Bearing Protection for AC Motors

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

A potential cause  for bearing damage in an AC motor is when motors are driven by VFDs (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, the bearings — their most vulnerable parts — are too often ignored.

Inverters, also known as VFDs and adjustable speed drives, can induce unwanted motor shaft voltages that, without adequate 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.

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 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 shallow 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.

Voltage Causes Degradation

Winding and bearing damage is caused by repetitive and rapid pulses applied to an AC 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.

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, 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 (either internally or externally) provides adequate protection against bearing currents for motor bearings and attached equipment.

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