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. 

Mike Howell
EASA Technical Support Specialist

Induction motors are most often applied to what are essentially constant speed drive applications. However, the use of induction motors in variable speed applications continues to grow, primarily due to technology advances in power electronics. This webinar will review speed control basics for induction machines.

• Wound-rotor motor speed control
• Squirrel-cage speed control by pole changing
• Squirrel-cage motor speed control by variable voltage, fixed frequency
• Squirrel-cage speed control by variable voltage, variable frequency

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

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.

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.

Charles LeMone
LeMone Technical Services
Roanoke, VA 

Is the induction motor the preferred answer in industrial drive applications? What is happening to the synchronous, DC and wound rotor motor (WRM) in those applications? This paper discusses:

• Observations as customers replace synchronous, WRM and DC machines with the induction motor
• Why synchronous motors are used in industrial applications
• Issues with WRM and soft starters
• A case study of a 12,000 hp 4-pole synchronous motor and a soft starter
• What needs to be done to modernize the older synchronous motor starter?
• How a brushless synchronous motor operates
• Brushless synchronous motor protection

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

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

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

Chuck Yung 
EASA Senior Technical Support Specialist

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