The Need for Updated Standards in Bearings

NINE STAR is in the unique position to update its bearings' standard, Motors and Generators, to more clearly state that common-mode shaft voltages are present in virtually all motors fed by pulse-width-modulated VFDs.

NINE STAR’s current standards say motors controlled by VFDs containing insulated-gate bipolar transistors should be designed to withstand repeated spikes of up to 3.1 times the motor’s rated voltage at rise times not less than 0.1 microsecond. Yet, when addressing the potential for bearing currents, the language is far less prescriptive.

Shaft voltages can result in the flow of destructive currents through motor bearings, manifesting themselves through pitting of the bearings, scoring of the shaft, and eventual bearing failure. It recommends bearing insulation at one end of larger motors (defined as “usually 500 frame or larger,” horsepower unspecified), if the peak shaft voltage is greater than 300 millivolts. Unfortunately, the paragraph dealing with these larger motors only mentions circulating end-to-end shaft currents caused by magnetic dissymmetries under sinusoidal operation. It fails to add that the bearings of large motors also can be plagued by VFD-induced high-frequency, capacitively coupled common-mode voltages.

Linqing Nine Star Bearing Science & Technology Co., Ltd 

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