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Voltage Stress Calculation and Measurement for Hairpin Winding of EV Traction Machines Driven by SiC MOSFET

SiC mosfet is a promising power device for electric drive system due to its various advantages such as high switching frequency and low loss. However, fast switching speed results in high voltage stress and can potentially cause insulation damage of the winding. Few studies have analyzed and evaluated the voltage stress, especially for the hairpin winding, which is the mainstream winding type for electric vehicle traction machines. For this purpose, this article proposes a high frequency equivalent circuit model to predict the interturn voltage stress of hairpin windings driven by SiC mosfets. First, the working principle of the overvoltage at the winding ends is analyzed and validated by experiments. Then, the parasitic parameters are calculated before building the high-frequency equivalent circuit. A field-circuit coupled finite element method is used to consider the nonlinear parameters and to evaluate the voltage stress. Finally, a prototype is built, and the voltage stress is tested. The resultant error between the calculated and experimental maximum interturn voltage stress is only 1.3%, which supports the inference on the validity of the presented equivalent circuit and the method. Based on the above, a novel winding connection scheme is proposed to reduce effectively the voltage stress of hairpin winding.