Comparison of Surface Permanent Magnet Coaxial and Cycloidal Radial Flux Magnetic Gears

Magnetic gears transform power between low-torque, high-speed rotation and high-torque, low-speed rotation using magnetic fields instead of interlocking teeth. This paper uses an extensive parametric finite element analysis study to quantitatively compare the optimal performances of two of the most promising magnetic gear topologies, radial flux coaxial magnetic gears and radial flux cycloidal magnetic gears, in terms of volumetric torque density and permanent magnet gravimetric torque density. At low gear ratios, optimal coaxial gears generally achieve higher torque densities than optimal cycloidal gears. However, at medium and high gear ratios, the cycloidal topology can generally outperform the coaxial topology in both of these metrics unless very thick magnets are used. Additionally, the cycloidal magnetic gear can realistically achieve much higher gear ratios than the coaxial magnetic gear, but the optimal gear ratio for a cycloidal design varies with other design parameters, such as outer radius. However, cycloidal designs suffer from significant fabrication challenges because one rotor's axis orbits the axis of the other rotor. Additionally, cycloidal magnetic gear rotors experience strong magnetic forces, which must be supported by the bearings, whereas the net magnetic forces on each rotor in a coaxial magnetic gear can be canceled out using symmetry.