System-level investigation of multi-MW direct-drive wind power PM vernier generators

Surface mounted permanent magnet Vernier (SPM-V) machines are known for their high torque density but relatively poor power factor compared to conventional SPM machines. The high torque density feature of the SPM-V machines is desirable for direct-drive offshore wind power applications as it leads to reduced generator size, mass and cost. However, their poor power factor can negatively affect the converter cost and efficiency. This paper compares the system-level performance, including generator active and structural components and converter, between the SPM-V and the conventional SPM generator systems. Four different power ratings, i.e. 0.5MW, 3MW, 5MW and 10MW, have been considered to study the trend of system-level performance with increasing power rating. The study shows that the SPM-V generators can be lighter and cheaper than their conventional SPM counterparts. However, after the consideration of converter cost and efficiency, the conventional SPM generator exhibited slightly better overall performance. Nonetheless, with the development of novel Vernier topologies and reduction in converter costs in the future due to emerging technologies, the Vernier generators can still be competitive for direct-drive offshore wind power applications

Effect of airgap length on electromagnetic performance of surface mounted permanent magnet Vernier machine

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 SPMV 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 machines for direct-drive offshore wind power: benefits and challenges

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

Space harmonic cancellation in a dual three-phase SPM machine with star-delta windings

This paper proposes a dual 3-phase SPM machine with star-delta windings. The machine is based on the widely adopted 12s/10p configuration and employs a number of methods to suppress the two largest core loss causing MMF harmonics. Firstly, stator shifting is used to reduce the 7th order harmonic, which is then completely suppressed through the use of a second 3-phase converter operating at a 15° phase shift with regard to the first one. Secondly, each of the winding sets employs 3-phase star-delta winding that are able to completely cancel the 1st order MMF harmonic. Additionally, these methods increase the amplitude of the torque producing harmonic and so yield a machine with better torque performance than a conventional 3-phase machine. Analytical modelling is used to demonstrate the cumulative impact on winding MMF harmonics. FEA is then used to validate the analytical predictions and to investigate other machine performance. The proposed machine demonstrates comparable average torque and efficiency to a 12-slot dual 3-phase machine, but offers a substantial reduction in torque ripple and PM eddy current losses. This helps improve the machine performance and reduce the risk of thermal demagnetization. A prototype machine has been manufactured and EMF and static torque measurements validate the expected performance of the proposed machine

Scaling effect on electromagnetic performance of surface mounted permanent magnet Vernier machine

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

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

This study investigates the scaling effect on power factor of surface mounted permanent magnet Vernier (SPM-V) machines with power ratings ranging from 3 kW, 500 kW, 3 MW to 10 MW. 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 two-dimensional finite-element analysis 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. (i) magnet leakage flux due to large coil pitch to rotor pole pitch ratio, (ii) magnet inter-pole leakage flux and (iii) stator slot leakage flux, reduces the ratio of armature flux linkage to permanent magnet 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

Effect of airgap length on electromagnetic performance of permanent magnet Vernier machines with different power ratings

This paper 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 3kW machine as an example, its performance is comprehensively compared with a conventional surface mounted permanent magnet (SPM) machine with the same airgap lengths using 2D Finite Element Analysis (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. 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. () (ratio of rotor pole pitch to electromagnetic airgap length). The results show that for 3kW machines, ()>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. 3MW and 10MW, the criterion ()>2.2 still results in achieving a performance closest to their optimal capability. However, when ()>2.2, special consideration should be paid to avoid potential irreversible magnet demagnetization at multi-MW power levels

AC losses in form-wound coils of surface mounted permanent magnet Vernier machines

Surface mounted permanent magnet Vernier (SPM-V) machines, because of their high torque density and multi-pole structure, are promising candidates for low speed direct-drive applications. To achieve high torque density, the SPM-V machines are generally designed with high gear ratios. Therefore, their operating frequencies can be much higher than those of the conventional SPM machines. This potentially increases the alternating current (AC) winding losses, especially those with form-wound coils proposed for high power applications. This paper investigates the AC losses (including skin effect, proximity effect, rotor PM induced and circulating current losses) in form-wound stator coils of a 3 MW direct-drive SPM-V machine with different slot/pole number combinations. The study reveals that the SPM-V machines have significantly higher AC winding losses than their conventional SPM counterparts for similar operating conditions. To reduce the AC winding losses in SPM-V machines, a novel flux shunt concept is proposed along with other conventional techniques such as increasing the number of turns/coil (with terminal voltage being kept constant) and parallel strands/turn, providing extra clearance at slot opening, etc. With the loss reduction techniques implemented, the SPM-V machines can achieve comparable efficiency but much higher torque density than their conventional counterparts. Moreover, the proposed flux shunt was also found to be very effective in reducing the potential risk of permanent magnet (PM) irreversible demagnetization, a key issue for SPM-V machines at high power ratings