Comparison of NdFeB and Ferrite Radial Flux Focusing Coaxial Magnetic Gears

Magnetic gears offer a contactless alternative to mechanical gears. However, the high price of rare earth magnets makes material costs a key concern for magnetic gears. This work compares the impact of NdFeB and ferrite magnets on optimized radial flux focusing coaxial magnetic gears. These results are then compared against a similar analysis of surface permanent magnet (SPM) designs from a previous paper. The results show that the flux focusing topology significantly increases torque density compared to SPM designs for both magnet materials. However, the flux focusing topology only improves the costeffectiveness of ferrite designs, not NdFeB designs, based on the assumed material prices of $10/kg for ferrite, $50/kg for NdFeB, and $3/kg for steel. While comparing flux-focusing with both materials using 2D and 3D finite element analysis (FEA), NdFeB designs remain more cost-effective based on 2D FEA. However, the NdFeB designs suffer more significant end effects, as they need shorter stack lengths to achieve a target torque; thus, when using 3D FEA for a target torque, both NdFeB and ferrite flux focusing designs achieve similar active material costs, which are higher than those for NdFeB SPM designs. Nonetheless, NdFeB still yields significantly higher torque densities. Additionally, in flux focusing designs using ferrite, the active mass is lower and more cost-effective than in SPM ferrite designs. Furthermore, flux focusing designs can achieve higher efficiencies than SPM designs, especially at higher gear ratios. Contour plots are also provided to demonstrate how changes in material prices affect the costs of optimized designs.