The output frequency spectrum of a thyristor phase-controlled cycloconverter using digital control techniques
The principle of operation dictates that the output of a cycloconverter contains some harmonics. For drive applications, the harmonics at best increase losses in the motor and may well cause instability. Various methods of analysing the output waveform have been considered. A Fortran 77 program employing a modified Fourier series, making use of the fact that the input waveforms are sinusoidal, was used to compute the individual harmonic amplitudes. A six pulse three phase to single phase cycloconverter was built and a Z-80 microprocessor was used for the control of firing angles. Phase locked loops were used for timing, and their effect upon the output with changing input frequency and voltage were established. The experimental waveforms are analysed by a FFT spectrum analyser. The flexibility of the control circuit enables the following investigations not easily carry out using traditional analog control circuit. The phase relationship between the cosine timing and reference wave in the cosinusoidal control method was shown to affect the output waveform and hence the harmonic content. There is no clear optimum value of phase and the T.H.D. up to 500Hz remains virtually constant. However, the changes of individual harmonic amplitudes is quite significant. In practice it may not be possible to keep the value of phase constant but it should be considered when comparing control strategies. Another investigation involves the changing of the last firing angle in a half cycle. It shows that the value of firing angles produced by the cosinusoidal control method is desirable. Operation at theoretical maximum output frequency was also demonstrated.