SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER

(AC REGULATOR)

Single phase full wave ac voltage controller circuit using two SCRs or a single triac is generally used in most of the ac control applications. The ac power flow to the load can be controlled in both the half cycles by varying the trigger angle 'α ' . The RMS value of load voltage can be varied by varying the trigger angle 'α ' The input supply current is alternating in the case of a full wave ac voltage controller and due to the symmetrical nature of the input supply current waveform there is no dc component of input supply current i.e., the average value of the input supply current is zero.A single phase full wave ac voltage controller with a resistive load is shown in the figure below. It is possible to control the ac power flow to the load in both the half cycles by adjusting the trigger angle 'α ' . Hence the full wave ac voltage controller is also referred to as to a bi-directional controller.The thyristor T1 is forward biased during the positive half cycle of the input supply voltage. The thyristor T1 is triggered at a delay angle of 'α ' (0 ≤α ≤ϖ radians).Considering the ON thyristor T1 as an ideal closed switch the input supply voltage appears across the load resistor RL and the output voltage O S v = v during ωt =α to ϖ radians. The load current flows through the ON thyristor T1and through the load resistor L R in the downward direction during the conduction time T1from ωt =α to ϖ radians.At ω t =ϖ , when the input voltage falls to zero the thyristor current (which is flowing through the load resistor RL ) falls to zero and hence T1naturally turns off . No current flows in the circuit during ωt =ϖ to (ϖ +α )The thyristor T2 is forward biased during the negative cycle of input supply and when thyristor T2 is triggered at a delay angle (ϖ +α ), the output voltage follows the negative halfcycle of input from ωt = (ϖ +α ) to 2ϖ. When T2 is ON, the load current flows in the reverse direction (upward direction) through T2during ωt = (ϖ +α ) to 2ϖ radians. The time interval (spacing) between the gate trigger pulses of T1 and T2is kept at ϖ radians or 1800. At ωt = 2ϖ the input supply voltage falls to zero and hence the load current also falls to zero and thyristor T2 turn off naturally

Performance parameter:

1. Input supply voltage

2. Output voltage across the load resistor R L ;

3. Output load current

Application of AC voltage controllers (Regulators)

1. Light dimmers

2. Industrial heating application

3. Speed control of induction motor

4. On load tap changing of a transformer

5. In starting of induction motor

6. Control of induction motor

7. Fan Regulator

8. Heater Control