Design and development of axial-field air-cored brushless DC motors

Abstract

This thesis is concerned with the principle and operation of axial-field brushless dc motors. It also describes the development of a brushless dc drive system which consists of three system elements: a disc-motor, an electronic power regulator, and a microprocessor-based controller. The principle of axial-field machines is discussed, and attention is given to the effect of the air-gap flux distribution on the emf waveform. By controlling the flux distribution, the induced emf is optimised for inverter-fed operation. The aim of the optimisation is to increase the motor's power density, and to simplify the interfacing between the control electronics and the motor. The designs and operations of three prototype motors are described, and certain problems relating to brushless dc motors, and to disc-motors in particular are discussed. These problems include undercommutation, and the effect of the drive configuration on the armature current. The design of the electronic power regulator and the selection of a suitable pulse-width modulation < r*WM) strategy for current control are presented. The features of the 3-phase 4-quadrant regulator, which capitalised on the special characteristics of the disc-motor, include the use of power MOSFETs as the PWM devices, and the use of an inverter bridge of which only the bottom-half is PWMed. A model of the switching regulator is also presented. The microprocessor-based controller sub-system controls the commutation sequence and the switching pulse-width of the power regulator to provide a constant torque output from the drive system. Both the commutation and the pulse-width controls are implemented by using the look-up table technique. The commutation signals are derived from a specially developed rotor position detector which can be used to provide automatic commutation advancing