Code No: 310204

III-B-Tech I-Semester Regular Examinations

November-2003
Set No. 4

POWER ELECTRONICS

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks:80

Answer any FIVE questions

ALL questions carry equal marks

1. a) Explain the turn-on and turn-of methods for SCR,BJT and power MOSFET with neat waveforms.

b) Discuss the various current ratings of SCR.

2. a) Show that the effect of source inductance on the performance of single phase fully controlled converter, is to present an equivalent resistance of ωLs/π ohms in series with the internal rectifier voltage.

b) A single phase fully controlled converter is supplied at 220V, 50Hz. Determine the average load voltage for the following cases when the firing angles is 45⁰ for purely resistive load.

3. a) Describe in detail the operation of dual converter in non-circulating current mode. b) Two three phase full converters are connected in anti parallel to form a three phase dual converter of the circulating current type. The input to the dual converter is 3phase, 400V, 50Hz.If the peak value of the circulating current is to b e limited to 20A, find the value of inductance needed for the reactor for firing angle of 60⁰.

4. A 1Φ 230V, 50Hz source connected to a anti parallel connected thyristor circuit; controlling power to the following loads, when α=90⁰. Calculate output voltage and output current and load power factor for a) R=10 OHMS; L=0 H b) R=10OHMS; L=60mH.

5. Explain single phase step down cycloconverter with output frequency of ¼ of input frequency with the help of Bridge type for RL load continuous conduction with neat waveforms.

6. a) Describe class B self commutation by an LC circuit employed for a thyristors circuits.

b) A two thyristor class C turn off circuit is required to be designed for use as a blinker to turn on and off a lamp of constant resistance of 10 ? from a DC supply of 100V. If the SCR used are converter grade with turnoff time 50μsec find the value of commutation capacitor so that the commutation failure may not occur.

7. a) Explain why output voltage control is required in Inverters.

b) Briefly list out the merits and demerits of any three types of voltage control techniques used at the input of Inverter.

8. A three phase fully controlled converters has R-L load of 1.5Ω & 4.5mH respectively and back Emf of 12V, the input voltage is 150V (rms) at 60Hz. Give the SPICE representation of the model circuit to calculate average and RMS thyristor current and instantaneous o/p current at ωt = α = 30⁰.