Fan Motor VFD Advice on Avoiding Bearing, Insulation Failures
Download the guide released by Colmac Coil, Colville, Wash., in PDF format on its website.

A technical bulletin, “VFD Application Guidelines,” describes the proper application of variable-frequency drive (VFD) speed control to fan motors. Released by Colmac Coil, Colville, Wash., the guide explains how to avoid potential bearing current failure and insulation failure when designing and installing VFD systems.

Improperly configured VFD electrical systems can contribute to excessively high shaft voltages, which can result in electric discharge machining (EDM), explains the team at Colmac. This occurs when voltage levels on the rotor/shaft exceed the dielectric rating of the bearing lubrication, and an arc is drawn across the bearings to ground. Every time this happens, a pit is created in the bearing race. As the EDM continues to deteriorate the bearing surfaces, the motor will experience vibration and noise levels above normal, overheating, hard starts, overloads and eventual bearing failure.

VFDs work by converting an AC voltage to DC voltage and then pulsing the DC voltage to simulate an AC sine wave at the required frequency to control motor speed. These DC voltage pulses travel down the conductors to the motor and can be reflected back to the drive. The reflected wave can increase in magnitude to the point where a partial discharge can occur (corona). This corona effect falls short of an actual insulation breakdown but can act to produce ozone, which leads to carbon tracing and insulation degradation. Left uncorrected, the corona effect will eventually result in insulation failure and equipment damage.

For more information about the application guidelines, e-mail Jeremy Olberding at