Bearing Protection of Inverter-duty Motors


Although it is now common knowledge that inverters—also known as variable frequency drives—often induce unwanted motor shaft voltages, many customers who purchase three-phase alternating current (AC) induction motors do not realize that models labeled “inverter-duty” or “inverter-ready” might not prevent inverter-caused bearing damage. While many such motors have inverter-rated insulation to protect their windings, their bearings are too often ignored. If they are to be truly ready for use with inverters, these motors also need bearing protection.

Without effective mitigation such as a good shaft grounding ring, inverter- induced shaft voltages can destroy bearings, causing premature motor failure. The National Electrical Manufacturers Association (NEMA) and the Canadian Standards Association (CSA) have yet to recommend that new motors have bearing protection against damaging electrical discharges.

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