Development of high force dense linear generators for wave energy converters

Abstract

The main concern of this thesis is the development of force dense linear generators for a Direct Drive Wave Energy Converter. Linear machines for direct drive power take-off systems are required to deliver very high force in order to harness the significant amount of power from the low velocity oscillation of an ocean wave. Therefore, the linear Vernier Hybrid Machine is investigated for its simple design structure and high force density at low speed, due primarily to the inherent magnetic gearing. Attention is focused on improving the performance of the existing linear Vernier Hybrid Machine and developing new variant topologies with higher force density. An improved E-core stator design, optimised permanent magnet dimensions and new segmented translator structure have been proposed which improve the machine performance in terms of mass and magnet utilisation. The implementation of a pole shifting method is shown to provide a significant reduction in the cogging force. Two cylindrical variants with three-dimensional flux paths are also developed from the improved E-core Vernier Hybrid Machine, which further improves the force with similar magnet mass and current density. Furthermore, a new combination of Halbach magnets arrays and Consequent Pole topology are employed in the flat E-core structure, known later as Halbach Consequent Pole Vernier Hybrid Machine, which significantly improves the flux density by reducing the inherent pole-to-pole leakage and thus further improve the force density and power factor of the machine. A cylindrical variant of the flat Halbach Consequent Pole topology has been designed and analysed to prove the performance improvement of the cylindrical versions compared to the flat. The flat Halbach Consequent Pole and two small scale cylindrical variants of the E-core Vernier Hybrid Machines have been built and tested in the laboratory. The flat prototype is built from laminated steel and both the cylindrical machines are made of Soft Magnetic Composites to allow the three-dimensional flux path. All the experimental results are shown to provide good agreement with the static and dynamic generator performance predictions. Finally, this thesis compares the performance of three flat and three cylindrical Vernier Hybrid Machine topologies for various axial lengths and air-gap diameters and investigate the feasibility of using them for a wave energy device