Reflected Wave Phenomenon in SiC Motor Drives: Consequences, Boundaries, and Mitigation

In voltage source inverter-based motor drives, the fast switching speeds of the power devices result in the reflected wave phenomenon. Besides the overvoltages at the motor end, there are overcurrents at the inverter end. The overvoltages increase stress on the motor and cable insulation, and the overcurrents affect the power devices. Wide bandgap (WBG) devices have lower switching losses and enable high switching frequencies. However, the reflected wave phenomenon is more pronounced in converters using WBG devices and could potentially increase the switching losses. This article presents a comprehensive analysis of the influence of the reflected wave phenomenon on the switching characteristics of SiC devices in motor drives. Notably, the effect of undamped reflected waves and different cable lengths on both the inverter end and motor end is studied. The analysis and experiments in this article uncover that, depending on the switching instance, the undamped reflected wave causes up to a 30% increase or up to a 15% decrease in the switching losses. A 12% reduction in the switching losses is observed with no cable to up to 20-m long cable. The last part of the article presents the design tradeoffs involved in designing a dv/dt filter. Five different dv/dt filters are compared in terms of their impact on the switching losses, dv/dt at the load, filter loss, and damping time of the reflected wave. The L filter and the RLC filter have around a 9% reduction in switching losses. The L filter suffers from increased damping time and higher reflected wave magnitude, making it unsuitable for high switching frequency applications. Overall, L // R filter offers the lowest filters losses with a 6% reduction in switching loss, making it more suitable for WBG devices-based high-speed motor drives.