Ultra-high efficiency in electrical machines for hybrid and all-electric propulsion

This thesis reports on a study into the efficiency of electrical machines with a particular focus on MW scale machines for aerospace propulsion. It considers several aspects of machine design and performance including aspects which become more prominent as the rating of the machine increases. The thesis establishes the aircraft level trade offs between the efficiency and mass of electrical machines and demonstrates that there are significant benefits in terms of aircraft range from marginal improvements in efficiency even if this is gained at the expense of an increase in mass. The thesis reviews several aspects of the efficiency of electrical machines including a description of the various loss mechanisms in electrical machines.
A detailed design study is described on a 2.5MW aerospace propulsion machine which considers various machines with different winding types, slot and pole combinations. This design study highlights the increasing role that core loss has on efficiency as the power rating increases. This design study established a preferred pair of designs with efficiencies >99%. The preferred designs from this design study are analysed in detail using finite element analysis in combination with a custom post-processor which considers several aspects of loss performance, including localised flux density waveforms and the extent of rotational fields in the stator core. This analysis culminates in an investigation into the benefits of incorporating cooling ducts into the stator at locations that correspond to high loss or high degree of rotationality.
A comprehensive set of experimental measurements of the properties of Cobalt-Iron are presented and discussed. These are the variation in electrical conductivity with temperature, the degree of anisotropy in specific loss with angle from the rolling direction and the temperature dependency of core loss which culminated in a set of temperature dependent core loss coefficients. The thesis concludes by summarising many of the key findings and identifying topics for further research