Research database

To reduce reliance on RE materials, Electrically Excited Synchronous Machines (EESMs) have emerged as a promising alternative, adopted by major car-makers such as BMW and Renault. Permanent magnets are replaced with a rotor excitation coil, enabling active and optimized field control. However, rotor excitation leads to significant Joule losses in the rotor, reducing efficiency and increasing thermal stress, imposing dedicated rotor heat extraction. Additionally, EESMs typically require twice as much the copper content of PMSMs, also raising environmental and supply concerns. Another alternative, although not yet ready for mass-market, are Variable Flux Machines (VFMs) utilizing low-coercivity permanent magnets such as iron nitride (FeN), which are free of RE materials and can be demagnetized and remagnetized through dedicated current pulses injected by the traction inverter. While attractive in theory, VFMs require that the inverter current rating (and so its cost cost) is oversized by 150% to 400% to sustain the very high current pulses needed for re-magnetization. For these reasons, VFMs are currently not adopted for high power applications. This project proposes a novel disruptive motor concept that merges the benefits of EESMs and VFMs while addressing their respective limitations. The proposed Rotor Excited Variable Flux Machine (REVFM) features a hybrid rotor with FeN magnets and a minimized excitation coil dedicated to intermittently regulate the magnetization state via dedicated current pulses. This eliminates continuous rotor losses, enhancing efficiency and removing the need for active rotor cooling. Moreover, because excitation is limited to short-duration pulses, lightweight and low-cost aluminum windings can replace copper in the rotor coil, significantly lowering both environmental impact and manufacturing costs. The lower density of aluminum also reduces centrifugal stresses, simplifying rotor mechanics at high speeds. Even more important, the REVFM overcomes the need for inverter oversizing and maintains stable magnetization during normal operation, allowing for high torque performance and improved reliability. The controlled magnetization strategy enables efficient, precise field modulation with reduced pulse current requirements, simplifying control and hardware design. In summary, the REVFM introduces a breakthrough solution for RE-free traction motors with minimal use of critical raw materials. By combining high efficiency, environmental sustainability, and manufacturability, this technology presents a compelling pathway toward next-generation electric mobility.