Experimental comparison of motor bearing currents with PWM hard- and soft-switched voltage-source inverters

This paper compares motor bearing currents due to pulsewidth modulation (PWM) hard- and soft-switched inverters. The mechanisms for bearing currents are first identified using an approach based on direct small-signal excitation of the motor bearing with sinusoidal and square-wave signals to characterize the bearings. It is shown that many of the motor models that have been proposed in the literature to explain bearing currents do not adequately explain the observed higher frequency effects. The paper also outlines some of the important phenomena which need to be considered for a more complete description of bearing currents. Finally, the paper compares the performance of hard and soft-switched inverters with respect to bearing currents and shaft voltage. Experimental results are provided for PWM hard- and soft-switched insulated gate bipolar transistor (IGBT) inverters which have exactly the same power circuit layout, identical chassis, and rating of 70 kVA.