# Q3. A 3-phase SCR bridge is supplied from a 3-phase 220Vms, 50HZ AC source, and directly feeds a 220

Q3. A 3-phase SCR bridge is supplied from a 3-phase 220Vms, 50HZ AC source, and directly feeds a 220 load resistor Sketch the circuit diagram for this rectifier [4 marks] 3.a. Refer to lecture notes] 3.b. For a firing angle of a = 0° (ie. diode operation), calculate the average DC output [4 marks] voltage, and the minimum instantaneous DC output voltage VpC (avg) 297V VpC (min) 269.44V Sketch over one fundamental cycle, the waveform of the DC output voltage and the current flowing into the load resistor for a firing angle of a = 75°. 3.c. [6 marks [Refer to lecture notes] If a very large capacitor is now connected in parallel with the load resistor, calculate the average current that now flows into the load resistor for the same firing angle of a = 75° (assume that the AC source has sufficient series inductance to adequately limit any surge charging current that may flow into the capacitor when the SCR is fired). [6 marks] 3.d. iR(avg)= 10A Total Marks for Question 3 = 20 marks

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