Application of bi-state magnetic material to automotive offset-coupled IPM starter/alternator machine

This paper investigates a new approach to designing high-speed interior permanent-magnet (IPM) synchronous machines using a bi-state soft magnetic material. The bi-state material can have its normally high magnetic permeability permanently reduced in localized regions to that of air by means of heat treatment. This new work significantly expands a previous investigation by considering offset-coupled IPM machines that make it possible to significantly increase the rotor speed while retaining all of the other specifications of the 6-kW starter/alternator application. Lumped-parameter models, Monte Carlo optimization, and both electromagnetic and structural finite-element analysis are used to develop new offset-coupled IPM machine designs with the new material at speeds of 40 000 r/min or higher. Results from this work demonstrate that the bi-state material offers a promising approach for designing high-speed IPM machines that offer weight and volume advantages compared to their lower speed counterparts at comparable system cost.