Motor Overvoltage Mitigation by Active Cancellation of Reflections Using Parallel SiC Devices With a Coupled Inductor

Employing high-switching-speed wide-bandgap devices improves the efficiency of industrial drive inverters but can result in motor overvoltages for shorter lengths of cable between the inverter and the motor. These overvoltages are caused by high-frequency impedance mismatches resulting in reflection of the voltage pulse edges of the inverter output. Forward and backward traveling reflections interfere and, in the worst case, cause double the dc-link voltage to appear across the motor terminals. In this article, this doubling of the motor voltage stress is observed with only a 10-m-long cable. To mitigate the motor overvoltage, a novel method using a differential-mode coupled inductor between the paralleled half-bridges and the phase output is proposed. By adding a delay time between the half-bridges at every switching event, a quasi-three-level output is produced that can actively cancel reflected voltage waves, eliminating the cause of motor overvoltages. The method can be utilized for three-phase inverters and, when using independently driven paralleled devices, only requires one additional inductor per phase. A design process for the coupled inductors is given, which aims to minimize the circulating current between the half-bridges and, therefore, give minimal increase in conduction losses due to imbalanced current sharing. These additional conduction losses and other limitations of the proposed method are analyzed. An inverter utilizing this proposed method is implemented and compared to a passive filter designed for overvoltage mitigation. The novel method achieves near perfect overvoltage mitigation with much smaller additional volume and much lower losses than that of the filter. Furthermore, the losses when using the active mitigation method are very similar to when using the inverter with no overvoltage mitigation.