Modulation of polyphase inverters

Modulated inverters are used to produce a variable AC output from a fixed DC power supply, for such purposes as the speed control of AC motors and emergency power. Previous work on inverter modulation has concentrated on the development of techniques applicable to single and three phase inverters. The current research effort is directed at the conversion of existing analogue modulation techniques to a form suitable for digital technology. This thesis seeks to extend the study of inverter modulation to the broader class of polyphase inverters, and to develop a digital modulation technique specifically designed for polyphase inverters. The basic principles of inverter modulation are discussed, from which it is concluded that for the output of an inverter to be of most use, it must have a variable fundamental amplitude and a low harmonic content. To understand the behaviour of harmonics in polyphase systems, a set of mathematical transformations is established which reduces the analysis of harmonics to that of a single phase system. Using these transformations, any polyphase inverter modulation scheme can be designed and analysed by considering only the conceptually simpler single phase case. An additional investigation is made into the effect of inverter configuration on the level of total harmonic distortion present in the inverter output, and a quantitative expression of the minimum total harmonic distortion achievable for any inverter configuration is given.