Motor Overvoltage Mitigation by Active Cancellation of Reflections Using
Parallel SiC Devices with a Coupled Inductor
Abstract
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 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 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.