Fractional Slot Concentrated Winding Machines for use in Offshore Wind Turbine Generators

This thesis investigates methods to improve the effectiveness of machines equipped with fractional slot concentrated windings (FSCW) for use in offshore wind turbine generators. These machines offer many potential advantages over conventional integer slot winding (ISW) machines but suffer from a large number of armature magneto-motive force (MMF) harmonics that induce excess eddy current losses in the rotor and PMs. Furthermore, surface-mounted PM (SPM) machines with FSCW have almost no machine saliency, making them incompatible with sensorless control schemes currently employed in offshore wind turbines. For FSCW to be an attractive alternative for offshore wind turbine generators, these downsides must be mitigated.
This work proposes a novel machine topology that utilises dual multiphase hybrid star-polygonal connected windings to eliminate unwanted MMF harmonics and improve machine torque performance and efficiency. In the particular case of a 3-phase machine, machines with a coil pitch of 1 and 2 are proposed and studied in detail. For the machine with a coil pitch of 1, all harmonics that are not torque producing either directly or through modulation by the stator teeth are eliminated. This results in greater torque performance, flux weakening capability, and efficiency than comparable concentrated winding machines. For a coil pitch of 2, the proposed winding structure is able to completely eliminate both the parasitic 1st subspace harmonic and the largest loss-causing higher order space harmonic such that the rotor and PM eddy current losses are substantially reduced. These machines are also scaled to 3MW and found to be competitive with the conventional ISW topology presently employed in offshore wind turbine generators. Furthermore, the first method for dq-analysis of star-delta wound FSCW machines is developed which allows for these machines to be used with modern control techniques like field oriented and sensorless control in the future. Finally, an investigation is carried out into improving the saliency of FSCW SPM machines in offshore wind turbines but it is found to be difficult to achieve without a notable reduction in machine electromagnetic performance. The results of this thesis are validated with analytical modelling, FEA simulations, and experimental measurements of a number of prototypes.