Code: A-26 Subject: POWER ELECTRONICS
Time: 3 Hours Max. Marks: 100
NOTE: There are 11 Questions in all.
Question
1 is compulsory and carries 16 marks. Answer to Q. 1. must be written in the
space provided for it in the answer book supplied and nowhere else.
Answer
any THREE Questions each from Part I and Part II. Each of these questions
carries 14 marks.
Any
required data not explicitly given, may be suitably assumed and stated.
Q.1 Choose
the correct or best alternative in the following: (2x8)
a.
The
maximum permissible frequency of the a.c. voltage source whose voltage is to be
rectified by a thyristor with 
of 175
is _____ Hz.
(A) 2500 (B)
2857
(C) 3334 (D) 4572
b. As compared to the thyristors, the merit of power transistors is that they have ___________ but their disadvantage is that they have ________.
(A)
a
requirement of continuous base drive, a positive temperature coefficient.
(B)
a
low switching frequency, a low conduction drop.
(C) a high switching frequency, a
negative temperature coefficient.
(D) a low conduction drop, a low
switching frequency.
c. A 3-phase controlled rectifier feeds a purely
resistive load 
. If the supply
voltage is 
at 50 Hz, 
is 
and the firing angle
is 
, then the duration of conduction of a device (viz.
thyristor) will be _________.
(A)
. (B) 
.
(C) 
. (D) 
.
d. In the step-down chopper the load current ripple can be reduced by ________.
(A) increasing the load
resistance.
(B) decreasing the chopper
frequency.
(C) increasing the load
inductance.
(D) using a freewheeling diode
with high reverse voltage capability.
e. If the load resistance of the single phase bridge
rectifier shown in Fig.1 is 4
, then the average load current with an 
of 
will be __________.
(A)
59.7
A. (B) 30.2 A.
(C) 61.0
A. (D) 49.5 A.
f. The bridge type of voltage source inverter has the advantage namely ______, whereas the PWM type of inverter has the advantage namely ___________.
(A) switching losses are low,
harmonic content of any order can be eliminated.
(B) low order harmonics can be
eliminated, switching losses are low.
(C) high order harmonics can be
eliminated, only low order harmonics can be eliminated.
(D) harmonics of any order can be
eliminated, switching losses are low.
g. For
the AC controller of Fig.2 the RMS load current with an
of 
rad will be nearly
___________.
(A)
28.3
A.
(B)
30.8
A.
(C) 21.2 A.
(D) 41.6 A.
h. A 3 phase six pole induction motor is having a supply frequency of 50 Hz and a slip of 0.02, then the speed of its rotor in mechanical radians per sec will be nearly
(A)
153.9. (B) 128.8.
(C) 104.7. (D) 138.2.
PART I
Answer
any THREE Questions. Each question carries 14 marks.
Q.2 a. Enumerate the advantageous features of a GTO over
:
(i) a
thyristor and (ii)
a power transistor.
Also give a typical gate control circuit for a GTO
and explain its operation. (6)
b. Design
the UJT triggering circuit of Fig.3 with the following data :
and 
The frequency of
oscillation is f =7.5 Hz and the width of the triggering pulse is 
(8)
Q.3 a. Draw the circuit and sketch the relevant waveforms
for a bipolar half-wave circuit.
Determine the mean values of load voltage and current for a bi-phase
half-wave circuit having the following data : 
at 50Hz, load
resistance 
and firing angle 
. (10)
b. With the help of the equivalent circuit of a
typical rectifier that is operated with load, describe the main sources that
contribute to loss of output voltage. (4)
Q.4 a. Give the waveforms and describe the operation
of a step-down chopper with an RL load.
Derive an expression for the peak-to-peak ripple current for the same. (6)
b. The
boost regulator of Fig.4 has an input voltage
of 6V. The average output voltage 
is 16V and the
average load current 
is 0.45A. The switching frequency is 30 KHz. If 
and C=200
determine the ripple current 
of inductor and the ripple
voltage 
of the filter
capacitor. (8)
Q.5 a. With
the help of a circuit and waveforms explain the operation of a single phase
cycloconverter constituted by two single phase bridge type controlled
converters. (6)
b. Give the diagram of a line-side commutation circuit and derive an expression for the circuit turn-off time. How does this operation differ from that of the load-side commutation circuit? (8)
Q.6 a. Explain the operation of a single phase AC controller with an RL type of load. Derive the expressions for :-
(i) the RMS output voltage and
(ii) the average value of the thyristor current. (6)
b. The AC voltage controller of Fig.5 has a
resistive load 
of 12
and an RMS input voltage 
of 125 V at 50
Hz. The delay angles of thyristors 
and 
are 
Determine
(i)
the RMS output voltage 
.
(ii) input power factor.
(iii)
RMS current of thyristors. (8)
PART II
Answer
any THREE Questions. Each question carries 14 marks.
Q.7 a. Give the circuit and describe the operation of
a three phase inverter with 120-degree conduction. Compare its features with an
inverter having 180-degree conduction. (6)
b. The single phase full-bridge inverter in
Fig.6 has a resistive load 
of 
and a DC input
voltage, 
of 54V. Determine :
(i)
The
RMS output voltage 
at fundamental
frequency.
(ii)
The
average and peak currents of each transistor.
(iii)
The
peak reverse blocking voltage of each transistor. (8)
Q.8 a. Explain the terms :
(i)
current-limit
control and
(ii)
time-ratio
control as applied to the firing schemes of DC drives. (6)
b. With the help
of a circuit diagram describe the operation of a chopper- controlled DC
motor. Give the advantages of this
drive as compared to a phase-controlled converter drive. (8)
Q.9 a. Give the circuit for the speed control of an
induction motor by variation of rotor resistance. Also draw the relevant speed-torque characteristics. (7)
b. A three phase 410V, 50 Hz, four pole, star-connected induction
motor is having the following data : 
, 
all parameters being
referred to the stator. If the angular
speed of the rotor is 153.41 mech. rad /s, determine :
(i) the
slip (ii) torque developed by the motor (iii)
max. torque. (7)
Q.10 a. Explain the terms carrier signal and
modulation index as applied to the PWM technique. Also describe the multiple PWM method of voltage control of a
single phase inverter. (6)
b. Give the
circuitry and waveforms and expressions for the load current of the class
B-type of choppers. (8)
Q.11 Write notes on any TWO of the following :
(i)
Chopper
circuit design.
(ii)
Blocked
group operation of a cycloconverter.
(iii)
Transformer
ratings for rectifier circuits. (2x7)