Comparison of Coaxial Magnetic Gears Using Rare Earth and Nonrare Earth Permanent Magnets

Abstract

Coaxial magnetic gears (CMGs) enable contactless torque transmission, addressing the noise and maintenance drawbacks of mechanical gears. However, the heavy reliance on rare earth permanent magnets (PMs) presents a challenge owing to their unstable supply and fluctuating costs. To mitigate dependence on rare earth PMs, this study proposes and evaluates four CMG configurations combining NdFeB and Ferrite. Each configuration is optimized using two-dimensional finite element analysis (FEA), with axial leakage considered by an equivalent magnetic circuit model instead of three-dimensional (3D) FEA. The suitability of bridge geometry for each PM combination is first assessed, revealing that irrespective of the PM material, the inner bridge provides superior torque density. Comparative analyses of the electromagnetic performance and active material costs were then conducted to evaluate the feasibility of each PM configuration. The results indicate that the difference in residual flux density between the inner and outer rotor PMs influences torque density, material costs, peak-to-peak torque, and efficiency. This difference was also observed in an appropriate gap between the active part and the end cover, at which end cover loss becomes negligible. Ferrite-Nd CMG showed the highest efficiency and lowest price among the combinations of nonrare earth PMs. Finally, electromagnetic performance comparisons of NdFeB-based and Ferrite-Nd CMGs using 3D FEA and experimental validation show that while the Ferrite-Nd CMG has a lower torque density than the NdFeB-based CMG, it offers promising advantages in terms of reduced rare earth dependency, negligible housing loss, and high efficiency.