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

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

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

[EasyDNNnews:PaidContentStart]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[EasyDNNnews:PaidContentEnd

11 presentations

$55 for EASA members

 

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.

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.

Presented by Chuck Yung
EASA Senior Technical Support Specialist

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

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

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

Mike Howell, PE
EASA Technical Support Specialist 

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

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.

Chuck Yung
Especialista Sénior de Soporte Técnico de EASA

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

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

Mike Howell
EASA Technical Support Specialist

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

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

Mike Howell
EASA Technical Support Specialist

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

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

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

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

Mike Howell, PE
Especialista de Soporte Técnico de EASA 

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

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.

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.

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

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

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

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

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

LEARN MORE

This presentation covers the following topics:

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

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

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

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

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

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

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

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

Topics covered include:

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

Mike Howell
Especialista de Soporte Técnico de EASA

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

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

Mike Howell
EASA Technical Support Specialist

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

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

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

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

David Sattler
L&S Electric

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

Chuck Yung
EASA Senior Technical Support Specialist

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

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

Chuck Yung
EASA Senior Technical Support Specialist

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

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

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

La resistencia de aislamiento se calcula  de la siguiente forma:

R = E / I_T  

donde

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

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

R = E / I T  

where

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

Chuck Yung
EASA Technical Support Specialist

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

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

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

This webinar recording covers: 

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

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

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

This webinar recording covers: 

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

This webinar covers:

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

Steve Skenzick
HPS Electrical Apparatus Sales & Service

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

Tom Bishop
EASA Technical Support Specialist 

Did you ever wonder if the preheating instruc­tions from solvent varnish and solventless resin (hereafter we’ll use the term “resin” when it ap­plies to both) manufacturers were really all that important? The short answer is, yes, they are. Here we’ll expand on some of the reasons that preheating is a key step in the winding process. 

One of the first benefits of preheating is that it drives out moisture that may have settled on sur­faces, or been absorbed by insulation material. A little known aspect of pre­heating is that it relieves the mechanical stress cracks, termed “crazing,” on the magnet wire insulation coat­ing that occur during coil winding and insertion. Epoxy B-stage materials can be set by preheating, provided the preheating time and tempera­ture meet the epoxy’s curing requirements. Random wind­ings typically don’t use many B-stage materials; however, many of the lacing products for endturns are thermoset­ting. Form coil windings often have B-stage surge ropes, and some felt packings used for endturn coil bracing are B-stage epoxy loaded. 

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

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

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

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.

David Sattler
L&S Electric

The goal of vacuum pressure impregnation is complete saturation of a winding with insulating resin. As resin penetrates the insulating materials, it causes them to darken. When VPI resin is drained from the winding, all end-turn insulation should be darkened to a uniform shade. Connection insulation should also show uniform darkening. If some insulation is a lighter shade, the coil or jumper is not fully saturated. The winding is not properly protected, and if the problem is not addressed, it is likely to cause premature failure of the unit. This could result in costly warranty work, or at a minimum, will be a failure to provide your customers with the quality they expect and deserve.

12 presentations

$60 for EASA members

 

A special discounted collection of 12 webinar recordings focusing on AC motor windings.

Once purchased, all 12 recordings will be available on your "Downloadable products purchased" page in your online account.

Downloadable recordings in this bundle include:

The Basics: Taking Motor Data
Presented September 2016

This presentation covers:

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

---

Taking Three-Phase Winding Data
Presented October 2012

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

---

The Basics: Motor Connections
Presented November 2016

This webinar covers:

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

---

Tips and Techniques for Winders
Presented August 2015

This webinar covers:

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

---

Rewinding Tips for Premium Efficient Motors
Presented June 2016

This webinar recording covers: 

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

---

Windings & Connections
Presented December 2015

This webinar recording focuses on the internal connections of AC motors, including:

• Wye or delta?
• Parallel circuits
• Dual voltage - delta connected, wye connected and wye/delta connected
• Tri-voltage - 2D2Y1D and others

---

Concentric or Lap? Considerations for the 2-Pole Stator Rewind
Presented September 2014

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

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

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

---

Stator Rewinds: When Things Get Tight
Presented June 2015

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

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

---

Ensuring Success With VPI
Presented June 2014

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

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

---

Induction Motor Rotor Windings: Squirrel-Cage and Wonld Rotor Basics
Presented January 2018

This presentation covers the following topics:

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

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

---

2-Speed, 2-Winding Pole Group Connections
Presented September 2018

The topics covered included in this webinar recording:

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

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

---

Minimizing Risk With High-Voltage Rewinds
Presented February 2014

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

• Stator winding design
• Insulation system validation
• Process control

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