Electromagnetic field analysis of induction motors by finite element method and its application to phantom loading

Abstract

A precise estimation of electromagnetic fields of skewed induction motors when high-order harmonic fields need to be considered continues to be a challenging problem for researchers. The thesis presents a time stepping finite element method (FEM) of common skewed rotor bar induction motors. The method being developed consists of two parts: (1) 2-D multi-slice circuit-torque-field coupled time stepping FEM model; and (2) 3-D eddy-current time stepping FEM model. In the 2-D model, the system equations will directly include the effects of saturation, eddy-current, non-sinusoidal quantities and the movements of the rotor. The end effects of the motors together with the external circuit conditions are considered collectively by coupling FEM with circuit equations. The torque equation is also coupled in the system equations so that the starting process of the motors can be simulated precisely. To estimate the inter-bar current loss, the system equations of 2-D FEM and 3-D FEM are solved simultaneously by using time stepping method. Hence the exact waveforms of currents, magnetic vector potentials and torque in the time domain can be obtained directly. In the thesis the following new methods are presented: (1) In time stepping FEM models, special techniques to reduce computing time and to increase computing accuracy are presented. The techniques developed will take into account the FEM mesh rotation, the solution of the system performance when torque equation is to be coupled, an automatic control of time step size within the program. An incremental method is also developed for finding the solution of steady-state performance of the motor. (2) New forms of the governing equations for a multi-slice model are derived. This approach allows the meshes of multi-slices to be regarded as one 2-D mesh so that the algorithm is very similar to that of general 2-D problems. The fields of the multi-slices are being solved en bloc simultaneously. (3) The 3-D eddy-current FEM of induction motors is solved by using time stepping method. A new iterative method for the solution of the 3-D complex FEM model is presented so that the computing time can be greatly reduced. (4) Formulas for the direct computation of iron losses to incorporate the non-sinusoidal characteristic of variables are presented. The developed FEM program, as a precise dynamic model of induction motors, has been used to predict the induction motor's behaviour in different operating conditions. In particular, it has been used to compute the starting process and to estimate the stray losses, which are difficult to be dealt with by using traditional methods for induction motors. It has also been used to simulate and assess the operation of phantom loading of induction motors. The results obtained by using the program being developed have very good correlation with the test data for a large number of cases studied.