Common mode active filtering effects in induction motor drives for application in 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, typical of vehicle applications. The CM voltage at the motor terminals, creating a shaft voltage through the motor air-gap with a possible arising of bearing current, can endanger the motor reliability and reduce its lifetime. Therefore the CM voltage filtering is desirable. A detailed description of a common mode active filter (CMAF) is reported. An analysis of the motor CM voltage and current on the motor ground connection, before and after the introduction of the CMAF, is performed. Furthermore the electromagnetic interference (EMI) towards the vehicle DC power supply line is investigated. All phenomena are studied both in simulation and experimentally. The simulated results are obtained developing a high frequency circuit model of the drive system, including the CMAF, implemented in PSpice simulator. In order 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 solutions for the increase of the motor reliability and of the drive electromagnetic compatibility. Moreover the CMAF does not worsen the EMI towards the DC supply line. Therefore its presence do not imply the need for additional filters, 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.