Motor drive common-mode EMI reduction by passive noise cancellation

A new method to reduce common-mode (CM) 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 to cancel the CM current produced by the motor drive. In the proposed approach, a start-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 that cancels with the CM current produced by the inverter. Furthermore, a Wheatstone impedance bridge is identified in the CM equivalent circuit of the system after these passive components are added. By properly selecting the added passive components, the Wheatstone bridge can be balanced, resulting in near-ideal cancellation of the input CM current. The development of this impedance balancing method is presented in this paper and an experimental setup is used to demonstrate the effects of the proposed method. A comparison with conventional EMI filtering methods in terms of overall filter volume and effectiveness is also presented.