High-Switching-Frequency SiC Power Converter for High-Speed Switched Reluctance Machine

Abstract

This article reports design and testing of silicon carbide) devices-based three-phase asymmetric H-bridge converter capable of switching at 50 kHz and intended to drive high-speed switched reluctance machine. Electromagnetic designs of a 7.5-kW, 30 000-r/min and a 20-kW, 50 000-r/min Switched reluctance machine are presented. The 30 000-r/min machine is fabricated and characterized. A machine model is obtained from the measured characteristics. Based on the machine model, as well as drive operating variables and device data-sheet parameters, two methods are proposed for evaluating the losses in the power devices. The first method relies on machine model to obtain machine current and converter duty ratio, and thereby estimate the device losses. A second simplified method provides closed-form expressions for device losses. The proposed analytical expressions relate the machine characteristics such as inductance and saturation current with converter losses. The estimated loss is further used to predict device junction temperature. The power converter and device ratings are chosen so that the converter is suitable for both machines. The capacitor selection and sizing are discussed. Further, the power converter architecture and other subsystems of the converter are described. Test results are presented for the asymmetric H-bridge converter at 800 V dc bus voltage, 50 A of load current, and 50 kHz of switching frequency. Thermal image is presented for validation of estimated heat-sink temperature. The predicted heat-sink temperature is found to agree well with the measured temperature. The power converter is used to operate the 30 000-r/min machine at various speeds.