Practical approach to model electric motors for electromagnetic interference and shaft voltage analysis

This study presents a practical method to find a high-frequency model of AC machine in order to predict leakage current and shaft voltage. A voltage-source inverter with hard switches generates pulse width modulated voltage with high dv/dt and common-mode voltage, which causes leakage current and shaft voltage due to stray capacitances in an electric motor. At fundamental frequency (a few hundred hertz), an equivalent circuit of an electric motor consists of inductances and resistances without considering stray capacitances. This study presents a practical approach in extracting high-frequency parameters of an electric motor for leakage current and shaft voltage analysis. Input impedance (magnitude and phase values) of several configurations of an AC motor is measured using a network analyser. All capacitive couplings between windings, stator and rotor are extracted based on theoretical analysis and measurement results. Simulations are compared with test results to verify the proposed method and model.