Bearing Current Protection for AC Motors with VFDs

Motor Bearing Protection

All bearings are subject to wear and damage, and the bearings inside of your AC induction motor are no exception. But are you aware of the fact that there is a potential electrical cause for bearing damage in a motor? There is – when motors are driven by VFDs (Variable Frequency Drives). Fortunately, appropriate motor shaft grounding can help prevent this problem.

All major manufacturers of 3-phase ac induction motors offer “inverter-duty” or “inverter-ready” models, but while these motors have inverter-rated insulation to protect the windings, the bearings — their most vulnerable parts — are too often ignored. The National Electrical Manufacturers Association (NEMA) has yet to recommend that new motors have bearing protection against damaging electrical discharges.   It is common knowledge that inverters, also known as VFDs and adjustable speed drives, can induce unwanted motor shaft voltages that, without effective mitigation, can destroy bearings, causing premature motor failure.

 Whether they are used to control a motor’s speed or torque, VFDs can induce voltages and currents that can damage bearings. In fact, the costly repair or replacement of failed motor bearings can wipe out any energy savings yielded by your VFD, and severely diminish the reliability of an entire system. In nearly every case, the most reliable and cost-effective way to minimize electrical bearing damage (and make these systems sustainable) is to install a motor shaft grounding ring, combined with insulation for motors greater than 100 horsepower.

Unlike older, single-point contact brushes, new grounding rings encircle a motor’s shaft with contact points. These rings safely bleed damaging currents to ground by providing a very low impedance path from shaft to motor frame, bypassing a motor’s bearings entirely. While many motor manufacturers will add shaft grounding rings to their motors upon request before shipping them, motors that are already in service will require you to retrofit these rings.

Winding and bearing damage is caused by repetitive and rapid pulses  that are applied to a motor from a modern VFD’s non-sinusoidal power-switching circuitry. This phenomenon is known as harmonic content, parasitic capacitance, capacitive coupling, electrostatic buildup and common mode voltage. Regardless of what you call it, high peak voltages and fast voltage rise times can cause cumulative degradation to insulation, bearings, coil varnish, etc. If the load impedance is higher than the line impedance, current is reflected back toward the VFD, creating voltage spikes at the motor terminal that can be twice as high as the DC bus voltage. What’s more, VFD-induced current damage is often overlooked until it is too late to save your motor.

For motors up to 100 horsepower, where common mode voltages could cause bearing damage, adding a shaft grounding ring to the motor (either internally or externally) provides effective protection against bearing currents for motor bearings and attached equipment.

For assistance selecting a shaft grounding ring for an existing motor, or to select a motor for a new application, please contact your local Kaman representative by commenting below, or using the form on this page.

Estimated Reading Time: 2 minutes

Pete Stollberger

Join the Conversation