Winding loss optimisation for high power density permanent magnet machines for aerospace propulsion

This thesis reports on an investigation into the winding losses of high power density brushless permanent magnet machines. The research is performed in the context of a design study for a 350kW, 6000rpm machine for aerospace propulsion with an emphasis on methods for increasing the electric loading that can be sustained in the stator. A particular focus of the research is the use of solid bar conductors although this type of conductor tends to give high level of AC losses if not properly designed.
A design study is undertaken to establish a machine design capable of meeting the 350kW specification which includes initial sizing and sensitivity studies on various design features and dimensions. This yields a design based on a large cross-section solid bar winding, which on the basis of quasi-static considerations, was capable of meeting the specification. However, consideration of AC losses in the stator winding caused by induced eddy currents result in almost an order of magnitude increase in conductor loss which would be unsustainable in practice.

A series of strategies to reduce AC loss were investigated specifically the re-positioning of conductors, magnetic slot-wedges, paralleling and transposition, consideration of aluminium and hybrid copper / aluminium windings and the optimisation of core geometry to alleviate flux in the stator slot. The optimum combination of these strategies result in a near 7-fold reduction in the conductor loss, making a solid bar winding competitive with a traditional wound machine in terms of loss while retaining the heat-transfer advantages.

The role of Litz wire as an alternative conductor is investigated in detail, including a comparison in both two- and three-dimensions of various methods for calculating eddy current losses in Litz. Machine level simulations of the reference design equipped with Litz wire are reported and comparison drawn with bar conductors.
The thesis culminates with a series of experimental measurements on a motorette for both strip wound bar conductor and Litz wire, which provide a level of validation for the simulations and highlight the challenges of measuring AC loss in Litz winding in representative stator cores.