Date of Award

Winter 1990

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

Charles K Taft


This work is concerned with the magnetic analysis of brushless DC motors, and, with the development of a method of designing brushless DC motors which have symmetric windings and rotors without any iron saliency. A two-dimensional nonlinear finite element magnetic model is used to analyze the magnetic behavior of the motor. Methods of using the finite element solution(s) to predict motor characteristic performance parameters, such as the torque-angle curve, detent torque-angle curve, torque constant, back emf constant and inductance are discussed in detail. It is shown that three-dimensional effects can make significant contributions to the motor inductance, making inductance prediction difficult. It is also shown that the prediction of detent torque can be extremely sensitive to the permanent magnet model.

A simple means of understanding the effects of rotor magnet arc width and winding distribution on the stator torque-angle curve shape is presented. In addition, methods of reducing detent torque by altering magnet arc width are discussed.

Finally, simple lumped models that allow one to predict motor performance characteristics as a function of important dimensions, magnet residual flux density, and phase current are developed and checked against finite element results. These models are used as a basis for an approach to designing brushless DC motors.