Performance Comparison of Surface and Spoke-Type Flux-Modulation Machines With Different Pole Ratios

In this paper, performance comparison between surface and spoke-type flux modulation (FM) permanent magnet (PM) machines is presented. Generally, spoke-array magnet arrangement is capable of increasing the torque density of PM machines due to its flux-focusing effect. Nevertheless, a flux-barrier effect is found when this magnet topology is applied in FM machines, which may offset the advantage in torque capability when the pole ratio is high. By flux distribution comparison of these two-machine topologies, the flux-barrier effect is visually explained. Through numerical FEA, this effect is further investigated in spoke-type vernier PM machines with a series of pole ratios. Finally, compared with surface-type FM machines, considerable reduction in modulated magnetic field as well as output torque capability is verified in high pole ratio, spoke-type FM machines.