Sunday, April 19, 2009


AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value of the alternating voltage applied to a load circuit by introducing Thyristors between the load and a constant voltage ac source. The RMS value of alternating voltage applied to a load circuit is controlled by controlling the triggering angle of the Thyristors in the ac voltage controller circuits.
In brief, an ac voltage controller is a type of thyristor power converter which is used to convert 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 ‘a’

There are two different types of thyristor control used in practice to control the ac power flow

· On-Off control
· Phase control

These are the two ac output voltage control techniques.
In On-Off control technique Thyristors are used as switches to connect the load circuit to the ac supply (source) for a few cycles of the input ac supply and then to disconnect it for few input cycles. The Thyristors thus act as a high speed contactor (or high speed ac switch).

In phase control the Thyristors are used as switches to connect the load circuit to the input ac supply, for a part of every input cycle. That is the ac supply voltage is chopped using Thyristors during a part of each input cycle.
The thyristor switch is turned on for a part of every half cycle, so that input supply voltage appears across the load and then turned off during the remaining part of input half cycle to disconnect the ac supply from the load.

By controlling the phase angle or the trigger angle ‘a’ (delay angle), the output RMS voltage across the load can be controlled.

The trigger delay angle ‘a’ is defined as the phase angle (the value of wt) at which the thyristor turns on and the load current begins to flow.

Thyristor ac voltage controllers use ac line commutation or ac phase commutation. Thyristors in ac voltage controllers are line commutated (phase commutated) since the input supply 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 the ON thyristor naturally turns off, when the device current falls to zero.

Phase control Thyristors which are relatively inexpensive, converter grade Thyristors which are slower than fast switching inverter grade Thyristors are normally used.

For applications upto 400Hz, if Triacs are available to meet the voltage and current ratings of a particular application, Triacs are more commonly used.

Due to ac line commutation or natural commutation, there is no need of extra commutation 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 for performance parameters are not simple, especially for the phase controlled ac voltage controllers with RL load. But however most of the practical loads are of the RL type and hence RL load should be considered in the analysis and design of ac voltage controller circuits.

The ac voltage controllers are classified into two types based on the type of input ac supply 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 at 50Hz in our country. Three phase ac controllers operate with 3 phase ac supply of 400V RMS at 50Hz supply frequency.
Each type of controller may be sub divided into
· Uni-directional or half wave ac controller.
· Bi-directional or full wave ac controller.
In brief different 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-directional controller).
· Three phase full wave ac voltage controller (bi-directional controller).

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 motor control).
AC magnet controls.

The basic principle of on-off control technique is explained with reference to a single phase full wave ac voltage controller circuit shown below. The thyristor switches and are turned on by applying appropriate gate trigger pulses to connect the input ac supply to the load for ‘n’ number of input cycles during the time interval . The thyristor switches and are turned off by blocking the gate trigger pulses for ‘m’ number of input cycles during the time interval . The ac controller ON time usually consists of an integral number of input cycles.

R=RL= Load Resistance
Fig.: Single phase full wave AC voltage controller circuit

Referring to the waveforms of ON-OFF control technique in the above diagram,
Two input cycles. Thyristors are turned ON during for two input cycles.
One input cycle. Thyristors are turned OFF during for one input cycle

Fig.: Power Factor

Thyristors are turned ON precisely at the zero voltage crossings of the input supply. The thyristor is turned on at the beginning of each positive half cycle by applying the gate trigger pulses to as shown, during the ON time . The load current flows in the positive direction, which is the downward direction as shown in the circuit diagram when conducts. The thyristor is turned on at the beginning of each negative half cycle, by applying gating signal to the gate of , during . The load current flows in the reverse direction, which is the upward direction when conducts. Thus we obtain a bi-directional load current flow (alternating load current flow) in a ac voltage controller circuit, by triggering the thyristors alternately.

This type of control is used in applications which have high mechanical inertia and high thermal time constant (Industrial heating and speed control of ac motors). Due to zero voltage and zero current switching of Thyristors, the harmonics generated by switching actions are reduced.

For a sine wave input supply voltage,V
RMS value of input ac supply = = RMS phase supply voltage.
If the input ac supply is connected to load for ‘n’ number of input cycles and disconnected for ‘m’ number of input cycles, then

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