Motion System Design

Need for update to NEMA standards

The National Electrical Manufacturers Association (NEMA) has yet to recommend that new motors sport protection against electrical discharges. Current NEMA standards highlight the possible need for extra bearing protection for VFD-driven motors, but the language isn’t specific enough to guide motor manufacturers and doesn’t yet reflect the effect of new innovation in shaft grounding. Stronger standards calling for effective mitigation would go a long way toward cautioning motor users of the need for such mitigation.

In its key role as an industry leader, NEMA is in the unique position to update its MG1 standard to more clearly state that common mode shaft voltages are present in virtually all motors fed by pulse-width-modulated (PWM) VFDs. No other entity is in such a position of authority, so NEMA may at the same time address the overall problem of electrical bearing damage more directly.

The association’s current standards acknowledge the potential damage from VFD-induced voltage spikes: They state that motors controlled by modern VFDs containing insulated gate bipolar transistors (IGBTs) should be designed to withstand repeated spikes (at the terminals) of up to 3.1 times the motor’s rated voltage, at rise times not less than 0.1 microsecond. When addressing the potential for bearing currents, the language is less prescriptive.

NEMA Standard MG1-2009 (Revision 1-2010), Section IV, Part 31, Definite-Purpose Inverter-Fed Polyphase Motors, correctly states: “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.”
Subsection of Part 31 recommends bearing insulation at one end of a larger motor (defined as “usually 500 frame or larger,” horsepower unspecified) if the peak shaft voltage is greater than 300 mV.

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 can also be plagued by VFD-induced, high-frequency capacitively coupled common mode voltages.

In future standards, it could be noted that for motors above 100 hp, in which both circulating currents and common mode voltages could cause bearing damage, combining an insulated bearing on one end with a shaft grounding ring on the opposite end provides optimal protection from electrical bearing damage.

Please refer to the August 2012 article from Motion System Design from Adam Willwerth, in which this topic is discussed in full.
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