Systematic Common-Mode Filter Design for Inverter-Driven Motor System Based on In-Circuit Impedance Extraction

Inverter-driven induction machines often suffer from premature bearing failure due to high-frequency common-mode (CM) voltage and induced CM current. One effective remedy is adding a CM filter to suppress the CM current. Optimal CM filter design requires prior knowledge of both noise source and termination impedances. With an improved in-circuit impedance measurement setup, the CM noise source impedance of a high-power inverter-driven motor system with strong background noise can be extracted. The setup also protects the measurement instruments from surges and transients from the harsh operating environment. The extracted noise source impedance allows a CM filter to be designed systematically without overdesigning. In the evaluation of the first-order single-inductor and second-order LC filters, it is found that the second-order CM filter is prone to differential-mode to common-mode (DM-to-CM) conversion, especially at the resonant frequency caused by the leakage inductance of the CM choke and the equivalent DM capacitance of the line-to-ground capacitors. Therefore, an additional DM inductor is necessary to curb the mode conversion, which may not necessarily result in final size reduction as compared to that of first-order single-inductor filter.