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

Employing high switching speed wide bandgap (WBG) devices improves the efficiency of industrial drive inverters, but can result in motor overvoltages for shorter lengths of cable between the inverter and motor. These overvoltages are caused by high frequency impedance mismatches resulting in reflection of the voltage pulse edges of the inverter output. Forwards and backwards travelling reflections interfere and in the worst case cause double the DC-link voltage to appear across the motor terminals. In this paper this doubling of the motor voltage stress is observed with only a 10 metre 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 which can actively cancel reflected voltage waves, eliminating the cause of motor overvoltages. The method can be utilised 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 minimise 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 analysed. An inverter utilising 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 the filter. Furthermore the losses when using the active mitigation method are very similar to using the inverter with no overvoltage mitigation.