Abstract: Power delivered to a load by a three-phase supply is controlled by six thyristors which are controlled by a control signal generator comprising a programmed microcomputer (2), a zero-crossover detector (3), a clock pulse generator (6) and a counter (7) so that the thyristor conduction angles are determined by a digital signal applied to an input (17) of the microcomputer. In order to obtain a fast response to changes in the supply frequency and to take into account any unbalance between the supply phases, the microcomputer repeatedly measures the actual length of each nominally 60.degree. portion of a complete period of the supply by starting the counter at a predetermined point within each portion, the counter subsequently being stopped by the output of the detector when the next zero-crossover occurs and then being read by the microcomputer.

Abstract: A waveform crossing detector circuit for a pair of waveforms of a three-phase AC supply applied to respective input terminals (7,8,9). The circuit includes a pair of opto-couplers (1,3; 2,4) for isolating an output terminal (21) from the supply. In order to obtain an accurate indication of the instants of crossing the light-emitting diodes (1,2) of the couplers are driven by respective transistors (5,6) which act as the actual crossing detectors. The collector circuits of these transistors are driven from the third phase, the potential of which is at a maximum when the relevant instants occur. Two further such arrangements may be connected to the taps (22,23,24) of an input potential divider in a cyclical manner and the output signals of the three resulting arrangements combined by means of logic gates to produce pulses which occur once every 60.degree. of the input waveform.