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3. Single Phase AC Voltage R_ RL Load

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					  CIRCUIT DIAGRAM FOR 1-PHASE A.C. VOLTAGE CONTROLLER
                     WITH R-LOAD:




  CIRCUIT DIAGRAM FOR 1-PHASE A.C. VOLTAGE CONTROLLER
                     WITH RL-LOAD:




TRIGGERING CIRCUIT FOR 1-PHASE A.C. VOLTAGE CONTROLLER:
Expt. No:                                                            Date :


           SINGLE PHASE AC VOLTAGE CONTROLLER
                    WITH R AND R-L LOAD
AIM:

        To observe the working of 1   AC voltage controller with R and RL Loads
and to observe and plot the voltage waveforms across load and SCR for different
firing angles.

APPARATUS:

S.No.             Apparatus                     Range              Type        Quantity
  1.      Ammeter                               (0-1A)              MI            1
  2.      Voltmeter                            (0-35 V)             MI            1
  3.      CRO                                     ….                ….            1
  4.      Load Rheostat                        50 / 5A            Wire           1
                                                                  wound
     5.   Inductive Load                    0-150 mH/2A             ….             1
     6.   SCRs                               12 A/600 V          TYN 616           2
     7.   1   centre tap transformer    100VA, 230/24-0-       Shell type        1
                                                24V
     8.   1   A.C. voltage controller          ….                 ….             1
          firing circuit
  9.      CRO Probe                               …..               ….            1
  10      Connecting wires                        …..               …..        Required
                                                                                 No.


THEORY:

R – Load:
1.        A single phase half wave circuit is one which produces only one pulse of load
          current during one cycle of source voltage.
2.        At some delay angle  , a positive gate signal is applied between gate and
          cathode turns on the SCR. Immediately full supply voltage is applied to the
          load as Vo .
3.        It is a resistive load current I 0 is in phase with V0 .
4.        Once the SCR is ON, load current flows until it is turned OFF by reversal of
          voltage at t   , 3 .
5.        By varying the firing angle  , the phase relationship between the start of the
          load current and the supply voltage can be controlled.
RL – Load:

1.     In RL Load due to the presence of inductance L, the load or output current
       rises gradually. After sometime it reaches maximum value and then begins to
       decrease.
2.     At t   , V0 is zero but it is not zero because of the load inductance, L.
3.     After t   , SCR is subjected to reverse anode voltage but it is not turned
       OFF as load current is not less than holding current.
4.     After some angle    , it reduces to zero and SCR is turned OFF as it is
       already reverse biased.
5.     After t   , V0  0 and is  0 . At t  2   , SCR is triggered again. V0 is
       applied to the load and load current develops as before.  is called extinction
       angle.

PRECAUTIONS:

1.   CRO should not be used in dual trace mode.
2.   Use fixed terminals of load rheostat.
3.   An isolation transformer should be placed between power circuit and input.

PROCEDURE:

1.   Switch ON the power supply.
2.   Observe the triggering outputs of the firing circuit for different firing angles
     using CRO.
3.   Make the connections as per the circuit diagram.
4.   Switch ON the triggering pulses to respective SCRs in the power circuit.
5.   Plot the output voltage waveforms across the load and SCRs for different firing
     angles.
6.   Repeat the above procedure for R-L load (with firing angle being greater than
     power factor) and draw the output waveforms.
7.   Disconnect the supply.

RESULT:

       Hence studied the working of a 1   A.C. voltage controller with R and RL
loads and observed the waveforms across load and SCR.


                           Firing angle       Voltage across load, V0 rms 
                              (degrees)                   (Volts)
          R- load                720              21.69              18.5
                                1260               8.16               5
          RL – load              450              23.44               22
CONCLUSION:

      From the experiment, the output voltage of the AC voltage controller for RL
load can’t be controlled for    where  is the firing angle and  is the power
factor angle. Therefore A.C. output powers can’t be controlled only for    . Thus
the control range of firing angle is     1800 .

				
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posted:12/31/2010
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