Wide range speed control of A.C. machines

• Main Author: Martin, D.
• Published: University of Bath 1983
• Subjects: 621.31042; Electronic devices & electromechanical devices
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.347614

This thesis is concerned with the development of an a.c. drive suitable for directly driving the rollers of a glass conveyor at low speeds. A variable frequency three phase current source inverter has been developed, which operates on the slitwidth technique and uses power transistors as the switching elements. Open loop control of the machines was investigated initially but was rejected, due to the inadequate speed regulation obtained. Consequently, a digital speed and position measuring transducer was developed using an inexpensive, commercially available slotted disc. Strategies for controlling a.c. machines were then considered. This resulted in the development of a system which allowed a synchronous machine to be controlled with constant torque angle and an induction machine to be controlled with constant slip frequency. These control strategies allow maximum torque to be obtained from the machines over their full speed range. The synchronous machine becomes self starting and cannot lose synchronism even when large loads are applied. The control strategies developed also allow both the synchronous and induction machines' output speeds to be frequency locked to a demand input. A microprocessor interface and appropriate software was then developed to apply ramp demand speed profiles to the machine, simulating the speed profiles required by the conveyor drive. This was followed by an investigation into the speed matching of drives of similar and different types. The frequency locking nature of the control strategies allowed excellent speed matching between drives to be achieved. The same machine was used as both the synchronous and induction machine, enabling a comparison between the two different machine types to be drawn. Both machine types produced similar maximum torque outputs and speed responses. Consequently, the inexpensive and easier to control induction machine is suggested for use as the conveyor drive.