Design of a permanent magnet axial flux high-speed generator

Abstract

Electrical generating sets powered by gas turbines are required for many applications, in particular for emergency situations due to their critical attributes; high reliability, lightweight, small size, multi-fuel capabilities, low maintenance, low noise and low gas emissions. This research contends that a permanent magnet axial flux (PMAF) high-speed generator with a small gas turbine engine offers advantages over the radial flux permanent magnet generators. Higher power densities can be achieved with the axial flux configuration when compared to their counter parts of the radial flux machines of similar output power. The attributes of the PMAF machines were certainly appealing; lightweight, small size, high efficiency and ease of construction. In this research, a design approach for the PMAF high-speed generator which accounts for the mechanical and electrical aspects was provided. The machine's key components such as retainment ring was carefully designed and the materials utilised in their structures were appropriately selected to insure high mechanical integrity, ease of construction and low manufacturing cost. The generator's principle dimensions were determined from a theoretical model which was derived from the machine's main design parameters. This theoretical model was then correlated by some empirical coefficients determined through the manipulation of the experimentally validated finite element (FE) results. The analytical results have shown that with the appropriate design considerations, PMAF high-speed generators can be designed with high power densities in the range of 6-8 kW/kg and high efficiencies ideally in the range of 94 - 96 %. The mechanical integrity and the steady state electrical performance of the machine were analysed using three-dimensional (3D) FE models. More in this research, a parametric study was carried out on the most influential parameters of the machine to improve its electrical performance through minimise rotor and stator eddy current losses. In addition, the total harmonic distortion in the output waveform was minimised through the appropriate and careful design of the magnet shape and topology with the aid of 3D electromagnetic FE analysis. Furthermore, using FE it was possible to design, optimise and analyse the rotor back-iron disc through the selection of best material, shape and size for use in the PMAF high-speed generator. A prototype of the PMAF high-speed generator was constructed and tested preliminary at low speed for the purpose of the evaluation of the electrical performance of the machine. Experimental results have shown that the machine was capable to meet the design requirements. For the mechanical integrity of the machine, the rotors were safely tested on a cold run test rig at the speed of 47,000 rpm. This thesis describes also the trends and the technical details in the manufacturing, construction and experimental setup for the PMAF high-speed generator.