Investigation of Stator Mounted Permanent Magnet Machines

This thesis investigates the electromagnetic performance of flux-switching permanent magnet (FSPM) machines within the context of electric vehicle (EV) drivetrains. It investigates the various aspect of design and electromagnetic behaviour of this type of machine which influence torque density, with a focus on the effects of scaling and magnetic saturation.
The thesis describes a series of design, scaling and optimisation studies, including benchmarking against equivalent sized surface-mounted permanent magnet (SMPM) machines. These studies demonstrate the influence of various design parameters on torque capability and the significant non-linearity of the torque versus current relationships in this type of machine due to the onset of significant magnetic saturation, even at modest electric loadings. It is also shown that FSPMs have relatively poor short-term overload capability compared to SMPM machines and that to achieve useful short-term overload capability it is necessary to optimise the design for this overload condition with an inevitable penalty in performance at the continuous operating point. The scaling studies demonstrate that particular attention must be paid to the selection of the split-ratio as the machine diameter increases, with a tendency for the optimal split ratio to increase in larger machines as magnetic saturation in the stator core tends to limit the benefits of increasing stator mmf. The various studies show that from a torque density perspective, FSPM have a lower torque density that an equivalent SMPMs particularly at higher electric loadings.
The nature of the magnetic saturation in two FSPM machines is explored in detail and various modifications to the stator geometry to reduce localised flux density are investigated. The thesis then explores the benefits of a machine based on a series of single-phase modules and demonstrates that although these are competitive on a torque per unit length basis, end-effects and the additional end-winding result in an uncompetitive solution overall. The issue of magnet eddy current loss is investigated using three-dimensional finite element simulations in the magnets and the benefits of different segmentations of the magnets are quantified. The thesis concludes be reporting on the build of a 150mm diameter prototype.