VFD Motor Bearing Currents and Shaft Grounding

What causes bearing currents in a VFD-driven motor? The inverter's PWM output does not sum to zero at every switching instant, producing a common-mode voltage that capacitively couples across the air gap and appears as a voltage on the rotor shaft; per IEC TS 60034-25 guidance on converter-fed motors, that shaft voltage discharges through the thin lubricant film in the bearing once it exceeds the film's breakdown point, and the resulting micro-arcs pit the raceway through electrical discharge machining (EDM). Left unmanaged, repeated pitting turns into fluting, a washboard pattern on the ball or roller track, and the bearing fails years before its rated L10 life. This article covers the common-mode voltage mechanism, EDM pitting and fluting, which motors carry the most exposure, and the three practical countermeasures: shaft grounding rings, insulated bearings, and common-mode or dV/dt output filtering.

Where Does the Shaft Voltage Actually Come From?

On a sine-wave utility supply, the three phase-to-ground voltages sum to zero at every instant, so no common-mode voltage exists. A PWM inverter switches each leg independently between the DC bus rails, and at most switching instants the three instantaneous phase voltages do not cancel. The result is a stepped common-mode voltage waveform, hundreds of volts in magnitude, switching at the carrier frequency with dV/dt in the hundreds of volts per microsecond.

This common-mode voltage reaches the motor terminals and couples to the rotor through the parasitic capacitance every induction motor has between winding, frame, and rotor. The ratio of shaft voltage to common-mode voltage, the bearing voltage ratio, is typically a few percent. Small ratio, large source voltage: the shaft voltage that results can still exceed the few volts it takes to break down a well-lubricated bearing film.

How EDM Pitting and Fluting Actually Damage a Bearing

The oil or grease film between ball and race is thin enough to act as a capacitor and a dielectric at the same time. While shaft voltage stays below the film's breakdown point, no current flows. Once it exceeds that point, the film punctures and a nanosecond current pulse arcs across the gap, vaporizing a microscopic volume of steel on the race. That is the identical physics behind the electrical discharge machining process used to cut hardened tool steel, just at a scale nobody wants inside a bearing.

From Isolated Pits to Fluting

A single discharge event leaves a crater a few micrometers across. On its own, harmless. A VFD switches tens of thousands of times per second, so the motor accumulates millions of discharge events per day of operation. As the rotor turns, balls repeatedly track over the same damaged zones, and craters densify into a periodic, washboard-like pattern spaced to the rolling element pitch. That pattern is fluting. Vibration and audible noise rise first, at a frequency tied to ball-pass rate, then cage wear accelerates and the bearing seizes or spalls.

Which Motors Are Actually at Risk?

Not every VFD-fed motor needs intervention. Risk scales with three factors working together, not any single one.

Frame Size and Power

Larger frames carry more parasitic capacitance between stator and rotor, so common-mode current has an easier path. Many motor manufacturers recommend shaft grounding as standard practice once frame size crosses roughly IEC frame height 280 mm or thereabouts, though the exact cutoff varies by design and nobody should treat it as a hard line — a smaller frame on a long unshielded feeder can still see damaging shaft voltage.

Carrier Frequency

A drive running an 8-16 kHz carrier for quieter operation switches far more often per second than one at 2-4 kHz. Every switching transition is a common-mode voltage step, so the quieter setting that reduces motor noise also raises the rate of EDM discharge events. See carrier frequency and switching losses for the full trade-off against IGBT heating.

Cable Length — A Different Mechanism, Often Confused

This depends on what's actually being asked. Reflected-wave voltage doubling at the motor terminals, covered in VFD output filters for dV/dt and cable length, stresses phase-to-phase and phase-to-ground winding insulation and gets worse with longer cable runs. Bearing current is a common-mode phenomenon that exists even on a motor mounted directly to the drive with almost no cable at all. Cable length changes reflected-wave severity; it does not meaningfully change bearing current risk.

Shaft Grounding Rings: The Standard Fix

A shaft grounding ring is a conductive microfiber brush that encircles the shaft and contacts its full circumference at low pressure, providing a milliohm-level path from shaft to motor frame. Induced shaft voltage discharges through that low-impedance path instead of arcing across the bearing film, because current takes the path of least resistance.

Installation location matters. The ring typically goes at the drive end, near the coupling, paired with an insulated bearing at the non-drive end. That combination forces current onto a single controlled path rather than splitting across both bearings — or worse, finding a return path through a connected gearbox or driven equipment on the far end of the shaft, which moves the damage instead of stopping it.

Other Mitigation Options and Their Trade-offs

Three alternatives cover most applications, each intercepting the problem at a different point.

What we see in the field: conductive grease gets tried as a stop-gap fix because it is cheap and needs no parts change, but it degrades within months and never holds the low impedance a ring or an insulated bearing maintains for years. It buys time, not a fix.

A common-mode or dV/dt output filter attacks the root cause rather than draining the discharge current, which helps when the same installation also needs cable-length protection. It adds cost, size, and weight at the panel, so most sites reserve it for large frames or long cable runs rather than fitting it as a default.

Selecting and Installing a Shaft Grounding Ring

Sizing to the Shaft

Rings are sold by shaft diameter and must seat with full 360-degree brush contact; a gap or an out-of-round fit leaves part of the circumference undrained and defeats the purpose.

Verifying It Works

An oscilloscope reading of shaft-to-frame voltage, taken with a shaft-riding probe before and after installation, is the standard check. A properly grounded shaft typically reads under a few volts peak-to-peak; anything still spiking to tens of volts points to a poor brush contact, a missing insulated bearing at the other end, or a ground path that bypasses the ring entirely through a coupled load.

Maintenance

The microfiber brush wears like a carbon motor brush. It should go on the same inspection interval as the bearing itself, not be forgotten once installed — a worn-out ring provides no protection while looking untouched from the outside.

Brand practice differs mainly in how the option is packaged rather than in the physics: ABB ACS880 and Siemens SINAMICS-fed motors at larger frame sizes commonly ship with shaft grounding as a factory option, and Schneider Altivar-fed installations follow the same IEC TS 60034-25 guidance through the motor OEM. See the variable frequency drives collection for drives across ABB, Schneider Electric, and Siemens ranges. For the broader PWM picture this sits inside, see how a VFD works with PWM and V/f control and the VFD engineering guide.

Conclusion

Bearing currents come from the same PWM waveform that makes VFD speed control possible; the common-mode voltage it produces has to go somewhere, and an unprotected motor lets it go through the bearing film. A shaft grounding ring at the drive end, an insulated bearing at the non-drive end, and carrier frequency chosen with switching losses in mind cover the large majority of installations. Specify shaft grounding by frame size and duty, not by habit, and verify it with a scope reading rather than assuming the nameplate option is doing its job.