Transverse Flux Permanent Magnet Machines for Wind Power Application

The high torque density of a transverse flux permanent magnet (TFPM) machine makes it a promising solution for the mass and cost reduction in direct-drive wind power application. However, its poor power factor has detrimental effects on the power converter rating and overall cost of the direct-drive system. Therefore, it is interesting and valuable to evaluate the feasibility of high torque but poor power factor in the TFPM machine for wind power application while considering several other electromagnetic characteristics. Initially, the phenomenon of magnetic coupling is observed between adjacent phases due to flux leakage in axially stacked multiphase TFPM machine topologies at MW and kW power levels. This causes asymmetric and unbalanced phase electromotive forces (EMFs) and torques, consequently, lowering the total three-phase torque in the TFPM machines. The intensity of magnetic coupling is found to be dependent on the distance and type of material between neighbouring phases, the level of magnetic saturation, and the polarities of magnets in the TFPM machines. Furthermore, a small-scale prototype has been built and tested to verify the magnetic coupling and clearly explain the manufacturing complexities associated with the TFPM machine. Then, the effect of the air gap length variation on the electromagnetic performance of surface-mounted TFPM (SMTFPM) machines in comparison to the conventional surface-mounted magnet (SPM) machines is comprehensively examined for both power levels. It is found that the air gap lengths larger than threshold levels are disadvantageous to the MW and kW TFPM machines, respectively, making their average torque, torque/mass ratio, and torque/cost ratio lower than those of the SPM machines. Finally, the 3-dimensional finite element method (FEM) predicted electromagnetic performances of various optimized TFPM machine topologies are summarized and compared with the benchmark SPM machines under the same specific conditions for both power ratings. The highest torque produced by the MW SMTFPM machine among all TFPM machines is higher than that of the SPM machine with a relatively poor power factor. Besides, the TFPM machines are less efficient with high losses, decreased torque density, and low torque/mass and torque/cost ratios. On the other hand, the kW TFPM machines perform better than the MW TFPM machines because of lower flux leakage, insignificant magnetic saturation, and smaller electrical loading. The performance of the kW TFPM machine is comparable to the SPM machine if magnetic coupling and manufacturing complexities are ignored.