Scalability and Key Tradeoffs of Variable Flux PM Machines for EV Traction Motor Systems

Methodologies for the design and control of variable flux permanent magnet synchronous machines (VF-PMSMs) to meet electric vehicle traction requirements with significantly reduced driving cycle losses have been proposed recently. The effectiveness of the proposed methods was demonstrated by a systematic design based on finite element analysis (FEA) and experimental driving cycle loss evaluation of a full scale VF-PMSM prototype. In this paper, simplified analytical models are developed to estimate the properties and key performance metrics of VF-PMSMs across a range of power and are verified using FEA. Fundamental tradeoffs between the normalized high-speed power capability and the range of magnetization state $(\boldsymbol{M}\boldsymbol{S})$ variation are identified. The relationship between $\boldsymbol{M}\boldsymbol{S}$ variation range and driving cycle loss reduction is evaluated quantitatively across the design space. A detailed analysis of the scalability of VF-PMSMs including loss reduction capability and system cost (including active materials in the machine, inverter power electronics, and battery cost) is presented.