Investigation of scaling effect on power factor of permanent magnet Vernier machines for wind power application

Abstract

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.

Permanent Magnet Vernier Machines for Direct-Drive Offshore Wind Power: Benefits and Challenges

“…The variations of power factor with electrical loading between the conventional SPM and SPM-V machines calculated using the analytical model are shown in FIGURE 18. Even with a relatively small 𝐺 𝑟 = 5, the power factors of the multi-MW SPM-V machines (with electrical loading in the range 50-60 AT/mm) are in the range of ~0.4-0.5, which is already very low [23]. The conventional SPM machines could still achieve a relatively good power factor of about 0.8-0.9 at these MW power levels.…”

“…In general, 𝑘 𝑐 ≪ 𝐺 𝑟 , and in most literature this reduced value of 𝑘 𝑐 is due to high inter-pole PM leakage fluxes. But it was revealed in [23] that the leakage fluxes due to the large coil pitch to rotor pole pitch ratio of PM-V machines are much more dominant than the inter-pole PM leakage fluxes. Overall, the PM fluxes in the PM-V machines are largely unutilized, resulting in poor power factors compared to the conventional SPM machines.…”

“…Accordingly, the power factor achieved across slot/pole number combinations will vary. Hence, the power factor is better towards lower slot/pole number combinations or high 𝜏 𝑟 ̅ , due to reduced leakage fluxes [20], [23]. In addition to these leakage fluxes, it was also revealed that there is a significant amount of PM flux that actually enters the stator core back iron and still does not contribute to the induced EMF due to the large coil pitch to rotor pole ratio [21].…”

“…An analytical model, considering all the leakage fluxes has been presented in [23] for the SPM-V machines. Through the analytical modeling and FEA simulation, it was revealed that, unlike the conventional SPM machines, the power factor of the SPM-V machines significantly drops with increasing electrical loading.…”

“…This reduces the overall system-level efficiency by 2-3% at mutli-MW power levels [22]. Since the power factor of the PM-V machines is largely driven by the electrical loading of the machine at multi-MW power levels, the scope for further improvement is small [23]. So if comparing for the same grid power between these two types of machines, the SPM-V machine will have to be designed for larger generator power to offset the excess converter losses.…”