High frequency model of an electric motor based on measurement results

Abstract

In a motor drive system, a voltage source converter with hard switches generates high dv/dt, which causes leakage currents due to stray capacitances in an electric motor. A high frequency model of an electric motor is an important issue for power electronic engineers, which helps them analyse leakage and bearing currents, and design EMI filters. At low frequency ranges, an equivalent circuit of an electric motor consists of inductances and resistances without considering stray capacitances and skin effects. These issues become more important at high frequencies, particularly in fast switching applications due to high dv/dt. An impedance of a 5.5 kW motor has been measured in terms of frequency using an impedance analyser and the results have been used to extract high frequency parameters. It is recommended to estimate the behaviour of the motor in a frequency range when the motor acts as a capacitive or an inductive load according to the phase values of the impedance. Then, the parameters can be calculated directly from the magnitude and the phase values of the impedance. A simple model of an electric motor at high frequency used for high speed switching applications is discussed in this research work.