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.
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.
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.
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”.
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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