Q. Explain AC voltage controllers?

Common applications for these controllers are found in fan, pump, and crane drives. Figure shows three-phase symmetrical ac voltage-controlled circuits for wye-connected and delta-connected stators, in which the thyristors are fired in the sequence that they are numbered, with a phase difference of 60°. The four-quadrant operation with plugging is obtained by the use of the typical circuit shown in Figure. Closed-loop speed-control systems have also been developed for single-quadrant and multiquadrant operation. Induction motor starters that realize energy savings are one of the ac voltage controller applications. However, one should look into the problems associated with harmonics.