Effects of Fast Switching Semiconductors Operating Variable Speed Low Voltage Machines

Abstract

The use of SiC MOSFETs in modern power electronic causes transient voltage overshoot effects in the winding system of an electrical machine due to the fast switching operation of the semi conductors. This overshoot leads in combination with an increased voltage level to higher electrical stress within the insulation system. Based on the requirements for conventional winding insulation systems, the emerging electrical field can generate partial discharges (PD). Since the PD depends on various quantities, a study of the basic relationships is performed here. This contribution discusses the necessity of a high frequency (HF) model for the electrical machine A hybrid, coupled approach of analytical and numerical simulations determine the parameter of this HF - model. In order to verify the HF - model, the predicted transient voltage distribution along the lap of a winding is compared to experimental data obtained from a motorette. The HF - model predicts the transient voltage with respect to the traveling time and location. In our contribution an approach will be discussed, in which the locally simulated voltage distribution and the occurrence of PD is predicted. 2D Finite Element simulations of the electric field are employed. To proof the approachs reliability to accurately predict PD for different topologies of the insulation system, different motorettes are studied. The different motorettes are developed using results of the method of design of experiments (DoE) to cover a wide variety of possible insulation system combinations. The model factors include a variation of the wire grade, the geometry and the winding scheme. The results obtained show, that PD prediction is within the standard deviation of the experimental data. Besides the validation of the simulation approach, the DoE demonstrated the influence of the different quantities on PD.