Phase Control of Induction Motor:
Figure 11.44 shows the schematic representation of 3 Phase Control of Induction Motor. Two thyristors are connected in antiparallel in each line. The firing sequence of the thyristors for all modes of operation is 1, 3′, 2, 1′, 3 and 2′ as shown in Fig. 11.45. The interval between successive firings is 60° and the firing frequency is six times the input frequency. It is seen from Fig. 11.45 that as 3′ is fired 1 and 3′ conduct for 60°; then 2 is fired and 3′ and 2 conduct for the next 60° and so on. Each
thyristor after being fired conducts for 120°. As long as a thyristor conducts the corresponding phase voltage will be known. When a phase gets open circuited, its voltage will be that induced due to currents flowing in the other two windings of the motor. As a result of phase control, the effective ac rms voltage applied to motor terminals decreases. This indeed is a method of voltage control.