Feasibility study of rare earth free electrically-excited machine technology for EV powertrain: Electromagnetic-Thermal performance evaluation with model-based design approach

This study investigates the feasibility of rare-earth-free electric machine technologies for electric vehicle powertrains by comparing the electromagnetic and thermal performance of an electrically excited synchronous machine (EESM) and an interior permanent magnet synchronous machine (IPMSM). A model-based design approach is applied to define motor specifications under system level constraints, including battery voltage variation due to state of charge (SOC), inverter efficiency, power factor, and thermal limits of the battery system. Temperature dependent efficiency maps are analyzed to reveal how variations in rotor field winding temperature of the EESM and magnet temperature of the IPMSM affect performance characteristics across base speed and high-speed regions. This analysis provides insight into efficiency degradation trends and their implications for electric drive control strategies. To capture dynamic machine behavior, a coupled electromagnetic and thermal transient co-simulation framework is employed under two standardized driving cycles: WLTP Class 3 and Artemis Motorway 150. The simulation evaluates the influence of different cooling strategy and various inlet oil temperatures on thermal responses. Results indicate that with proper electromagnetic design and rotor side cooling using a housing water jacket and direct axial oil spray combination, the EESM can maintain thermal safety while reducing structural complexity. Furthermore, the EESM demonstrates up to 1.14 % higher energy efficiency than the IPMSM under specific dynamic driving conditions. These findings support continued exploration of rare earth free electric machine design as a viable solution for next generation electric traction systems.