Use of sampling ambiguity in convertor firing control systems

In a previous paper it was demonstrated that considerable reduction in the harmonic content of a circulating current cycloconvertor could be obtained by regularly sampling the reference waveforms which control the firing of the thyristors. The paper describes how this sampling process also allows the reference waves, needed for a cycloconvertor with a multiphase output, to be produced from a single sinusoidal waveform. The difficulty of producing a polyphase set of sinusoidal reference waveforms balanced in amplitude and equally displaced in phase is thus eliminated. This is of particular advantage when the amplitude andy frequency of the reference waveforms are to be voltage controlled. The paper demonstrate show this principle may be extended to the general design of multiphase signal generators. Methods are described for determining the frequency, the phase displacement and the phase sequence of sampled signals. The paper concludes that the introduction of regular sampling in cycloconvertors permits the use of a high-frequency reference waveform in the place of low-frequency waveforms and the use of a single-phase wave generator instead of the usual 3-phase waveform generator with a consequent saving in the cost of control equipment.