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An equivalent model of a four-winding transformer for a feed-forward-type active filter is presented in this paper and a three-phase inverter-fed motor drive system can he described as a single-phase system for the convenience of analysis. After a passive common-mode voltage detection method is analysed theoretically, a resistor-capacitor network is used to detect common-mode voltage generated by a PWM inverter; a push-pull circuit, in which the complementary transistors have no maximal rating voltage limitation, is used to amplify current for driving a four-winding transformer which is used to impose a compensating voltage with the same amplitude but with the opposite polarity to the common-mode voltage generated by the PWM inverter. Some design considerations of the filter are also given in detail. In a 380 V/3 kW motor drive system, simulation analysis and experimental results confirm the validity of the proposed filter that the common-mode voltage at motor terminal is eliminated almost completely and the negative effects of the common-mode voltage, such as shaft voltage and bearing current, are unobvious.