Multi-Objective Design Optimization of Multi-Layer Hybrid Magnet PMSM Considering Irreversible Demagnetization Conditions

This paper presents a novel design and optimization method for hybrid magnet permanent magnet synchronous motors (HM-PMSMs) that strategically combines low-cost ferrite and high-energy rare-earth permanent magnets (RE-PMs). Each PM type exhibits distinct electromagnetic and thermal properties, directly influencing machine performance. To leverage these characteristics, a multi-layer HM-PMSM structure is proposed, using the magnetic circuit method and the B-H curves of both PM types as a first step. Furthermore, a multi-objective optimization framework is formulated by incorporating maximum torque per ampere (MTPA) operation and irreversible demagnetization (I-D) constraints, ensuring an optimal balance between torque density and safe electromagnetic-thermal performance. To address the computational complexity of the optimization process, a surrogate-based approach combining the kriging method and non-dominated sorting genetic algorithm II (NSGA-II) is employed. As a result, four Pareto-optimal solutions are identified. To comprehensively assess the extracted optimal designs, their performance is evaluated under MTPA and field-weakening (FW) operations. The comparison highlights the effectiveness of the proposed optimization in improving torque density and constant power speed range (CPSR). Finally, a forced I-D analysis utilizing the Demagnetized Area Ratio (DMAR) index confirms the demagnetization resistance of the optimized designs, ensuring stable operation under currents up to twice the rated value.