Main Components Of A Circuit

MAIN COMPONENTS OF CIRCUIT: • • • • • • • • •

Delta-wye transformer Phase shifter Zero crossing detector Integrator Comparator Pulse generator Jk flip flop Driver circuit Power circuit

WORKING: The three separate identical cards(R,Y,B) are used for controlling the triggered angles of thyristors T1,T2,T3 respectively. The phase shifter shift the phase voltages R’.Y’.B’ in reverse direction. Adjust 100K pots of the phase shifter for shifting the synchronised signal voltages by 60 deg. The output waves Ro,Yo,Bo of the phase shifters are used as synchronised voltages for R,Y,B phases respectively. The zero crossing detector generates square wave pulses(R1,Y1,B1). The triangular waves(R2,Y2,B2) are obtained at the output of integrator. Comparator compares the triangular wave forms with a voltage Vc which varies between +Vcc to –Vcc and is controlled by the 47k pot. The output of the comparators forms input to the pulse generator(monostable). The monostable outputs(R4,Y4,B4) are used as a clock pulses for J-K flip flops. Outputs of the flip flops are the required 120 deg conduction pulses for the thyristor. The obtained outputs(R5,Y5,B5) are ANDED with a high frequency carrying signal to produce the triggered signals. The high frequency is essential to reduce the gate power dissipation and size of pulse transformer. When switch ‘s’ is closed (x=0) the pulse widths are almost same as waves R5,Y5,B5 respectively. And when switch ‘s’ open (x=1) are almost same as R5,Y5,B5 if angle less than 60deg. These modified pulse signals are applied to thyristors respectively through pulse transformer. The control circuit is built and tested to give the appropriate gate pulses. It is necessary to check and mark the phase sequence of the power supply and connect properly the phases with the power circuit and control circuit.

TEST PROCEDURE: Check whether the firing circuit is working properly or not for all three phases. If it is working as expected check the polarity of the output pulses of the control circuit. Use proper thyristors and diodes and make three phase half controlled bridge converter power circuit. See that the incoming power supply is properly connected to the bridge and the control circuit so that proper phase sequence is maintained. For safety of power circuit it is routed through properly rated switch and breaker. Again check the triggered pulses for the thyristor of R,Y,B phases respectively before breaker is switched ON. The triggering pulses are delivered to the gate of the thyristors.

After connecting the above procedure connect required load and release the three phase power supply and firing pulses to thyristors.the circuit will ready for operation as a three phase half controlled converter.