All bearings are subject to wear and damage, and the bearings inside your AC induction motor are no exception.

A potential cause of bearing damage in an AC motor is when VFDs drive motors (Variable Frequency Drives).

Fortunately, appropriate motor shaft grounding can help prevent the problem by aiding in bearing protection.

Inverters or VFDs

All major 3-phase AC induction motors manufacturers offer "inverter-duty" or "inverter-ready" models. Still, while these motors have inverter-rated insulation to protect the windings, we tend to ignore vulnerable bearings.

Other terms for inverters are VFDs and adjustable speed drives. They can induce unwanted motor shaft voltages that can destroy bearings, causing premature motor failure.

VFDs control a motor's speed or torque. However, they can induce voltages and currents that damage bearings.

The costly repair or replacement of failed motor bearings can wipe out any energy savings yielded by your VFD, severely diminishing the entire system's reliability.

Minimize Bearing Damage

In nearly every case, the most reliable and cost-effective way to minimize electrical bearing damage and increase long-term bearing protection is to install a motor shaft grounding ring with insulation for motors greater than 100 horsepower.

New grounding rings encircle a motor's shaft with contact points, unlike older, single-point contact brushes.

These rings safely bleed damaging currents to the ground by providing a shallow impedance path from the shaft to the motor frame, bypassing a motor's bearings entirely.

Many motor manufacturers will add shaft grounding rings to their motors upon request. But, motors already in service require you to retrofit them.

Voltage Causes Degradation

Repetitive and rapid pulses applied to an AC motor from a modern VFD's non-sinusoidal power-switching circuitry causes bearing damage. This phenomenon is known as harmonic content, parasitic capacitance, capacitive coupling, electrostatic buildup, and standard mode voltage.

High peak voltages and fast voltage rise times can cause cumulative degradation to insulation, bearings, coil varnish, etc. Suppose the load impedance is higher than the line impedance. In that case, the current is reflected 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 (Find an additional resource, "11 Ways to Help Maximize Bearing Life," from Motion).

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

Next Steps

Following best practices will increase bearing life. For assistance selecting a shaft grounding ring for an existing motor or to choose a motor for a new application, don't hesitate to get in touch with your Motion Automation Intelligence specialist.