This paper investigates the impact of scaling on the electromagnetic performance of Surface Mounted Permanent Magnet Vernier (SPM-V) machines with a main focus on open circuit induced EMF. Three different power ratings, i.e. 3kW, 500kW and 3MW, have been chosen for this study. For each power rating, the SPM-V machines are analyzed for different slot/pole number combinations to compare their optimal performance with a conventional SPM machine. Step by step development of an analytical equation is presented for the prediction of induced EMF taking into account the inter-pole leakage of rotor permanent magnets. 2D Finite Element Analysis (FEA) has been used to validate the analytical equation across different power ratings. The analytical equation is thereafter utilized to study the influence of different geometric parameters on the performance of the SPM-V machines. It reveals that the back EMF and torque of SPM-V machines, for a given normalized pole pitch (rotor pole pitch to magnetic airgap length), is unaffected by the increase in airgap length due to scaling. However, the power factor of SPM-V, unlike the conventional SPM, reduces significantly with increase in electrical loading due to scaling effect. The analytical model for induced EMF and the 2D FEA predicted results are validated by experiments using conventional SPM and SPM-V machine prototypes.

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This paper investigates the scaling effect on power factor of surface mounted permanent magnet Vernier (SPM-V) machines with power ratings ranging from 3kW, 500kW, 3MW to 10MW. For each power rating, different slot/pole number combinations have been considered to study the influence of key parameters including inter-pole magnet leakage and stator slot leakage on power factor. A detailed analytical modelling, incorporating these key parameters, is presented and validated with 2D Finite Element Analysis (FEA) for different power ratings and slot/pole number combinations. The study has revealed that with scaling (increasing power level), significant increase in electrical loading combined with the increased leakage fluxes, i.e. (a) magnet leakage flux due to large coil pitch to rotor pole pitch ratio, (b) magnet inter-pole leakage flux and (c) stator slot leakage flux, reduces the ratio of armature flux linkage to PM flux linkage and thereby has a detrimental effect on the power factor. Therefore, unlike conventional SPM machines, the power factor of SPM-V machines is found to be significantly reduced at high power ratings.

This paper investigates the effect of airgap length on the electromagnetic performance of 3kW surface mounted permanent magnet Vernier (SPM-V) machine. The performance is compared with a conventional surface mounted permanent magnet (SPM) machine with same airgap length using 2D Finite Element Analysis (2D FEA). 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. The results show that the SPM-V machine can achieve much higher torque capability than the conventional SPM machine at smaller airgap length. However, there is an optimal airgap length beyond which the torque performance of SPM-V machines drops below the conventional SPM counterparts. Moreover, unlike the conventional SPM machines, the power factor of SPM-V machines drops significantly with increase in airgap length.

Permanent magnet Vernier (PM-V) machines, at low power levels (few kWs), have shown a great potential to improve the torque density of existing direct-drive PM machines without much compromising on efficiency or making the machine structure more complicated. An improved torque density is very desirable for offshore wind power applications where the size of the direct-drive machine is an increasing concern. However, the relatively poor power factors of the PM-V machines will increase the power converter rating and hence cost. The objective of this paper is to review the benefits and challenges of PM-V machines for direct-drive offshore wind power applications. The review has been presented considering the system-level (direct-drive generator + converter) performance comparison between the surface-mounted permanent magnet Vernier (SPM-V) machines and the conventional SPM machines. It includes the indepth discussion on the challenges facing the PM-V machines when they are scaled up for multi-MW offshore wind power application. Other PM-V topologies discussed in literature have also been reviewed to asses their suitability for offshore wind power application.

This is a repository copy of Effect of airgap length on electromagnetic performance of permanent magnet Vernier machines with different power ratings.

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