Thiagarajar College of Engineering
Department of EEE

Power Electronics Virtual Laboratory

AC voltage controllers are thyristor based devices which convert fixed AC directly to variable AC without a change in frequency.
Thyristor based voltage controllers have high efficiency, flexibility in control and compact size.
The thyristors will be naturally commutated by the AC supply so external commutation is not required.
There are majorly two types of control methods used in AC voltage controllers
1. Phase Control (more common)
2. Integral Cycle Control

Half wave voltage controller	Full wave voltage controller


It has one SCR and one diode. So anyone of the half cycles can only be controlled.	It has two SCRs so both the half cycles can be controlled.
Since both the half cycles are not symmetrical, a dc component is introduced, which is undesirable.	DC component will not be introduced if the firing angles of both the SCRs are equal.

Integral Cycle Control

Integral cycle control consists of switching on the supply to load for an integral number of cycles and then switching off the supply for a further number of cycles
This control technique is used in applications where mechanical time constant or thermal time constant is of the order of several seconds.
For example, mechanical time constant for many of the speed-control drives or thermal time constant for many of the heating load is usually quite high. For such applications, almost no variation in speed or temperature will be noticed if control is achieved by connecting the load to source for some on-cycles and then disconnecting the load for some off-cycles
The equation of output RMS voltage is given below.

where, n indicates the number of on-cycles, m indicates the number of off-cycles and V_s is the RMS value of supply voltage.

Other possible configurations of AC voltage controllers

Note: The below are the other configurations of phase controlled full wave AC voltage controller.

This configuration is suitable for low-power applications where the load is resistive and has only a small inductance.

This configuration uses 4 power diodes and 1 SCR.
In the positive half cycle, diodes D1, D3 and SCR T1 will conduct. During negative half cycle, diodes D2, D4 and SCR T1 will conduct.
This configuration is cheaper as only one SCR is used (only one firing pulse circuitry is required)
The disadvantage of this configuration is that, at any time, 3 devices (2 diodes and 1 SCR) will be conducting. This causes more voltage drop.