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Extraction of Common-Mode Impedance of an Inverter-Fed Induction Motor

In a variable speed drive (VSD) system, high-frequency switching of power devices will cause large common-mode (CM) voltage during operation. While the CM voltage acts on the parasitic capacitance of the system, it will generate serious CM current on the motor side. It is necessary to model the VSD system so as to analyze and forecast the CM current theoretically. However, an essential condition of modeling CM current is to extract the CM impedance on the motor side of the system. In this paper, a novel extraction method for the CM impedance of an inverter-fed induction motor is proposed. First of all, time-domain waveforms of the CM voltage and current on the motor side of the VSD system are measured, and then mathematical calculations are performed to obtain the spectrum characteristics of the CM impedance on the motor side of the system. Compared to the traditional extraction methods of the impedance parameter by an impedance measurement or numerical calculation, the method proposed in this paper does not require laborious modeling of the motor or cable, and the extraction process is much simpler and easier. Moreover, in traditional methods, the motor impedance parameters are usually obtained in a nonworking state, while the proposed extraction method can be conducted in working state. Thus, the CM impedance extracted by the proposed method will be more accurate. Through comparing measurement and the simulation results of the CM current on motor side, the proposed CM impedance extraction method for an inverter-fed induction motor is validated.