The 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