AC-VOLTAGE REGULATORSM.KaliamoorthyPSNACET
AC VOLTAGE CONTROLLER CIRCUITS(RMS VOLTAGE CONTROLLERS)
AC voltage controllers (ac line voltage controllers) are employed to vary the RMS valueof the alternating voltage applied to a load circuit by introducing Thyristors between the load anda constant voltage ac source. The RMS value of alternating voltage applied to a load circuit iscontrolled by controlling the triggering angle of the Thyristors in the ac voltage controllercircuits.In brief, an ac voltage controller is a type of thyristor powerconverter which is used toconvert a fixed voltage, fixed frequency ac input supply to obtain a variable voltage ac output.The RMS value of the ac output voltage and the ac power flow to the load is controlled by
varying (adjusting) the trigger angle ‘
’
ACVoltageController V
0(RMS)
f
S
Variable ACRMSO/P Voltage ACInputVoltagef
s
V
s
f
s
There are two different types of thyristor control used in practice to control the ac power flow
On-Off control
Phase controlThese are the two ac output voltage control techniques.In On-Off control technique Thyristors are used as switches to connect the load circuit to theac supply (source) for a few cycles of the input ac supply and then to disconnect it for few inputcycles. The Thyristors thus act as a high speed contactor (or high speed ac switch).
PHASE CONTROL
In phase control the Thyristorsare used as switches to connect the load circuit to theinput ac supply, for a part of every input cycle. That is the ac supply voltage is chopped usingThyristors during a part of each input cycle.
AC-VOLTAGE REGULATORSM.KaliamoorthyPSNACETThe thyristor switch is turned on for a part of every half cycle, so that input supplyvoltage appears across the load and then turned off during the remaining part of input half cycleto disconnect the ac supply from the load.
By controlling the phase angle or the trigger angle ‘
’ (delay angle), the output
RMSvoltage across the load can be controlled.
The trigger delay angle ‘
’ is defined as the phase angle (the value of
t) at which thethyristor turns on and the load current begins to flow.Thyristor ac voltage controllersuse ac line commutation or acphase commutation.Thyristors in ac voltage controllers are line commutated (phase commutated) since the inputsupply is ac. When the input ac voltage reverses and becomes negative during the negative half cycle the current flowing through the conducting thyristor decreases and falls to zero. Thus theON thyristor naturally turns off, when the device current falls to zero.Phase control Thyristors which are relatively inexpensive, converter grade Thyristorswhich are slower than fast switching inverter grade Thyristors are normally used.For applications upto 400Hz, if Triacs are available to meet the voltage and currentratings of a particular application, Triacs are more commonly used.Due to ac line commutation or natural commutation, there is no needof extracommutation circuitry or components and the circuits for ac voltage controllers are very simple.Due to the nature of the output waveforms, the analysis, derivations of expressions forperformance parameters are not simple, especially for the phase controlled ac voltage controllerswith RL load. But however most of the practical loads are of the RL type and hence RL loadshould be considered in the analysis and design of ac voltage controller circuits.
TYPE OF AC VOLTAGE CONTROLLERS
The ac voltage controllers are classified into two types based on the type of input acsupply applied to the circuit.
Single Phase AC Controllers.
Three Phase AC Controllers.Single phase ac controllers operate with single phase ac supply voltage of 230V RMS at50Hz inour country. Three phase ac controllers operate with 3 phase ac supply of 400V RMS at50Hz supply frequency.Each type of controller may be sub divided into
AC-VOLTAGE REGULATORSM.KaliamoorthyPSNACET
Uni-directional or half wave ac controller.
Bi-directional or full wave ac controller.In briefdifferent types of ac voltage controllers are
Single phase half wave ac voltage controller (uni-directional controller).
Single phase full wave ac voltage controller (bi-directional controller).
Three phase half wave ac voltage controller (uni-directionalcontroller).
Three phase full wave ac voltage controller (bi-directional controller).
APPLICATIONS OF AC VOLTAGE CONTROLLERS
Lighting / Illumination control in ac power circuits.
Induction heating.
Industrial heating & Domestic heating.
Transformer tap changing (on load transformer tap changing).
Speed control of induction motors (single phase and poly phase ac induction motorcontrol).
AC magnet controls.
PRINCIPLE OF ON-OFF CONTROL TECHNIQUE (INTEGRAL CYCLE CONTROL)
The basic principle of on-off controltechnique is explained with reference to a singlephase full wave ac voltage controller circuit shown below. The thyristor swi