Squirrel Cage Rotor Testing Methods
Squirrel Cage Rotor Testing Methods (photo credit:

Introduction to testing //

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. There are a wide variety of rotor tests that can be applied both in the service center and at the end user site that can aid in assessing rotor condition.

Further, there are tests that can be performed with the motor assembled, and others that require disassembly.

The main purpose of the information that will be presented here is to describe many of the available tests that can be utilized under these different circumstances. In addition to conventional squirrel cage rotor testing methods such as the growler test, also covered will be techniques such as the use of a core loss tester, high current excitation, and spectrum analysis of vibration.

Almost all squirrel cage rotors have bars and end rings made of alloys of either aluminum or copper, or pure copper.

The rotor cage consists of the bars and the end rings. Copper or copper alloy rotors are usually of fabricated design. That is, the bars and end rings are fabricated prior to assembly into the rotor, and then brazed or welded together. Far less common are copper rotors with cast bars that were manufactured over 50 years ago, although there is new technology that may make these commercially available in the near future.

Typical die cast squirrel cage induction motor
Figure 1 – Typical die cast squirrel cage induction motor

Aluminum rotors are predominantly of diecast construction (Figure 1 above), with the bars and end rings being cast in one machine operation. Larger motors, typically above NEMA frame size, may use fabricated aluminum rotors that have the bars (usually made by extruding) welded to the end rings.

In general, the following discussion applies to both fabricated and die cast rotor construction, unless indicated otherwise.

Rotor principles

Testing of a squirrel cage rotor requires some understanding of how the rotor functions. The rotor of an induction motor is like the secondary winding of a transformer, with the motor stator being the primary. This is easiest to visualize at motor startup, when the rotor is not yet turning.

Currents and voltages are induced in the bars and end rings, which make up the cage, of the rotor (Figure 2). The rotor cage is similar in appearance to pet rodent exercise wheels from over a century ago, thus the name “squirrel cage”.

Squirrel cage rotor | It consist of consists of the bars and end rings.
Figure 2 – Squirrel cage rotor | It consist of consists of the bars and end rings.

There are other types of rotors used in AC motors such as synchronous and wound rotor, however, the focus here will be on the squirrel cage induction rotor.

The bars in a squirrel cage rotor form parallel paths, joined at their ends by end rings. The stator winding poles divide the rotor bars into parallel circuits equal to the number of stator poles. The number of rotor poles is always equal to the number of stator poles. A 2-pole winding divides the rotor into 2 parallel circuits that continuously move around the rotor cage as the rotor revolves.

The greater the number of poles, the greater the number of rotor circuits.

The end rings complete these circuits, thus a 2-pole winding end ring will be subject to more current than with a higher number of poles in the winding. This factor makes end ring integrity more critical as the number of poles decrease (and speed increases).

Title:Squirrel Cage Rotor Testing – Tom Bishop, Technical Support Specialist – Electrical Apparatus Service Association, Inc.
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Squirrel Cage Rotor Testing Methods
Squirrel Cage Rotor Testing Methods

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  1. Method Statement HQ
    Aug 24, 2016

    The book is very useful and details everything required for the testing.. Thumbs Up.

  2. Chandramouli Chakraborty
    Jan 30, 2015

    Please give more details against each subject or you can add more useful links against each articles like they do in Wikipedia

  3. Chandramouli Chakraborty
    Jan 30, 2015

    I do like the articles published in your webpage ,they are quiet useful and interesting.

    • Edvard
      Jan 30, 2015

      Thank you Chandramouli!

      • Chandramouli Chakraborty
        Jan 30, 2015

        I am privileged to be a subscriber of your article.Thank you too.

  4. Geets
    Feb 03, 2014

    Is the link above working for anybody else? I keep receiving an Explorer error message.

    • Edvard
      Feb 03, 2014

      It works just fine. It’s IE issue. Try with Chrome or Firefox.

  5. youssef
    Feb 02, 2014

    In the context of technological research to solve the problems of rotating electrical machines after several years of experience and training that about is has positive results obtained by a prototype electric motor already realize, I’m looking for a sponsor to have tests certified by a testing laboratory and study. this project has a relationship with the project of making the electric car engine

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