This presentation focuses on the steps to take before rewinding to avoid the undesirable situation of high no-load motor amps after the rewind.

The presentation covers the following steps that should be performed on every AC stator rewind:

• Inspect the stator bore and rotor outside diameter for evidence of machining or damage
• Record the original winding data exactly as found
• Verify the winding data
• Test the stator core before and after rewinding removal

This presentation is most useful for service center mechanics and winders with any level of experience, and service center supervisors and managers.

Tom Bishop
Especialista Sénior de Soporte Técnico de EASA

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

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

The most frequent concern about a motor with high current is high no-load current with a 3-phase motor. That topic has been addressed in three prior Currents articles beginning with “Test Run Tips: Common Causes For High No-Load Current On Rewound Motors” (June 2002); “Avoiding High No-Load Amps On Rewound Motors” (February 2004); and “Taming Those Misbehaving Motors” (December 2009). This article will cover the broad topic of high no-load current with 3-phase motors and the issue of high current with load with 3-phase motors. Also considered will be cases of lower than expected no-load current.

Presented by Tom Bishop, PE
EASA Senior Technical Support Specialist

This recording covers the broad topic of high no-load current with 3-phase motors and the issue of high current with load with 3- phase motors. Also covered are the cases of lower than expected no-load current. 

Primary topics are:

• High no-load current – motor not rewound
• High no-load current – rewound motor
• High current with load

This presentation is intended for mechanical technicians, winders, supervisors, engineers and managers. 

Chuck Yung
Especialista Sénior de Soporte Técnico de EASA

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

Chuck Yung
EASA Senior Technical Support Specialist

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

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

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

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

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

Mike Howell
Especialista de Soporte Técnico de EASA

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

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

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

Chase Fell
Precision Coil and Rotor

An ideal insulator allows no leakage current to flow. The power factor of an insulator is defined as the cosine of the phase angle between voltage and current. For an ideal insulator, the current leads the voltage by exactly 90 degrees and the power factor for this ideal system would be zero. Coil systems in electric motors and generators have inherent losses causing capacitive and resistive current flow. For this insulation, the power factor cannot be zero. 

The power factor (PF) tip-up test is commonly used as a quality measurement for new coils and windings manufactured for AC motors and generators rated 6 kV and higher. For modern stator winding insulation systems, the power factor and the dielectric dissipation factor are very nearly the same. PF tip-up testing can be useful to verify the quality of the winding manufacturing process, insulation material performance, consolidation of conductors, uniformity of groundwall taping and state of resin curing. Once an insulation system reaches corona inception voltage (CIV), partial discharge (PD) will effectively short out some of the capacitance of the insulation and the power factor will increase. PF testing is applicable to individual vacuum pressure impregnation (VPI) coils and resin-rich coils as well as cured complete windings. Power factor tip-up testing is not applicable for bench testing of green VPI coils or evaluating pre-processed complete VPI windings. 

The power factor tip-up test can be useful in the rewind shop to verify the quality of a newly-installed coil system including the effectiveness of VPI processing. PF testing of in-service windings can set a baseline measurement for maintenance trending. The in-service PF tip-up test can potentially identify groundwall insulation aging since the capacitance between the copper conductor and the core is generally reduced as delamination and/or air pockets become present in the insulation between the coils and the core.

The challenge for every motor repair firm is twofold: to repair the equipment properly; and to demonstrate to their customers by means of adequate testing and documenta­tion that rewound motors retain their operating efficiency. Following the guidelines in the “DOs” and “DON’Ts” below will help you accomplish both.

Numerous studies have been done to determine the effect rewinding has on motor efficiency. These studies identified several variables that can impact the efficiency of a rewound motor, including core burnout temperature, winding design, bearing type, air gap and winding resistance. The following guidelines were developed as a result of those studies, which found that the efficiency of both standard and energy efficient electric motors can be maintained during rebuilding and rewinding. 

To ensure that motors retain their efficiencies when rewound, EASA also strongly recommends that electric motor repair centers comply with ANSI/EASA Standard AR100: Recommended Practice For The Repair Of Rotating Electrical Apparatus and strictly adhere to the “DOs” and “DON’Ts” that follow. These guidelines, which contain safe values (based on available data) and correct procedures, apply to both energy efficient and standard motors. Further study of the matter continues, and these guidelines will be revised if additional information warrants.

Chase Fell
Technical Education Committee Chair
Jay Industrial Repair

High potential (hipot) testing procedures for motor and generator windings are usually performed with 50/60 Hz AC or DC as the power source. Hipot testing is a critical step in validating the quality of new windings. AC and DC hipot tests are also useful to understand the condition of aged insulation for machines in service. DC hipot testing is widely used in motor repair because the equipment is portable, and the steady state test current comes mostly from leakage through the insulation.

When a breakdown occurs, DC causes less damage to material adjacent to the fault when compared to AC tests. A disadvantage of DC testing is the voltage is not distributed in the same way as what the winding sees with AC. Specifically, the DC test stresses the end turns much higher.

AC hipot testing is much more consistent with the voltage stress of the machine in service. Studies have shown that the AC hipot test can reveal insulation defects that are left undetected with DC tests. An AC test can better detect voids and delamination inside the insulation system. The disadvantage of the AC test at power frequency is when the size of the test set and/or complexity of the setup becomes problematic in motor repair and in the field.

High-potential testing is routinely used to assess the ground insulation of AC stator windings in-process, after completion of a rewind and post-delivery. This webinar covers:

• Differences between AC and DC high-potential tests
• Sizing AC test sets when testing large windings
• What relevant standards address (and what they don’t)
• Communicating test requirements to all stakeholders
• When to test and at what levels
• How to evaluate results

Beneficial for service center managers, supervisors and technicians responsible for high-potential testing.

There is much discussion in the industry about how to properly electrically test AC stator and wound rotor windings. Topics include test voltage, procedure, frequency and when to perform the various tests. This article describes how the following standards address these questions:

• NEMA MG 1-2011
• (MG1) IEEE 43-2000
• (IEEE 43) IEEE 62.2-2004
• (IEEE 62.2) IEEE 522-2004
• (IEEE 522) IEEE 1068-2009
• (IEEE 1068) ANSI/EASA AR100-2010
• (AR100) CSA C392-2011 (C392)

These standards are regularly reviewed and coordinated, so some of the information may not match the old yellowed reference taped to your toolbox lid. These updated references should replace anything dated previous to the dates indicated on the standard. AR100 Section 4.3.1 lists the recommended tests for stator and wound rotor windings. They are insulation resistance (IR), winding resistance, growler, phase balance, surge comparison, polarity and ball rotation tests. This article covers the IR, winding resistance and surge tests. Noticeably absent from this list is the ever popular high potential (hi-pot) test. Topics covered also include:

• IR (or megohm) test
• Polarization index test
• Winding resistance test
• Surge comparison test
• Hi-pot test.

Presented by Carlos Ramirez
EASA Technical Support Specialist

Is the motor drawing high no-load amps and winding data are correct? Are you experiencing unusual heating of the stator under load? Those common questions can be answered by checking the stator core condition. This presentation will discuss how to perform a stator core test using a loop test. It also will explain how to analyze the results, providing information about the associated equipment, tips for repairing core damage and explain other alternatives for stator core testing.

The presentation covers:

• Loop test theory
• Testing procedure
• Acceptable limits for losses and core temperatures
• Associated equipment
• Tips for repairing core damage
• Alternative stator core test

This presentation will be useful for supervisors, winders and test technicians.

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

LEARN MORE

Chuck Yung
EASA Senior Technical Support Specialist

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

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

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

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

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

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

Topics covered include:

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

Cyndi Nyberg Esau
Former EASA Technical Support Specialist

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

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

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

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

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

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

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

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

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

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

This webinar recording covers:

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

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

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

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

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

This webinar covers:

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

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

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

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

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

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

• Wave form
• Displacement
• Velocity
• Demodulation

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

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

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

Topics discussed include:

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

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

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

Chase Fell
Precision Coil and Rotor

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

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

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

In a previous article in Plant Engineering ("A systematic approach to AC motor repair," Plant Engineering, April 2015), EASA highlighted the good practices for electrical repair found in ANSI/EASA Standard AR100 Recommended Practice for the Repair of Rotating Electrical Apparatus, and the significant impact they can have on motor efficiency and reliability. But that was only part of the story, because mechanical repairs—and even documentation, cleaning, and inspection—can also markedly affect motor reliability and efficiency.

This latest article focuses on the mechanical and "other" repair good practices prescribed by AR100 that are mandatory in EASA's motor-repair accreditation program, including lubrication, bearings, and repair of frames, shafts, and bearing fits.

Items discussed include:

• Identification and labeling
• Identification of cause of failure
• Cleaning and inspection
• Cooling system check
• Exterior finish
• Packaging and transportation
• Mechanical repairs including items such as shafts, bearings, lubrication, frames, etc.
• Mechanical tests and instrument calibration

READ THE FULL ARTICLE

Chuck Yung
Especialista de Sénior de Soporte Técnico de EASA

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

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

This webinar discusses:

• Temperature rise
  • Method of detection
  • Insulation class
  • Enclosure
  • Service Factor
• Increasing winding life
  • Insulation class
  • Cooling system
  • Winding redesign

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

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

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

The article covers:

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

READ THE FULL ARTICLE

Mike Howell
Especialista de Soporte Técnico de EASA 

La pregunta sobre los niveles de tensión utilizados en la prueba de impulso para comprobar el aislamiento entre espiras de los estatores de pletina surge muy a menudo. ¿Y por qué nos la hacemos? Muchos centros de servicio tienen buenos fabricantes de bobinas en sus listados de proveedores calificados y varios clientes que poseen motores de pletina. No es improbable que cada fabricante de bobinas y cada cliente puedan especificar niveles de prueba diferentes. Aún más, la mayoría de las normas o guías disponibles proporcionan un rango de niveles de prueba. Esto es debido a que no todos los sistemas de aislamiento están diseñados con la misma capacidad para soportar impulsos. Además, las propiedades del aislamiento entre espiras de un sistema en particular pueden variar ampliamente, dependiendo del grado de procesamiento alcanzado al momento de la prueba.

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

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

Mike Howell
EASA Technical Support Specialist

The open stator impedance test (a.k.a, ball test or dummy rotor test) is used by many service centers as a quality control check before winding treatment and/or a troubleshooting test. This webinar recording reviews test procedures, expected outcomes and incorporating thermal camera imaging. Topics covered include:

• Understanding the principles of the test
• Performing the test safely and consistently
• Expanding the value of the test with thermography 

This webinar recording is intended for personnel responsible for testing stator assemblies.
 

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

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

Tom Bishop
EASA Senior Technical Support Specialist

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

This article covers:

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

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

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

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

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

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

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

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

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

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

Chase Fell
Presidente del Comité de Educación Técnica
Jay Industrial Repair

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

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

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

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

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

Pat Douglas
Kirby Risk-Mechanical Solutions & Service

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

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

Chuck Yung
EASA Technical Support Specialist 

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

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

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

La resistencia de aislamiento se calcula  de la siguiente forma:

R = E / I_T  

donde

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

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

R = E / I T  

where

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

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

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

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

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

EASA’s Rotor/Armature Core Test Form provides a step-by-step procedure for calculating the number of turns and cable size required for a loop test. The form also has provision for recording the meter and temperature readings obtained during the test. Core sketches that show the location of measured dimensions and a wiring diagram of instrument connections are also included.

For more details on rotor/armature core testing, see Section 7 of the EASA Technical Manual.

Con el aumento de la popularidad de los variadores de frecuencia electrónicos (VFDs o drives), es probable que casi todos los centros de servicio miembros de EASA hayan reparado algún motor que funcione con un variador de frecuencia en las instalaciones de un cliente. Lo más conveniente después de reparar estos motores es probarlos, generalmente en vacio, utilizando un drive. Esto nos permite simular la aplicación real del cliente variando la velocidad así como también tener la certeza que el motor funciona bien mecánicamente dentro de un rango de velocidades. Esto incluye descubrir problemas de vibraciones, identificar alguna velocidad de resonancia dentro del rango de operación y otros problemas. Además, si el motor está trabajando por arriba de la frecuencia de red (ej. 50 ó 60 Hz), deberá funcionar hasta alcanzar su velocidad máxima para comprobar sus niveles de vibración y que los rodamientos puedan expulsar el exceso de lubricación y se asienten por sí mismos en su posición de trabajo (break-in).

Los temas cubiertos incluyen:

• desafíos de selección
• Tensión, potencia y frecuencia
• VFD nuevos o usados para las pruebas?
• Parámetros de operación

Art Godfrey (retired)
Birclar Electric & Electronics
Romulus, Michigan
Technical Services Committee Member

With the growing popularity of variable frequency drives (VFDs), it is likely almost every EASA service center has repaired motors powered by one in a customer’s installation. For these motors, it is best that after repair they are tested using a VFD, typically at no-load. This will provide operation mimicking the actual customer applica­tion, at varying speeds, and will help ensure proper mechanical operation throughout the speed range. This will include detecting vibration problems, identifying any resonant speeds within the operating range, and more. Also, if the motor is used above utility line fre­quency (i.e., 50 Hz or 60 Hz), it should be operated up to top speed for vibra­tion measurement and a good break-in of the bearings.

Pat Douglas
Kirby Risk-Mechanical Solutions & Service

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

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

Tom Bishop, P.E. 
EASA Technical Support Specialist 

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

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

By Gene Vogel
EASA Pump & Vibration Specialist

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

Topics covered in this article include:

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

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Determining whether or not a squirrel cage rotor is defective is an issue that is a challenge to every service center as there is often no simple way to determine the integrity of a rotor. The primary focus of this session is to describe many of the available tests that can be utilized in the service center or at the motor installation site. In addition to conventional squirrel cage rotor testing methods such as the growler test, techniques that will also be covered are the use of a core loss tester, high current excitation, and spectrum analysis of vibration.

Target audience: This presentation will be most useful for service center and field technicians with at least 2 years experience, service center supervisors and managers, engineers, or anyone with previous experience dealing with suspected open rotor issues.

EASA’s Stator Core Test Form provides a step-by-step procedure for calculating the number of turns and cable size required for a loop test. The form also has provision for recording the meter and temperature readings obtained during the test. Core sketches that show the location of measured dimensions and a wiring diagram of instrument connections are also included.

For more details on stator core testing, see Section 7 of the EASA Technical Manual.

This presentation covers:

• The importance of the stator core test 
• Simple theory to share with technicians and customers 
• Practical approach for testing small stators demonstrated 
• Eliminating pen + paper; loop test calculations for any device 
• Assessing the results

Mike Howell
EASA Technical Support Specialist

The question of surge test voltage levels used for testing the turns insulation of form wound stators comes up often. And, why wouldn’t it? Many service centers have several good coil suppliers in their qualified supplier lists and quite a few customers who own form wound motors. It’s not unlikely that each of the coil suppliers and each of the customers could be specifying different test levels. What’s more, most authoritative standards or guides available provide a range of test levels. This is because not all insulation systems are designed to have the same surge withstand capability. Also, the turn insulation properties of one particular system can vary widely depending on the extent of processing complete at the time of test.

Mike Howell
EASA Technical Support Specialist

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

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

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

Cyndi Nyberg
Former EASA Technical Support Specialist

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

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

Cyndi Nyberg 
Former EASA Technical Support Specialist 

In the April 2007 issue of CURRENTS, we covered surge testing anomalies, speci.cally for AC windings. The surge test can be used for DC windings as well. It can be a useful tool for evaluating armatures and some DC fields. 

A note of caution:  If a winding does not have a minimum insulation resis­tance per ANSI/EASA AR100-2006, it is not safe to apply an overpotential test (surge or high potential). 
Surge testing shunt .elds may not provide meaningful results if the surge pulse decays too quickly — if it dissipates through only the .rst few hundred turns. To obtain a test voltage high enough to test every turn would require too high a voltage. That high voltage would overstress the ground-wall insulation. 

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

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

Topics discussed are:

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

Cyndi Nyberg
EASA Technical Support Specialist 

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

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

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.

Art Godfrey (retired)
Birclar Electric & Electronics

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

This webinar covers:

• The reasons for performing core testing and why they are important
• An explanation of the two core testing methods:
• Loop testing
• Use of a core tester
• How to properly perform a core test
• How to assess the results

Steve Skenzick
HPS Electrical Apparatus Sales & Service

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

Chuck Yung
EASA Senior Technical Support Specialist

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

Topics covered in this article include:

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

Cyndi Nyberg
Former EASA Technical Support Specialist

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

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

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

Mike Howell
Especialista de Soporte Técnico de EASA

El núcleo del estator de un motor de inducción se puede fabricar utilizando laminaciones de una sola pieza (vea la Figura 1 a la izquierda) de hasta un diámetro exterior de 48 pulgadas (1200 mm) aproximadamente. Para estatores más grandes, o cuando se minimiza el material de desecho, las láminaciones del estator son segmentadas (vea la Figura 1 a la derecha). El espacio circunferencial típico entre las láminaciones segmentadas es de solo unas 0,012 pulgadas (0,3 mm), por lo que se exagera en las cifras incluidas. El número de segmentos elegidos por un fabricante para un diseño determinado puede depender de varios factores, algunos técnicos y otros económicos. Para la mayoría de las actividades de reparación del centro de servicio, las máquinas con estatores con laminaciones segmentadas se procesan de la misma forma que aquellas con laminaciones de una sola pieza. Sin embargo, hay algunas áreas que vale la pena explorar que podrían ser útiles cuando se trabaja con estatores con laminaciones segmentadas.

Mike Howell
Especialista de Soporte Técnico de EASA

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

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

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

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

Outlines the basic steps and procedures commonly used for testing three-phase, AC motors rated 600 volts or less, including preliminary insulation resistance, continuity and no-load tests, as well as the single-phase test for rotors. Also explains how to test components once the motor has been disassembled; these procedures include surge-comparison, polarization index, growler and DC high-potential tests, as well as the “loop” test for stator cores. Additionally, examines performance of final insulation resistance, noload, vibration and dynamometer tests of repaired or rewound motors.

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

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

Mike Howell
EASA Technical Support Specialist

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

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

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

By Jane Alexander
Managing Editor of Maintenance Technology

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

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

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

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

Chuck Yung
EASA Technical Support Specialist

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

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

Bill Colton 
Baldor Electric Co. 
Commerce, California 
Technical Services Committee Member 

Today’s cost of energy has become a major consideration in most businesses. This is certainly true of EASA service centers as it is with most industry. We are all trying to find ways to make our facilities more efficient to either become more competitive, or gain greater profits – perhaps both. 

One of the tools that may make an EASA facility more attractive to potential customers is the ability to put a motor under load or even tested to a specified load. 
This is often done with a dyna­mometer. A dynamometer is defined as a device for measuring mechani­cal power, especially one that measures the output or driving torque of a rotating machine. 

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

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

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

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

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

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

Mike Howell
EASA Technical Support Specialist

The two primary reasons for performing stator core testing in the service center are (1) to verify that the stator core is acceptable for continued use and in the event of a rewind, and (2) to verify that the repair process has not adversely changed the stator core condition.

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

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

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

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

Mike Howell
EASA Technical Support Specialist

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

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

Mike Howell
EASA Technical Support Specialist

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

Chuck Yung 
EASA Senior Technical Support Specialist

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

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

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