VFD Bearing Currents: EDM Fluting Causes and Shaft Grounding

VFD-induced bearing currents are stray electrical discharges that pass through a motor's bearings because a variable frequency drive's fast switching leaves a voltage on the shaft with nowhere clean to go. Left alone, that discharge slowly spark-erodes the bearing races until a motor that should run for years fails in months, and the fix is boring, well understood, and cheap compared to the failure.

Why a VFD Puts Voltage on the Shaft

A variable frequency drive does not output a clean sine wave. It builds one from thousands of rapid DC pulses (pulse width modulation, or PWM) switched by IGBT transistors, typically at a carrier frequency somewhere in the 2-16 kHz range. Each pulse edge rises from zero to the full DC bus voltage in a fraction of a microsecond, compared to the several milliseconds it takes utility-grid sine power to swing from zero to peak. That fast edge is the whole problem: it is not the motor's operating frequency that damages bearings, it is the switching speed of the drive's output stage.

How That Turns Into Common-Mode Voltage

In a healthy three-phase system the instantaneous sum of the three phase voltages is zero. PWM switching does not hold that sum at zero at every instant, so a residual "common-mode" voltage appears between the stator windings and the grounded motor frame. That common-mode voltage couples across the small air gap between stator and rotor through parasitic capacitance, the same way a capacitor couples an AC signal across an insulating gap. The result is a voltage that appears on the rotor and, because the rotor and shaft are the same electrically continuous piece of steel, on the shaft itself.

Why the Bearing Becomes the Discharge Path

A rolling-element bearing is not a solid electrical connection between shaft and frame. A microscopically thin film of lubricant separates the balls or rollers from the races, and that film behaves like a small capacitor and a dielectric barrier at the same time. While shaft voltage stays low, the film insulates. Once the voltage across the film exceeds its dielectric breakdown strength, the film punctures and a brief, intense spark jumps through it, exactly like a tiny electrical discharge machining (EDM) event. That is why the industry calls this failure mode EDM bearing current damage: the physics is the same process used deliberately in EDM machining, just unwanted and uncontrolled.

What the Damage Looks Like

Each discharge removes a microscopic amount of metal from the race and ball surface. On its own, one spark is nothing. Repeated thousands of times per second of run time, the damage accumulates in a recognizable progression:

• Frosting: a dull, matte, sandblasted-looking finish on the race, visible mainly when the bearing is disassembled and inspected. This is early-stage pitting and is often mistaken for normal wear.
• Fluting: the pitting organizes into regularly spaced grooves running across the race, resembling a washboard. Fluting shows up as an unmistakable, often audible, vibration signature at frequencies tied to the bearing's rolling-element pass frequencies.
• Advanced failure: increased noise, heat, and vibration accelerate mechanical wear, and the bearing fails well before its rated L10 bearing life. A vibration route that already checks ISO 10816-3 severity bands will often flag this well before teardown.

Because fluting has a directional, patterned signature rather than the random pitting typical of contamination or lubrication failure, it is one of the more distinguishable entries among common bearing failure modes and symptoms.

Which Motors Are Most at Risk

NEMA MG1 Part 31 recommends bearing insulation be considered once peak shaft voltage exceeds 300 millivolts, and it specifically calls this out for motors in frame sizes usually 500 and larger. IEC TS 60034-25 gives similar guidance from the international side, recommending an insulated bearing at the non-drive end for larger inverter-fed machines to break the circulating current path. Risk climbs with motor frame size, with longer motor cable runs (which increase the reflected-wave voltage the motor terminals see), and with higher PWM carrier frequency settings, since more switching events per second means more discharge opportunities per second.

Shaft Grounding Rings

A shaft grounding ring mounts around the shaft and provides a low-impedance path from shaft to motor frame that bypasses the bearing entirely, using conductive microfibers or a similar continuous contact element rather than relying on the bearing's lubricant film to carry current. Properly installed and maintained, it gives the common-mode current an easy way home before it can build up enough voltage to punch through the oil film. It is one of the lowest-cost defenses and is typically added at the motor's non-drive end, which is also where motor insulation class and bearing selection get evaluated together during a VFD retrofit.

Insulated Bearings

An insulated bearing uses a ceramic coating or ceramic (hybrid) rolling elements to electrically break the circuit at one bearing, usually the non-drive end, so current cannot complete a path through that bearing. This does not eliminate the common-mode voltage; it just denies it a path through the protected bearing. Because current will still find the path of least resistance, insulating one bearing without addressing the shaft voltage at the source can push more current through the other, unprotected bearing, which is why insulated bearings are usually paired with a shaft grounding ring rather than used alone.

dv/dt Filters and Output Reactors

A dv/dt filter sits between the drive and the motor and slows the rate of voltage rise on each switching edge, reducing the fast dv/dt that drives capacitive coupling into common-mode voltage in the first place. Output reactors offer a lighter-weight, lower-cost version of the same idea for shorter cable runs. Because these devices address the voltage at its source rather than diverting it after the fact, they are the most complete fix, particularly on new installations with long cable runs where reflected-wave effects compound the problem, and they pair well with checks on overall power quality across the feeder.

Getting Ahead of the Failure

None of these defenses require guesswork once a plant knows which motors are at risk: any VFD-fed motor at NEMA frame 500 or larger, any motor on a long cable run, and any motor already showing that washboard vibration signature. Shaft voltage can be measured directly with an oscilloscope and a grounding brush, and periodic vibration and thermal checks (see thermography vs. vibration analysis) catch fluting before it becomes a failure. Fabrico reads machine condition and OEE straight from the line and auto-routes a work order the moment a loss like early-stage fluting shows up, catching what a fixed sensor threshold alone would miss, and it runs EU-built with EU data residency under ISO 27001, ISO 20000-1, and ISO 9001. Book a Fabrico demo to see it on your own line.

Frequently Asked Questions

Do all VFD-driven motors get bearing currents?

Every PWM-driven motor develops some common-mode voltage on the shaft. Whether it becomes damaging depends on shaft voltage magnitude, motor frame size, cable length, grounding and bonding quality, and carrier frequency. Small motors on short, well-bonded cable runs at low carrier frequencies often run for years without issue.

Can a standard motor be retrofitted with a shaft grounding ring?

Yes. Shaft grounding rings are designed as an add-on that mounts around the exposed shaft, typically at the non-drive end fan cover, without requiring the motor to be rewound or the bearing replaced.

How do I know if fluting is already happening without disassembling the bearing?

Vibration analysis is the primary non-invasive route. Fluting produces a distinctive vibration signature at frequencies tied to the bearing's ball pass and cage frequencies, which shows up clearly on a spectrum before the bearing is pulled apart for visual confirmation.

Does grounding the motor frame to earth solve the problem by itself?

No. Frame grounding is necessary for safety and for giving common-mode current a path back to the drive, but it does not stop that current from also taking a shortcut through the bearing. Shaft grounding, insulated bearings, and dv/dt filters address the bearing-specific path directly.