Investigation of torque and torque ripple in dual three-phase permanent magnet synchronous machines for electric vehicles

Permanent magnet (PM) synchronous machines (PMSMs) are featured with high torque density, high power density, and high efficiency. Compared with conventional single three-phase (STP) PMSMs, dual three-phase (DTP) PMSMs exhibit some additional advantages, such as better fault-tolerant, power sharing, and torque capabilities, as well as better efficiency performance. This thesis investigates the electromagnetic performance of DTP PMSMs, with particular reference to torque capability and torque ripple for electric vehicle applications. A generic winding configuration technique of DTP PMSMs is firstly developed based on the star of slots. Different slot/pole number combinations, phase shift angles, and coil pitch numbers are all considered. Thus, for PMSMs with any slot/pole number combination, all feasible DTP winding configurations can be quickly obtained with the proposed method. For 48-slot/8-pole PMSMs, which are widely utilized in electric vehicles, two feasible DTP winding configurations, i.e., single-layer full-pitched one and double-layer short-pitched one, are proposed and compared under healthy and three-phase open-circuit conditions. It is found that the double-layer short-pitched DTP winding configuration is the preferred DTP winding configuration. Further, the concept of “attenuation factors”, which is utilized in STP PMSMs with stator shifting, to describe the effects of stator shifting angle on different magnetomotive force (MMF) orders, is extended to DTP PMSMs by considering spatial and time shifting angles together. Similarly, the instantaneous torque separation method in STP PMSMs is also extended to DTP PMSMs by considering the cross-coupling effects within and between the two winding sets. In addition, the effects of AIPM rotors, which are under extensive investigation in STP PMSMs, on the electromagnetic performance of DTP PMSMs are evaluated. It reveals that the application of AIPM rotors to DTP PMSMs can enhance average torque but increase iron loss and decrease overall efficiency in various operating conditions. Hence, AIPM rotors are not recommended for DTP PMSMs in electric vehicles. Finally, a simplified approach to estimate torque performance of PMSMs with rotor skew is developed. With the simplified approach, the effectiveness of rotor skew in STP and DTP surface PMSMs (SPMSMs) and interior PMSMs (IPMSMs) are investigated. The optimal skew angles for STP and DTP PMSMs accounting for the effect of load can be obtained, which are especially important for IPMSMs.