Novel Permanent Magnet Synchronous Machines with Reduced Rare-earth Magnets for Electric Vehicles

The rapid shift toward electrified transportation has driven the demand for traction motors that combine high torque density, efficiency, and reliability, while reducing dependence on rare-earth permanent magnets (REPMs). This thesis addresses these challenges through a systematic investigation of advanced machine topologies, multiphase configurations, rotor design strategies, and reliability-enhancing methodologies for electric vehicle (EV) powertrains.
The feasibility and performance potential of multiphase machines are first established through comparative analysis of three-, five-, six-, and nine-phase configurations. Results show that multiphase systems deliver 3-6% higher torque per REPM volume, inherently lower torque ripple, and up to 55% reductions in DC-link capacitance, with the dual-three-phase configuration achieving the best performance-to-cost balance.
Building on this foundation, three hybrid rotor topologies, spoke-V, spoke-delta, and spoke-II, are proposed and optimised using multi-objective evolutionary algorithms. These designs reduce NdFeB usage by up to 40% while maintaining torque output, resulting in up to 67% improvements in torque per unit rare-earth volume. Four-quadrant performance and torque decomposition analyses provide deeper insight into the distinct roles of hybrid magnet arrangements.
To improve torque quality, six rotor core shaping techniques are investigated. Asymmetrical shaping proves most effective, suppressing dominant harmonics and reducing torque ripple significantly, while partial shaping offers comparable benefits at lower computational and manufacturing costs.
Finally, robust design methodologies are developed to prevent demagnetisation in ferrite-based PM-assisted synchronous reluctance machines. A Kriging-based surrogate modelling framework accurately predicts and mitigates demagnetisation risk, enabling cost-effective, reliable alternatives to REPM-based machines.
Collectively, the contributions of this thesis provide a pathway to sustainable, high-performance, and reliable EV traction machines. By integrating multiphase operation, hybrid rotor topologies, torque ripple suppression, and demagnetisation prevention, the work establishes a comprehensive framework for next-generation electric powertrain design.