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 (PW M) hard- and soft-switched inverters. The mechanisms for bearing currents a re first identified using an approach based on direct small-signal excitati on of the motor bearing with sinusoidal and square-wave signals to characte rize 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 e xplain 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 performanc e of hard and soft-switched inverters with respect to bearing currents and shaft voltage. Experimental results are provided for PWM hard- and soft-swi tched insulated gate bipolar transistor (IGBT) inverters which have exactly the same power circuit layout, identical chassis, and rating of 70 kVA.