Low Frequency Modulation properties of Thyristor circuits

Load voltage waveforms corresponding to symmetrical phase-angle triggering and integral-cycle triggering in single-phase thyristor circuits, and also the waveform due to half-wave rectification, are all discrete forms of amplitude modulation. In each case the modulated output voltage is obtained from a sinusoidal (supply) carrier signal by use of a rectangular modulating function, dependent on thyristor switching.The use of amplitude modulation (AM) techniques for frequency conversion in high power applications is subject to certain severe restrictions of performance. These restrictions can be largely overcome by the use of phase modulation (PM) methods which involve two channels of AM in each electrical supply line.Appropriate waveforms may be realised by the use of controlled switching of thyristors. These are arranged in combinations of inverse-parallel connected pairs forming subtractor modulators. Certain thyristor commutation problems arise in PM systems at high power levels. These problems can be overcome by producing so-called AM/PM waveforms that combine the separate advantages of AM and PM systems.