Question

Distinguish between on-off control and phase control of ac voltage controller.

Answer 1

An on-off control is designed for any type of input signal. The thyristor switch establishes a connection between the load and the AC source for some cycles of the input voltage and forms a disconnection for few cycles of the same voltage.  

A phase control is meant for only sine wave signals. The thyristor switch sets up a connection between the load and the AC source for only part of each cycle of the input voltage.