Effects of Demagnetization on Zero Sequence Current in Open-Winding Permanent Magnet Synchronous Machines

The open-winding permanent magnet synchronous machines (OWPMSMs) with a common DC bus have a simple structure, low cost, enhanced fault tolerance, and power quality of a 3-level converter system. They have been proposed for many high-performance and safety-critical applications. The common DC bus topology leads to zero sequence current (ZSC) circulation in the stator windings. However, several different control techniques exist to reduce the ZSC to an acceptable level in practical applications. Like any other permanent magnet machine, the OWPMSM is susceptible to irreversible demagnetization when subjected to harsh thermal and electromechanical conditions. The localized partial demagnetization induces additional harmonics in the ZSC, resulting in a higher torque ripple. The demagnetization fault-tolerant control can only be developed once a theoretical framework for the zero-sequence current and torque ripple under demagnetization becomes available. This paper aims to develop such a framework, focusing on fractional-slot concentratedwinding OWPMSMs. The paper analyzes how magnet faults induce lower-order harmonics in ZSC and torque and shows that the slot/pole combinations primarily govern the orders of induced harmonics. These analytical findings have been verified through finite element analysis (FEA) and experimentation on prototype OWPMSMs.