SINGLE PHASE AC VOLTAGE CONTROLLER WITH R & RL LOADS
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To Verify the operation of single phase AC Voltage controller with R and RL Loads and
to observe the output and input waveforms
S.No EQUIPMENT Qty
1. I-φ Transformer 1
2. I-φ AC voltage controller
power circuit with firing unit
3. Voltmeters(MI meters) 2
4. Rheostat 1
5. Inductive load 1
6. CRO with (1:10) Probe 1
7 Patch cards
AC voltage controller’s are thyristor based devices ,which converts the fixed Ac
voltage into variable AC voltage with same frequency .The circuit diagram of Single phase AC
voltage controller is shown in figure .It consists of two SCR’s connected in anti parallel. The
input and output voltage waveforms are also shown. The SCR’s are gate controlled and gate
pulses are obtained from firing unit.
For the first half cycle of input voltage waveform SCR T1 conducts and gives
controlled output to load. During the other half cycle of input voltage waveform SCR T2
conducts .During the Positive half cycle T1 is triggered at a firing angle of wt= α .T1 starts
conducting and source voltage is applied to the load from α to π. At wt= π both Vo and Io
falls to zero. Just after wt= π, T1 is reverse biased and therefore it is turned off by self
For RL –Load:
During the first half cycle wt = 0 to π SCR T1 is forward biased and is
triggered at wt=α and output current starts building up through load .At wt=π, load and
source voltage are zero. But the output current is not zero because of inductive load. At
wt=(β>π), the load current reduces to zero, angle β is called extinction angle. After wt =
π, SCR T1 is reverse biased, but does not turn off because the output current is not zero.
At wt=β, only when output current is zero T1 turns off.
During the negative half cycle SCR T2 is forward biased and is triggered at
wt = π+α. The output current flows through the load in reverse direction. The operation
of SCR T2 is similar as that of SCR T1 during the period wt = π+α to wt = (2β-α) but in
the negative direction. At wt= ( 2β-α) the SCR t2 is commutated and the next positive
half cycle will be regulated by SCR T1. In this way the AC Voltage controller will be
useful for regulating the AC voltage.
A) For R-Load:
1. Connect the circuit as shown in figure.
2. Verify the connections from the lab instructor before switch on the supply.
3. Keep the rheostat position value given by the lab instructor
4. Switch ON the CRO and calibrate it with the input voltage.
5. Switch on the power circuit and firing circuit.
6. Observe the output voltage waveform in the CRO.
7. Note down the reading of α from the CRO and Vo from the voltmeter
8. Also calculate the theoretical value of output voltage from the formula and
compare it with the practical value of the output voltage, which is observed from
9. Repeat the above process from step 6 to 8 for various firing angles.