Abstract
In present-day variable speed a.c. drives the power system is usually of the D.C. link type. Here a diode bridge rectifier feeds a large capacitor to produce the D.C. link voltage, which in turn feeds an inverter and machine. In such a system, the input current waveform is severely distorted, degrading the power factor, and under regenerating operation of the machine the returned energy is usually dissipated in protection resistors. These drawbacks can be overcome by using active switches and PWM control in the front-end converter, which may then be called a REVERSIBLE RECTIFIER. With proper choice of control strategy power conversion between the mains and the machine, together with excellent waveform and power factor, can be achieved simultaneously. In this dissertation, various control methods have been fully studied and critically compared, together with discussions of the special problems of each control scheme. It is shown that direct control of the a.c. current, in the so-called current-forced schemes, offers good stability in control of the power flow. Voltage-forced schemes have also been studied for comparison. Conditions for stability in both rectifying and regenerating modes have been analysed. This has led to guidelines for selecting a suitable control strategy in practical applications and to a method of designing a suitable control system. The design and operation of a low power model reversible rectifier has been presented and two control systems have been built for implementing different control schemes. One control system is a simple analogue system which provides a cheap but still elegant approach to the control of reversible rectifiers. The other is by means of a transputer system, which allows the more refined schemes to be implemented and more importantly provides the possibility of integrated control of the reversible rectifier with that of the machine in the future. The experimental results obtained have shown to be entirely consistent with the theory.