Motor Bearing Currents Under PWM Variable Speed Drive Operation

The causes and effects of  VFD-supplied motor shaft and bearing currents, as well as measures to counteract them, have been exhaustively covered in literature. However, as a drive and motor supplier, we frequently receive requests for information on this topic.

Basically, shaft currents are induced because of the high frequency of the voltage pulses sent to the motor from the VFD. Recall that a pulse-width modulated VFD creates a synthesized sine wave by firing its output transistors many thousands of times per second. These pulses form high-frequency waves sent along the motor cable to the motor. Since impedance is inversely proportional to frequency, the capacitances of the cable and motor present little or no impedance to these high-frequency pulses. As a result, circulating currents can readily flow in the motor shaft. With sufficient magnitude,  and in the absence of corrective measures, these shaft currents can pass through the bearings and races to the motor frame, causing bearing pitting or “fluting” – regular, tightly spaced grooves on the bearing races – via a process referred to as Electric Discharge Machining (EDM). Ultimately, these irregularities will cause bearing failure.

Linqing Nine Star Bearing Science & Technology Co., Ltd            

(Insulated bearing, Hybrid ceramic bearing and  Brass cage DGBB)

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