un tablero de prueba de 60 Hz, un motor clasificado para 400 V 20 Hz (20 V/Hz) sin una velocidad máxima segura publicada tendría que operarse a 1200 V 60 Hz para alcanzar los voltios por hercio nominales; superando por mucho la tensión nominal y la velocidad base (3x nominal).

Si un motor funciona por debajo de la frecuencia de línea máxima (velocidad de rotación), la resonancia y las velocidades críticas del eje que podrían ocurrir a frecuencias más altas no serán evidentes. Por esta razón, efectuar pruebas de vibración en motores por debajo de la velocidad máxima no garantiza el cumplimiento de las tolerancias cuando el motor funciona a la frecuencia de línea y velocidad máxima.

Si un motor funciona por debajo de los voltios por hercio nominales, los problemas asociados con el ruido electromagnético y la vibración podrían enmascararse. Las ondas de fuerza que causan la vibración electromagnética son casi proporcionales al cuadrado de los voltios por hercio. Por ejemplo, si una máquina de 400 V 80 Hz (5,0 V/Hz) funciona a 230 V 60 Hz (3,8 V/Hz), cualquier vibración electromagnética se reduciría aproximadamente al 60 % de su valor normal.

Otro problema con el funcionamiento por debajo de los voltios por hercio nominales es que la corriente en vacío se reducirá y, aunque la relación será aproximadamente lineal en algún rango, la reducción severa dará como resultado un deslizamiento notable. La relación entre los voltios por hercio y la corriente será menos predecible y difícil de evaluar. Además, si hay otros problemas que habrían sido evidenciados al probar con los voltios por hercio nominales (por ejemplo, error en el devanado, conexión incorrecta), es posible que no sean fáciles de detectar.

Operar por arriba del 10% de los voltios por hercio nominales excede los límites permitidos de la mayoría de las máquinas. Dado que la relación entre los voltios por hercio y la corriente de magnetización se vuelve no lineal a medida que los núcleos del estator y del rotor se acercan a la saturación magnética, no es factible evaluar la corriente en vacío. Por ejemplo, un motor de 200 V 100 Hz (2,0 V/Hz) que funciona a 208 V 60 Hz (3,5 V/Hz) podría acercarse más a la corriente de rotor bloqueado que a la corriente de vacío normal. Cuando se comete este tipo de error en la prueba, el devanado del estator puede dañarse muy rápidamente.

Los centros de servicio que reparan regularmente motores accionados con inversores deben considerar invertir en equipos con capacidad de prueba a voltaje y frecuencia variables. Para obtener información adicional sobre la prueba de motores accionados con inversores y adquirir un VFD para su centro de servicio, los siguientes artículos publicados en la revista Currents deberían ser útiles y están disponibles en la biblioteca de recursos de easa.com:

• “Testing Methods for Induction Motors for Use in VFD-Powered Applications” de noviembre del 2014
• “Selección y uso de un variador de frecuencia electrónico para hacer pruebas en un centro de servicios” de enero del 2016.

La prueba de balanceo de fases no está estandarizada y tiene muchos nombres que incluyen, pero no se limitan a: Prueba de impedancia de estator abierto, prueba de estator bobinado, prueba del balín y prueba con rotor falso. Muchos centros de servicio utilizan esta prueba de alguna forma para solucionar problemas y como control de calidad antes de barnizar/resinar el devanado. El enfoque típico es aplicar un voltaje trifásico balanceado en los terminales del devanado del estator con el rotor afuera y luego evaluar el balanceo y la magnitud de las corrientes resultantes. Los criterios de aceptación difieren, pero es razonable que el desequilibrio de corriente esté dentro del 10 % de la corriente media. Además, con la impedancia más baja que resulta al tener el rotor fuera del estator, se debería medir la corriente nominal al aplicar entre el 12 y el 20 % de los voltios nominales (voltios por hercio). Por ejemplo, si un motor tiene una potencia nominal de 460 V 60 Hz (7,7 V/Hz), normalmente se obtendría una corriente nominal si se aplican entre 55 V y 90 V a 60 Hz. Si los datos de placa son 460 V 200 Hz (2,3 V/Hz), se esperaría la corriente nominal a 60/200 = 30 % de ese rango o a 17 V y 27 V, si se prueba a 60 Hz.

A special discounted collection of 6 webinar recordings focusing on DC motor electrical procedures.

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: Understanding DC Motor Tests
Presented October 2016

• Ampere turns of the armature, field and interpole data
• Determining the best armature coil pitch
• Verifying interpole circuits
• Importance of brush angle
• Equalizers and armature windings

Adjusting Brush Neutral
Presented June 2011

The webinar covers:

• How to set brush neutral in DC machines.
• Several methods of setting brush neutral along with the benefits and drawbacks of each.
• Tips for permanent magnet and series-would machines.
• Tips on how to recognize problems and settings that affect brush neutral, and what to check if the neutral adjustment seems higher than usual.

Target audience: This presentation is most useful for service center and field technicians involved in the repair of DC machinery, service center managers engineers, or anyone involved in DC motor or generator repair, as well as those who are simply looking to expand their understanding.

Carbon Brushes, Current Density and Performance
Presented June 2019

The lowly brush is underrated and misunderstood. The brush grade, brush pressure and spring tension, as well as the effect of load and humidity are each important to brush performance in DC machines, wound rotor motors, and synchronous machines.

This presentation covers:

• Importance of brush grade
• Effect of humidity and load (current)
• Best practice method for removing brushes to improve performance
• Brush pressure & spring tension by application
• Supplemental cooling of slip ring / brush enclosures

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

Drop Testing of Fields and Synchronous Poles: Tips to Interpretation
Presented November 2011

This presentation covers:

• The basics of drop testing, as well as offers tips for interpreting the results.
• Both the AC and DC drop test are described as well as the advantages and drawbacks for each.
• For those cases where the drop test results are out of tolerance, this material will guide the technician in determining the reasons for the variation-how to recognize the difference between shorted coils and differences in iron, airgap or other influences.
• Rewind and assembly tips will also be discussed, where they influence the results of the drop test.

Target audience: This presentation is most useful for service center and field technicians with at least 5 years experience, service center managers, engineers, or anyone involved in DC motor or generator repair, as well as those who are simply looking to expand their knowledge.

Final Testing of DC Machines
Presented September 2011

To assure a quality repair, there specific tests (such as neutral-setting and interpole-armature polarity) that should routinely be performed on every DC machine. When done correctly, the simple procedures presented will prevent scenarios such as that late night phone call from an irate customer whose DC machine is "arcing like a fireworks show."

Target audience: Technicians with at least a moderate lever of experience in DC machine repair will benefit from this session.

Advanced DC Testing
Presented April 2012

This presentation shares tips that are not covered in “Fundamentals of DC: Operation and Repair Tips,” such as:

• Tips for interpreting armature and interpole tests
• Finding that ground in the newly rewound armature
• Interpreting questionable drop test results

It also covers final assembly tests including how to determine whether the cause of sparking is the interpoles or the armature.

Target audience: This presentation is aimed at the experienced technician and supervisor.

Chuck Yung 
EASA Technical Support Specialist 

EASA’s Engineering and Technical Support Department has received many calls over the past few months concerning repairs on DC motors. Most callers have asked for tips on troubleshoot­ing to make sure repairs are handled correctly. 

Prior to assembly, all windings should be tested for shorts, grounds and correct polarity. 

After a DC motor is assembled and ready to test run, a few simple checks will greatly reduce the chance of motor problems.  The following procedures should be especially helpful to those shops that don’t have a dyna­mometer. 

Chuck Yung
EASA Senior Technical Support Specialist

More than 400 people have at­tended EASA’s Fundamentals of DC Operation and Repair Tips seminar since it was introduced in 2003. Even the most experienced and well-trained DC technicians will appreciate picking up some more testing tips. 

Richard Huber, P.E.
Richard Huber Engineering, Ltd.

The intent of this article is to provide an overview of the more common techniques used for dissolved gas analysis (DGA) of mineral oil.

This presentation covers:

• The basics of drop testing, as well as offers tips for interpreting the results.
• Both the AC and DC drop test are described as well as the advantages and drawbacks for each.
• For those cases where the drop test results are out of tolerance, this material will guide the technician in determining the reasons for the variation-how to recognize the difference between shorted coils and differences in iron, airgap or other influences.
• Rewind and assembly tips will also be discussed, where they influence the results of the drop test.

Target audience: This presentation is most useful for service center and field technicians with at least 5 years experience, service center managers, engineers, or anyone involved in DC motor or generator repair, as well as those who are simply looking to expand their knowledge.

Kirk Kirkland
Electrical Repair Service Co.
Birmingham, Alabama
Technical Education Committee Member

Progressive end users/motor own­ers these days have a greater apprecia­tion for and understanding of motor reliability.

However, many no longer accept the premise that a quality rebuild is accomplished because experienced, well trained technicians serviced a motor and applied static and/or run testing which resulted in a motor with improved reliability. Many end users have developed stringent motor re­pair specifications to which the motor rebuilder must comply. 

More frequently, end users are requiring that the service center pro­vide proof of compliance. This often equates to a series of qualifying tests that may include having the service provider perform a load test on the serviced motor. The load test can be the “acid test” which validates that the repair services rendered meets with the end user’s specifications and/or the original manufacturer’s design criteria. Some end users are exposed to non-compliant issues when their driven equipment is the “load test.” 

REVISED September 2022!

The EASA Technical Manual, containing more than 900 pages of information specific to electric motor service centers, is available FREE to EASA members as downloadable PDFs of the entire manual or individual sections. The printed version is also available for purchase. Each of the 13 sections features a detailed table of contents.

VIEW, DOWNLOAD OR PURCHASE

Chuck Yung
EASA Senior Technical Support Specialist

Although the rotating equipment repair industry has been around for over a century, technology continues to introduce new test instruments and procedures. Some of these are good: surge test, growler, core loss test; some are bad: core testing a rotor at 60 times its operating frequency, or performing a Hipot at several times the prescribed value; and some are just plain ugly.

This paper will help you to sort out which are which, and help educate your customers as to the reasons why. Standards organizations (IEEE, ANSI, IEC) have developed specific tests, with much scientific thought as to how stringent a test should be. ANSI/EASA AR100: Recommended Practice for the Repair of Rotating Electrical Apparatus consistently references the relevant standard(s) for each test.

This paper, presented at the 2013 EASA Convention, summarizes the accepted and other electrical tests required by motor and generator end users. It covers:

• Various standards (IEEE, IEC, NEMA, ANSI and API) that describe and legitimize most of the tests used by our industry
• Other tests, not supported by any recognized standards, that end users request repairers to perform
• An outline of these tests, with supporting standards, which should be useful when discussing testing requirements with end users

A special discounted collection of 7 webinar recordings focusing on various aspects of electromechanical repair.

Once purchased, all 7 recordings will be available on your "Downloadable products purchased" page in your online account.

Downloadable recordings in this bundle include:

Time-Saving Repair Tips
Presented August 2014

This webinar shares:

• The secrets used by other service centers to gain a competitive edge in the repair process.
• Mechanical, winding and machining tips reduce repair time, help avoid unnecessary rework, and decrease turn-around time.

Target audience: Supervisors, machinists, mechanics, winders, and sales personnel who interact with the end user.

Repair Best Practices to Maintain Motor Efficiency
Presented June 2012

There are certain repair processes, such as winding removal and replacement, that can impact the efficiency and reliability of electric motors. Prudent repair practices must not increase overall losses, and preferably should maintain or reduce them.

This presentation explains how those repair processes affect efficiency and reliability, and gives the best repair practices in order to maintain or improve efficiency.

Target audience: This presentation is most useful for service center inside and outside sales representatives, customer service personnel, engineers, supervisors and managers. The content will be beneficial for beginners through highly experienced persons.

Practical Problem Solving for the Entire Service Center
Presented August 2013

This presentation focuses on a report format developed by Toyota for a simple, yet methodical approach to document improvement. Whether you're dealing with problems related to sales, purchasing, repair or testing, if all team members can learn to speak the same, simple problem-solving language, they can tackle problems efficiently and effectively.

Target audience: This presentation is best suited for executives, managers, team leaders and front line supervisors from the office and service center who want to understand and implement such a program.

Induction Motor Speed Control Basics
Presented March 2019

Induction motors are most often applied to what are essentially constant speed drive applications. However, the use of induction motors in variable speed applications continues to grow, primarily due to technology advances in power electronics. This webinar will review speed control basics for induction machines.

• Wound-rotor motor speed control
• Squirrel-cage speed control by pole changing
• Squirrel-cage motor speed control by variable voltage, fixed frequency
• Squirrel-cage speed control by variable voltage, variable frequency

AC Motor Assembly and Testing
Presented August 2018

This webinar recording focuses on:

• Motor assembly issues
• Electrical and mechanical inspection
• Static and run testing
• AC motors with ball, roller and sleeve bearings

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

On-Site Testing & Inspection of Electric Motors
Presented July 2015

This webinar covers electrical testing and inspection of installed electric motors, including:

• Condition assessment for continued service
• Diagnostic fault testing and interpretation
• Physical inspection key points

Selecting Replacement DC and 3-Phase Squirrel Cage Motors
Presented September 2019

On many occasions, a different motor type is desired or needed. In these cases it is essential that the replacement motor provides the required performance, and do so reliably.

This presentation focuses primarily on the electrical aspects of selecting replacement motors. It also addresses speed and torque considerations.

• DC motor to DC motor
• DC motor to 3-phase squirrel cage motor
• AC motor to 3-phase squirrel cage motor

Target audience: Anyone involved with selecting replacement motors or diagnosing issues with replacement motor installations.

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
Especialista Sénior de Soporte Técnico de EASA

El problema más frecuentemente relacionado con la corriente alta en un motor, es la elevada corriente en vacío de los motores trifásicos. Este tema ya había sido tratado en tres artículos publicados anteriormente en la revista Currents de EASA, empezando con “Test Run Tips: Common Causes For High No-Load Current On Rewound Motors” (Junio de 2002); continuando con “Avoiding High No-Load Amps On Rewound Motors” (Febrero de 2004); y finalizando con “Taming Those Misbehaving Motors” (Diciembre de 2009). Este artículo cubrirá el amplio tema  de la corriente alta en vacio y el problema de la corriente con carga elevada en los motores trifásicos. También en él se consideran casos en los cuales existe una corriente en vacio menor que la esperada.

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. 

Chuck Yung
Especialista Sénior de Soporte Técnico de EASA

Cuando se prueban armaduras de CC, ya sea al entrar a reparación o una vez rebobinadas, una pregunta que escucho muy a menudo incluye la interpretación de los resultados de la prueba de impulso (barra-barra de alta frecuencia).

Chuck Yung
EASA Senior Technical Support Specialist

When testing DC armatures, whether incoming for repair or after completing a rewind, one question I often hear involves interpreting the surge test (or the high-frequency bar-to-bar test) results. There is a lot to our interpretation of the bar-bar test or surge test.

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

This article addresses electrical testing and inspection of installed 3-phase squirrel cage motors. The main purposes of testing installed electric motors are condition assessment for continued service or to diagnose suspected faults. The emphasis here will be on diagnostic electrical testing and interpretation, as well as physical inspection key points. Note: Most of the tests and inspections described in this article can also be performed on 3-phase wound rotor motors, and induction and synchronous generators.

The pump repairs are completed! Now the pump needs to be tested. This presentation discusses the procedures for the basic tests that can be performed on pumps that have been repaired in the service center.

Final testing of pumps can include:

• Operational tests
• Seal leakage test
• Motor chamber leakage test (submersibles)
• Casing pressure test

While some of these tests are not difficult to perform, knowing the methods and limits will help service centers to confidently deliver quality pump repairs.

To assure a quality repair, there specific tests (such as neutral-setting and interpole-armature polarity) that should routinely be performed on every DC machine. When done correctly, the simple procedures presented will prevent scenarios such as that late night phone call from an irate customer whose DC machine is "arcing like a fireworks show."

Technicians with at least a moderate lever of experience in DC machine repair will benefit from this session.

This book ws developed in conjuction with EASA's two-day Fundamentals of DC Operation & Repair Tips seminar.

This book is not meant to replace the many good texts that cover the theory and design of DC machines, but to supplement them. Its purpose is twofold: to help the technician understand DC machine theory without complex formulae; and in a larger sense, to record in one place the repair procedures and tips usually learned the hard way during a long career of DC machine repair. It may take a decade or longer for a technician to become proficient and knowledgeable. We hope this book will cut many years from that timeline.

The text begins with DC theory (no math, we promise!), and then follows the logical progression of a DC machine through the service center. Disassembly, inspection and testing are covered in the initial chapters. 

Subsequent chapters are organized around the main parts of a DC machine. The final chapters cover assembly, final testing and application issues. Sections focusing on components explain how those parts work, how they are made and how they can best be repaired.

Repair tips gleaned from EASA members’ decades of experience are liberally sprinkled throughout the book. While many texts about DC machines explain how they should work, this is the first (to our knowledge) to discuss all the exceptions that a repairer is liable to run across during a lifetime of working with DC machines. These might otherwise be labeled “lessons learned the hard way,” except that the reader can benefit from having all these special cases collected in one source. When possible, it is better to learn by reading than by trial and error; otherwise, the first encounter with a unique design can result in a painful “learning experience.”

A DC machine can be used interchangeably as a motor or generator, simply by changing the connection. Any DC motor can be driven and used to produce power, and any DC generator can be motorized to provide mechanical power. Although this text predominately refers to “motor;” the material applies to both motors and generators.

As with the other EASA publications—Principles of Large AC Motors, Mechanical Repair Fundamentals of Electric Motors, and Root Cause Failure Analysis—each section is designed to stand alone. The small amount of duplication is intentional, to save the reader from flipping back and forth between sections.

Table of Contents - (Download the complete Table of Contents)

• Nomenclature and Nameplate Information
• DC Motor Theory
• Disassembly and Inspection
• Testing
• Armatures
• Commutators
• Frames
• Ventilation and Accessories
• Motor Assembly and Final Testing
• On-Site Troubleshooting
• Failure Analysis

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Mike Howell
Especialista de Soporte Técnico de EASA

La mayoría de los centros de servicio realizan algún tipo de prueba por comparación de impulsos, aunque la terminología y la metodología pueden variar. En términos simples, se comparan las respuestas o formas de onda obtenidas al aplicar  pulsos con tiempos rápidos de subida y si existe una diferencia excesiva, la unidad bajo prueba podría estar defectuosa. La forma de onda producida por el pulso es exclusiva de la unidad bajo prueba, que por ejemplo podría ser el devanado de un estator. La forma de onda será función de la resistencia, capacitancia e inductancia del circuito bajo prueba y un buen número de variables pueden afectar esas características.

Una dificultad o reto de la prueba por comparación de impulsos ha sido su subjetividad. Es decir, que para quienes realizan la prueba no siempre es fácil obtener la misma conclusión al comparar dos formas de onda. Durante las dos últimas décadas, varios fabricantes de equipos han comenzado a utilizar métodos analíticos para evaluar los resultados de la prueba por comparación de impulsos. El objetivo es eliminar tanto como sea posible la mayor cantidad de subjetividad, de tal forma que el operador pueda decidir de forma sencilla lo que debe hacer con la unidad. El método de análisis más utilizado, en diferentes formas, es el Error de Relación de Área (EAR) .

¿Necesita estar capacitado en EAR para poder realizar las pruebas por comparación de impulsos de forma satisfactoria? No, pero si usted cuenta con dicha formación, un conocimiento básico de los datos reportados por el equipo le puede ayudar a tomar una decisión bien fundamentada.

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.

Mike Howell, PE
EASA Technical Support Specialist

The phase balance test is briefly described in section 4.2.8 of ANSI/EASA AR100-2020. Other names for this test include open stator impedance test, ball test, small rotor test and dummy rotor test. The phase balance test is used in some form by many service centers both as a troubleshooting test and a quality control check before winding treatment. The typical approach is to apply a reduced and balanced three-phase voltage to the stator winding terminals with the rotor removed and then to evaluate the resulting current balance and magnitude. Acceptance criteria differ, but it is a reasonable expectation that the current should be balanced within 10% of the average current.

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.

Gene Vogel
EASA Pump & Vibration Specialist 

There are many common causes of high vibration on rotating machinery; too many to list here. But often, what would otherwise be an acceptable level of vibration is amplified by resonance. All machines are susceptible to resonance. Resonance occurs when the natural frequency of some machine component coincides with an exciting force. When resonance occurs it is the combination of exciting force and a natural frequency that results in high vibration; both must be present at the same frequency for resonance to occur. When resonance does cause excessive vibration, it is important to identify the natural frequency and the mode shape of the vibration. A simple bump test, conducted with the machine not running, is a good first step in identifying the natural frequency (Figure 1).

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.

Chuck Yung
EASA Senior Technical Support Specialist

One of the more mundane things we as repairers must be concerned with is motor storage. For many of us, storing large motors for major customers is its own profit center. For all of us, being aware of how our customers store the motors we repair and send to them is critical to customer satisfaction. A poorly stored motor is likely to suffer winding or bearing failure, and we don’t want unrealistic warranty claims over something outside our control.

Our primary concerns when storing motors, especially long-term, are windings, bearings and shaft sag.

Tom Bishop, P.E., and Chuck Yung
EASA Senior Technical Support Specialists

The typical service center repairs at least 300 motors per technician annually. Saving 8 minutes (0.133 hours) per job equates to: 300 x 0.133 = 40 man-hours per year—a full week of labor per employee. It is not unrealistic to expect twice that much savings, just by implementing some of these timesaving tips.

We all know that seemingly small time savings can significantly improve the bottom line. For a service center with a 12% return on investment (ROI), shaving a few minutes off each job is the equivalent of adding 2 manmonths of billing per productive employee.

For a 10-man service center, with a shop rate of $75 per hour, 20 man-months times 75 = $258,000. To add a quarter-million dollar account usually means adding personnel, sales maintenance, and risk of bad debt/warranty expense. However, steps that streamline efficiency continue to pay dividends.

Topics covered include:

• Layout and workflow
• Time killers
• Time: Is every hour on the job billable?
• Time-saving equipment
• Attitude and productivity
• Communicating effectively
• Training
• Lighting
• Calibration
• Storage/handling/procurement
• Parts storage
• Examples from real service centers

Doug Moore
Kentucky Service Co., Inc.

Many EASA members perform service on pumps of some type and have had the customer return the pump or call back to say it leaks or it still will not pump. We solved this by making a very inexpensive test center for all types of pumps: flooded suction, immersion lube, submersible, centrifugal, deep well and many others.

Cyndi Nyberg Esau
Former EASA Technical Support Specialist

In today’s economic climate, service centers look for opportunities to expand their services to their customers, and therefore profits. End users are also looking to reduce costs as well as downtime. Predictive and preventative maintenance has become increasingly important in industry.

Infrared thermography (IR) has traditionally been associated with inspection of switchgear and motor control centers (MCCs), a service that has become highly competitive. This paper will focus on niche opportunities for the service center – offering or using IR for more specialized services. 

Monitoring the condition of electric motor systems can detect problems that otherwise would not appear until a catastrophic failure occurs. By using thermography, abnormal heat sources that are invisible to the naked eye can be detected and remedied before a failure. This monitoring is not only limited to the motor. The motor system includes the driven equipment, MCC, cable runs, protective devices and the power supply. 

While IR will be the focus of this paper, there are other tests and technologies that will be necessary to complete the evaluation of a motor system. Although a thermal image may indicate that excessive temperatures are present, more information is usually necessary to fully assess a problem. Tools like vibration analysis, current signature, insulation resistance testing, and even visual inspection, all work in conjunction with thermography to paint a complete picture. 

The use of thermography is not limited to field service. The service center can employ this technology during the repair of rotating machinery. Applications include but are not limited to testing for open rotors and assessing the condition of stator and armature cores, windings, bearings and shop equipment.

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

Este artículo cubre las pruebas eléctricas y la inspección de motores trifásicos jaula de ardilla que han sido instalados. Los principales objetivos al probar los motores en el sitio de trabajo son: Evaluar su condición para garantizar su funcionamiento continuo odiagnosticar presuntos fallos. Aquíharemos énfasis en las pruebas y enla interpretación de los resultados, asícomo también, en la inspección física de los puntos clave. Nota: La mayoría de las prue­bas descritas también se pueden realizar a los motores con rotor bobinado y en los gen­eradores sincrónicos y de inducción. 

Luther (Red) Norris 
Quality Solutions Co. LLC 
Greenwood, Indiana 
Technical Services Committee Member
 
Brushless servo motors ARE electric motors; therefore many of the tools needed to test them are already available in an electric motor service center. In this article, I have listed some instruments and tools that will be needed to service servo motors.

For the purpose of simplifying the instruments and tools needed for brushless servo motor repair, I am going to break them into two groups. 

• Group 1—those usually found in an electric motor service center.
• Group 2—those that may not usually be found in an electric motor service center. 

This webinar recording covers:

• Insulation system versus insulation materials
• Stresses imposed on insulation systems
• Insulation system components / functions
• Typical testing of system components / functions

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

Note: This "encore" technical article first appeared in the September 2004 issue of Currents.

ll of us in the electrical appara­tus service industry test the winding ground insulation resistance of ma­chines such as motors and generators. A frequent question is: What is the minimum acceptable megohm (M.) value for this winding? The good news is that there is a standard that identi­fies minimum values for insulation resistance of rotating machines. 

That standard is the “IEEE Recom­mended Practice for Testing Insulation Resistance of Rotating Machinery,” IEEE Std 43-2000. The EASA “Recom­mended Practice For The Repair Of Rotating Electrical Apparatus,” ANSI/EASA AR 100-2010, uses IEEE 43 for its insulation resistance test references. Note that IEEE 43 only applies to rotat­ing machinery. There is no equivalent standard for non-rotating electrical machinery such as transformers. In this article we will delve into determining minimum insulation resistance for rotating electrical machinery. 

This webinar covers:

• Types of tests: Insulation resistance, Polarization index, High potential, Surge, Partial discharge test
• Testing of machines: Motors (AC and DC), Generators (AC and DC)

Target audience: This webinar will be most useful for service center supervisors, electromechanical technicians, winders and field service personnel.

Gene Vogel
EASA Pump & Vibration Specialist
and
Walter Barringer
Mobius Institute, Knoxville, TN

Temperature is hot or cold, pressure may be high or low and a tank may be full or empty. But vibration cannot be adequately described by a single parameter. Vibration is composed of amplitude, frequency and phase. Overall amplitude may be used as a simplistic indicator of machinery condition; much like a noise could be described as loud or soft, even though there is a big difference between the scream of a siren and the roar of a train. And so it is with vibration.

The siren sounds different than the train because they are different frequencies. In the same way, the vibration from a failing rolling element bearing can be distinguished from coupling misalignment. This combination of vibration amplitude and frequency is the most common and useful vibration data for determining machinery condition, and analyzing machinery vibration problems. The phase angle of the vibration plays an important role in dynamic balancing and advanced analysis. The analysis of vibration amplitude and frequency as represented in the vibration spectrum, is the topic of this paper.

This paper covers how to get the vibration spectrum and what it means, including:

• Wave form
• Displacement
• Velocity
• Demodulation

Vicki Warren, Iris Power
Mississauga, Ontario
Brian F. Moore, Georgia Power
Atlanta, Georgia

Surveys have shown that stator winding insulation failure account for about 40% of motor failures in motors rated 2300V and above. In addition, the work force in general is losing its technical experience. This impacts both the customers we serve and our own internal work force that fixes the equipment.  Lastly, there seems to be a shift toward a more political type customer base that is less likely to own up to their contribution to motor failures. These reasons combine to force motor shops into better testing to know that a more reliable product is being shipped.

Several old and new test methods have recently gained popularity with AC induction motor maintenance specialists.  This paper, presented at the 2013 EASA Convention, will examine Power Factor Tip-up and Partial discharge testing to assess stator winding conditions for motors rated 2300V and above. Both tests will be evaluated for: effectiveness; which windings/types of machines the test is effective; set-up; interpretation and limitations.

Topics discussed include:

• Brief review of stator winding failure mechanisms
• Brief review of power factor and power factor tip-up
  • The theory/math behind it
  • Georgia Power’s use as a sorting tool
• Partial discharge terms that apply to power factor and tip-up testing
  •   Inception and extinction voltage
  •   Magnitude
  •   Polarity
• Reading actual power factor and test data sheets
  •   Advantages and limitations of off-line tests
  •   Deciding if there is a problem or not
• Case studies: What to do next (if you suspect a problem)
  • Partial discharge testing (brief theory and expected results)
  • Dynamometer testing or full-load testing at the customer’s plant us

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

The most common method of load testing motors is dynamometer testing. In this article, we will address the reasons for performing load tests as well as methods of load testing motors. Further details on dynamometer testing are given in Tech Note 5 "Dynamometer Testing Electric Motors," which can be found in the EASA Technical Manual. The primary reason for load testing is to verify that the motor produces torque corresponding to the nameplate parameters such as horsepower/kilowatts, speed, voltage and current. If it is a DC motor, another key consideration would be commutation (i.e., to verify that there is no sparking at the brushes).

Chase Fell
Precision Coil and Rotor

Ground faults, short circuits and bad connections in interpole coils, series coils and compensating windings cause performance problems in DC machines, including brush sparking, flashover, stalling and catastrophic failure. Shunt coils have many turns of relatively small wire and are usually excited by a DC source independent of the armature. Series, interpole and compensating coils in the armature circuit usually are wound with a few turns of heavy wire as these coils carry armature current.

For accurate test results, make sure windings are clean and dry. Verify connections of low resistance fields by visual inspection. Apply DC voltage to an assembled field frame and perform a thermography scan to detect problems including uneven heating and loose or corroded connections. Verify that the terminal lead markings are correct. Lead marking should conform to the original equipment manufacturer (OEM) nameplate, NEMA MG1 or IEC 60034-8, whichever is applicable.

Chuck Yung
Especialista de Sénior de Soporte Técnico de EASA

Una de las cosas más mundanas de las que debemos preocuparnos como reparadores es el almacenamiento de los motores y para muchos, almacenar motores grandes para clientes importantes representa ganancias. Para todos nosotros, ser conscientes de cómo nuestros clientes almacenan los motores que les reparamos es crítico desde el punto de vista de la satisfacción del cliente. Es probable que un motor mal almacenado sufra fallos en el devanado o en los rodamientos, y no queremos reclamos por garantía poco realistas sobre algo que está fuera de nuestro control.

Nuestras principales preocupaciones al almacenar motores, especialmente a largo plazo, son los devanados, los rodamientos y el pandeo del eje.

This webinar covers :

• Quantities/qualities of measuring and test equipment utilized in the industry
• How to locate/source qualified calibration suppliers
• Costs of internal / external calibration for some measuring and test equipment

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.

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

It’s impossible to balance line-to-line voltages perfectly in three-phase circuits, so they typically differ by a few volts or more. However, if voltage unbalance exceeds 1%, it can markedly decrease the performance and energy efficiency of three-phase motors while increasing the likelihood of premature failure. Avoiding these issues requires a proactive approach that includes installing adequate protective devices and periodically checking for voltage unbalance at the motor terminals.

The article covers:

• What it is
• Common causes
• Effects of unbalanced voltage
• Testing for unbalanced voltages and single-phasing
• Ways to correct unbalanced voltages

READ THE FULL ARTICLE

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.

Mike Howell, PE
Especialista de Soporte Técnico de EASA 

La prueba de equilibrio de fases se describe brevemente en la sección 4.2.8 de la norma ANSI/EASA AR100-2020. Otros nombres para esta prueba incluyen prueba de impedancia de estator abierto, prueba del balín, prueba de rotor pequeño y prueba de rotor artificial. La prueba de equilibrio de fases se utiliza de alguna forma en muchos centros de servicio, tanto para solucionar problemas y también como control de calidad antes del tratamiento final del devanado. El enfoque típico es aplicar un voltaje trifásico reducido y balanceado en los terminales del devanado del estator sin instalar el rotor y luego evaluar el balance y la magnitud de la corriente resultante. Los criterios de aceptación difieren, pero una expectativa razonable es que la corriente debe estar equilibrada dentro del 10% de la corriente promedio.

This webinar covers electrical testing and inspection of installed electric motors, including:

• Condition assessment for continued service
• Diagnostic fault testing and interpretation
• Physical inspection key points

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.
 

Bram Corne
Miembro del Comité Técnico de Servicios
Orbits
Kor trijk, Belgium 

Sin exagerar, los sistemas de motores eléctricos desempeñan un papel fundamental en nuestras operaciones industriales, ya que actúan como el corazón de numerosos procesos. Las bombas, compresores, ventiladores y otros tipos de movimientos mecánicos son cruciales para nuestras actividades industriales. En consecuencia, la salud de estos motores influye directamente en la confiabilidad operativa y la longevidad del ecosistema industrial más amplio. En una cultura que valora la confiabilidad, la previsibilidad del tiempo de inactividad, la disponibilidad de los repuestos, la empleabilidad de los técnicos y la limitación de los daños consecuentes son primordiales. Reconocer esto resalta la necesidad de evaluar la salud de los motores eléctricos en operación. Siguiendo el ejemplo del campo médico, nuestro objetivo es predecir de forma proactiva fallos catastróficos antes que ocurran. Al hacerlo, es posible evitar que se produzcan o progresen. Por lo tanto, la integración de las mediciones y los diagnósticos adecuados para los motores eléctricos puede mejorar significativamente el rendimiento general de los activos.

Bram Corne
Technical Services Committee Member
Orbits
Kor trijk, Belgium 

Without exaggeration, electric motor systems play a pivotal role in our industrial operations, serving as the heartbeat of numerous processes. Pumps, compressors, fans and other types of mechanical movement are crucial to our industrial activities. Consequently, the health of these motors directly influences the operational reliability and longevity of the broader industrial ecosystem. In a culture that values reliability, the predictability of downtime, the availability of spare parts, the employability of technicians, and the limitation of consequential damage are paramount. Acknowledging this underscores the necessity for performing online health assessments of electric motors. Mirroring the medical field, we aim to proactively predict catastrophic failures before they occur. By doing so, it is feasible to prevent failures from occurring or progressing. Hence, integrating the appropriate measurements and diagnostics for electric motors can significantly enhance overall asset performance.

Asset Health Assessment goes beyond traditional maintenance approaches by offering a holistic perspective on an electric motor’s condition. This method employs a comprehensive suite of diagnostic tools and methodologies, beginning with the creation of an exhaustive overview of the entire system. This is achieved by gathering information, briefing operators and conducting visual inspections. Beyond these fundamentals, technologies such as Electrical Signature Analysis (ESA), Vibration Analysis, Ultrasound Analysis, Thermal Analysis, among others, can furnish deeper insights into the condition of the electric motor (Figure 1). This article primarily focuses on the benefits of incorporating ESA into the existing diagnostic toolkit.

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

Tom Bishop
EASA Senior Technical Support Specialist

When considering building a large growler for testing armatures and rotors, the initial decision typically is to select a kVA rating. A primary reason for this is that the growler will need to be connected to a power supply of sufficient ampacity at the supply voltage. To help simplify a complex design process, four kVA ratings have been selected for this article. One of the selected ratings should fit the needs of most service centers.

This article covers:

• A design example
• Determining turns and wire size
• Building the core
• Determining coil dimensions

Kirk Kirkland
Electrical Repair Service Co. 
Birmingham, Alabama
Technical Education Committee Member

Editor's Note: This article is similar to a July 2006 Currents article titled "Tips for Test Running Motors With Roller Bearings." These two articles complement and supplement each other.

End users frequently demand that EASA service centers provide an array of test data at the conclusion of the service/repair process. These tests are normally to validate compliance with the customer’s motor repair specifications. It is also a good idea to have your own in-house specifications so you can prove that you’re compliant with EASA motor repair guidelines such as those found in the Recommended Practice for the Repair of Rotating Electrical Apparatus (ANSI/ EASA AR100-2006).

One of the more common tests involves running the motor no-load and providing the motor owner with electrical test information and vibration spectrums covering various frequency bands. No-load run tests are commonly applied to AC induction motors. In many cases, these motor types are designed for a belted-duty application. That means they may have a roller bearing in the drive end of the motor. The most common roller bearings utilized in belted applications are the two-piece NU type that consists of an inner race mounted on the bearing shaft journal and the rollers caged on the outer race.

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

We often hear the terms preventive maintenance (PM) and predictive maintenance (PdM) of electric motors. However, it’s far less often that we give consideration to the tasks associated with these methods of maintaining motor operation and extending operating service life. This article will address some of the more common activities associated with PM and PdM, with the focus on three-phase squirrel cage motors.

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

Tom Bishop, P.E.
EASA Technical Support Specialist
 
Prior to rewinding it is advisable to assess the condition of the core iron of stators, armatures and wound rotors. The assessment is performed by a core test, which magnetizes the core to a pre­scribed magnetic flux density. The predominant tests used to determine core condition are the hot spot test and the core loss watts test. The hot spot test compares the hottest spot in the core to either ambient temperature or core average temperature. The watts loss test compares the core loss test watts prior to winding removal to the same test af­ter the windings have been removed and the core prepared for rewind. 

Core testing traditionally was performed by the use of the loop (ring) test. That required multiple turns of wire to be passed through a core in order to magnetize the core and test for shorted laminations. Mag­netic strength is related to the ampere-turns (amperes x turns) of the magnetizing coil. Mod­ern core testers make it possible to test a core with a single turn of wire, by using high current. Thus the core tester uses one turn and many amperes, whereas the loop test typically uses many turns and a relatively low current. 

Scott Clark, Ph.D., P.E.
Brandon & Clark, Inc.

Los sistemas tradicionales para la detección de defectos en las barras de un rotor a menudo son insuficientes en sensibilidad, capacidad de registro y análisis numérico de los resultados de la prueba. Muchos medios para la detección de los defectos de un rotor requieren que el motor esté completamente ensamblado con un estator funcional. Estas limitaciones han establecido las bases para desarrollar una nueva técnica de detección de defectos en el rotor. Un nuevo método denominado Rotor Magnetic Field Analysis (RMFA), amplía las técnicas tradicionales de prueba del rotor del motor de inducción con mediciones precisas de campo magnético y herramientas de procesamiento de datos. Por más de tres décadas, esta la primera nueva tecnología para la prueba de rotores.

Chase Fell
Presidente del Comité de Educación Técnica
Jay Industrial Repair

Por lo general, las pruebas de Alto Potencial (hipot) para devanados de motores y generadores son realizadas utilizando una fuente de 50/60 Hz o de CC. La prueba de hipot es un paso crítico para validar la calidad de los devanados nuevos. Las pruebas hipot CA y CC también son útiles para entender el estado del aislamiento envejecido de las máquinas que están en servicio. La prueba hipot CC es muy usada en la reparación de motores ya que el equipo es portátil y la corriente de prueba de estado estable proviene principalmente de las fugas a través del aislamiento.

Cuando ocurre un disparo, la prueba CC causa menos daños al material adyacente al punto de fallo en comparación con la prueba CA. Una desventaja de la prueba CC es que el voltaje no se distribuye en el devanado de la misma forma que cuando se aplica CA. Específicamente, la prueba CC esfuerza mucho más las vueltas finales.

La prueba de hipot CA es mucho más consistente con los esfuerzos de voltaje a los que se ve sometida la máquina en servicio. Los estudios han demostrado que esta prueba puede revelar defectos en el aislamiento que no se detectan con la prueba CC. Una prueba CA puede detectar mejor los huecos (voids) y la delaminación del sistema de aislamiento. La desventaja de la prueba CA a frecuencia industrial es cuando el tamaño del equipo y / o la complejidad de la configuración de la prueba se vuelven problemáticos para la reparación del motor y en el campo.

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

En este artículo discutiremos las pruebas de los condensadores usados en los motores eléctricos en general, así como también las pruebas asociadas al uso específico de condensadores empleados para la corrección del factor de potencia y en el arranque de los motores eléctricos (Ver Figuras 1 y 2). Para obtener información de como calcular la capacidad de los condensadores para corrección del factor de potencia y en un motor eléctrico, consulte las Subsecciones 2.10 y 2.11 del Manual Técnico de EASA.

Pat Douglas
Kirby Risk-Mechanical Solutions & Service

Un encoder es un tipo de dispositivo de retroalimentación que a menudo se instala en un motor para monitorizar el movimiento (óptico sencillo). Un encoder de cuadratura indica tanto el movimiento como la dirección del eje de salida del motor.

La Figura 1 muestra una forma de onda digital buena y la Figura 2 una forma de onda digital con ruido. Ambas han sido escaneadas desde un osciloscopio utilizado en un centro de servicio.

A special discounted collection of 8 webinar recordings focusing on various aspects of pump repair.

Once purchased, all 8 recordings will be available on your "Downloadable products purchased" page in your online account.

Downloadable recordings in this bundle include:

Troubleshooting Pump Performance Problems
Presented May 2017

This presentation covers:

• Not enough pressure (head) or not enough flow – how do you respond?
• How to determine if a pump is operating properly
• Differentiating a pump problem from a system problem
• Determining pump load and power requirements
• The effect of fluid parameters and cavitation on pump performance. 

Target audience: This will be most useful for service center technicians and engineers. The content will also be beneficial for supervisors and managers who are responsible for pump failure analysis and testing. 

Pump Failure Case Study
Presented December 2013

This presentation covers:

• Brief overview of disassembly and evidence of failure
• Discussion of possible failure scenarios
• Review of actual repairs, modification and reassembly
• Update of machine's present operation

Repair Tips for Submersible Pumps
Presented February 2013

This presentation focuses on:

• Types of submersible pumps
• Tips on seal arrangements
• Common repair procedures
• Cables and cable entries
• Testing submersibles in the service center

Assessing Impeller Damage
Presented May 2019

The impeller is generally the most difficult pump component to repair and the most expensive to replace. This session will look at case histories of failed pumps and the steps to determine the cause of failure. Topics covered include:

• Erosion, corrosion, cavitation or wear: What happened to this impeller?
• How to spot the tell-tale signs
• What operational conditions led to impeller damage

Repairing Impeller Damage
Presented May 2016

We’ve covered how to assess impeller damage. Now learn how to fix that damage. This presentation covers: 

• Replacing/repairing wear rings
• Repairing cavitation damage
• Impeller replacement options
• Dynamic balancing impellers

Techniques for Straightening Pump Shafts
Presented March 2011

The slender dimensions of many pump shafts make them susceptible to distortion, which affects pump performance and reliability. This recording presents a methodical approach and effective techniques for measuring and correcting shafts which are bent or twisted.

Target audience: This presentation is intended for service center supervisors, managers and machine shop technicians.

Vertical Turbine Pump Repair Tips
Presented February 2012

Vertical turbine pumps are used extensively in every segment of industry. Although they are not complex, repairing them in the service center can present a few challenges. This presentation gives some approaches and procedures that experience has shown will make the job easier.

Final Testing for Pumps - An Overview
Presented November 2014

The pump repairs are completed! Now the pump needs to be tested. This presentation discusses the procedures for the basic tests that can be performed on pumps that have been repaired in the service center.

Final testing of pumps can include:

• Operational tests
• Seal leakage test
• Motor chamber leakage test (submersibles)
• Casing pressure test

While some of these tests are not difficult to perform, knowing the methods and limits will help service centers to confidently deliver quality pump repairs.

Chuck Yung
EASA Technical Support Specialist 

Ever rebuilt a motor, only to have the customer call and complain that the motor "isn't right?"  Maybe it vibrates or draws too many amps, or "It never used to run this hot."

The response is usually a costly trip for a salesman and service technician to check it out. 

This presentation focuses on:

• Types of submersible pumps
• Tips on seal arrangements
• Common repair procedures
• Cables and cable entries
• Testing submersibles in the service center

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

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

The EASA/AEMT Rewind Study was published in 2003, prior to the introduction of premium efficient (IE3) motors. The recently completed follow-up study evaluated motors with premium efficiencies to confirm that, as with the earlier study, the efficiency of these motors can be maintained during rewind and repair by using established good practices.

This webinar covers the results and the technical details of this most recent study.

It will benefit service center managers, customer service representatives, sales representatives, supervisors and technicians.

EASA’s Rotor/Armature 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 rotor/armature core testing, see Section 7 of the EASA Technical Manual.

Approximately three million workers in the United States on a daily basis, face extreme risk from uncontrolled energy when servicing machinery.  Serious injury or death can be the result. Typical non-lethal injuries include fractures, lacerations, contusions, amputations, puncture wounds, electric shock, and falls. The average lost time for injuries runs approximately 24 days. The Occupational Safety and Health Administration (OSHA) estimates that approximately 120 fatalities and approximately 28,000 serious and 32,000 minor injuries each could be prevented if proper lockout/tagout procedures at job sites are initiated. This poses a serious problem for exposed workers and their employer. The OSHA Control of Hazardous Energy Sources Standard establishes uniform requirements to ensure that the hazards of uncontrolled energy in U.S. workplaces are evaluated, safety procedures implemented, and that the proper hazard information is transmitted to all affected workers.

BUY THE WEBINAR RECORDINGS
(Both Management & Employee presentations are included)

Pat Douglas
Kirby Risk-Mechanical Solutions & Service

An encoder is a type of feedback device that is often installed on a motor to monitor the motion (simple optical). A quadrature encoder indicates both motion and direction of the motor output shaft. 

Figure 1 indicates a good digital waveform.  Figure 2 indicates a noisy digital waveform. Both are scans from an oscilloscope used in a service center.

Tom Bishop, P.E. 
EASA Technical Support Specialist 

There are many tests that can be performed on a DC motor to verify the integrity of windings, correct coil polarities and proper running perfor­mance. What we will address in this article are a select few simple tests that can help assure a motor operates properly when the customer applies it. Our intent is not to oversimplify and suggest that performing these tests alone is all that is required for an effective repair. Rather, the intent is to highlight some tests that give a maximum return for the time invested in testing. 

The tests we will cover are drop testing fields, checking interpole polarity, checking compound field polarity, brushholder spacing, setting neutral and two-way run testing. All of these tests can usually be per­formed with the motor assembled, although in some cases the end bracket on the commutator end may need to be removed to access the field lead connections. 

By Gene Vogel
EASA Pump & Vibration Specialist

High vibration is a common problem for motors that are installed on top of vertical pumps. Its source can be a mechanical issue with the pump, motor or coupling or even hydraulic forces from the pump. Structural issues involving “reed frequency” resonance often amplify the problem, but effective diagnosis must begin with an understanding of the underlying vibratory forces. Although the general vertical pump category includes submersibles, this article focuses solely on the ones that most commonly exhibit high-vibration conditions: surface-mounted pumps with the motor bolted to a pedestal on top.

Topics covered in this article include:

• Mass unbalance
• Coupling type and alignment
• Mechanical action of pump shaft & impeller
• Hydraulic action of fluid
• Resonant frequencies
• Basic frequency analysis
• Trim balancing
• Other possibilities
• Vertical pump troubleshooting checklist

READ THE ARTICLE

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.

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

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.

Mike Howell
EASA Technical Support Specialist

Most service centers perform some form of surge comparison testing, though terminology and methodology may vary. In simple terms, two winding responses or waveforms from a fast rise-time surge are compared and if there is an excessive difference, the unit under test may have a defect. The waveform that is produced by the pulse is unique to the unit under test, which for example, could be a stator winding. The waveform will be a function of the resistance, capacitance and inductance of the test circuit and quite a few variables can affect those characteristics.

One difficulty or challenge with surge comparison testing has been its subjectivity. That is, it is not always easy for operators to reach the same conclusion when comparing two waveforms. Within the last few decades, several equipment manufacturers have begun to utilize analytical methods to evaluate the surge comparison test results. The goal is to remove as much subjectivity as possible so that disposition of the unit under test is a simple decision for the operator. The analytical method that has become most popular, in various forms, is use of the Error Area Ratio (EAR).

Do you have to have EAR capabilities in order to perform surge comparison testing satisfactorily? No, but if you have the capability, a basic understanding of the data reported by the equipment can help you make an informed decision.

Cyndi Nyberg
Former EASA Technical Support Specialist

The surge test is used to detect winding faults in AC and DC windings. If there is a turn-to-turn short, the surge test will show that. The surge test is an important step in the initial inspection of a machine, as well as a final test to ensure a proper rewind. 

For this article, we will only focus on three-phase windings. The surge test is typically run by applying a high voltage across each of two phases of a three-phase motor. The decaying resonance patterns of the two phases are superimposed upon one another on an oscilloscope. If the two phases are identical, as they should be, then the patterns will be identical. A perfect match will yield only one apparent pattern (Figure 1a) while a variance or difference, as shown in Figure 1b, represents an apparent problem. Testing continues until all phases have been compared to one another – 1 to 2, 2 to 3 and 3 to 1. 

Richard Hughes (deceased)
Pump & Motor Works, Inc.

Proper testing of a synchronous motor is crucial for validating repair, reconditioning or remanufacture. There are several types of synchronous motors and a brief description of each will be given prior to discussing testing.

Topics discussed are:

• Brush type motor
• Watch for resistance value extremes
• Brushless motors
• Performing insulation resistance test
• Performing AC voltage drop test

Cyndi Nyberg
EASA Technical Support Specialist 

When a motor is test run without a load after it has been rewound, one of the questions we are asked is why the amps are too high, even higher than the nameplate full load. Here are a few of the common factors to consider. 

First, if there is a problem with the data, it is im­portant to realize that not every service center has access to EASA’s extensive database and technical knowledge. Of course, some motor failures are so catastrophic that it is impossible to determine the connection or even the turns per coil for a particu­lar winding. Without access to the EASA database, if factory data cannot be obtained, the service cen­ter is left to make assumptions. If this is the case, you may have inherited someone else’s problem if they misinterpreted the data. 

Art Godfrey (retired)
Birclar Electric & Electronics

No-load testing of repaired motors is common in most if not all repair service centers. ANSI/EASA AR100-2010 Recommended Practice specifies, “for AC motors, no-load running tests should be made at rated voltage and rated frequency.” For sine-wave powered motors, this statement is straightforward. For motors used on variable frequency drives (VFDs), there are a number of possibilities service centers may employ with the motor supplied by line (utility) power, and in some cases the tests may be less than ideal.

This webinar covers:

• 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

This webinar covers: 

• Ampere turns of the armature, field and interpole data
• Determining the best armature coil pitch
• Verifying interpole circuits
• Importance of brush angle
• Equalizers and armature windings

Chuck Yung
EASA Senior Technical Support Specialist

While the majority of EASAns don't get involved with field construction of motor bases, most do have an important motor base in the service center. The baseplate used in conjunction with the test panel is important, especially when measuring vibration levels of a running motor. Vibration levels recorded in the shop should correlate to those taken when the customer installs the motor in the plant. A discrepancy gives valuable clues as to the cause of vibration, allowing quick identification of the problem. For this reason, the design and installation of that baseplate are important if the in-shop vibration readings are to have value. Since that is important, let's review a few basic steps to make sure that the baseplate functions correctly.

Topics covered in this article include:

• Importance of baseplate design and installation
• Maximizing bond area
• Use of anchor bolts

Cyndi Nyberg
Former EASA Technical Support Specialist

Editor's Note: This article is similar to a February 2010 Currents article titled "Preloading roller bearing motors for no-load run testing." These two articles complement and supplement each other.

Ball and sleeve bearing motors can always be test run without any type of external load on the motor and bearings. 

However, when repairing a motor equipped with roller bearings that is used in an application with a radial load, such as a belted load, it is not advisable to perform the standard no-load test run for any length of time. Yet the no-load test run is a crucial step in the repair process to ensure proper operation. Without that radial load, the bearings can be damaged. This article will describe two ways to put a load on the shaft of a motor and therefore the roller bearing, so that it can be test run to ensure that it has been properly repaired. 

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.

Mike Howell
Especialista de Soporte Técnico de EASA

La prueba a rotor bloqueado de los motores trifásicos de jaula de ardilla se usa para validar un diseño y para el control de calidad; esta prueba también puede ser una herramienta de diagnóstico valiosa. Pero esta prueba no forma parte de las actividades rutinarias de la mayoría de los centros de servicios.

Dos retos que a menudo enfrentan los centros de servicio son: La capacidad de par del dinamómetro y la potencia del tablero de pruebas.

Generalmente, esto se soluciona haciendo las pruebas a voltaje reducido, lo que presenta otro reto- como extrapolar los datos de prueba al voltaje nominal, asegurando una precisión razonable. Si la extrapolación está muy desviada, corremos el riesgo de rechazar un motor bueno o aceptar uno malo.

El propósito de este artículo no es proporcionar procedimientos detallados para realizar las pruebas a rotor bloqueado, sino presentar un enfoque práctico para analizar los datos a voltaje reducido, empleando herramientas a las que la mayoría de los centros de servicio tienen acceso en sus instalaciones. Adicionalmente, mientras este artículo se centra en los datos de la prueba a rotor bloqueado, la metodología empleada seguramente puede ser utilizada en otras pruebas donde existan condiciones y relaciones similares.

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.

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.

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

This presentation covers:

• Not enough pressure (head) or not enough flow – how do you respond?
• How to determine if a pump is operating properly
• Differentiating a pump problem from a system problem
• Determining pump load and power requirements
• The effect of fluid parameters and cavitation on pump performance. 

Target audience: This will be most useful for service center technicians and engineers. The content will also be beneficial for supervisors and managers who are responsible for pump failure analysis and testing. 

Mike Howell
EASA Technical Support Specialist

Las dos razones principales para probar el núcleo de un estator son (1) comprobar que es apto para continuar en servicio y en el evento de un rebobinado, (2) verificar que el proceso de reparación no ha afectado negativamente su estado. Esta prueba se puede efectuar con un probador de núcleo comercial o de forma manual, utilizando una fuente de C.A. adecuada, cables e instrumentos de prueba. Algunas de las razones para realizar la prueba manualmente son:

• El cliente o el centro de servicio la prefieren / especificaciones 
• No hay un probador de núcleo comercial disponible 
• El tamaño del estator es inapropiado para el probador de núcleo comercial disponible 

Además, algunos centros de servicio se abstienen de realizar las pruebas de núcleo en estatores pequeños por diferentes razones. Estas incluyen, dificultades con la configuración de la prueba, cálculos, costos e incluso imagen. El propósito de este artículo es el de explorar una configuración de bajo coste para probar los núcleos de estatores pequeños.

By Jane Alexander
Managing Editor of Maintenance Technology

According to EASA’s technical experts, changes in motor/system vibration readings provide the best early warning of developing problems in a motor or system component. Other parameters to monitor may include operating temperature of critical components, mechanical tolerances, and overall system performance, including outputs such as flow rate, tonnage, and volume.

Motor-specific baselines incorporate records of electrical, mechanical, and vibration tests performed when units are placed in operation or before they’re put in storage. Ideally, baselines would be obtained for all new, repaired, and in situ motors, but this may not be practical for some applications. These baselines typically include some or all of the following:

• Load current, speed, and terminal voltage
• Motor current signature analysis (MCSA)
• Mechanical tests
• Vibration
• Infrared thermography
• New motor baselines
• Repaired motor baselines

READ THE ARTICLE

The US Department of Energy (DOE) has issued rulemaking on test procedures for small electric motors for more than a decade. The present “final rule,” effective February 3, 2021, is the culmination of those efforts. The final rule will be mandatory for product testing beginning July 6, 2021. If you want to view the complete detail of the final rule that was published in the Federal Register on January 4, 2021, it can be found at https://beta.regulations.gov/.  For further reading about the final rule, see this DOE site.

Chuck Yung
EASA Technical Support Specialist

Insulation resistance is affected by several variables: the type of insulation, age of the material, surface area, moisture and contamination. Insulation resistance can be described as being made up of four components: Leakage, capacitance, conduction and absorption. Capacitance normally only affects the first few seconds of the Polarization Index (PI) test; conduction should be essentially zero if the windings are dry; and leakage current is constant over time.

The PI test is useful because the remaining variable – absorption current – indicates the health of the insulation.

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

For most service centers the traditional repair services such as electric motor rewinding have been and will continue to be in a state of decline over time. Among the factors leading to this reduction in business are conversions to more efficient motors, improved maintenance of existing motors, incentives to replace with more efficient motors and in some regions a reduction in the industrial customer base. A consequence of this is that there is more competition for a “shrinking pie”. Service center reaction can be to make a comparable reduction in size or become pro-active and seek new business. The objective of this paper is to suggest and detail some of these alternatives, namely value-added repair and service opportunities for service centers that carry with them the added benefit of contributing to optimizing motor reliability.

The opportunities for value-added repairs and services are ever-increasing. Topics covered here are:

• Bearing isolators, increased winding wire area, ball-to-roller/roller-to-ball bearing conversions
• Preventive and predictive maintenance (PM & PdM) services: vibration analysis, condition monitoring, bearing lubrication, electrical testing (IR, amps, volts, kW)
• Motor management
• New premium efficient motors vs. repair and retrofitting of existing motors

This 48-page manual was developed following EASA’s 12-part Vibration for Service Centers webinar series. It serves as an introductory training resource for service center technicians and supervisors involved in measuring, evaluating, and correcting vibration and balancing issues on machines under repair – as opposed to the in-plant predictive maintenance tasks covered in most general classes on the subject.

This document is intended as basic introductory training material for anyone who may be involved in evaluating or correcting vibration issues on machines repaired in the service center. Only certain sections may be of interest depending on the area and amount of involvement in vibration issues.

For a technician with responsibility for analyzing and correcting vibration and balancing issues, a general understanding of all of the information is essential. For technicians who will conduct field vibration and balancing services in customers' facilities, additional training is strongly recommended. A Level 1 vibration analysis class (usually 4 or 5 days) is a first step toward the competence needed for conducting field services. A Level 2 class is recommended. A number of providers offer ANSI-certified Level 1 and Level 2 vibration analysis classes, which normally include an opportunity for certification.

Major sections in the document include:

• Introduction and Overview
• Vibration Basics: Amplitude, Frequency and Phase
• Vibration Tolerances
• Basic Vibration Analysis
• Dynamic Balancing Basics
• Resonance
• Rolling Element Bearing Vibration
• Demodulation and High Frequency Band Measurements
• Field Analysis Techniques
• Field Balancing—Problems and Solutions

DOWNLOAD THE COMPLETE TABLE OF CONTENTS

LEARN MORE ABOUT THE COMPANION WEBINAR RECORDINGS

This 12-part recording (15 hours) covers a wide range of topics on vibration.

Members rely on EASA to provide technical assistance and training in all areas related to machinery repair. In the area of machinery vibration, there are training providers that offer general classes in vibration analysis and balancing, but the content is geared to plant maintenance personnel who would be conducting in-plant predictive maintenance services. Some key areas important to EASA service center technicians is not covered adequately, and much of the content does not apply to vibration testing conducted in the service center. This course is designed to address those shortcomings and provide fundamental training in vibration analysis and balancing that directly applies to technicians working in the service center.

Main goals of the series
This webinar series was designed to:

• Provide EASA service center technicians with the technical knowledge they need to effectively measure and diagnose vibration on machines being tested in the service center.
• Provide the foundation understanding necessary to use vibration data as an indicator of machinery condition
• Provide the fundamental knowledge of dynamic balancing necessary to use common balancing instruments in the service center

LEARN MORE ABOUT THE COMPANION BOOK

Who would benefit from watching this series?
Service center technicians who measure and analyze machinery vibration, and those who must evaluate the vibration data will benefit greatly from the fundamental understanding and knowledge provided by this training series. Service center engineers who may be involved in writing, interpreting and applying vibration and balancing specifications and tolerances will gain a practical understanding of the terms, definitions and parameters encountered in those specifications.

As with any technical subject, fundamental math skills will allow attendees to quickly comprehend concepts and apply techniques. Vibration results from the mechanical and electrical forces at work in machinery, and a fundamental understanding of those forces, and machine components that cause them, will aid in the understanding and application of the subject matter.

Price
Downloadable version - $745 for members, $1,885 for non-members
DVD-ROM version (for viewing on a computer) - $795 for members, $1,985 for non-members

Dan Patterson 
Flanders Electric Motor Service, Inc. 
Evansville, Indiana 
Technical Services Committee Member 

Ensure A Quality Product 
In the previous article referenced above, I covered methods and criteria for testing motors in the service center. Service centers make every effort to ship a quality rebuild. On occasion, the test-run data may have suspicious characteristics. Even though the motor meets the vibration standards, further investigation is warranted. The motor may exhibit a noise, rumble, or exces­sive bearing temperatures. Spectral data might contain harmonic families, or wave-form data contains impacting. In this circumstance, I will make the statement: “It will never be as close to a motor repair service center as it is now.” Accurate test-run docu­mentation can prove invaluable as a comparison tool when judg­ing the performance of a motor once it is installed. After all, you did ship a quality product, didn’t you? 

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, and (2) to verify that the repair process has not adversely changed the stator core condition.

The purpose of this article is to discuss how we determine, assess and compare stator core test results. It is extremely important to understand that variance in test procedures may invalidate comparison.

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

The Institute of Electrical and Electronics Engineers (IEEE) standard for insulation resistance testing of mo­tor and generator windings that was published in 2002 has been revised. The 2013 edition was published in March 2014. 

The first change in the new docu­ment is a slight change in the title. It has changed from “IEEE Recom­mended Practice for Testing Insulation Resistance of Rotating Machinery” to “Recommended Practice for Testing Insulation Resistance of Electric Ma­chinery.” The reason for the change was to use the more prevalent IEEE term for motors and generators. Significant changes to clauses of the standard that affect service center repairs and testing are described in this article.

Mike Howell
EASA Technical Support Specialist

Locked-rotor testing of three-phase squirrel cage induction motors is used for design validation and qual­ity control; it also can be a valuable diagnostic tool. But, this testing isn’t a common task for most service centers. Two challenges service centers often face are dynamometer torque capac­ity and test panel electrical capacity. The work-around is usually reduced-voltage testing, which presents another challenge – how to extrapolate the test data to rated voltage with reasonable assurance of accuracy. If the extrapo­lation is too far off, we run the risk of either rejecting a good motor or accept­ing a bad one. 

The purpose of this article isn’t to provide detailed procedures for per­forming locked-rotor tests, but rather to present a practical approach for analyzing the reduced-voltage data us­ing tools that most service centers have access to at their facilities. Additionally, while this article will focus on locked-rotor test data, the methodology used can certainly be extended to other tests where similar conditions and relation­ships exist.

Mike Howell
EASA Technical Support Specialist

Induction motor stator cores can be manufactured using single-piece laminations (see Figure 1 left) up to an outside diameter of about 48 inches (1200 mm). For larger stators, or when minimizing scrap material, the stator laminations are segmented (see Figure 1 right). The typical circumferential gap between segmented laminations is only around 0.012 inches (0.3 mm), so it is exaggerated in the included figures. The number of segments chosen by a manufacturer for a given design can depend on several factors, some technical and some economic. For most service center repair activities, machines with segmented lamination stators are processed no differently than those with single-piece laminations. However, there are a few areas worth exploring that could be helpful when working with segmented lamination stators.

Chuck Yung 
EASA Senior Technical Support Specialist

Wound rotor (WR) motors represent only a small fraction of all electric motors in service. In reviewing the EASA Technical Support call logs, one would conclude that there are many more wound rotor motors in service. Because many of us do not work on wound rotor motors often, it is understandable that not everyone has a clear understanding of how they differ from a squirrel cage motor. The purpose of this article is to dispel some misconceptions about how they work and to offer valuable tips for failure analysis, repair and testing. Other topics covered include:

• Secondary voltage
• Crane applications
• Testing tips, after assembly