Bearing currents in variable-speed motors — why a motor bearing can fail prematurely with no mechanical cause

The guide on VFD earthing and EMC covers the general earthing and shielding measures around a variable-frequency drive. This article covers a specific, often overlooked consequence of that same switching technique: electrical bearing currents that can damage the bearing of the driven motor itself, independent of any mechanical cause.

The phenomenon: a voltage between shaft and housing

A variable-frequency drive switches its output voltage at a high switching frequency (typically several kHz) using the PWM principle (pulse-width modulation). These fast switching edges bring with them a common-mode voltage, which, via the parasitic capacitances between the stator, rotor and shaft of the motor, builds up a — usually small, but high-frequency — voltage between the shaft and the motor housing.

Why this leads to current flowing through the bearing

The lubricant in a motor bearing behaves electrically as a thin, insulating film between the rolling elements (balls or rollers) and the bearing raceway. As long as the shaft voltage stays below the breakdown strength of that lubricant film, no current flows. Once the built-up shaft voltage locally exceeds that breakdown strength, however, the film punctures at that point and a brief current discharges through the bearing — similar to a miniature spark discharge. These repeated discharges cause small pits on the raceway surfaces over time, a wear pattern known as fluting, visible as a series of parallel grooves.

Why this causes premature bearing wear without an obvious mechanical cause

Because bearing currents are an electrical, not a mechanical phenomenon, the resulting damage occurs without there being any overload, insufficient lubrication, or alignment fault — the classic causes that are usually investigated first for bearing failures. A motor that shows an unexplained, premature bearing failure relatively soon after a variable-frequency drive is put into service, while the mechanical load and lubrication are in order, is an indication to consider bearing currents as the cause.

Countermeasures per IEC TS 60034-25

IEC TS 60034-25 (guide for the design and performance of AC motors specifically intended for supply from converters) names, among others, the following common countermeasures:

Electrically insulated bearings at the non-drive end (NDE) of the motor — often applied from a certain frame size onward (in practice typically around frame 280 and larger), so that the common-mode circuit is broken via the bearing.
A shaft grounding brush (shaft grounding ring/brush) that continuously connects the shaft to the motor housing via a low-impedance path that doesn't depend on the bearing, so that the built-up shaft voltage is discharged before it can puncture the bearing film.

Note: the precise frame-size threshold and the choice between insulated bearings, a grounding brush, or a combination of both depends on the motor power, the drive's switching frequency and the cable length between drive and motor; for a specific design, consult the full text of IEC TS 60034-25 and the motor and drive manufacturer's specifications.

Practical relevance

When combining an existing motor with a newly installed variable-frequency drive — or when selecting a new variable-speed drive train — it is worthwhile to check in advance whether the motor is fitted with an insulated bearing or a shaft grounding brush, especially for larger motors and with a relatively long motor cable (see also the guide on motor cable length and reflection). For an unexplained, premature bearing failure on a variable-speed motor, a bearing-current measurement (with an oscilloscope and a shaft voltage probe) is a targeted diagnostic step before other, less likely causes are investigated.

Common mistakes

Attributing a premature bearing failure on a variable-speed motor solely to lubrication or misalignment, without considering bearing currents as a possible cause.
Fitting a shaft grounding brush without periodically checking or cleaning it — the brush is a wear part that, when worn or contaminated, gradually loses its low-impedance function.
Replacing an insulated bearing on the wrong side with a standard bearing during maintenance, without realising that it was precisely that insulation that broke the common-mode circuit.
Assuming bearing currents only matter on very large motors — while the risk increases with power and frame size, the phenomenon can also occur on smaller variable-speed motors, depending on the drive settings and cable length.