Synchronized scheme of continuous space-vector PWM with the real-time control algorithms

This paper describes in details the basic peculiarities of a new method of feedforward synchronous pulsewidth modulation (PWM) of three-phase voltage source inverters for adjustable speed ac drives. It is applied to a continuous scheme of voltage space vector modulation. The method is based on a representation of the pulse patterns as a function of the fundamental and switching frequencies of the drive system. Accurate trigonometric algorithms of vector PWM have been analysed, and also compared with the algebraic ones based on linear approximation of the durations of active switching states from their position inside clock-intervals. In order to provide smooth shock-less pulse-ratio changing and quarter-wave symmetry of the voltage waveforms, special synchronising signals are formed on the boundaries of the 60° clock-intervals. The process of gradual transition from continuous to discontinuous synchronous PWM at higher values of the fundamental frequency has also been described. Results of analysis of the spectral characteristics of the output voltage of the inverter show advantage of synchronous PWM in comparison with conventional asynchronous modulation at low indices of the frequency ratio between the switching and the fundamental frequency. Special attention has been given to the analysis and comparison of the computational effectiveness of the proposed algorithms of synchronized modulation.