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.
