Commutation of a Brushless Motor, Using Power Transistors

Abstract

The design and performance of a d.c. brushless motor and the application of power transistors to the commutation of the winding are considered in this thesis. The arrangement of a conventional d.c. motor is inverted and tappings from the commutated-winding are connected to bipolar switching circuits. The motor has a double-stator feature with identical windings in two lamination stacks. The field winding is also stationary. The interface between the motor and the power electronics comprises position sensors (opto-switches) connected to logic circuitry to decode the switching pulses. The supply voltage is provided by a thyristor bridge with control circuitry capable of performing IR compensation as well as starting the motor with an adjustable rate of rise of voltage. The brushless motor has shunt characteristics. Speed control is very smooth and the response to changes in the supply voltage is quick. Maximum output torque is about 8.5 Nm when operating at 200 V with an efficiency of about 55%. Successful operation of the motor depends largely on the angle between the pole and winding axes at switching. This angle is optimised for maximum developed torque and for minimum peaks in the supply current. Safe operation of the transistor is also dependent upon the switching angle in limiting the current peaks. The switching losses are very small and reliability is enhanced by additional components in the bipolar circuit. This work shows that a truly brushless d.c. motor can be satisfactorily operated with an electronic commutator consisting of power transistors at medium values of voltage and current.