Bearing currents
Bearing currents come in several different appearances. However, while modern motor design and manufacturing practices have nearly eliminated the low frequency bearing currents induced by the asymmetry of the motor, the rapid switching in modern variable frequency drive systems may generate high frequency current pulses through the bearings. If the energy of these pulses is sufficiently high, metal transfers from the ball and the races to the lubricant. This is known as electrical discharge machining or EDM. The effect of a single pulse is insignificant, but a tiny EDM pit is an in continuity that will collect more pulses and expand into a typical EDM crater. The switching frequency of modern VFD is very high and the vast number of pulses causes the erosion to quickly accumulate. As a result, the bearing may need replacing after only a short time in service.
