Conducted Common-Mode EMI Reduction by Impedance Balancing

A new method to reduce common-mode (CM) electromagnetic interference (EMI) emission at the dc input of variable-speed motor drives is presented. Unlike conventional passive or active filtering techniques that rely on impedance mismatch or active noise cancellation, the proposed method uses a passive circuit with matched impedance to cancel the inverter CM current. In the proposed approach, a star-connected three-phase RC circuit is placed at the output of the inverter to extract the CM voltage. An inductor is connected between the star point of the RC circuit and the middle of the dc bus capacitor to inject a CM current into the dc input. A Wheatstone bridge is then identified in the CM equivalent circuit of the system with these additional components. By properly selecting the inductor, the Wheatstone bridge can be balanced, resulting in near-ideal cancellation of the input CM current. Development of the method is presented and an experimental setup is used to demonstrate its effects. A comparison with conventional EMI filtering methods in terms of overall filter volume is also presented.