# Power Electronics

Ηλεκτρονική - Συσκευές

24 Νοε 2013 (πριν από 4 χρόνια και 5 μήνες)

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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

. Assume that the load current is continuous. If
the firing angle of the SCR is
45º, draw the waveforms of the output

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
-
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

torque is 50 N
-
m at 1000 r
pm. The voltage constant is 0.8 V/A
-
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.

ii.

iii.

Explain the role of converter (s) placed at either end of the HVDC

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