Gene Vogel
EASA Pump & Vibration Specialist

A customer specifies that the rotor is to be balanced to 4W/N. Is that the 4W/N Military specification, or the 4W/N API specification?It could make a big difference. And, how do they compare to the ISO 1940/1 specification (G2.5, G1, etc.)? Fortunately, for symmetrical rotors, comparing the various standards is only a matter of a bit of easy algebra. For non-symmetrical rotors, the process gets a little more difficult because each of the specifications handles these cases differently. The other good news is that there are on-line references that provide graphic and tabulated comparisons.

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

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.

Presented by Chuck Yung
EASA Senior Technical Support Specialist

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

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

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

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

Gene Vogel
EASA Pump & Vibration Specialist

La especificación ISO para balancear rotores rígidos (ISO 1940-1) fue innovadora cuando fue introducida hace varias décadas. Esta norma estableció los Grados de Calidad de Balanceo basada en la velocidad teórica que el centro de masa de un rotor se encontraría en espacio libre, girando a la velocidad de funcionamiento normal del rotor. Esta es terminología técnica difícil de expresar, pero un entendimiento práctico de la naturaleza de las fuerzas de desbalanceo es importante para aplicar las tolerancias de balanceo en rotores de diversas máquinas. Esto también ayuda a entender el impacto de los cambios fundamentales en la reciente norma de reemplazo: 21940-11: 2016.

Primero, vamos a clarificar la diferencia entre desbalanceo y vibración. Si una máquina tenía cierta cantidad de desbalanceo y fue asentada sin restricciones sobre un acolchado suave (una almohadilla de caucho), existirá cierta cantidad de vibración a 1x rpm. Atornille esa misma máquina a una fundación maciza y la vibración a 1x rpm será mucho menor. Así que no hay conversión directa de desbalanceo a vibración y viceversa.

Por consiguiente, para maquinaria en funcionamiento, las unidades de amplitud de vibración comunes de desplazamiento y velocidad no son medidas directas del desbalanceo, La cantidad de desbalanceo del rotor se puede describir como una cantidad de masa (peso) en un radio determinado.

El artículo continúa cubriendo:

• Unidades de desequilibrio
• Dos posibles aproximaciones para el uso de planos de cojinetes para evaluar la tolerancia de equilibrio
• Desplazamiento del centro de gravedad

Gene Vogel
EASA Pump & Vibration Specialist

The ISO balancing specification for rigid rotors (ISO 1940-1) was innovative when it was introduced decades ago. It established Balance Quality Grades based on the theoretical velocity the mass center of gravity of a rotor would encounter in free space, spinning at the rotor’s normal operating speed. That’s a mouthful of technical jargon, but a practical understanding of the nature of unbalance forces is important in applying balance tolerances to various machine rotors. It is also helpful in understanding the impact of fundamental changes in the recent replacement standard, 21940-11: 2016.

First, let’s clear up the difference between unbalance and vibration. If a machine had a certain amount of unbalance and was sitting unrestrained on a soft pad (a durometer pad), there would be a certain amount of vibration at 1x rpm. Bolt that same machine to a massive foundation and the vibration at 1x rpm would be much less. So there is no direct conversion from unbalance to vibration or vice versa.

Consequently, the common vibration amplitude units of displacement and velocity are not direct measures of unbalance for operating machinery. The amount of rotor unbalance can be described by an amount of mass (weight) at a certain radius.

The article goes on to cover:

• Unbalance units
• Two possible approaches to using bearing planes to evaluate balance tolerance
• Displacement of center of gravity

Mike Howell
Especialista de Soporte Técnico de EASA

Para aquellos que trabajan casi exclusivamente con estatores trifásicos con devanados imbricados o concéntricos, las conexiones de los rotores bobinados con devanados ondulados pueden ser un reto. Esto es especialmente cierto, cuando los datos de conexión se pierden o cuando el fallo en el bobinado provoca daños en la conexión existente.

En estos casos, es conveniente contar con un método práctico que nos permita diseñar un diagrama de conexiones válido.

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.

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.

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

Occasionally an end user wants to take a motor designed for horizontal mounting and use it in a vertical position. In this article, we will address some of the key mechanical factors that should be considered when applying a horizontal ball bearing motor in a vertical mounting position. Figure 1 illustrates a horizontal motor in a vertical shaft down position.

These key factors include:

• Axial thrust load capacity of bearing supporting rotor weight
• Rotor weight
• Weight of output shaft attachments
• Axial thrust from direct connected driven equipment
• Bearing lubrication paths
• Bearing lubricant retention
• Shaft up or shaft down orientation
• Ingress protection
• Locking axial thrust bearing

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

De vez en cuando un usuario final quiere utilizar un motor diseñado para montaje horizontal en posición vertical. En este artículo, trataremos algunos factores mecánicos clave que deben ser considerados cuando se utiliza un motor horizontal con rodamientos de bolas en una aplicación en la que trabaja en montaje vertical. La Figura 1 ilustra un motor horizontal en posición vertical con el eje hacia abajo.

Los factores clave incluyen:

• Capacidad de carga axial del rodamiento que soporta el peso del rotor.
• Peso del rotor
• Peso de los elementos acoplados al eje de salida
• Empuje axial de los equipos de impulsión acoplados directamente
• Trayectorias de lubricación de los rodamientos
• Retención del lubricante de los rodamientos
• Orientación del eje: Hacia abajo o hacia arriba
• Protección contra ingreso
• Fijación axial del rodamiento de empuje

Tom Bishop, P.E.
EASA Technical Support Specialist 

This article describes machine balancing of the rotating components of motors and generators, primarily rotors and armatures. The methods described here, in general, can be applied to on-site balancing if the rotating component is accessible. The intent is to describe the methods of attaching balance weights, not determining acceptable balance level or the location and amount of correction weight. 

The advent of computerized balancing machines has made the latter steps rather straightforward. However, the challenge of how to attach a weight in such a way that it will remain secure and not negatively affect machine operation remains at times a vexing problem. 

What is the purpose of dynamic balancing a rotating part? It is to reduce unbalance and consequently to bring vibration to acceptable levels to allow for normal bearing and other component life. The acceptable levels of vibration are described in EASA Tech Note 32, “Standards For Dynamic Balancing,” thus we won’t explain them here.

Chuck Yung
Especialista Sénior de Soporte Técnico

Solía bromear con que si mencionas la palabra armónicos, los ingenieros se emocionan mientras que los ojos de los que no lo son se nublan. La verdad es que los armónicos se pueden entender fácilmente cuando se explican en términos sencillos. Estos son simplemente múltiplos de la frecuencia fundamental, con secuencia positiva, cero o negativa. La frecuencia fundamental es la frecuencia de línea, también llamada armónico de primer orden, que es de 60 Hz en América del Norte o de 50 Hz en la mayor parte del resto del mundo.

Otros armónicos (quinto, séptimo, etc.) se pueden ver como ese orden multiplicado por la frecuencia fundamental, o visualizarse como si tuvieran ese número de formas de onda en la misma distancia que una sola forma de onda de la frecuencia fundamental. Entonces, en un sistema de 60 Hz, el quinto armónico es 5x60 o 300 Hz. Habrá 5 formas de onda completas en el lapso de una sola forma de onda de 60 Hz. Cuando las porciones positiva y negativa de la onda sinusoidal son simétricas, los armónicos de números pares no existen.

Cualquier armónico que sea múltiplo de tres, en el mundo trifásico, es un armónico de secuencia cero; y, cuando estamos considerando un sistema de potencia sinusoidal, se cancela (a excepción de los alternadores síncronos, que están fuera del alcance de esta discusión).

Chuck Yung
EASA Senior Technical Support Specialist

I used to joke that if you mention harmonics, engineers get excited while the eyes of non-engineers glaze over. The truth is that harmonics can be easily understood when explained in layman’s terms. Harmonics are simply multiples of the fundamental frequency, with positive, zero or negative sequence. The fundamental frequency is line frequency – also called the first order harmonic -- that being 60 Hz in North America or 50 Hz in most of the rest of the world.

Other harmonic numbers (5th, 7th, etc.) can be viewed as that order times the fundamental frequency, or visualized as having that number of waveforms in the same distance as a single waveform of the fundamental. So in a 60 Hz system, the 5th harmonic is 5x60 or 300 Hz. There will be 5 complete waveforms in the span of a single 60 Hz waveform. When the positive and negative portions of the sine wave are symmetrical, even number harmonics are non-existent.

Any harmonic that is a multiple of three, in the three-phase world, is a zero-sequence harmonic; and, when we are considering a sinusoidal power system, cancels out (except for synchronous alternators, which are outside the scope of this discussion).

Cyndi Nyberg 
Former EASA Technical Support Specialist 

Stator windings in three-phase motors are de­signed to have the amount of flux that the core needs to produce the desired output. The number of turns and size of wire are limited by the core dimensions. However, in the squirrel cage rotor, there are many more variables in the design. One of the variables is the shape of the rotor slot. Many rotor designs are skewed. So, why are ro­tors skewed? 

As the rotor turns, discontinuities on the sur­face of the rotor and stator disrupt the magnetic flux path of the motor. The flux path variation shows up in the form of harmonics that affect the performance of the motor. The difference be­tween the number of stator slots and rotor slots has a significant impact on the harmonics. The motor may be noisy, or there may be stray torques that lower the torque during starting or acceleration. The stator-rotor slot difference is why a motor winding that is redesigned for a dif­ferent speed may have problems, and why it is important to check the stator-slot-rotor-bar com­bination before proceeding with the redesign. 

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

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

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

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

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. 

By Chuck Yung
EASA Technical Support Specialist 

Most EASAns are familiar with the basics of rotor rebarring.  We know that the endrings and bars are not always the same material, and understand the importance of maintaining the cage resistance.  For those of us who only occasionally rebar rotors, here are a few tips to assure a quality repair. 

En toda reparación mecánica, la capacidad para desmontar, reparar y volver a montar el motor de forma apropiada sin dañar innecesariamente ninguna de sus piezas es fundamental. Esto suena sencillo, sin embargo, durante el proceso de desarme se cometen demasiados errores costosos.

Si todos los motores entraran “como nuevos”, la tarea sería más simple, aunque esto no sería garantía de que el montaje del motor fuera adecuado.

Cuando un centro de servicio recibe un pago por reparar un equipo, quiere que este permanezca en funcionamiento, ya que, si el equipo falla dentro del período de garantía, deberá asumir el costo de volver a repararlo. Por lo que tiene sentido realizar la reparación correcta la primera vez.

Los procedimientos de reparación, así como los propios motores, son afectados por los cambios en la tecnología. Este libro intenta incluir las últimas tecnologías comprobadas. En muchos casos, los métodos de reparación tradicionales aún pueden ser la alternativa más práctica. Las opciones presentadas a lo largo de este libro están destinadas a ayudar a los técnicos a seleccionar el método de reparación correcto, reconociendo que la decisión final recae en el propietario del equipo.

Algunas veces los métodos de reparación pierden popularidad, no porque aparezcan métodos mejores sino debido a técnicas deficientes. Otros métodos de reparación son adecuados para algunas aplicaciones, pero no para otras. Es trabajo del reparador decidir cuál será el mejor método para cada caso.

Este libro se encuentra dividido en secciones para los componentes básicos del motor con métodos de reparación y consejos dispersos por todas partes. Donde resulte práctico, se discuten también las causas de fallo. Esto ayudará a los técnicos a seleccionar el método de reparación más apropiado para cada aplicación en particular. La información presentada se basa en publicaciones de EASA y en revistas técnicas y literatura suministrada por fabricantes de motores, proveedores y centros de servicio establecidos.

COMPRAR DESCARGAR COMPRAR VERSIÓN IMPRESA

Tabla de contenido

• Terminología del motor
• Aplicaciones del motor y encerramientos
• Procedimientos de inspección y prueba
• Consejos para desmontar motores
• Rodamientos
• Alojamientos de rodamientos, orificios de eje, sellos y ajustes
• Ejes
• Rotores
• Ensamble del motor
• Accesorios y cajas de conexiones del motor
• Dinámica del motor
• Vibración y geometría del motor
• Corrientes por el eje/rodamientos
• Consideraciones especiales para motores a prueba de explosión
• Fallos en las componentes mecánicas
• Reparaciones misceláneas

Esta obra contiene muchas sugerencias sobre el manejo apropiado de las diferentes partes de un motor para minimizar los daños durante el proceso de reparación. Sin embargo, es imposible desarrollar un listado que las incluya todas.

En cambio, el principio básico de tomarse el tiempo para usar la herramienta adecuada y por lo general el procedimiento apropiado guiará a los técnicos por el camino correcto.

Mike Howell
EASA Technical Support Specialist

For those who work almost exclu­sively with lap or concentric wound three-phase stators, wave wound rotor connections can be a challenge. This is especially true if connection data gets lost or if an existing winding con­nection is damaged during a failure. In these cases, it is useful to have a practical method for laying out a valid connection diagram.

Chuck Yung
EASA Technical Support Specialist 

Occasionally, someone asks how much current a squirrel cage rotor bar carries. That’s an interesting question, and the answer depends on several factors. The rotor kVA* of a wound rotor motor is typically about 0.8 times the stator kVA. 

The rotor rated voltage is open circuit—a condition than cannot exist in a functional squirrel cage rotor— and the amps are at rated-load; the two don’t “coincide,” thus the 0.8 factor. For a squirrel-cage induction rotor, a multiplier of 0.96 is used because the magnetizing current comes from the stator rather than from the rotor. 

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.

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

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. 

Mike Howell
EASA Technical Support Specialist

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

Fundamental to every good mechanical repair is the ability to disassemble, repair and reassemble the motor correctly without unnecessary damage to any of the motor parts. This sounds simple, and yet too many costly mistakes are made in this process of taking things apart. If every motor repaired was in “as new” condition, the task would be much simpler. But this would be no guarantee that the reassembly would be correct.

​There is usually an easy way and a hard way to remove and install parts. Brute force is seldom the easiest or the correct way. The old saying of “don’t force it, get a bigger hammer” is seldom the best way.

When a service center is paid to repair equipment, the service center wants it to stay in operation. If the equipment fails again—within the warranty period—the service center pays to repair it again. It makes sense to repair it correctly the first time.

In order to improve equipment, it is important to know how and where it operates. Without understanding why a motor fails, it is impossible to deliberately improve its mean time between failures.

To do this, there must be communication between the service center and the motor user. Not only does this help the repairer decide the best course of action, but it helps the user appreciate the professionalism of the service center.

Repair procedures, like motors themselves, are affected by changes in technology. This book attempts to include the latest proven technologies. Time-honored methods of repair, in many cases, may still be the most practical option. Options presented throughout this book are intended to help the technician select the appropriate repair method, recognizing that the ultimate decision rests with the equipment owner.

Repair methods sometimes fall into disfavor, not because better methods are introduced, but because of poor techniques. Other repair methods are well-suited to some applications but not to others. It is the job of the repairer to decide what is the best method for each case.

This book is divided into sections for basic motor components with repair methods and tips dispersed throughout. Where practical, reasons for failures are also discussed. These will aid the technician in selecting the most appropriate method of repair for each unique application.

The information presented draws from EASA publications, IEEE publications, technical journals and literature supplied by vendors, motor manufacturers and established service centers.

This book contains many suggestions on how to correctly handle the various parts of a motor during the repair process so as to minimize damage. However, it is impossible to develop an all-inclusive list. Instead, the basic principle of taking the time to use the correct tool and correct procedure will usually lead the technician down the right path. Always remember, if it has to be forced beyond reason, it might be that neither the proper tool or procedure is being used or something is wrong with the parts. Step back and ask “What am I overlooking?”

Table of Contents

1. Motor Nomenclature
2. Motor Applications and Enclosures
3. Test and Inspection Procedures
4. Motor Disassembly Tips
5. Bearings
6. Bearing Housing Repair, Shaft Openings, Seals and Fits
7. Shafts
8. Rotors
9. Motor Assembly
10. Motor Accessories and Terminal Boxes
11. Motor Dynamics
12. Vibration and Motor Geometry
13. Shaft/Bearing Currents
14. Special Considerations for Explosion-Proof Motors
15. Failures in Mechanical Components
16. Miscellaneous Repairs

This book is available as part of EASA's Fundamentals of Mechanical Repair seminar.

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Eugene Vogel
EASA Pump & Vibration Specialist

On occasion, service centers are asked to balance fan blades that are designed for an overhung mounting. The fan blade may be received mounted on the shaft, or without any shaft. The decision has to be made about how to mount the rotor in the balancing machine. One solution is to fabricate a mandrel to balance the fan blade between the machine pedestals. The other alternative is to mount the fan blade on the end of the shaft in an overhung configuration, with the fan blade outboard of both balancing machine pedestals. This would be the more expedient method if the fan blade is already mounted on the shaft in the overhung configuration.

As long as the fit of the fan blade to the shaft doesn’t change (when using a mandrel), it can be mounted in either configuration for balancing without affecting the results. If the fan blade is balanced in one configuration, it is balanced for the other. 

How the fan blade is mounted doesn’t change the balance, as long as the fit to the shaft doesn’t change. So the question is, “Which is easiest?” Often it is easiest to mount the rotor in the overhung configuration, but balancing in that configuration presents some challenges. Those challenges are addressed here.

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

Jim Bryan
Especialista de Soporte Técnico de EASA

 El motor de inducción de jaula de ardilla funciona cuando se aplica tensión al bobinado del estator que induce, a través del entrehierro un voltaje en el circuito del rotor. El rotor de jaula de ardilla está formado por un núcleo o paquete magnético con ranuras que alojan un determinado número de barras conductoras y anillos de cortocircuito que fijan todas las barras entre sí. La jaula de ardilla está hecha con  barras conductoras y anillos generalmente fabricados con aleaciones de cobre o aluminio. En la Figura 1 se puede apreciar una jaula de ardilla sin el núcleo o paquete magnético.

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

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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

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BOOK DOWNLOAD CD-ROM BOOK & CD-ROM

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

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.

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

Being accustomed to rewinding AC stators, we may not realize that there is an equivalent repair service that can be performed on some rotors…namely, rebarring. Our focus in this article will be the rebarring of fabricated copper squirrel cage rotors. Redesign of rotors is outside the scope of this article.

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

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

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

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

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

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

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

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

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

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

Sections in the manual include:

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

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

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DISCOUNTED BLACK & WHITE BOOKS!
Prices are now DISCOUNTED on remaining black & white books while supplies last! If you have already purchased a black & white manual and are interested in the color version, please contact EASA Customer Service (+1 314 993 2220).

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Cyndi Nyberg 
Former EASA Technical Support Specialist 

When manufacturers design a motor, there are many variables. For a given stator, the winding has to conform to some fairly rigid rules, and there is not a lot of variance, even among different manufacturers. However, the rotor de­sign is wide open. The variables in the rotor design include: the number of bars, bar material, bar shape, end ring design, skew, air gap, and construction (cast or fabricated). All of these will ultimately have an impact on the speed torque characteristics of a motor. 

Unfortunately, because of the rotor design, it is difficult to alter the basic shape of the speed torque curve of a motor in for repair by modifying the stator winding. Modifications can be made to the rotor such as changing the bar material or size, and changing the end ring design, but it is difficult to determine the actual effect they will have on the operating characteristics. 

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.

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.

The first step in test running a wound rotor motor is to apply approximately half-rated voltage to the stator, with the rotor circuit open (leads open or brushes lifted).

Check the rotor ring-to-ring voltage. It should also be approximately half-rated rotor voltage. Typically it will be slightly higher than the ratio of rated stator to rotor volts.

For example, if the stator is rated 460 volts and the rotor 300 volts, with 230 volts applied to the stator, the open circuit rotor voltage should be about 157-160 volts.

With the rotor open and energized for the above test, the rotor may “crawl” or most often will remain stationary.

If the rotor immediately accelerates to speed when the stator is energized, the rotor is either shorted or misconnected internally (or the rotor has an unusually high number of parallel circuits).

To test run the motor, short the rotor ring leads and apply reduced voltage to the stator. If the rotor remains stationary, disconnect power to the stator.

Next, hand-rotate (spin) the rotor and energize the stator with the rotor rotating. It should then start.

The reason that the wound rotor may tend to lock-up or not rotate (i.e., cog) is that the stator-rotor slot combination makes it sensitive to rotor position.

In many cases, simply slightly rotating the rotor will allow it to start.

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

Chuck Yung
EASA Senior Technical Support Specialist

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

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.

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

Although unbalanced magnetic pull can affect other rotating electric machines such as DC motors and generators and single-phase motors and generators, our focus in this article will be on three-phase squirrel cage induction motors. The two main topics are unbalanced magnetic pull (UMP) and rotor pullover. For clarity we will begin by defining and briefly explaining these terms.

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

The slip ring or wound rotor induction motor (WRIM) has been used in a variety of applications. Many of these applications use the WRIM’s high starting torque capabil­ity while limiting current to start and run very high inertia loads such as hammer mills, rolling mills, centri­fuges, and rotary kilns. Other applica­tions utilize the variable speed capability of the WRIM. Probably the most common use of WRIMs for variable speed is in crane and hoist service. Other variable speed uses for the WRIM include wiredraw ma­chines, fans, blowers, pumps and refrigeration compressors. 

Variety of solutions, options 
Many of these applications, if designed today, would utilize a standard induction motor and variable frequency drive (VFD), particularly those where speed control is the desired end result. When older WRIMs or their controllers fail, the best solution often is to replace both motor and control. There are situations, however, where the best solution may be to replace the old controller with a VFD and continue to use the WRIM. 

Chuck Yung
EASA Senior Technical Support Specialist

Vertical motors differ from horizontal motors in numerous ways, yet some view them as “just a horizontal motor turned on end.” The obvious differences are the (usually) thrust bearings, with arrangements varying from single- to three-thrust bearings with different orientations suited for specific load, rpm and applications.

Less obvious differences are in the ventilation arrangements, shaft stiffness, degrees of protection and runout tolerances. This session will include:

• Bearing systems: Single, double or more?, Thrust direction, Angle of contact and rpm, Spherical thrust bearings, hydrodynamic
• Ventilation and cooling
• Operating environment, and enclosures: Enclosures (degrees of protection), ODP, TEFC, WPI, WPII (IP equivalents)
• Oil types and quantity: Bearing load and operating temperature, Consideration of speed, Sizing and adding cooling tubes
• Runout tolerances and repair methods: Upper bearing housing, Bearing carrier and shaft, Bottom bracket flange, Best practice methods for re-machining

This recording will benefit the service center owner, supervisor, technicians, sales personnel and customer.

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.

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

Chuck Yung
EASA Senior Technical Support Specialist

Even though they comprise a small portion of electric motors in service, wound rotor motors are disproportionately represented in EASA’s tech support call volume. There are several misconceptions about how they work. This paper will describe how they are applied and explain several simple but critical tests for the repairer.

• What are the rotor leads used for?
• What is the purpose of the steps/resistance changes?
• How should you evaluate the completed repair?
• Common causes of failure and how to prove them to your customer
• Considerations and cautions for retrofitting a wound rotor motor with a VFD
• Identifying wave wound rotor connections