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.,
$\overline {{{\boldsymbol{\tau }}_{\boldsymbol{r}}}} $
(ratio of rotor pole pitch to electromagnetic airgap length). The results show that for 3 kW machines,
$\overline {{{\boldsymbol{\tau }}_{\boldsymbol{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
$\overline {{{\boldsymbol{\tau }}_{\boldsymbol{r}}}} $
>2.2 still results in achieving a performance closest to their optimal capability. However, when
$\overline {{{\boldsymbol{\tau }}_{\boldsymbol{r}}}} $
>2.2, special consideration should be paid to avoid potential irreversible magnet demagnetization at multi-MW power levels.