Design of a brushless PM starter-generator for low-cost manufacture and a high-aspect-ratio mechanical space envelope

This paper presents results from a design study into a brushless PM starter-generator. The automotive application demands a low-cost solution within a highly constrained mechanical space-envelope. Material costs are reduced through the use of ferrite permanent magnets and aluminium conductors. Design challenges include the selection of an appropriate machine topology to realise a high specific output given the lower performance materials and managing the additional losses associated with the higher resistivity of an aluminium winding. The problem is further exacerbated by the demanding space-envelope, operating requirements and the necessity of a design suited to volume manufacture. The packaging arrangement suits an internal-rotor radial-flux topology and the limited mechanical space-envelope means that the aspect ratio of the stator outer diameter to the machine active length is high. The design approach outlined in the paper combines numerical electromagnetic analyses with a lumped parameter thermal model and is calibrated with tests on machine subassemblies. The methodology allows for a more informed design process, where the manufacture and assembly nuances affecting the performance of the starter-generator are identified and accounted for prior to the prototyping of a complete machine assembly.