Analysis of Shaft Voltage and Bearing Current in an Inverter-Fed Nongrounded Motor

Abstract

This paper analyzes shaft voltage and bearing current in a nongrounded motor driven by a voltage-source PWM inverter. First, the shaft voltage of the nongrounded motor is measured. The experimental results indicate that a bearing current, which can not be explained by the conventional theory, exists, and that the bearing current may shorten the life of the motor bearing. To investigate the mechanism, some experiments and parameter measurements are done, and simulation models are built. As a result, it is shown that this issue is caused by unbalance in the inductance and stray capacitance of the motor windings and these resonant phenomena.