Monday, October 29, 2012


Fifth Semester
Electrical and Electronics Engineering
(Common to Instrumentation and Control Engineering)
(Regulation 2008)
Time : Three hours Maximum : 100 Marks
Answer ALL questions
PART A — (10 × 2 = 20 Marks)
1. Draw the turn-on characteristics of TRIAC and mark the timings td, tr and   ton?
2. What are the factors that influence the turn-off time of a thyristor?
3. Draw the circuit diagram of single phase dual converter.
4. Define the term voltage ripple factor, current ripple factor.
5. Mention any two disadvantages of frequency modulation control strategy.
6. What are the advantages of ZVS when compared to ZCS?
7. Differentiate VSI from CSI.
8. List various applications of phase controlled converters.
9. What is meant by cycloconverter?
10. Write the output R.M.S. voltage for single phase AC voltage controller with
resistance load.
PART B — (5 × 16 = 80 Marks)
11. (a) Explain the operation of IGBT with the help of neat structural diagram and suitable wave forms. (16)
(b) (i) Draw the turn-off characteristics of an SCR and explain the mechanism of turn-off. (8)
            (ii) What are the different methods for turning off an SCR? Explain all methods in detail. (8)
12. (a) (i) Describe the operation of a single phase two pulse bridge converter using 4 SCR'S with relevant waveforms. (10)
           (ii) Discuss the working of above converter in the converter mode with RLE load. (6)
       (b) (i) A single phase semi converter is operated from 120 V 50 Hz ac supply. The load current with an average value Idc is continuous and ripple free firing angle α =π 6 . Determine.
 (1) Displacement factor
(2) Harmonic factor of input current
(3) Input power factor. (10)
            (ii) Write a note on battery charger. (6)
13. (a) (i) Discuss the principle of operation of DC-DC step down chopper with suitable waveform. Derive an expression for its average DC output voltage. (8)
            (ii) A step-down dc chopper has a resistive load of R = 15 and input voltage Edc = 200 V. When the chopper remains ON, its voltage drop is 2.5 for a duty cycle of 0.5.
                 Calculate :
(1) Average and r.m.s value of output voltage
(2) Power delivered to the load. (8)
      (b) Draw the circuit of CUK regulator and explain its working principle with   necessary waveform in detail. (16)
14. (a) (i) Describe the working of a 1φ pull bridge inverter with relevant circuit and waveforms. (8)
           (ii) What is PWM? List the various PWM techniques and explain any one of them. (8)
(b) Explain the Harmonic reduction by transformer corner lines and stepped  wave inverters. (16)
15. (a) (i) Describe the operation of single phase full wave a.c voltage  controller with the help of voltage and current waveform. Also derive the expression for average value of output voltage. (10)
            (ii) For a single phase a.c regulator feeding a resistance load, show that power factor is given by the expression ( ) 2ππx . (6)
      (b) (i) Describe three-phase to three phase cycloconverter with relevant circuit arrangement using 18 thyristors. (8)
            (ii) Show that the fundamentals RMS value of per phase output voltage  of low frequency for an m-pulse cycloconverter is given by π πm n E Epn or sin . (8