Extensions to the finite element method for the electromechanical analysis of electric machines

Abstract

This thesis deals with the electromechanical analysis of induction machines using the finite element method. The primary objective is the computation of magnetically induced forces and force distributions in the machine that produce vibration and noise, given the geometry, winding layout, material characteristics, and source voltage waveform of the machine. Contributions are made in two general areas: the computation of forces from finite element solutions for electromagnetic fields, and the modelling of electric machines using finite element analysis.

Then the model is used to study the effects of rotor eccentricity, parallel connection of stator poles, magnetic saturation, and stator slot closure on vibration inducing forces and force waves. The finite element-based model provides the first opportunity to model these effects in quantitative detail. The forces and force waves produced by rotor eccentricity are determined, and the reduction of these forces by XIV parallel pole connection is demonstrated. Detrimental force waves produced by magnetic saturation are calculated. Stator slot closure is shown to reduce force waves due to slotting, but increase force waves due to saturation.

The modelling of electric machines is performed using a transient, finite element model of induction machine electromechanical dynamics. The airgap flux density and force distribution data produced by the model are decomposed into sinusoidal travelling waves in space and time using a two-dimensional Fast Fourier Transform technique. The modelling method is verified using a base case study of an induction motor by comparing the computed travelling waves with those predicted by established theory.

The investigation of force computation from finite element field solutions concerns both global (net) forces and force distributions (sets of local forces). The difficulties of global force computation are analyzed, and an original method for determining local forces is introduced. The method is specifically adapted for finite element analysis, and can be used to compute magnetically-induced forces within or on the surface of linear or nonlinear magnetic material.