21-04-2014, 12:08 PM
AC VOLTAGE CONTROLLERS
AC VOLTAGE.ppt (Size: 836 KB / Downloads: 19)
Introduction
The power flow into a load can be controlled by varying the rms value of the load voltage.
This can be accomplished by thyristors, and this type of power circuit is known as ac voltage controllers.
The most application of ac voltage controllers are:
Industrial heating
On-load transformer tap changing
Light controls
Speed control of induction motors
AC magnet controls
Principle of On-Off Control
The principle of on-off control can be explained with the following single-phase full-wave controller.
This type of control is applied in applications which have high mechanical inertia and high thermal time constant.
Typical examples are industrial heating and speed control of motors.
If the input voltage is connected to load for n cycles and is disconnected for m cycles, the output load voltage is found from:
Principle of Phase Control
Due to the presence of diode D1, the control range is limited.
The rms output voltage can only be varied between 70.7 to 100%.
The output voltage and input current are asymmetrical and contain a dc component.
This circuit is a single-phase half-wave controller and is suitable only for low power resistive loads, such as heating and lighting.
Since the power flow is controlled during the positive half-cycle of input voltage, this type of controller is also known as unidirectional controller.