Rotor Bar Monitoring via Motor Current Analysis
The condition of the rotor bars in an induction motor can be measured by performing a high-resolution frequency analysis on the input current to the motor.
The presence of a defective rotor bar will cause the motor torque to be reduced slightly every time a pole of the rotating magnetic field passes by it. This happens at twice the slip frequency, for both the north and south poles of the field cause a momentary reduction. This reduction in torque also results in a reduction in the input current to the motor at the same rate -- this is a result of conservation of energy. This periodic reduction in the motor current is actually an amplitude modulation of the motor current. The amount of modulation is related to the severity of the rotor bar problem.
A good way to detect motor current modulation is to perform a frequency analysis on the current, and look at the sidebands around 60 Hz spaced at twice the slip frequency. This can be done with a current clamp placed around one phase of the input line and connecting it to a spectrum analyzer.
For this test, the motor must be operating under load, for with no load, the slip will be very slow, and no appreciable torque is being developed.
Only one phase of a three-phase motor need be measured.
The spectrum analyzer must be capable of generating a high-resolution spectrum from 0 Hz to about 70 Hz, or a zoom spectrum from 50 Hz to 70 Hz. A frequency resolution of 1600 lines is desirable in order to separate the 2X slip sidebands from other sidebands caused by load variations, etc.
The high resolution and the zoom spectrum are desirable because the slip frequency sidebands will be very close in frequency to the 60 Hz line frequency. For instance, for a motor turning 1760 RPM, the slip frequency will be 1800 - 1760 = 40 RPM, which is equivalent to 0.667 Hz. The sidebands will be spaced at twice this frequency, or 1.334 Hz.
If the sidebands are 55 to 60 dB (1,000:1) down from the 60 Hz peak, the rotor bars are considered good, but if they rise to 40 dB (100:1) below the 60 Hz peak, damaged rotor bars are indicated. It is possible to calibrate a system like this to relate the actual number of open bars to the sideband level if the number of bars in the rotor is known.
The spectrum above is from a 1760-RPM motor with rotor bar problems.
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