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Jul 26, 2018 7:04:00 AM

Detecting Cracked Rotor Bars

Do you suspect that you have a cracked rotor bar in your electric motor?  Broken rotor bars rarely cause immediate failures, especially in large multi-pole (slow-speed) motors. Broken rotor bars precipitate deterioration in other components as well and can result in time-consuming repairs and loss of motor efficiency. 

Here are some methods to help detect cracked or broken rotor bars:

GROWLER TEST - A growler is an electrical device used for testing insulation of a motor for shorted coils. A growler consists of a coil of wire wrapped around an iron core and connected to a source of alternating current. When placed on the armature or stator core of a motor the growler acts as the primary of a transformer and the armature coils act as the secondary. A "feeler", a thin strip of steel such as a hacksaw blade or metal filings can be used as the short detector. For best results the rotor should be heated.

SINGLE PHASE ROTOR TEST - Single phase power is applied to the motor while the rotor is slowly rotated.  Using an analog meter, any fluctuation in amp draw is monitored. Lower or increased stator current is a sign that one or more cracked rotor bars are present. This test is best performed with the rotor heated and while the motor is still intact.

HIGH CURRENT ROTOR TEST - This test is performed by applying high current through the shaft of the rotor thermally scanning the rotor searching for shorted laminations. (Laminations are the steel portions of the stator and rotor consisting of thin lamination sheets stacked together.)

Hot spots can be caused by shorted laminations. This can result in uneven heating of the rotor which then causes the rotor to bow and go out of balance. Stress to the bars and premature bar cracking and fatigue are most often the result. A loose rotor to shaft interference fit can also be detected with this test.

INDUCTION MOTOR CURRENT ANALYSIS (CURRENT SPECTRUM TEST) - This test is considered the most accurate and reliable. It is performed while the motor is under 50 - 100% load. The rotor induces currents back in the stator windings.

These appear as side bands around the supply line frequency (60 HZ) peak, and are a function of the motors number of poles and slip frequency (the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm, or in percentage or ratio of synchronous speed.) An estimate of the number of broken rotor bars can be determined by comparing the sideband amplitudes.


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