Exact common-mode and differential-mode equivalent circuits of inverters in motor drive systems taking into account input rectifiers

In this paper, exact common-mode and differential-mode equivalent circuits of PWM inverters in motor drive systems are derived by taking the input rectifiers into account. The proposed equivalent circuits are derived by viewing the whole system as a chain of networks. The input rectifier and the inverter are represented by switching functions, while other parts are represented by impedance circuits. The obtained equivalent circuits reveal clearly that conduction of diodes in the rectifier and switching of the inverter are the causes of the coupling between the common-mode and differential-mode circuits. The transient characteristics of the common-mode current can be then obtained from the derived equivalent circuits with better accuracy as compared to the conventional one. Additionally, the DC bus charging phenomenon can also be explained from the new equivalent circuits, and the conditions which lead to such phenomenon are investigated. Simulation and experiment are finally given to confirm the validity of the theoretical results.