Ac voltage controller

AC VOLTAGE
CONTROLLER
JOSIN HIPPOLITUS A, ASSISTANT PROFESSOR, SRM UNIVERSITY

INTRODUCTION
AC Voltage controllers are thyristor based devices which convert fixed alternating voltage
to variable alternating voltage without a change in frequency
Since AC Voltage controllers are phase controlled devices, thyristors and TRIACs are line
commutated.
The main disadvantage of ac voltage controllers is the introduction of objectionable
harmonics in the supply current and load voltage waveforms, particularly at reduced output
voltage levels.
Applications:
Domestic and Industrial heating, Transformer tap changing, lighting control, speed control
of single phase and three phase ac drives, starting of induction motors.
AC Voltage controls are adaptable for closed loop control systems.

CONTROL STRATEGIES
Two control strategies are used to control the power flow in ac voltage regulators.
a) Phase Control:
The switching device is so operated that load gets connected to ac
source for a part of each cycle of the input voltage.
b) Integral cycle control:
Switching on the supply to the load for an integral number of cycles
and switching off the supply for a further number of integral cycles.

1 PHASE AC VOLTAGE
CONTROLLERS WITH R LOAD

1 PHASE AC VOLTAGE
CONTROL USING RL LOAD

GATING SIGNAL
REQUIREMENTS
In a half wave controller, a pulse signal is enough to trigger the thyristor. Whereas in
a full wave controller with RL load a pulse signal is not enough.
This is because when thyristor T2 is triggered at π+α, the current through the
thyristor T1 is non-zero which prevent the thyristor from turning off. At β+α, the
current through the thyristor T1 becomes zero. But at this time the pulse signal in T2
for firing would have been zero if it is a pulse signal.
Hence a continuous gating signal is used in case of RL loads.

Ac voltage controller

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