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

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

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

Chuck Yung
EASA Senior Technical Support Specialist

Occasionally a customer wants a spare DC machine, and you find a replacement that is almost – but not quite – identical. Often, either the original or the spare is compound wound. The customer then asks: “What do the series fields do?” and “Can we just isolate the series leads?” 

There are a couple of considerations, but the answer is that “it depends.” If the nameplate is factory marked “Stabilized shunt” or “Stab shunt,” that tells us that the series contribution to total field flux is relatively small. In many applications (e.g., extruder), the relatively small series is not essential to successful operation of the motor.

Topics covered in the article include:

• Shunt and series field
• Flux contribution
• Higher percent compounding
• Inductive kick
• Shunt field failure

Carlos Ramirez
Especialista de Soporte Técnico de EASA

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

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

Este webinario está enfocado a bobinadores, supervisores e ingenieros.

Aluminum to Copper Conversion: What You Need to Know Presented by Carlos Ramirez EASA Technical Support Specialist Have you received a vintage machine that has been wound with aluminum wire?  This presentation explains how to perform a proper conversion from aluminum to copper wire in AC and DC machines, including examples for rewinding stators and shunt fields.  Topics covered include:  Aluminum and copper wire area  AWG and metric wires  AC motor windings  Shunt field coils  Examples of conversion  This presentation is intended for winders, supervisors, and engineers.

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

Want to test your knowledge after watching the webinar?

TAKE THE QUIZ

This inspection report helps record a DC motor's connection as it enters the service center. It is intended for use with 2-, 4- and 6-pole DC machines and includes space to:

• Draw and number the leads and jumpers
• Number of poles in series or in parallel
• Number of interpoles
• Number of series fields

This form will aid in collecting all needed information regarding a DC machine recieved for repair: nameplate data, armature coil data, armature dimensions, field winding data, field coil dimensions, general winding information as well as job and customer details.

This fillable PDF conveniently helps you save DC machine data for future reference. SImply copy the file or "Save As" to create a form for each motor you repair. The PDF includes a convenient button that can help you easily send DC data to EASA technical support.

This incoming inspection report provides a place to record basic DC motor conditions and test values, including:

• Customer information
• Armature voltage and amps
• Field voltage, amps ,etc.
• Electrical test information for the armature, fields, interpoles and series windings
• Brush and brushholder information

6 presentations

$30 for EASA members

 

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.

Mike Howell
EASA Technical Support Specialist

When rewinding the shunt fields of a DC machine, it is important to avoid making changes that could negatively impact performance. The recommended practice is to maintain the manufacturer’s winding configuration during the repair. That is, the field circuit connection, turns per coil, mean or average length of turn (MLT) and wire size should not be changed. However, service centers do sometimes encounter issues around wire size availability. The purpose of this article is to provide some guidance for making wire size substitutions when the original size is unavailable.

Chuck Yung
EASA Senior Technical Support Specialist

When rewinding field coils, there are a couple of common problems that make life difficult for the service center. One problem occurs when the newly rewound set of shunt fields is returned to the service center, roasted again. What causes this, and whether we can improve our winding proce­dure, has been the subject of much discussion. 

We all realize that something caused that first failure. If the subse­quent failure looks a lot like it — the shunt fields were charred in both cases — then it seems logical that the underlying cause could be the same. Sometimes it is. 

Let’s look at why the coil is burnt. Well sure, it got too hot. But why? Is it because the shunts are energized when the machine is otherwise shut down? If so, the fields can be automatically de-energized after “x” minutes of machine inactiv­ity. Installing a timed relay in the controls will avoid future problems. It may be that the customer is able to trace the problem to a new operator, who needs more training. 

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

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

 

Helpful tools

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

Mike Howell, PE
EASA Technical Support Specialist

Unlike their AC counterparts, DC machines do not have rotating magnetic fields. Rather, there are fixed magnetic field axes for the field (direct axis) and armature (quadrature axis). Even though the armature is rotating, the magnetic field axis in the armature is fixed thanks to commutation, which allows the direction of current in an armature conductor to change as it passes from the region under one main field pole to the next.

Mike Howell, PE
Especialista de Soporte Técnico de EASA

A diferencia de sus homólogos de CA, las máquinas de CC no tienen campos magnéticos rotativos. Más bien, existen ejes de campo magnético fijos para el campo (eje directo) y la armadura (eje en cuadratura). Aunque la armadura esté girando, el eje del campo magnético de la armadura está fijo gracias a la conmutación, lo que permite que la dirección de la corriente en un conductor de la armadura cambie a medida que pasa debajo de un polo de campo principal al siguiente.

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.

Illustrates procedures for manufacturing random-wound interpole and main field coils for DC machines. Covers everything from removing old coils and taking data to installing and connecting new coils, including how to construct winding forms and jigs, how to shape coils to conform with the curvature of the field frame and how to insulate field coils. Both “wet” and “dry” winding techniques are illustrated.

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.

Covers all necessary steps for layer-winding DC field coils, how to layer-wind a shunt field coil, how to reinsulate an interpole field coil, and how to install large field coils in the frame.

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.

Cyndi Nyberg Esau
Former EASA Technical Support Specialist

Trickle heating is another very practical option to heat AC stator windings or DC field windings while they are not energized. In this method, a low voltage is supplied to the winding. The advantage of trickle heating on a winding is that you are heating the winding somewhat evenly, rather than relying on radiant heat from an "outside" source. In addition, the maximum surface temperature attained is much lower due to the lower watts per square inch of surface area.

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.