Insulated bearings— New solution for VFD motors

The use of variable-frequency drives (VFDs) to control alternating-current (AC) motors has increased dramatically in recent years. In addition to featuring low operating costs and high performance, VFDs save energy. The challenge facing system designers and engineers is to minimize damage to AC motors from shaft current. From its first minute of operation, a VFD induces destructive voltages that build up on a motor shaft until they find discharge paths to the frame (ground). In most cases, motor bearings present the path of least resistance. Once voltage is sufficient to overcome the resistance of the oil-film layer on a bearing, shaft current discharges, causing electrical-discharge-machining pits and fusion craters in the race wall and ball bearings.

This phenomenon continues until the bearings become so severely pitted that fluting, excessive noise, and failure occur. Mitigation of this damage is possible through various strategies. Some are narrow in application, and most are costly. Many are not technically feasible. However, a new technology employs insulation material by plasma- spraying method to prevent harmful currents and provide a low-cost solution to the problem. That is INSULATED BEARINGS.