High-Frequency Modeling of an Adjustable Speed Drive

The use of high frequency switching power devices in adjustable speed drives (ASDs) induces high voltage variations (dv/dt) that excite the parasitic elements of the power circuit, leading to conducted emissions at high frequencies. The advent of these devices has thus generated several unexpected problems, such as premature deterioration of motor ball bearings and high increases in the electromagnetic interference (EMI) levels, caused by the circulation of the high frequency parasitic currents. This paper deals with high frequency modeling of an ASD system. This will be used to study the influence of the PWM inverter commutations on the level of the common mode and the differential mode currents between the power converter and the motor. First a shielded 4-wire energy cable model is presented. Then, a high frequency model of the PWM inverter and a model of the AC motor are proposed. Finally, the ASD system is simulated and the obtained results are compared to the experimental measurements