APPLICATION OF SAMPLING TECHNIQUES TO THE PHASE-CONTROLLED THYRISTOR CYCLOCONVERTER

Abstract

Low frequency distortion components of the output voltage of a cycloconverter are largely responsible for the restriction on its practical range: of frequencies, and the object of this thesis is to show that these components can be attenuated by the application of sampling techniques to the control system. After a general description of the operation and control of the cycloconverter, the distortion of the output waveform due to low frequency components is discussed. Under these circumstances, the fundamental repetition frequency of the waveform is less than the wanted output frequency, and two methods of determining it for given input and output frequencies are developed. The characteristics and properties of the low frequency distortion components, and the requirements for attenuating them are analysed. The particular effects on the magnitudes of these components due to operation of the cycloconverter in the inhibited mode, rather than the circulating-current mode, are examined and the requirements for attenuating them are identified. It is shown that the communications engineering processes of pulse width modulation and of natural sampling can be identified in the control of the cycloconverter. Regular sampling is more widely used in communications engineering, and its effect on the low frequency distortion components in the cycloconverter output is compared with natural sampling. A modified control method for the inhibited cycloconverter is then developed to attenuate these components. Digital computer programs were written to test the effect of introducing modifications to the control of the cycloconverter, and the more significant results are given in graphical and tabulated form. An experimental cycloconverter, with an inhibition control circuit designed for this project, was constructed to check the validity of the computer programs. The design details are described, and the experimental results are discussed.

Date of Award	1976
Original language	English
Awarding Institution	University of Plymouth