Chuck Yung
EASA Senior Technical Support Specialist

Although the earliest practical DC motor was built by Moritz Jacobi in 1834, it was over the next 40 years that men like Thomas Davenport, Emil Stohrer and George Westinghouse brought DC machines into industrial use.

It’s inspiring to realize that working DC motors have been around for over 160 years. For the past century, DC machines over 30 or 40 kW have been cooled in the same manner – by mounting a squirrel cage blower directly over the commutator.

By Jim Bryan
EASA Technical Support Specialist (retired)

The effects of excessive temperature on motor performance are notorious. After moisture, they are the greatest contributor to bearing and winding failures. Understanding the source of increased temperature is key to correcting the problem and improving the reliability of your facility’s motor fleet.

Topics in this article cover:

• Overload and service factor
• Ventilation
• Voltage variation
• Electrical steel
• Current density
• Circulating currents
• Harmonics

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By Chuck Yung
EASA Senior Tecnical Support Specialist

The evolution of electric motor design as it pertains to cooling methods provides insights about better ways to cool machines in service. The array of methods engineers have devised to solve the same problems are fascinating yet reassuring because many things remain unchanged even after a century of progress. This article discusses how motors are cooled and how heat dissipation can be improved for applications that fall outside the normal operating conditions defined by the National Electrical Manufacturers Association (NEMA) Standard MG 1.

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Chuck Yung
EASA Senior Technical Support Specialist

I’ve often commented on how for­tunate we are to work on such a variety of electric motor designs. One day, you are working on a new design some designer has recently created, and the next day you are repairing a motor that could be in a museum. It’s fascinating to see the different ways engineers have devised to do the same thing, and yet reassuring to see how many things remain unchanged even after a century of electric motors. 

One aspect of electric motors that could be placed in both categories is the way an electric motor is cooled. This article takes a look at how motors are cooled and how we can improve cool­ing for some of the special applications we encounter.

Chuck Yung 
EASA Technical Support Specialist 

Most of us involved in the repair of electrical equipment have a good understanding of how an electric motor works–especially the stator and ro­tor. But the fan can appear deceptively simple. Fans are pretty interesting, once we learn a few “affinity laws”—rules that also apply to blowers and impellers. This article will review some basic facts about fans that explain how small changes to a fan can make a BIG difference in the following critical areas: 

• Volume of air moved
• Static pressure
• Load
• Losses (efficiency)

These rules hold true for fan applications, im­pellers in pumping applications, and cooling fans on electric motors. When applied to the external fan of a TEFC (IP-54) motor, these rules offer some real opportunities for efficiency improvement. 

Often an overhung fan is balanced in a single plane, only to find that the vibration has shifted to the outboard bearing. Attempts to use standard two-plane techniques may result in calculated correction weights that are very large and produce poor results. There are more effective ways to approach this common problem. This presentation shows a methodical approach and techniques for tackling this difficult balancing problem.

Target audience: This presentation is intended for field service balancing technicians, supervisors and managers.

Chuck Yung
EASA Senior Technical Support Specialist 

There is now a much-needed DC repair standard, created through the participation of EASA members and industry motor users. The scope, as written in the document, states: “This document covers general recommendations for the repair of DC electric motors and includes guidelines for both the user and the repair facility ... . This standard covers reconditioning, repair and rewind of horizontal and vertical wound-field direct current motors and generators. It applies to all ratings above 0.75 kW (1 hp).”

The standard was created by a group of like-minded industry professionals who recognized the need for a best practice guide specific to DC machines. The working group accomplished the creation of 2455 in fewer than three years, from project authorization request approval to final publication. This document was modeled after IEEE 1068, a widely used electric motor repair standard.

The working group participants were:

*Denotes EASA member 

The working group covered everything from the receipt of the machine, to inspection and testing, to all aspects of repair including rewinding. Beginning with “Diagnostics in user’s plant,” the document recommends specific information gathering by the motor owner/end user to provide as much information as practical to the repair facility. 

Incoming tests for the repair facility are described in detail, with many tests (e.g., voltage drop test) being very familiar to EASA service centers. Nearly a dozen figures will be familiar to EASA members because they were originally published in EASA documents. 

The standard includes specific rewinding tips for fields and armature, as well as assembly and final test recommendations. IEEE 2455 also includes annexes for an Evaluation and Repair form, turn and undercut guidelines, an explanation of commutation, Commutator Condition Guide, sleeve bearing clearance, ventilation and cooling, post-assembly electrical adjustments and bearing currents in DC machines.

Chuck Yung
Especialista Sénior de Soporte Técnico de EASA 

Existe ahora una norma para las reparaciones de CC muy necesaria, creada gracias a la participación de miembros de EASA y usuarios de motores de la industria. El alcance, tal como se describe en el documento, establece: "Este documento abarca recomendaciones generales para la reparación de motores eléctricos de CC e incluye directrices tanto para el usuario como para el taller de reparación... Esta norma cubre el reacondicionamiento, la reparación y el rebobinado de motores y generadores horizontales y verticales de corriente continua con devanado de campo y aplica a todas las potencias superiores a 0,75 kW (1 hp)." 

La norma fue creada por un grupo de profesionales de la industria con ideas afines que reconocieron la necesidad de una guía de buenas prácticas específica para máquinas de CC. El grupo de trabajo logró crear la norma 2455 en menos de tres años, contados desde la aprobación y autorización del proyecto hasta su publicación final. Este documento se basó en la norma IEEE 1068, una norma de reparación de motores eléctricos ampliamente utilizada.

Los participantes del grupo de trabajo fueron: 

*Indica miembro de la EASA

El grupo de trabajo abarcó todos los aspectos, desde la recepción de la máquina hasta la inspección y las pruebas, incluyendo todos lo relacionado con la reparación y el rebobinado. Comenzando con un"Diagnóstico en la planta del usuario", el documento recomienda que el propietario/ usuario final del motor recopile datos específicos para proporcionar la mayor cantidad de información posible al taller de reparación.

Las pruebas de entrada al taller de reparación se describen en detalle, y muchas de ellas (por ejemplo, la de caída de tensión) son muy conocidas por los centros de servicio de EASA. Casi una docena de figuras resultarán familiares para los miembros de EASA, ya que se publicaron originalmente en documentos de EASA.

La norma incluye consejos específicos para el rebobinado de campos e inducidos, así como recomendaciones para el montaje y las pruebas finales. La IEEE 2455 también incluye anexos con un formato de evaluación y reparación, pautas para efectuar el rectificado, ranurado y biselado, una explicación de la conmutación, una guía de condición del colector, holgura de cojinetes de deslizamiento, ventilación y enfriamiento, asi como también ajustes eléctricos posteriores al ensamblaje y corrientes por los rodamientos en máquinas de CC. 

Chuck Yung
Especialista Sénior de Soporte Técnico de EASA

Aunque el primer motor de C.C. práctico fue construido por Moritz Jacobi en 1834, fue durante los 40 años siguientes que hombres como Thomas Davenport, Emil Stohrer y George Westinghouse fabricaron máquinas de C.C. para uso industrial.

Es inspirador darse cuenta que los motores de C.C. han estado trabajando por más de 160 años. Durante el siglo pasado, las máquinas de C.C. con potencias por arriba de los treinta o cuarenta kW han sido refrigeradas de la misma forma, montando un soplador de aire de jaula de ardilla directamente en el colector.