Mike Howell
Especialista de Soporte Técnico de EASA

Las dos razones principales para realizar la prueba de núcleo del estator en el centro de servicios son (1) verificar que el núcleo del estator es apto para uso continuo y en caso de rebobinado y (2) verificar que el proceso de rebobinado no ha alterado de forma adversa la condición del núcleo del estator.

El propósito de este artículo es discutir como determinar, evaluar y comparar los resultados de la prueba de núcleo. Es muy importante comprender que la variación de los procedimientos de prueba puede invalidar la comparación.

David Sattler
L&S Electric, Inc.

A no ser que se tenga mucho cuidado, tirar del alambre magneto al desmantelar el estator de un motor a menudo deforma o dobla los dientes de las laminaciones. Estos dientes deformados comprometerán la calidad de la reparación y hay estudios que demuestran que este problema puede reducir la eficiencia del motor. Sin embargo, aunque esta reducción puede ser pequeña, genera altos costos y desperdicio de energía.

Aunque los clientes rara vez notan la merma del rendimiento, nuestro objetivo durante la reparación de los motores es siempre llevar a cabo rebobinados de la más alta calidad posible. Por lo tanto, hemos diseñado e implementado el uso de discos (platos) retenedores para mantener los dientes del estator en su lugar mientras se saca el alambre magneto de las ranuras. Los discos que se ven en las fotografías nos han ayudado a evitar y garantizar dañar los dientes del estator al sacar el alambre del estator.

Carlos Ramirez
EASA Technical Support Specialist

¿El motor consume mucha corriente en vacío, aunque los datos del bobinado son correctos? ¿El motor se calienta con carga de forma inusual? Estas son preguntas comunes que pueden ser resueltas verificando la condición del núcleo del estator. En este webinario discutiremos cómo realizar la prueba de lazo en el núcleo de un estator y cómo analizar los resultados obtenidos, proporcionando información sobre los equipos utilizados, consejos para reparar el núcleo del estator y otras pruebas alternativas.

El seminario incluye:

• Teoría de la prueba de lazo (“toroide”)
• Procedimiento de prueba
• Límites aceptables para las pérdidas y las temperaturas en el núcleo
• Equipo asociado
• Consejos para la reparación de núcleos dañados
• Pruebas alternativas

Este webinario es útil para supervisores, bobinadores y personal encargado de realizar las pruebas.

Chuck Yung
EASA Senior Technical Support Specialist

When a core loss test reveals localized hot spots, or visual inspection identifies physical damage, the ability to repair the damage in a cost-effective manner means the difference between repair or replacement.

Topics covered in this recording include:   

• What core loss flux level is correct?
• Clearing small localized hot spots
• What is the best way to clear surface shorting?
• Grinding versus spreading the laminations
• “Watt knocking” or physically manipulating the core
• Debunking the rusting myth — Coreplate, class F red insulator, waterglass
• Restack: complete or partial — Does lamination grade really matter? 

This presentation is intended for owner-managers, shop supervisors, machinists, service center technicians, and safety directors.

David Sattler
L&S Electric, Inc.

Unless great care is taken, pulling magnet wire from a motor stator often bends or splays the lamination’s end teeth. Bent teeth, or teeth that have been splayed outward at the ends of the core stack, will likely compromise the quality of the repair job. Studies¹ show that motor efficiency may be reduced by splaying end teeth. Even if that reduction in efficiency is slight, any reduction in efficiency results in higher costs and wasted energy.

Even though these performance reductions are seldom noticed by customers, our goal in motor repair is always to produce the highest quality rewind possible. Therefore, we have designed and implemented the use of disc clamps to hold the stator tooth tips in place while pulling magnet wire from the slots. The clamping fixtures described in the photos have helped ensure that we avoid damaging the stator teeth during the stripping process.

Cyndi Nyberg Esau
Former EASA Technical Support Specialist

Most rotor, stator and armature laminations used in industrial electric motors and generators are made from non-oriented, cold-rolled electrical steels. However, there is a lot of variation in the specific properties in these electrical steels. This article will discuss properties of electrical steels, as well as the methods used to optimize the magnetic characteristics of the laminations, specifically through the annealing process.

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.

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

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

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

Topics covered include:

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

Chuck Yung
EASA Senior Technical Support Specialist

You've just dismantled a special motor for a customer, and the core test indicates the watts loss/pound is excessive.  The high core losses are caused by shorts between the laminations.  This may be the result of a ground failure.  Or excessive temperatures may have caused the deterioration of inter-laminar insulation (called coreplate.)

Whatever the cause, a replacement is 16 weeks away, and your customer wants his motor repaired.  This motor sounds like a prime candidate for a restack, but you are hesitant.  Your company has a reputation for quality, and the finished product has to meet your usual high standards. 

You want to know the best procedure for repairing this core.  Here are some guidelines to help you do the best possible repair. 

Chuck Yung
Technical Support Specialist
Electrical Apparatus Service Association
St. Louis, MO

The paper "Stator Core Repair and Testing" by Chuck Yung, presented at the EASA Convention 2007, provides detailed guidance on the repair and testing of stator cores in electric motors. The paper aims to ensure quality restacks, identify key areas for quality control, and offer useful tips for special cases. It also debunks myths about alternative core repair methods and introduces a new effective alternative for repairing shorted laminations.

When a motor failure includes damage to the laminated core, the decision to repair or replace the motor depends on factors such as price and availability. If a replacement motor is not available, a labor-intensive restack of the stator core may be necessary. The goal of a restack is to dismantle the core, clean and recoat the laminations to minimize heating from eddy currents in shorted laminations, and reassemble the core with the correct geometry.

Core losses are categorized into hysteresis losses, notching stresses, and eddy-current losses. Hysteresis losses are minimized by the annealing process, while notching stresses are relieved during manufacturing. Eddy-current losses occur when adjacent laminations are shorted together, behaving as a single thicker lamination. The stacking factor, which describes the amount of actual lamination steel content over the core length, is crucial for minimizing core losses.

Proper stacking pressure, maintaining the original core length, and using low-loss silicon steel for replacement laminations are essential for reducing eddy-current losses. The paper emphasizes the importance of using laminations of the same thickness as the original to avoid increased losses. Eddy-current losses vary with the square of the frequency, so motors operating at higher frequencies require thinner laminations.

The paper debunks several myths about core repair methods, such as using water to create iron oxide coatings, sodium silicate, and acids. These methods are ineffective and can cause further damage. Instead, the paper recommends using suitable coreplate materials that can withstand burnout oven cycles.

The restack procedure involves drawing a detailed diagram of the stator core, sanding each lamination to remove burrs and residue, coating the cleaned laminations with coreplate, and restacking them to the original configuration. The paper highlights the importance of maintaining stack geometry and using a stacking fixture to minimize irregular lamination surfaces.

An alternative method for repairing cores with localized missing sections of teeth involves using magnetic wedge material to reconstruct the tooth geometry. This method is limited by the reduced ferrous material in magnetic wedges.

The paper introduces a new process called Cure-a-Core, which uses an aqueous solution of phosphoric acid with zinc, manganese, and iron in suspension to etch shorted laminations and form a durable coreplate. This process has successfully reduced core losses and improved power factor in several service centers.

Key Points Covered:

• Factors influencing the repair-replace decision for damaged stator cores
• Core losses: hysteresis, notching stresses, and eddy-current losses
• Importance of stacking factor and proper stacking pressure
• Myths about alternative core repair methods
• Detailed restack procedure
• Alternative repair methods using magnetic wedge material
• Introduction of Cure-a-Core process for reducing core losses

Key Takeaways:

• Repairing stator cores is labor-intensive but necessary when replacements are unavailable.
• Core losses must be minimized through proper stacking and use of suitable materials.
• Myths about core repair methods are debunked, emphasizing the need for effective techniques.
• The restack procedure requires careful attention to detail and maintaining stack geometry.
• Magnetic wedge material can be used for localized repairs but has limitations.
• The Cure-a-Core process offers a new, effective method for reducing core losses and improving power factor.

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.

Blake Parker
Technical Education Committee Member
High-Speed Industrial Service

When looking at a stator core that requires repair, it can be easy to jump to conclusions. There are many factors to consider when re-stacking a stator. Those include the materials, core compression, length of the core, vents, spacers, vent construction and more. This article focuses on taking proper measurements when re-stacking a stator core and how to go about stacking the stator to ensure those dimensions are met.

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

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.

Austin Bonnett
EASA Education and Technology Consultant
Gallatin, MO

In the paper "The Most Unlucky Things That Can Happen To A Customer’s Motor," presented at the EASA Convention 2004, Austin Bonnett explores the common causes of motor failures and provides insights into how these failures can be predicted, prevented, and repaired. The paper emphasizes the importance of understanding the root causes of motor failures, which are often predictable, repeatable, and preventable.

Bonnett outlines a methodology for identifying the root causes of motor failures, which includes examining the failure mode, failure pattern, appearance, application, and maintenance history. He stresses the importance of recording critical data and measuring results to benchmark performance and make necessary upgrades or revisions.

The paper identifies the most common sources of motor problems, including issues with bearings, stators, rotor cores, shafts, misalignment, and other factors. Bearing problems are often caused by improper lubrication, contamination, and excessive vibration and shock. Improper lubrication can result from using too much or too little lubricant, incompatibility of lubricants, or using the wrong type of lubricant. Contamination can occur due to moisture, foreign materials, and corrosion, leading to bearing damage. Excessive vibration and shock can be caused by rotor unbalance, coupling unbalance, system unbalance, sudden stops or loading, and environmental influences.

Stator problems are typically related to thermal overload, severe electrical abnormalities, and contamination of the insulation system. Thermal overload can result from horsepower overload, excessive ambient temperatures, load cycling, too many starts, or failure to accelerate. Electrical abnormalities include overvoltage, undervoltage, unbalanced voltage, single phasing, transients, and partial discharge. Contamination of the insulation system can be caused by moisture, condensation, abrasion, and foreign materials.

Rotor core failures are often due to poor geometry, out of balance, defective or damaged squirrel cages, and improper joining of bars to end rings. Common shaft failures include metal fatigue, rotational bending, torsional bending, extreme temperatures, residual stress, and environmental factors. Misalignment issues can arise from problems with the motor, coupling, driven equipment, mounting base, and other factors.

Bonnett also discusses other frequent causes of motor failures, such as misapplication, misuse, inappropriate repairs, alteration of the cooling system, hazardous terminal boxes, and coupling failures. He emphasizes the importance of proper maintenance and monitoring to prevent these failures and ensure reliable motor operation.

Key Points Covered:

• Root cause methodology for identifying motor failures
• Common sources of motor problems, including bearings, stators, rotor cores, shafts, and misalignment
• Causes of bearing problems, such as improper lubrication, contamination, and excessive vibration and shock
• Stator problems related to thermal overload, electrical abnormalities, and contamination
• Rotor core failures due to poor geometry, defective squirrel cages, and improper joining of bars to end rings
• Common shaft failures, including metal fatigue, rotational bending, torsional bending, and residual stress
• Misalignment issues and other frequent causes of motor failures

Key Takeaways:

• Motor failures are often predictable, repeatable, and preventable.
• Understanding the root causes of motor failures is essential for effective troubleshooting and repair.
• Proper lubrication, contamination prevention, and vibration control are crucial for bearing health.
• Thermal overload, electrical abnormalities, and contamination are common causes of stator problems.
• Rotor core failures can result from poor geometry, defective squirrel cages, and improper joining of bars to end rings.
• Shaft failures are often due to metal fatigue, rotational bending, torsional bending, and residual stress.
• Misalignment and other factors can lead to motor failures, emphasizing the importance of proper maintenance and monitoring.

Chuck Yung
Senior Technical Support Specialist
Electrical Apparatus Service Association
St. Louis, MO

In the paper "Time- and Money-Saving Devices for the Service Center," presented at the EASA Convention 2015, Chuck Yung shares practical tips and innovative ideas to help service centers improve productivity and save money. With the high costs of modern testing equipment, such as surge testers and test panels, it is essential for service centers to find cost-effective solutions. This paper compiles valuable insights from EASA members and others in similar industries, offering a range of tips to enhance efficiency and reduce expenses.

One of the key areas covered is electrical testing. Yung suggests building a large shop growler using a scrap stator or a DC field and pole, which can serve as a custom growler for testing rotors. For performing AC voltage drop tests on machines with inaccessible inter-coil jumpers, he recommends using a "pick up coil" made from a solenoid coil or a small field coil. Armature testing can be simplified by using the end bracket with the brush rigging to perform a voltage drop test between bars, avoiding the need for expensive devices.

Core loss testing can be enhanced by using a surplus 400 Hz generator, which quickly reveals hot spots. Welding machines can serve as low voltage, high current test panels for drop testing interpoles or large synchronous field coils. Additionally, a wound rotor motor can be repurposed as a variable output autotransformer or a variable frequency power supply, providing versatile testing capabilities.

In the machining section, Yung highlights the use of expandable collets to grip the inside diameter of shaft clearance fits, making it faster and safer to handle end brackets. For balancing, he suggests creating a stepped shaft from a scrap shaft to hold commonly balanced fans or impellers. To reduce eye strain and mistakes when chamfering a commutator, a page magnifier mounted on a magnetic base can be used.

For aluminum rotor repairs, Yung offers an alternative to rebarring by drilling through the end ring and inserting a replacement bar, which can be MIG welded to the end ring. This method allows for quick repairs without the need for a complete rebar.

Cleaning processes can be improved with the use of automatic parts washers, which offer significant labor savings and environmental benefits. Dry ice cleaning is recommended for its portability and eco-friendliness, especially for on-site motor cleaning. For removing varnish from machine surfaces, an oxygen-rich flame from a cutting torch can be used to burn off the resin without overheating the part.

Yung also provides tips for cleaning heat exchangers, such as using bottle brushes or taking radiator-type heat exchangers to a local radiator shop for boiling out. Painting the stator core a bright color improves visibility for winders, reducing eye strain and the chance of errors. An inexpensive pull-down shade can protect winding insulation materials from contamination.

Finally, Yung advises against building an induction bearing heater, as it is more cost-effective to purchase one. However, he suggests using the heater to test coils for shorted turns. For replacement parts, local marinas or sheet metal shops can fabricate air baffles and fan covers when the original manufacturer is unavailable.

Key Points Covered:

• Electrical testing tips, including building custom growlers and using pick up coils
• Enhancing core loss testing with surplus 400 Hz generators
• Using welding machines and wound rotor motors for versatile testing
• Machining tips for handling end brackets and balancing
• Alternative methods for aluminum rotor repairs
• Cleaning processes with automatic parts washers and dry ice cleaning
• Tips for cleaning heat exchangers and painting stator cores
• Advice on purchasing induction bearing heaters and sourcing replacement parts

Key Takeaways:

• Cost-effective solutions can significantly improve productivity and reduce expenses in service centers.
• Custom-built tools and repurposed equipment can enhance testing capabilities.
• Proper cleaning and maintenance practices are essential for efficient operations.
• Investing in the right equipment, such as induction bearing heaters, can save time and money in the long run.
• Local resources, such as marinas and sheet metal shops, can provide valuable support for sourcing replacement parts.

Blake Parker
Miembro del Comité de Educación Técnica
High-Speed Industrial Service

Al revisar el núcleo de un estator que requiere reparación, puede ser fácil sacar conclusiones precipitadas. Hay muchos factores que se deben considerar al re apilar el núcleo del estator. Estos incluyen los materiales, la compresión y largo del núcleo, los orificios de ventilación, los espaciadores, la construcción de los orificios de ventilación y más. Este artículo se centra en tomar las medidas adecuadas para volver a apilar el núcleo de un estator y cómo garantizar que se cumpla con esas dimensiones.

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
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.

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.