Optimization and Comparison of Permanent Magnet Synchronous Motors with Mechanically and Magnetically Coupled Magnetic Gears at Different Gear Ratios for Helicopter Applications

Permanent magnet synchronous motors (PMSMs) are suggested for applications with high torque density, reliability, and efficiency requirements. Integrating PMSMs with magnetic gears (MGs), magnetically geared motors (MGMs), can potentially increase the torque density and reliability of the system. There are two common radial flux inner stator MGM topologies, mechanically coupled and magnetically integrated MGMs. This study independently optimized PMSMs, mechanically coupled MGMs, and magnetically integrated MGMs using 2D finite element analysis (FEA) with a genetic algorithm (GA) to maximize the specific torque (ST), the maximum achievable torque divided by the active mass, for a thorough comparison. Multiple gear ratios and slot/pole-pair (SPP) combinations were considered. As expected the MGMs outperform the PMSMs in terms of ST and magnet usage. Additionally, the analysis shows a reduction in the performance of magnetically integrated MGMs at higher gear ratios as the pole pairs of the MG are dependent on the selected SPP and may not represent the optimal MG at a certain gear ratio. Nonetheless, at lower gear ratios, magnetically integrated MGMs achieve higher ST by utilizing a thin back-iron on the high-speed rotor (HSR). The 3D simulations demonstrate that the optimal MGMs experience more substantial torque drops compared to direct-drive motors, especially at higher gear ratios. These torque reductions are a result of end effects and axially escaping flux, inherent features of MGs. MGMs manage to achieve comparable efficiencies to direct-drive motors at lower gear ratios while offering the advantage of lower mass and volume.