Postfault Operation Strategy for Cascaded H-Bridge Inverters Driving a Multiphase Motor

Multiphase motor drives based on a cascaded H-bridge (CHB) voltage source inverter (VSI) are suitable for fault-tolerant applications because a damaged H-bridge can be safely bypassed without interrupting the drive operation. This bypass has the effect of an undesired dc-link voltage shortage that multiphase motors can be effectively tolerated if appropriate x-y components are injected into the stator voltages. These degrees of freedom, which are not available in three-phase motors, permit to reach higher speeds without field weakening nor torque oscillations during the fault. However, no specific strategies exploiting this particular advantage of multilevel multiphase drives have been proposed so far, albeit resorting to strategies devised for three-phase VSIs would reduce the drive performance. This article proposes a postfault strategy for multiphase CHBs with bypassed H-bridges that better uses its dc link by injecting appropriate x-y components in the modulation reference signals, without altering the \alpha-\beta ones. Experimental results, obtained with a five-level five-phase motor drive, show that the proposed strategy manages to run the motor at rated speed with negligible torque ripple when an H-bridge is bypassed. It is also recognized that the continuous operation in this situation requires using a circulating-current filter to mitigate the extra stator copper losses that arise.