Shaft Current In VFD Operation - Insulation Bearing

Bearing damage can occur if the fast switching VFD IGBT circuitry produces stray currents in the rotor. When the load Z is higher than the line Z, then current is reflected back to the VFD causing quite high voltage spikes to be induced in the rotor and stator metal.

The resultant stray currents in the stator are not a large problem as it is solidly earthed - although they can damage insulation, but those in the rotor must find their way to earth, and this is usually via the bearings.

Resultant arcing at the ball/raceway contact points in the bearing cause abnormal wear and pitting and also degradation of the lubricant which dramatically reduces effective bearing life.

Bearings are designed to operate with a thin oil film between contact surfaces, but once the dielectric properties of that oil is compromised, then current will flow, burning the oil and pitting the metal surfaces.

One problem with the use only of insulated bearings is that the shaft voltages are not mitigated and can eventually find other paths to earth (an attached appliance which can then suffer bearing damage) or even damage the insulated motor bearings if no other path is available. Another problem is the contamination of the bearing insulation over time, allowing the currents to flow through them.

You can safely use uninsulated bearings where these VFD drives are in operation provided you give those stray currents a more attractive earth path than that of the bearings A shaft grounding ring (SGR) on the rotor shaft is such a method, and is generally a cheaper and more effective option than insulated bearings.

Some of these grounding rings employ microfibre contact technology which provides a much greater earth contact area, wears very little, and is more effective even than insulated bearings.

Best practice normally recommends combining insulated bearings and a grounding ring for motors over 100Hp.

Failure to use effective stray current and common mode voltage protection/earthing when employing VFDs can result in bearings achieving less than 1% of their expected service life