Calculation of the electromagnetic parameters of a switched reluctance motor using an improved FEM-BIEM. Application to different models for the torque calculation

Abstract

This paper expands on a hybrid numerical method coupling the finite element method (FEM) and the boundary integral equation method (BIEM) developed to calculate magnetic fields in a switched reluctance motor (SRM). This hybrid approach is a solution to the issues stemming from the small size and odd shape of the SRM airgap, where meshing a FEM grid is difficult. In the past, the stator and rotor FEM domains kept their actual shapes, which resulted in problems with the definition of the unit outward normal vector n~ at the corner points of the SRM poles. So, the normal derivative of the magnetic vector potential, that is the tangential component of the flux density, was undefined at these points. In this paper, a new approach with new definitions of the stator and rotor FEM domains is presented. The new concept defines the SRM airgap as a ring in which circular boundaries permit the correct definition of the normal derivative quantities and exploits the natural Neumann condition. The electromagnetic parameters of a 7.5 kW SRM prototype machine were calculated using this new approach. They compare favorably with test data. Phase magnetization characteristics also have good precision and smoothness. These electromagnetic parameters were then used to assess and compare two different SRM models for the calculation of the torque on the motor shaft: the linear trapezoidal model; and the nonlinear model. The comparison between the measured torque and the torque as calculated with the two different models shows that the more simple trapezoidal model should be satisfactory in many cases, and could in particular be used in automated SRM designs.