Effect of Airgap Length on Electromagnetic Performance of Permanent Magnet Vernier Machines With Different Power Ratings

This article investigates the effect of airgap length on the electromagnetic performance of direct-drive surface mounted permanent magnet Vernier (SPM-V) machines with different power ratings. Using 3 kW machine as an example, its performance is comprehensively compared with a conventional SPM machine with the same airgap lengths using two-dimensional finite-element analysis. For each airgap length, the slot/pole number combination for the SPM-V machine is investigated to achieve the optimal performance compared to the conventional SPM machine. In order to make the study more generic, the slot/pole number and the airgap length variations are expressed as normalized pole pitch, i.e., (tau) over bar (r) (ratio of rotor pole pitch to electromagnetic airgap length). The results show that for 3 kW machines, (tau) over bar (r) >2.2 is a good design criterion for the SPM-V machines to achieve higher average torque and efficiency than the conventional SPM machines. In addition, a reasonably good power factor (>0.9 in this case) can be achieved. Although the power factor of SPM-V machines drops significantly at multi-MW power level, i.e., 3 and 10 MW, the criterion (tau) over bar (r) >2.2 still results in achieving a performance closest to their optimal capability. However, when (tau) over bar (r) >2.2, special consideration should be paid to avoid potential irreversible magnet demagnetization at multi-MW power levels.