ABSTRACT
As an alternative to the adjustable gate control of phase-controlled cycloconverters described in Chapter 12, the switches can operate continuously, like diodes. This requires that the gate control signal must fully be on during the whole of one half cycle and fully off during the other half cycle. In effect the rectifier valves, usually thyristors, act as onoff switches for the whole of a half cycle. As in phase control, the commutation of the conducting switches occurs naturally. The output voltage waveform follows the profile of the ac supply voltage, as in Fig. 13.1, leading to the name envelope cycloconverter. In Fig. 13.1 there are three half cycles of the supply voltage, which defines the frequency ratio ωin/ωo =3. As with rectifier circuit operation an increase of pulse number reduces the voltage ripple and the higher harmonic content. In envelope cycloconverter operation it is invariably necessary to use some form of input transformer connection. Conduction in successive phases of the transformer secondary windings has to occur, with respect to the supply voltages, so as to normally provide an output voltage envelope that is approximately a stepped waveform of the desired output frequency.
