· 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 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.
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 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).
APPLICATIONS OF AC VOLTAGE CONTROLLERS
Lighting / Illumination control in ac power circuits.
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