Investigation and results of eddy currents on DC motor bearings

Investigation into several similar bearing failures has led to the realization that the source of electrical current causing the failures is not fully understood and that the problem is widespread. Probable causes, an attempt to measure the currents, and possible solutions to the bearing failure are discussed. Several methods of bearing fluting due to eddy currents in the shaft of DC machines are discussed, and procedures to avoid applying the currents or to protect the machine from the currents are outlined. It is noted that there is no conclusive evidence that defines a cure for the problem of capacitive coupled eddy currents in the motor shaft; however, until the problem is better defined and a correction made, drive installations with 13.8 kV primary voltage on isolation transformers with multiple drives would do well to apply shaft grounding brushes to reduce the occurrence of fluted bearings on machines