A microcomputer-based data acquisition system for diagnostic monitoring and control of high-speed electric motors

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Virginia Tech

A microcomputer-based data acquisition and control system was designed for the diagnostic monitoring and control of high-speed electric motors. The system was utilized in high-speed bearing life-testing, using an electric motor as a test vehicle.

Bearing vibration and outer race temperature were continuously monitored for each ball bearing in the motor. In addition, the stator winding and motor casing temperature were monitored.

The monitoring system was successful in detecting an unbalance in the rotor caused by the loss of a small piece of balancing putty. The motor was shut down before any further damage occurred. In a separate test, excessive clearance between a bearing outer race and the motor caused high vibration readings. The motor was monitored until the condition began to deteriorate and the bearing outer race began to spin significantly. Again, the monitoring system powered down the motor before any significant damage occurred.

The speed of the motor tested is controlled by a PWM (pulse width modulation) technique. The resulting voltage and current waveforms are asymmetrical and contain high frequency components. Special circuitry was designed and constructed to interface sensors for measuring the voltage and current inputs to a spectrum analyzer. Using frequency and order analysis techniques, the real and reactive power inputs to the three·phase motor were measured.