Mike Howell
Especialista de Soporte Técnico de EASA

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

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

Mike Howell
EASA Technical Support Specialist

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

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.

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. 

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

BUY A COPY FOR YOUR OFFICE

PRINTED BOOK DOWNLOADABLE PDF

This book is also available focusing on NEMA Standards — in both English and Español.

NEMA - English NEMA - Español

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.

Chuck Yung
EASA Technical Support Specialist

Member Question: We recently received a rotating frequency converter for repair. It appears to be a wound-rotor motor coupled to an induction motor. The drive motor is not the same speed as the wound-rotor motor. How does this work?

The rotating frequency converter is exactly as you describe. Usually the drive motor has fewer poles than the wound-rotor motor, so the wound-rotor motor is driven faster than its synchronous speed to increase the frequency of the output.

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

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

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

12 presentations

$60 for EASA members

 

A special discounted collection of 12 webinar recordings focusing on AC motor windings.

Once purchased, all 12 recordings will be available on your "Downloadable products purchased" page in your online account.

Downloadable recordings in this bundle include:

The Basics: Taking Motor Data
Presented September 2016

This presentation covers:

• Photo documentation
• Paper documentation
• Measurements
• Winding data: turns, wire size, connection, core dimensions
• Keeping cause of failure questions in mind

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Taking Three-Phase Winding Data
Presented October 2012

This presentation stresses the importance of taking accurate winding data and explains and emphasizes the consequences of inaccurate data. Details are provided on how to take accurate electrical and mechanical data as well as how to verify the data is correct. It gives you and improved ability to "get it right the first time" so as to avoid the added cost and time of another rewind to correct errors.

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The Basics: Motor Connections
Presented November 2016

This webinar covers:

• Internal connections
• Connections in the outlet box
• Connections in the MCC Ladder diagrams

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Tips and Techniques for Winders
Presented August 2015

This webinar covers:

• Procedural tips for coil insertion
• Creating slot room where there is none
• Faster, easier separators
• Lacing technique to prevent phase paper pull-out
• Interspersed coil winding made simple
• Better braze joints

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Rewinding Tips for Premium Efficient Motors
Presented June 2016

This webinar recording covers: 

• Importance of core loss testing
• Methods to reduce core losses
• Slot fill improvement without reducing copper

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Windings & Connections
Presented December 2015

This webinar recording focuses on the internal connections of AC motors, including:

• Wye or delta?
• Parallel circuits
• Dual voltage - delta connected, wye connected and wye/delta connected
• Tri-voltage - 2D2Y1D and others

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Concentric or Lap? Considerations for the 2-Pole Stator Rewind
Presented September 2014

Two-pole motors present special rewind issues, especially when converting them from concentric to lap windings. The pitch is especially important as certain coil pitches will cause harmonics that have a negative impact on performance. Optimum pitches are often very difficult to wind and shorter pitches result in sacrificed conductor area.

This presentation explores sample redesigns and present some guidelines to assist in deciding between the concentric and lap winding.

Target audience: This webinar will be most useful for service center winders, engineers, supervisors and managers. The content will be beneficial for beginners through highly experienced persons.

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Stator Rewinds: When Things Get Tight
Presented June 2015

When preparing to rewind random or form wound stators, sometimes there just doesn’t seem to be enough room in the stator slot for the desired conductor area and insulation quantities. Common scenarios encountered are redesigns from concentric to lap, changes to higher voltages or aggressive designs from the OEM.

This webinar will look at balancing stator copper losses against insulation reliability.

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Ensuring Success With VPI
Presented June 2014

Global vacuum pressure impregnation is the most common insulation system processing method utilized for form wound stators today. A successful VPI depends on several variables including materials, methods and maintenance. This recording will provide information to assist the service center with ensuring success with form wound VPI projects.

Target audience: This recording will be most useful for service center winders, engineers, supervisors and managers. The content will be beneficial for beginners through highly-experienced persons.

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Induction Motor Rotor Windings: Squirrel-Cage and Wonld Rotor Basics
Presented January 2018

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. 

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2-Speed, 2-Winding Pole Group Connections
Presented September 2018

The topics covered included in this webinar recording:

• One circuit wye connection — Best, no parallel paths, turns per coil may prevent this
• Delta or multiple parallel circuits—Produces closed circuits, Circulating currents
• Open delta (4 wire connection)
• Permissible connections—Skip pole, adjacent pole
• Determined by speed combination

T​arget audience: This webinar recording will benefit service center technicians and supervisors.

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Minimizing Risk With High-Voltage Rewinds
Presented February 2014

This webinar presents a product quality planning process for industrial motor stator windings rated above 4 kV. Emphasis is placed on analyzing gaps between these projects and lower voltage rewinds as they relate to:

• Stator winding design
• Insulation system validation
• Process control

Target audience: This presentation is most useful for service center winders, engineers, supervisors and managers. The content targets beginners through highly experienced persons.

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