1
POKHARA UNI VERSI TY
Level: Bachelor
Semester
–
Spring
Year : 200
9
Programme:
BE
Full Marks: 100
Pass Marks : 45
Course:
Power Electronics
Time : 3hrs.
Candidates are required to give their answers in their own words as far
as practic
able.
The figures in the margin indicate full marks.
Attempt all the questions.
1.
a)
Name the different methods of turning on a thyristor. Explain the
gate triggering method in detail.
b)
An input ac source of 230 V (rms) is connected to the input terminals
of a bridge converter. The bridge converter consists of one SCR and
three diodes and the output of the converter is connected to a
resistive load of 10
. Assume that the load current is continuous. If
the firing angle of the SCR is
45º, draw the waveforms of the output
voltage and load current.
Also, calculate i) the average value of the dc output voltage, and
ii) the power delivered to the load.
7
8
2.
a)
Draw the circuit diagram of a thre
e

phase semi

converter feeding an
R

L

E load. Sketch the waveforms of the output voltage for i) the
angle (
) less than 60º, and ii) the firing angle(
) greater than 60º,
and also prove that
the output voltage
V
0
= 3 V
m
l (1+ cos
) / (2
)
Where V
ml
= the peak value of the line

to line voltage
= firing angle of the SCR.
b)
What is a multi

phase chopper? Why is multi

phas
e chopper
preferable in high power circuits? Explain, with appropriate
waveforms, the working of a multi

phase chopper operating in phase

shifted mode.
7
8
3.
a)
What is a cycloconverter? Draw the input and output waveforms for
a 16
2/3
cycloconverter
and explain how the device converts 50 Hz
input supply in to a 16
2/3
Hz output supply.
b)
Name the methods available to control the speed of a three

phase
8
7
2
induction motor. Briefly explain how an inverter can be employed to
control the speed of a three

phase
induction motor.
4.
a)
Describe the working principle of a three

phase inverter operating in
180
conduction mode. Draw the waveforms of the three line
voltages.
b)
A single

phase full

bridge inverter is connected to a resistive load R
= 10
.The input voltage to the inverter is V
s
= 100 V. Draw the
waveforms of the output voltage and current. Express the
instantaneous output voltage in the Fourier series terms.
7
8
5.
a)
Draw a circuit diagr
am, using converter(s), for the two

quadrant
operation of a dc motor. Explain how the circuit works.
b)
A single
–
phase 220 V, 50 Hz ac supply a separately excited dc
motor through two single

phase semi

converters, one for the field
circuit and another for t
he armature circuit. The firing angle for the
field semi

converter is set to zero. The total resistance of the field
circuit is 250
and that of the armature circuit is 0.2
. The load
torque is 50 N

m at 1000 r
pm. The voltage constant is 0.8 V/A

rad/s
and the torque
constant is 0.8 N

m/A
2
. Assuming the armature and
field currents to be continuous and neglecting losses, determine: i)
the field current, and ii) the firing angle of the converter of the
converter o
f the armature circuit.
7
8
6.
a)
Briefly example the following types of HVDC links :
i.
Monopolar Link
ii.
Bipolar Link
iii.
Homopolar Link
Explain the role of converter (s) placed at either end of the HVDC
link.
b)
What is a 12

pulse converter? Draw the circuit diagram
of a 12

pulse
converter and give its merits.
7
8
7.
Write short notes on (
Any Two
) :
a.
UJT based time delay relay
b.
Major components of a HVDC transmission system
c.
PWM techniques.
5×2
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