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Effects of VFD Motor Shaft Voltage and How to Prevent It

February 14, 2020

 

Shaft voltage is a phenomenon that commonly occurs in electric motors powered by variable frequency drives. The rapidly-switching voltage pulses of VFDs induce high voltages in shafts and bearings that may overcome insulating properties of bearing grease, and the resulting sparks can cause pitting, fluting, frosting, grooving, and cratering that lead to premature failure of the bearings and motor. Bearing damage due to shaft voltage can develop in as little as one week, and failure can occur within a few months. The damage can also extend to the bearings of other connected equipment such as tachometers and gear boxes. Early symptoms of motor shaft voltage are noise, vibration, darkened grease, and overheating. Bearings lose their original shape, sloughing off metal particles that mix with the grease and increase friction.

 

Shaft voltage only occurs when the motor is energized and rotating, and VFD-induced shaft voltage can be exceedingly brief. Measuring shaft voltage requires a specialized high-bandwidth instrument equipped with a carbon brush probe attachment to contact the rotating shaft. The best way to prevent shaft voltage and extend bearing life is to install a shaft grounding ring that channels current away from the bearings to ground. These devices are easy to install, and the cost is relatively small in comparison to the steep price you’ll pay in the event of bearing or motor failure.

 

About PJM

 

PJM Mechanical Contractors, Inc. is a multi-faceted mechanical contractor providing construction and installation, service and maintenance, and design/build services in a comprehensive range of mechanical disciplines that include HVAC, plumbing, controls, refrigeration, custom metal fabrication, piping, and backflow preventer testing. PJM primarily serves clients in pharmaceuticals, manufacturing, healthcare, institutional, and other various commercial and industrial sectors.

 

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