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.

This handy form provides fields to record all of the details needed to document and/or order replacement coils for an AC form-wound machine. The form also includes detailed descriptions/definitions of the data to be recorded in each field.

Mike Howell
EASA Technical Support Specialist

The October 2012 Currents article titled "How to properly test AC stator and wound rotor windings" provides a thorough explanation on the proper application of insulation resistance, winding resistance, surge testing and high potential testing for stators and wound rotors. The article emphasizes that NEMA MG 1-2011 specifies AC and DC high potential (hi-pot) test levels for new windings and does not recommend repeated application of the high potential test. This is reinforced in EASA's Recommended Practice for the Repair of Rotating Electrical Apparatus (ANSI/EASA AR100-2010) which calls for reduced voltage levels for repeated tests should they be required. This article is intended to provide additional information on the high potential test performed on new windings. Specifically, it addresses the advantages of AC high potential testing for new, form coil stator windings. Topics discussed include: A real-world example Destructive test? Sizing the AC test set Example calculation Bibliographic references to additional reference materials

Presented by Chuck Yung
EASA Senior Technical Support Specialist

This webinar recording shares some of the “best practice” rewind methods used by (and learned from) EASA service centers around the world: connection recognition, best insulating materials, wire choices and tips to save time and effort. Topics covered include:

• Slot liner, separators and phase insulation
• Managing voltage stresses
• Making the connection: solder, crimp fittings or silphos
• Lacing tips
• Testing the completed winding

This webinar is intended for experienced and prospective winders, and those who supervise winders.

Cyndi Nyberg
Former EASA Technical Support Specialist

It is very important to take accurate data when you rewind a form-wound motor or generator, especially if the coils will be made by an outside coil manufacturer. There are a couple of variations to the standard coil design that is not common, but that you may come across from time to time.

Brick-type design
A brick-type winding uses two different sizes of wire arranged as shown in Figure 1. Rather than use one large rectangular wire for each turn, this example uses four smaller wires. When more than one conductor is used, each individual conductor must be insulated. However, it is not necessary to insulate every wire to achieve the required separation.

Chuck Yung 
EASA Technical Support Specialist 

There are times when a winding cannot be processed through the burn­out oven, so it must be removed “cold.” The bond strength of most resins is approximately 8-10 psi (55-70 kPa), which means that a fairly large coil might have nearly 3,000 pounds (1350 kg) of bonding force with the slot. 

In those cases, there are some use­ful tips that can be used to reduce the difficulty in removing the coils. Many of the techniques in this article can be adapted for open slot wound rotors and armatures. 

Chuck Yung
EASA Senior Technical Support Specialist

When an electric motor is expected to be stored for an appreciable time before it is placed into service, certain steps should be taken to ensure that it will be suitable for operation when it is needed. The practical limitation we need to recognize is that much of what we do when putting a motor into long-term storage has to be undone when the same motor is moved into operation. This article addresses common recommendations for stored motors.

Chuck Yung
Especialista Sénior de Soporte Técnico de EASA

Con el aumento continuo de los tamaños de los motores CA y la obvia superioridad de los devanados con bobinas preformadas (pletina o solera), un área en la que podemos ayudar a mejorar la confiabilidad de los motores de nuestros clientes es rediseñando estos motores grandes de alambre redondo para que acepten bobinas preformadas. La mayoría de los reparadores estarían de acuerdo en que las máquinas de alambre redondo por arriba de 600 hp (450 kW) deberían rediseñarse con bobinas preformadas. Así mismo, aquellas con tensiones nominales superiores a 2 kV serían más confiables con bobinas de pletina.

Nadie quiere rebobinar un motor con 60 #14 AWG (62- 1.6 mm). Con la abundancia de proveedores especializados en laminaciones de estatores, el costo y la practicidad para convertir motores de alambre redondo a pletina está al alcance de casi todos los centros de servicio. Las laminaciones para reemplazar el núcleo pueden ser troqueladas o cortadas con láser o agua y entregadas en tiempos muy razonables.

Chuck Yung
EASA Senior Technical Support Specialist

With a steady increase in random wound AC motor sizes and the obvious superiority of the form coil winding, one area where we can help improve customers' motor reliability is by redesigning those large random wound motors to accept form coils. Most repairers would agree that machines rated larger than 600 hp (450 kW) should be designed as form coil machines. Likewise, those rated over 2 kV will be much more reliable as form coil machines.

No one wants to rewind a motor using 60 #14 AWG (62- 1.6 mm) wires in hand. With an abundance of niche suppliers of stator laminations, the cost and practicality of converting a random wound motor to form coil are available to nearly all service centers. Replacement laminations can be punched, laser-cut or water-cut, and supplied with very reasonable delivery times.

Presented by Mike Howell
EASA Technical Support Specialist

EASA’s Energy Policy states that members will strive to ensure that the methods, techniques and materials they use to service and rebuild rotating electrical machines will maintain or improve their energy efficiency, whenever possible. Controlling stator copper losses during rewinds is a significant part of that effort. This webinar recording looks at several aspects of winding design to prevent increased temperature rise and decreased efficiency.

• I²R losses and conductor area / length
• Eddy current losses and laminated conductors
• Circulating current losses and transposed conductors

This webinar recording will benefit service center managers, supervisors and technicians responsible for rewinds.

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.

Chuck Yung
EASA Technical Support Specialist 

When rewinding motors rated 6 kV and above, there are certain steps beyond the normal rewind procedures used for 2.3 kV/4 kV machines. Whether a machine is to be VPI processed makes a difference in how the winding should be treated. 

Aside from the obvious issues of insulation and higher voltages, there is also the possibility of partial discharge (PD), which brings its own unique set of problems. Air is an electrical insulator, albeit one of inconsistent quality. Increased humidity lowers the dielectric breakdown voltage of air, so an air gap that might be adequate under dry conditions may prove inadequate when the humidity is high (Table 1). Even though a form coil is fully taped, sealed and processed, the presence of air in voids within the coils, or between the coil and ground (i.e., in the slot) can cause problems with PD. 

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

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

Helpful tools

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

This presentation covers the following topics:

• Induction motor basics for operation
• Squirrel-cage
  • Conductor material
  • Deep-bar effect
  • Multiple-cage windings
  • Phase resistance
  • IEC/NEMA design letters
  • Speed-torque characteristics
• Wound-rotor
  • Winding construction
  • Wave-wound connections
  • Distribution factor and chord factor
  • Rotor phase voltage
  • Speed-torque characteristics

Target audience: This webinar will benefit service center technicians and supervisors. 

An increasing number of manufacturers are using magnetic wedges in their form-wound machines. When a winder fails to replace magnetic wedges in kind, the winding temperature rise can increase by 20°C, and the magnetizing current can increase by 20% or more.

This recording explains why they are used, provides a balanced review of the benefits and negative issues associated with their use, and explains how to avoid the problems.

• Why some manufacturers use magnetic wedges
• Benefits of magnetic wedges
• Downside of magnetic wedges
• Fitting and installation to prevent them from falling out in service

Target audience: This will benefit service center technicians and supervisors.

David Sattler
L&S Electric

El objetivo de la impregnación por vacío y presión (VPI) es saturar completamente un devanado con resina aislante. A medida que la resina penetra en los materiales aislantes, los oscurece y al drenar la resina VPI del devanado, todo el aislamiento de las cabezas queda oscuro en un tono uniforme. El aislamiento de la conexión también debe quedar oscuro de forma pareja. Si algún aislamiento muestra un tono más claro, la bobina o el puente no han quedado completamente saturados y el devanado no está debidamente protegido. Si no se soluciona el problema, es probable que esto provoque un fallo prematuro en el equipo. Esto podría generar una garantía costosa o, como mínimo, la reparación no brindará a sus clientes la calidad que esperan y merecen.

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

Shows how to determine loop dimensions, winding the loop, insulating it and spreading the coils, along with determining the dimensions of the spread coil.

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.

Teaches how to determine type of connection, number of parallel circuits, turns per coil, wire size, span and groups. Shows step-by-step way to properly record all information.

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.

Describes the procedures for inserting a formed coil into a stator. Shows proper tools and various wedging methods. Includes connecting the coils and insulating the connections.

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.

David Sattler
L&S Electric

The goal of vacuum pressure impregnation is complete saturation of a winding with insulating resin. As resin penetrates the insulating materials, it causes them to darken. When VPI resin is drained from the winding, all end-turn insulation should be darkened to a uniform shade. Connection insulation should also show uniform darkening. If some insulation is a lighter shade, the coil or jumper is not fully saturated. The winding is not properly protected, and if the problem is not addressed, it is likely to cause premature failure of the unit. This could result in costly warranty work, or at a minimum, will be a failure to provide your customers with the quality they expect and deserve.