Concentric magnetic gears: A review of topological advances, the persistent rare-earth magnet dilemma, and a hybrid excitation pathway

Concentric magnetic gears (CMGs) offer significant advantages for electric vehicle drivetrains, including contactless torque transmission, high efficiency, and built-in overload protection. Despite these benefits, commercial viability is hampered by a heavy reliance on rare-earth permanent magnets (REPMs), raising serious concerns about costs, supply chain security, and sustainability. This review critically examines strategies to mitigate this reliance. Analysis of recent topological innovations shows that while torque density has improved, the fundamental dependence on REPMs remains unchanged. Furthermore, direct reduction strategies, including system integration, material substitution, topological optimization, passive conductors, and complete electrification, often entail significant performance trade-offs. Consequently, hybrid excitation is identified as a key paradigm shift. The core contribution of this review is the development of a clear taxonomy distinguishing “auxiliary electromagnetic integrations” for added functionality and “true hybrid excitation,” where windings act as a co-primary source of magnetic flux. The study concludes that true hybrid excitation is the most strategic yet underexplored research area, uniquely enabling features such as variable gear ratios, overload resilience, and the capability to replace REPM volume directly. Therefore, focused research on true hybrid-excited CMGs is presented as the essential path toward developing sustainable, high-performance, next-generation magnetic gearing systems.