Analysis of High Gear Ratio Capabilities for Single-Stage, Series Multistage, and Compound Differential Coaxial Magnetic Gears

Magnetic gears perform the same fundamental power conversion as mechanical gears. However, magnetic gears have significant potential benefits due to their noncontact operation. This paper compares three different ways to achieve high gear ratios using coaxial magnetic gears by evaluating trends for radial flux coaxial magnetic gears with surface mounted permanent magnets. First, a single-stage design can be used, but the torque density and efficiency both decline as the gear ratio increases. Additionally, the gear ratio achievable with a single-stage coaxial magnetic gear is limited by practical considerations, such as the maximum number of modulators and pole pairs that can be used within the given space. Second, a multistage design can be formed by connecting single-stage designs in series. Multistage designs can achieve much higher net gear ratios with much less of a torque density penalty, especially as the number of stages increases, but this entails greater complexity. Third, the compound differential coaxial magnetic gear (CDCMG) is proposed. The CDCMG is formed by interconnecting two single-stage coaxial magnetic gears and can achieve gear ratios much higher than the product of the gear ratios of the individual stages. However, the circulating power in the CDCMG leads to poor efficiencies.