Theoretical design of surface-mounted permanent magnet motors with field- weakening capability

Abstract

This master thesis deals with the design of an permanent magnet (PM) motor for electric vehicles. An analytical model for surface mounted PM-motors (SMPM) is derived and verified with finite element analysis (FEM). Special attention is paid to the iron losses as they influence the performances. Today's motors for traction in electric vehicles are most often induction motors. In recent years, PM-motors have become interesting, as the efficiency can be increased. This is very important in battery applications. The first part of the project consisted of a literature study that aimed at building knowledge on machine design for field weakening applications. An analytical model for SPM-motors was deduced thereupon. An analytical model for the design of SMPM-motors was implemented in Matlab. It was verified with two-dimensional FEM calculations (Flux2D). It was noticed, that an optimal analytical design tool requires good means for predicting the iron losses. This is due to the fact that the iron losses form a significant fraction of the total losses in SMPMmotors and therefore have a big influence on the performances. Based on the results from the FEM analysis, an iron loss model for the stator teeth and the stator yoke was derived. This iron loss model is based on the description of the flux created by the magnets and the currents in the respective areas. The influence of the stator leakage is included as well. The iron loss model covers the complete operational range. An improved model attempting to include the leakage flux was also derived. Based on the computer program, some designs and the influence of certain parameters as the number of poles or the airgap length are discussed. The design that uses the stator of the induction motor that shall be replaced is of special interest. In addition, a compact design is presented.