NVH Analysis and Optimization of Electric Drive System in Over Modulation and Six-Step Regions

Pulse width modulation (PWM) and the corresponding modulation index (MI) value are of critical importance to the performance of electric drive systems for electric vehicle applications. For interior permanent magnet (IPM) machines, operating in overmodulation (OVM) and six-step modes increases the voltage output beyond the linear region, allowing the motor to achieve higher torque and power with reduced inverter loss. However, the resulting distorted current waveforms and higher current ripple harmonics lead to a notable increase in the motor noise. A multi-disciplinary approach has been developed to analyze the NVH performance of a three-phase 8-pole IPM motor when it operates in the OVM and six-step regions at high speed. The PWM current ripple harmonics induced by voltage-source inverters are predicted using different MIs and subsequently validated through experiments. The current ripple data are used for the prediction of electric motor efficiency, including an analysis of copper, iron and magnet losses. Furthermore, the dynamic electromagnetic forces are predicted using the distorted current waveforms in the OVM and six-step regions and integrated into a comprehensive finite-element model for the machine and the motor fixture. RMS vibration responses are measured in a motor dyno and also predicted using the analytical method. The test data and analysis results demonstrate that the 24th winding order and its second harmonics at the 48th order are the most sensitive to the MI. A true six-step operation can result in a more than 20 dB increase in motor whine, which presents a significant NVH risk for EV applications. The comprehensive analytical method allows the optimal MI parameters to be selected to balance the torque, efficiency and NVH performance of electric drive systems, with successful application to General Motors’ Ultium systems.