Effects of Common-Mode Active Filtering in Induction Motor Drives for Electric Vehicles

This paper deals with the active common-mode (CM) voltage compensation in an induction motor drive where the inverter is supplied by a dc source, which is typical of vehicle applications. The CM voltage at motor terminals, creating a shaft voltage through the motor air gap with possible rise in bearing current, can endanger motor reliability and reduce its lifetime. Therefore, CM voltage filtering is desirable. On the other hand, the operation of an active filter has an impact on drive efficiency due to its specific losses and can affect the electromagnetic interference (EMI) emissions that are generated by the drive. Such effects are investigated in this paper. A detailed description of a CM active filter (CMAF) is presented. An analysis of the CM voltage and current on the motor ground connection before and after the introduction of the CMAF is performed. The power losses due to the CMAF operation are analyzed and experimentally evaluated. Furthermore, the EMI toward the vehicle dc power supply line is investigated. All phenomena are studied by simulation and experimentally. The simulated results are obtained developing a high-frequency circuit model of the drive system, including the CMAF, which is implemented using the PSpice software. To accurately perform the experimental tests, a new dedicated high voltage dual dc line impedance stabilization network (LISN) is designed and set up on purpose. The CMAF is found to be an effective solution for the increase in motor reliability and drive electromagnetic compatibility, and its operation does not significantly reduce drive efficiency. Moreover, the CMAF does not worsen the EMI toward the dc supply line. Therefore, its presence does not imply the need for additional filters with respect to the case where no CMAF is used. Simulated results are in good agreement with the experimental ones, confirming the validity of the proposed modeling of the drive system.