What Causes Shaft Voltage in VFD Motors?

Variable frequency drives improve efficiency, but their switched output can capacitively couple voltage onto the motor shaft. The resulting shaft voltage may discharge through the lowest-impedance path—often across motor bearings.

When discharge events occur repeatedly, they can create pitting and fluting patterns on bearing races. Over time this increases noise, vibration, and the risk of unexpected failure.

Mitigation strategies include breaking the current path (insulated bearings), providing a controlled discharge path (shaft grounding devices), or a combination depending on system design.

For many industrial users, a shaft grounding ring offers a practical path to divert current away from bearings while keeping maintenance straightforward.