Download 3 Phase Fully Controlled Rectifier

3 Phase Fully Controlled Rectifier
Aim
The aim of Experiment is to analyze the operation (Switching) of three phase fully
controlled rectifiers with resistive load.
Apparatus
1. Power electronic trainer.
2. Connection wires.
3. CRO
Theory
Phase controlled AC-DC converters employing thyristor are extensively used for
changing constant ac input voltage to controlled dc output voltage. In phase
controlled rectifiers, a thyristor is tuned off as AC supply voltage reverse biases it,
provided anode current has fallen to level below the holding current. Controlled
rectifiers have a wide range of applications, from small rectifiers to large high
voltage direct current (HVDC) transmission systems. They are used for
electrochemical processes, many kinds of motor drives, traction equipment,
controlled power supplies, and many other applications
A three-phase fully-controlled bridge rectifier can be constructed using six SCRs
as shown in fig.1
The three-phase bridge rectifier circuit has three-legs, each phase connected to one
of the three phase voltages. Alternatively, it can be seen that the bridge circuit has
two halves, the positive half consisting of the SCRs S1, S3 and S5 and the negative
half consisting of the SCRs S2, S4 and S6. At any time, one SCR from each half
conducts when there is current flow. If the phase sequence of the source be RYB,
the SCRs are triggered in the sequence S1, S2 , S3 , S4, S5 , S6 and S1 and so on.
If SCRs are used, their conduction can be delayed by choosing the desired firing
angle. When the SCRs are fired at 0o firing angle, the output of the bridge rectifier
would be the same as that of the circuit with diodes. For instance, it is seen that
D1 starts conducting only after  = 30o. In fact, it can start conducting only
after  = 30o , since it is reverse-biased before  = 30o. The bias across
D1 becomes zero when  = 30o and diode D1 starts getting forward-biased only
after  =30o. When vR() = E*Sin (), diode D1 is reverse-biased before  =
30o and it is forward-biased when  30o. When firing angle to SCRs is zero
degree, S1 is triggered when  = 30o. This means that if a synchronizing signal is
needed for triggering S1, that signal voltage would lag vR() by 30o and if the
firing angle is , SCR S1 is triggered when  =  + 30o. Given that the
conduction is continuous, the following table presents the SCR pair in conduction
at
any
instant.
Period, range of 
 + 30o to  + 90o
 + 90o to  + 150o
 + 150o to  + 210o
 + 210o to  + 270o
 + 270o to  + 330o
 + 330o to  + 360o and  +
0o to  + 30o
SCR Pair in conduction
S1 and S6
S1 and S2
S2 and S3
S3 and S4
S4 and S5
S5 and S6
Procedure
1. Connect the three-phase full wave controlled rectifier circuit shown in
Fig.1 on the power electronic trainer.
2. Turn on the power
3. Plot the input and output waveforms on the same graph paper" same axis".
4. Measure the average and RMS output voltage by connect the AVO meter across
load resistance.
5. Turn off the power
Circuit Diagram
Fig.1
Calculation
The average output voltage of the bridge circuit is calculated as follows, with a
change in variable, where  =  + 60o.
In the expression above, U is the peak line-to-line voltage, whereas E is the
amplitude of phase voltage of 3-phase supply.
Graph
RESULT:
Conducted the experiment and analysed the operation of three phase fully controlled rectifier.