Current Jam

This article focuses on the latest technology, called the insulated gate bipolar transistor that has increased switching frequencies almost twentyfold from the last few years. It turns out that the newer variable-frequency drives, while working full-time shifts as speed controllers of ac induction motors, may be moonlighting with a little part-time electrical discharge machining (EDM). The new drives are suspected of setting up bearing currents. These currents, acting in a fashion identical to the mechanism of EDM—the manufacturing technique by which metal is removed from a workpiece through the use of high-energy sparks—have been eroding the material from the races of motor bearings. In some cases, stray currents are destroying bearings a few months after startup. ABB recently patented a motor winding designed to eliminate circulating bearing currents. The design divides the stator winding into an even number of equal parts per phase. The groups are then distributed uniformly between ac supply connections at both ends of the stator.