Protective provisions against bearing faults of converter supplied motors

Bearing deficiencies of converter supplied motors may be caused by induced shaft voltages or capacitively coupled bearing voltages. Bearing currents resulting from shaft voltages, circulate through both bearings, but pass only one bearing when driven by the bearing voltage. Previous investigations identified the common mode voltage of the converter as the main source of the bearing voltage. The common mode currents cause a shaft voltage of switching frequency, which is superimposed to the low-frequency components resulting from asymmetries of the stator yoke. The first part of this article demonstrates measures how to reduce the hazard potential by design features of the converter, of the grounding system and of the motor. The second part deals with protective provisions at the motor for those applications, in which a sufficient lifetime of the bearings cannot be guaranteed by primary measures only.