AC Voltage Controller (AC to AC)

Intorduction

An ac voltage controller is a converter that controls the voltage, current, and average power delivered to an ac load from an ac source.

Applications >> light-dimmer circuits and speed control of induction motors

How can we control the Lamp Luminance (brightness)?

By controlling the power delivered to the Lamp. once we can adjust input (voltage, current) we can control the amount of power (and so the Lamp Luminance) .

But How can we do that ?

1-what about Voltage Divider ?

Figure (1)

Disadvantage : Low efficiency , No automation !

2- What about transformer ?

Figure (2)

- High efficiency

- But low degree of flexibility (control), and also large size

Can we automate the control ?

>> 3- Switching Mode control

AC voltage controller Types

1- Phase Control

In this mode the input source is switched off from the load for some time and on for another during one cycle of input source so that the load rms voltage can be changed and so that the total average power delivered to the load can be controlled. this can be done by tiggering the circuit by a trigering signal at firing angle in the first half cycle and another firing angle in the negative half cycle as shown in Figure (3).

Figure (3)

2- Integral-Cycle Control

In this mode the input source is switched off from the load for N number of cycles and off for M number of cycles so that the total average power delivered to the load can be controlled as shown in Figure (4).

Figure (4)

Phase control for Resistive Load

Figure (5) show the resistive load phase control circuit AC source is switched by the help of two anti-parallel SCR thyristors in series with the load.

The circuit work as follow:

intiallay S1, S2 are off and no cuurent in the load.
In the positive half cycle a triggering signal is applied to the gate of S1 at firing angle 𝜶 and hence start to conduct (idealy being short circuit) and the current start to flow throw the load and again S1 is turned off when the current fall below its holding current (approximately at angle 𝝅).
The same for S2 in the negative half cycle is triggered at firing angle (𝜶+𝝅) and hence start to conduct (idealy being short circuit) and the current start to flow throw the load in negative direction and again S2 is turned off when the current fall below its holding current (approximately at angle 2𝝅).
and the steps is repeated for the next cycles as shown in Figure (5).