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4. Single Phase Fully Controlled Bridge Converter

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					 CIRCUIT DIAGRAM FOR 1-PHASE FULLY CONTROLLED BRIDGE
               CONVERTER WITH R LOAD:




 CIRCUIT DIAGRAM FOR 1-PHASE FULLY CONTROLLED BRIDGE
               CONVERTER WITH R-L LOAD:




TRIGGERING CIRCUIT FOR 1-PHASE FULLY CONTROLLED BRIDGE
                      CONVERTER:
Expt. No:                                                      Date :

SINGLE PHASE FULLY CONTROLLED BRIDGE CONVERTER
               WITH R AND RL LOAD

AIM:

        To observe the working and operation of single phase fully controlled bridge
converter with R and R-L Loads and to plot the voltage waveforms across load and
thyristors (with and without freewheeling diode) for different firing angles.

APPARATUS:

S.No.           Apparatus                    Range             Type       Quantity
  1.    Isolation transformer          1 KVA, 230V/ 115-       Shell         1
                                      60-30-0-30-60-115 V
  2.    Cathode                 Ray            …                …             1
        Oscilloscope
  3.    Load Rheostat                     145 / 2.8A         Wire            1
                                                              wound
  4.    Inductive Load with           0-25-50-100-150          …..            1
        tapings                       mtt,2A
  5.    Silicon Controlled                 16A/1200 V       16 TT 512         4
        Rectifiers
  6.    Power diode(s)                    16A/1200 V        SPR 16 PB         1
  7.    1   fully controlled               …..               …..            1
        bridge converter firing
        Kit
  8.    Voltmeter                           (0-30V)            MC            1
  9.    Ammeter                              (0-2A)            MC            1
 10.    CRO Probe                              ….               ….           1
 11.    MCB                                230V/6A              ….           1
 12.    Fuse                                   5A              Glass         1
 13.    Connecting wires                       …                …         Required
                                                                            No
TABULAR COLUMN:

S.No.         Time      Conduction         Load        Firing angle     Output     Output
              period     period for                        T / 2  Tc   Voltage,   current
             for half   half cycle,                    
                                                             T /2          V0         I0
              cycle       Tc ( ms )                     (degrees)        (volts)     (A)
               T 
                
               2
               (ms)
     1.         10            6                             720           6.2       0.07
     2.         10            3           R-Load                          2.7       0.03
                                                           1260
     1.         10            6          RL Load            720           6.0       0.07
                                           (with
     2.         10            3        freewheeling        1260           2.6       0.03
                                           diode)
     1.         10            6          RL Load            720           4.6      0.055
                                       (without free
     2.         10            3          wheeling          1260                    0.015
                                           diode)                         1.3


THEORY:

        A full converter or fully controlled converter uses thyristors only and there is a
wide control over the level of d.c. output voltage. The load is connected to the A.C.
voltage source through the thyristors. So, the load current is continuous always over
the working range. At t  0 and t   ; T1 and T2 are forward biased through
already conducting SCRs T3 and T4 block the forward voltage. For continuous
current, thyristors these are reverse biased. When forward biased SCRs T1 , T2 are
triggered at t   , they get turned ON. As a result Vm appears across thyristors T3
and T4 .
        At that time, load current flows continuously. If Vm Sin  is positive,
thyristors T1 , T2 conduct from t   to    , forward biased SCR’s T3 , T4 are
triggered. The supply voltage turns OFF T1 and T2 by natural commutation and the
load current is transferred from T1 , T2 to T3 , T4 .

PRECAUTIONS:

1.        Use the fixed terminals of load rheostat.
2.        Don’t connect CRO in dual trace mode.
3.        Initially keep MCB in ON position.
4.        The Power circuit and input must be electrically isolated.
5.        Carefully observe waveforms on CRO.
PROCEDURE:

1.   Observe the triggering circuit outputs using CRO.
2.   Disconnect the supply if the triggering pulses to the triggering circuit are proper.
3.   Connect the circuit as per circuit diagram and switched ON the supply.
4.   Supply the trigger pulses to the respective SCR’s
5.   Note the load voltage and load current for different firing angles.
6.   Draw the relevant waveforms with and without free wheeling diode.
                       T / 2  TC
     Firing angle,              1800
                          T /2
     Where T total time period.
             Tc conduction period.
7.   Repeat the above procedure for R-L load with and without freewheeling diode.
8.   Plot the output waveforms on a graph sheet.
9.   Disconnect the supply.

EXPRESSIONS FOR LOAD VOLTAGE AND LOAD CURRENT:

With R – load:
                                           
                                       1
Average load voltage,         Vdc         V          sin t  d t 
                                       
                                                   m
                                           
                                  Vm
                                         cos t 
                                                            
                              
                                  
                                  Vm
                                      1  cos  
                                  
                Vdc Vm
Load current, I dc 
                       1  cos  
                 R R
With R-L Load and without free wheeling diode:

                                               
                                       1
Average load voltage,         Vdc             V          sin t  d t 
                                       
                                                       m
                                               
                                  Vm
                                       1  cos t 
                                                                
                              
                                  
                                  Vm
                                       cos   cos  
                                  
                      Vdc Vm
     Load current, I dc       cos   cos  
                       R R
With R-L Load and with freewheeling diode:
                                   
                                 1
Average load Voltage,     Vdc   Vm sin t  d t 
                                          
                                  Vm
                                         cos t 
                                                            
                              =
                                  
                                  Vm
                              =        1  cos  
                                  
                                    Vdc
                 load current, I dc 
                                     R
                                   Vm
                                 =    1  cos  
                                   R

R-load:

                          T  TC
                                            10  6
(i)    From graph,   2           1800          1800  720
                             T                 10
                               2
       Vdc  m 1  cos    m 1  cos 720 
            V                 V
                              
                               26
                            =     1  0.3090  = 10.83 V
                               
                         10  3
(ii)   From graph,             1800  1260
                           10
       Vdc  1  cos    1  cos1260 
            Vm                26
                              
                               26
                           =      1  0.5877  = 3.4122 V.
                               

RL Load (Without freewheeling diode) :
                       10  6
i) From graph,              1800  720
                        10
                 = 2250
    Vdc  m  cos   cos     cos 720  cos 2250 
         V                      26
                                    
                                     26
                                 =         0.3090  0.7071
                                     
                                 = 8.409 V
                         10  3
ii)    From graph,            1800  1260 ;   2160
                           10
       Vdc  m  cos   cos     cos1260  cos 2160 
            V                     26
                                    
                                     26
                                 =         0.5877  0.8090 
                                     
                                 = 1.832V.
RL Load (With freewheeling diode) :

                      10  6
i)    From graph,          1800  720
                        10
           Vm
      Vdc  1  cos   
            
                           26
                             
                              1  cos 720 
                             26
                        =         1  0.3090    = 10.83V.
                             

                        10  3
ii)   From graph,            1800  1260
                          10
      Vdc  m 1  cos    1  cos 1260 
           V                26
                            
                                   26
                         =              1  0.5877    = 3.41V.
                                   

RESULT:

       Hence studied and observed the working of a 1   fully controlled bridge
converter with R and RL (with and without freewheeling diodes) Loads and voltage
waveforms across thyristors, load for different firing angles.

                             Firing angle              Voltage across load, V0 rms 
                Load            (degrees)                         (Volts)
                                                        Theoretical       Practical
          R- load                      720                10.83              6.2

                                     1260                  3.41               2.7
          RL – load                   720                   8.4               4.6
          without
          freewheeling               1260                  1.83               1.3
          diode
          RL Load                      720                 10.83               6
          With
          freewheeling               1260                  3.41               2.6
          diode


CONCLUSION:

        From the experiment, it is concluded that the output voltage of 1   fully
controlled bridge converter decreases when the load is inductive in nature and it is
improved by connecting a free wheeling diode across load.

				
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posted:12/31/2010
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