Configurations and Characteristics of Active Canceling and Compensating Circuits for Reducing Common-Mode Voltage Generated by Voltage-Source PWM Inverters

This paper deals with active circuits for cancellation and compensation of common-mode voltage caused by a PWM inverter. A non-negligible amount of leakage or ground current would flow through stary capacitors between stator windings and a motor frame due to a large step change in the common-mode voltage at every switching operation of the PWM inverter. First, it is shown experimentally that the active common-noise canceler (ACC), which has been proposed by the authors, is effective in reducing not only conducted EMI but also motor shaft voltage and bearing current. Next, a new ACC characterized by an isolated power supply is developed to enlarge an applicable voltage range in which it is possible to integrate the ACC with a PWM inverter. Furthermore, the authors proposes another type of ACC, the purpose of which is, not to achieve complete cancellation, but to restrain only a slope in a change of the common-mode voltage applied to the the motor. As a result, the core size of the common mode transformer used in the active compensating circuit becomes smaller considerably. Some experimental results show good effects of the proposed active circuits on both ground current and conducted EMI.