Scaling Effect on Electromagnetic Performance of Surface-Mounted Permanent-Magnet Vernier Machine

Abstract

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.

System-Level Investigation of Multi-MW Direct-Drive Wind Power PM Vernier Generators

“…Both the conventional SPM and SPM-V machines are globally optimized for maximum torque using the OPERA optimizer tool which uses a combination of deterministic and stochastic methods [17]. The geometric variables used for global optimization are the same as investigated in [4]. It is worth noting here that this paper adopts the same 2D FE model as in [4], which has already been experimentally validated using conventional SPM and SPM-V machine prototypes.…”

“…The geometric variables used for global optimization are the same as investigated in [4]. It is worth noting here that this paper adopts the same 2D FE model as in [4], which has already been experimentally validated using conventional SPM and SPM-V machine prototypes. The slot/pole number combinations used for different power ratings for the SPM-V machines are given in TABLE II.…”

“…In the recent years, Vernier permanent magnet (PM) machines have attracted increasing interest for low speed direct-drive applications because of their high torque density, inherent low torque ripple and relatively simple structure [2], [3]. But Vernier PM machines have a drawback of relatively low power factor compared to the conventional surface mounted PM (SPM) machines [4], [5]. The power factor becomes significantly lower (~0.4-0.5) for SPM Vernier (SPM-V) machines at high electrical loading, which is unavoidable for high power (multi-MW) offshore direct-drive generators [4].…”

“…But Vernier PM machines have a drawback of relatively low power factor compared to the conventional surface mounted PM (SPM) machines [4], [5]. The power factor becomes significantly lower (~0.4-0.5) for SPM Vernier (SPM-V) machines at high electrical loading, which is unavoidable for high power (multi-MW) offshore direct-drive generators [4]. This will result in a significant increase in converter rating and cost.…”

“…Such large generators will have significantly higher electrical loadings and magnetic saturation levels. As a result, they might have even lower power factor, as investigated in [4]. Moreover, the comparison in [5] neglects the cost and mass of the generator structural materials, which have been found to dominate the total generator cost and mass at high power ratings [1].…”