A voltage-source PWM inverter produces common-mode and normal-mode currents flowing through stray capacitors inside an ac motor every switching. Therefore, the currents may cause radiational EMI to electronic equipment, e.g., AM radio receivers, because its oscillation frequency ranges from 100kHz to several MHz. However, few technical papers related to the radiational EMI have been reported although attention has paid to the EMI.
This paper discusses relationships between radiational electromagnetic noises and the commom-mode and normal-mode currents. Reduction methods of both the commom-mode and normal-mode currents are also discussed on the basis of a motor model including the stray capacitors. It is shown theoretically and experimentally that both the currents can be damped by the combination of the common-mode transformer which has already been proposed by the authors and normal-mode filters proposed in this paper. Radiational EMI of a vector-controlled induction motor drive system of 3.7kW is measured according to the VDE 0877. Experimental results show not only that the radiational EMI results from the high-frequency oscillatory currents, i.e., the common-mode and normal-mode currents, but also that the combination of the common-mode transformer and the normal-mode filters is an effective, practical way of reducing the EMI.