Set No. 4
Code No: V3112/R07
III B.Tech I Semester Supplementary Examinations, May 2010 POWER ELECTRONICS (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) The SCR is rated to give an average power dissipation of 0.5W. Its gate voltage varies from 2.5V to 10V. Keeping average gate power dissipation constant, plot allowable gate characteristics of SCR. For a triggering gate pulses of duty cycle 0.6. Calculate the value of average gate dissipation.
m o
c . ld
(b) Explain the gate characteristics of a Thyristor.
[8+8]
2. Derive the expressions for the following performance factors of single phase fully controlled bridge converter
r o
(a) input displacement factor (b) input power factor
w tu
(c) voltage ripple factor (d) active power input
j. n
(e) Reactive power input
[5+3+2+3+3]
3. A three phase full converter is supplied from a three phase 230V, 60Hz supply. The load current is continuous and has negligible ripple. If the average load current Idc = 150A and commutating inductance Lc = 0.1mH, determine the overlap angle when
w w
w
0
(a) α = 10
(b) α = 300 and (c) α = 600
[6+5+5]
4. (a) Explain the principle of ON-OFF control used in a.c. voltage controller. (b) Derive the expression for the input power factor in an a.c. voltage controller using ON-OFF control. (c) Explain its application with the help of a circuit and waveforms.
[6+5+5]
5. Discuss the working of a single phase bridge type cyclo converter with RL loads and for discontinuous operations with relevant output waveforms and circuit diagram [4+6+6] for f0 = 1/2 fs. 6. Explain the operation of a basic dc chopper and obtain the following as a function of Edc , R and duty cycle δ. (a) average output voltage and current 1 of 2