A single-phase AC controller (voltage controller) is used to vary the value of the alternating voltage after it has been applied to a load circuit. A thyristor is also placed between the load and the constant source of AC voltage.
The root mean square alternating voltage is regulated by changing the thyristor triggering angle. In the case of phase control, the thyristors are employed as switches to establish a connection from the AC input supply to the load circuit during each input cycle. For every positive input voltage, chopping occurs and voltage is reduced.
During half part of the cycle, the thyristor switch is turned ON to enable the voltage input to appear across the load. This is followed by the OFF state during the last half cycle so as to disconnect the load from the source voltage.
When the triggering angle α is controlled, the RMS value of the voltage on the load is also controlled. The triggering angle α is therefore, defined as the value of ωt at which the thyristor switches ON.
When two or more sequence control stages are connected, it is possible to have an improvement in power factor and further reduction in THD (total harmonic distortion). An n-stage sequence control converter has n windings in the transformer secondary part with each rated es/n (the source voltage).
When two AC converters are placed parallel to each other, the zero sequence way is created. A little difference between the two converters causes a great zero sequence in circulating current. The diagram below shows the parallel system of a converter. The direction of the current is anti-clockwise with respect to that of the voltage system.