Put a VFD on a standard induction motor, and the bearings fail in six months. The motor nameplate says 40,000 hours. The grease looks fine. The balls and raceway are scored like someone dragged a wire through them. That’s electrical erosion, not mechanical wear.

How a VFD puts voltage on the shaft

A PWM drive switches the motor winding at several kilohertz, with voltage edges that rise in nanoseconds. The high dV/dt couples capacitively between the stator windings and the rotor. The rotor picks up a common-mode voltage that has nowhere to go, so it builds up on the shaft. When the voltage exceeds the grease film’s breakdown strength, it discharges through the bearing in a tiny spark.

That discharge is an electrical discharge machining (EDM) event. It pits the raceway. Over thousands of cycles, the pits merge into a fluted, washboard pattern you can feel with a fingernail on the bearing race. The motor gets noisy, then hot, then seizes.

Why you don’t see it until it’s late

Early EDM damage is invisible. The bearing runs normally, the grease looks clean, and the temperature is fine. The fluting develops over months. By the time the motor starts sounding like a bag of marbles, the bearing is already finished. Replacing the bearing without fixing the shaft voltage just resets the clock for another six months.

What actually fixes it

Shaft grounding. A grounding brush or a grounding ring rides on the shaft and gives the discharge current a low-resistance path to the motor frame. This is the cheapest and most effective fix. Install it on the drive-end or non-drive-end shaft, per the motor manufacturer’s guidance. It’s a wear part; inspect it annually.

Insulated bearings. Ceramic-coated or hybrid ceramic bearings block the current path through the bearing itself. They’re common on larger VFD motors, but they don’t solve the problem by themselves if the current finds another path through the driven equipment. Insulated the drive-end bearing and the current can still burn through the non-drive-end bearing or the pump seal.

Conductive coupling. Use a shielded motor cable with the shield bonded 360 degrees at both ends. This reduces the common-mode voltage that drives the current in the first place. It’s not a complete fix on its own, but it reduces the stress on the grounding device.

dv/dt filters or sine-wave filters. On long cable runs (more than 30 meters), a dv/dt filter at the drive output slows the voltage edges. A sine-wave filter reconstructs a near-sinusoidal voltage. These are the expensive option, but they also protect the motor winding insulation, which is the other thing fast edges damage.

What doesn’t work

Bearing grease with “EP” additives doesn’t stop EDM. The discharge punches through the grease film regardless. Adding more grease just delays the next failure. A better bearing, a tighter clearance, or a higher NEMA rating doesn’t help either; the current doesn’t care about the bearing load rating.

Grounding the motor frame better is also not the answer. The frame is already grounded. The problem is that the shaft isn’t grounded, and the bearing is the only path. You have to give the current a path that bypasses the rolling elements.

Which motors are at risk

Any motor on a VFD is at risk, but the damage shows up fastest on 10 to 100 kW motors running at high carrier frequencies. Small motors have low capacitance and less induced voltage. Very large motors often come with insulated bearings from the factory. The danger zone is the mid-range: fan and pump motors, conveyor drives, spindle drives, on standard motors that were never designed for inverter duty.

Diagnose it early

If a VFD-driven motor is failing bearings repeatedly, measure the shaft voltage with an oscilloscope and a differential probe. You should see repetitive voltage spikes of a few volts to tens of volts at the PWM frequency. If the voltage exceeds 30 V peak, bearing damage is likely. Measure before you replace the second bearing; that’s when you know the root cause is electrical, not misalignment or bad lubrication.

Bottom line

If you’re putting a VFD on a standard motor, budget for a shaft grounding ring from day one. It costs a fraction of a bearing replacement and a fraction of a motor replacement. The motor manufacturer’s “inverter duty” rating is a start, but it doesn’t replace the grounding path on every installation.