Woongkul Lee
The pursuit of sustainable electrification has reached new heights, with a growing emphasis on reducing reliance on rare earth magnets in electric machine design. To shed light on this exciting development, the 2023 IEEE Transportation Electrification Conference (ITEC), held at the Huntington Place Convention Center, in Detroit, MI, USA, recently hosted a panel session dedicated to high-performance rare earth magnet-free electric machine design. The session brought together academia, research, and industry experts to discuss the advancements, challenges, and opportunities in this transformative field. The invited panelists included Dr. Babak Fahimi, University of Texas at Dallas; Dr. Ayman EL-Refaie, Marquette University; Dr. Seungdeog Choi, Mississippi State University; and Dr. Gianmario Pellegrino, Politecnico di Torino (Figure 1). This article provides a comprehensive overview of the key insights shared during the panel session, highlighting the significance of rare earth magnet-free electric machine designs in shaping a sustainable future.
Figure 1. The 2023 ITEC panel session moderator and invited panelists discuss rare earth magnet-free electric machine design. From left: Dr. Woongkul Lee, Dr. Choi, Dr. Fahimi, and Dr. EL-Refaie.
The panel session commenced by examining the motivations behind pursuing rare earth magnet-free electric machines. The escalating costs, limited availability, increasing demand, and environmental concerns associated with rare earth magnets have necessitated the exploration of alternative solutions. The automotive industry needs 10 times the amount of rare earth metals it currently has to meet 2030 electric vehicle goals. In the renewable energy sector, a direct drive wind turbine generator requires 600–1,000 kg/MW of permanent magnets, out of which 35% are rare earth metals, which include neodymium, praseodymium, terbium, and dysprosium, as summarized in Table 1. By eliminating the dependency on these magnets, researchers and engineers aim to develop electric machines that are cost-effective, environmentally friendly, and readily available to meet the ever-increasing demand from the energy and transportation sectors.
Table 1. Rare earth materials used in the automotive and renewable energy sectors.
The panelists delved into the latest technological breakthroughs and innovations driving rare earth magnet-free electric machine design. Novel materials, advanced design techniques, and rare earth-free or -reduced machine topologies were explored as potential avenues for achieving comparable performance to traditional rare earth magnet-based machines. First, the panel discussed the switched reluctance machine (SRM) as one of the viable options to replace permanent magnet machines with robust rotor design and high reliability with redundancies. One of the panelists, Dr. Fahimi, pointed out that the noise and vibration issues of the SRM have been significantly improved due to recent advancements in control, novel slot–pole combination, and rotor tooth optimization. A nonconventional drive topology (i.e., the asymmetric half-bridge inverter) of the SRM can also be advantageous for fault-tolerant operation.
Efficiency and power density are critical factors in electric machine design for electrified transportation. During the panel session, the experts discussed various optimization strategies employed to enhance the performance of rare earth magnet-free electric machines, including ferrite-based internal permanent magnet (IPM) machines, axial flux machines, dual-stator/rotor structures, hybrid permanent magnet (e.g., NdFeB + ferrite) machines, and synchronous reluctance machines. A comprehensive comparison of an optimized rare earth magnet-based IPM machine with an optimized ferrite-based IPM machine was presented by Dr. Pellegrino. Due to the inherently low remanent flux density of ferrite in comparison with NdFeB, the ferrite-based IPM needs to operate with 50% increased phase current and increased stack length to meet the identical performance of the rare earth magnet-based IPM machine. Dr. Choi has also investigated different magnet materials, the number of phases, radial versus axial design, and inner versus external rotor design. Eight different motors have been comparatively studied and confirmed that ferrite-based electric machines tend to have more than 45% reduced torque density compared to that of rare earth magnet-based machines. Various electric machine designs that combine different types of permanent magnets, including NdFeB, Dy-free NdFeB, Ferrite, and FeN, were presented by Dr. EL-Refaie. The Dy-free NdFeB-based electric machine seems to be the closest option to compete with regular NdFeB-based machines.
To provide practical insights, an industry representative joined the panel session to share experiences and perspectives. The representative highlighted the importance of real-world implementation considering power density and efficiency, focusing specifically on electric vehicle applications. They discussed the challenges faced during the transition from rare earth magnet-based machines to magnet-free alternatives, including supply chain considerations, manufacturing scalability, and cost-effectiveness. Despite the challenges, the industry representative emphasized the potential benefits and encouraged continued collaboration between academia and industry to drive further progress in this domain. As a near-term feasible solution, most panelists agreed that the Dy-free, or heavy rare earth-free, electric machine could be promising compared to the other potential options.
Integrating rare earth magnet-free electric machines with power electronics and control systems emerged as a central theme during the panel discussion. The panelists acknowledged the challenges associated with seamless integration, such as thermal management, system-level performance, and control strategies. However, they also highlighted the tremendous opportunities for innovation, cross-disciplinary collaboration, new permanent magnet materials, and the development of intelligent control algorithms to overcome these challenges. Integrating magnet-free machine designs with emerging technologies, such as silicon carbide and gallium nitride power devices, further amplifies the possibilities for improved performance and efficiency.
The panel session on rare earth magnet-free electric machine design showcased the remarkable progress in advancing sustainable electrification. By challenging the traditional reliance on rare earth magnets, researchers and engineers are paving the way for cost-effective, environmentally friendly, and readily available electric machines. The insights shared during the panel session underscore the importance of collaboration, innovation, and optimization to unlock the full potential of rare earth magnet-free designs. As we look toward a greener future, the transition to magnet-free electric machines will play a pivotal role in accelerating the widespread adoption of sustainable electrification across industries and applications.
Digital Object Identifier 10.1109/MELE.2023.3320522
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