ZIMBABWE
TERTIARY EDUCATION,
MINISTRY OF HIGHER AND
TECHNOLOGY
SCIENCE AND
INNOVATION,
DEVELOPMENT
HIGHER EDUCATION
EXAMINATIONS COUNCIL
(HEXCO0)
NATIONAL DIPLOMA
IN
ELECTRICAL POWER ENGINEERING
SUBJECT:
Power Electronics
DURATION:
PAPER NO: 517/16/S02
3 Hours
NOVEMBER/DECEMBER 2 0 2 2 EXAMINATION
REQUIREMENTS
1.
Answer book
Non programmable caleulator
2
INSTRUCTIONS TO CANDIDATE
.
Answer any five (5) questions
2.
All questions carry equal
3.
Start a
n e w question o n a
marks
fresh page
This paper consists of 5 printed pages.
PAPER NO: 517/16/S02- POWER ELECTRONICSS
QUESTION 1
The buck-boost in
K
20.25 and the
Fig 1 has an input voltage of V,
=
12V.
The duty cycle
switching frequency is 25KHz. The inductance L
and filter capacitor C
=220uF, The average load current la
a)
b)
c
The average output
e
The entire values of L and C
voltage Va
The peak to peak output
voltage ripple, AV
The peak to peak
ripple current of inductor, Al
The peak current of the
transistor, 7p
=
=
150uH
1.25A. Determine
(3 marks)
(3 marks)
(3 marks)
(6 marks)
(5 marks)
Fig 1
t
L
2
PAPER NO: 517/16/S02 POWER ELECTRONICS
QUESTION 2
Draw the circuit diagram of a DIAC triggering circuit
Give that the DIAC breakdown voltage Vpo = 40V, C = 470nF, R = 1000 to
25000n
(4 marks)
V = 240Vrms, 50Hz. Determine the minimum and maximum firing angle.
(12 marks)
State any four (4) applications of the circuit above
(4 marks)
QUESTION 3
a
Draw the diagram of complementary communication
b)
A complementary communication in (a) has Ri = 200v, R2 = 10 and
(6 marks)
R2100.
Determine
i
Peat value of current through thyristor
i)
Value of capacitor,
if each
Ti and T2
(6 marks)
thyristor has turn off time is
404s. Take a factor of safety 2.
(8 marks)
QUESTION 4
a
b)
State two types of controllers
(4 marks)
A single-phase a.c voltage controller has a resistive load of R- 102 and
he input voltage is V, = 120V, 60Hz. The delay angle of thyristor is «=
"/2
Determine:
R.M.S value of output voltage (V%)
ii)
Input power factor (pf)
ii
Average input current
3
(2 marks)
(12 marks)
(2 marks)
PAPER NO: 517/16/S02 -POWER ELECTRONICSS
QUESTION 5
A single phase full converter
is supplied from 230V, 50Hz source, thhe
load consists of R = 102 and a large in inductance so as to render
load current constant.
the
For a firing angle delay of 30°, Detemine
a)
Average output voltage
(4 marks)
b
c
Average output current
(3 marks)
Average and r.m.s values of thrysitor current
(8 marks)
Power factor
(5 marks)
QUESTION 6
a)
The maximum junction temperature of a transistor is
Ti
=
150°C and
the ambient temperature is TA = 25°C. If the thermal empendances
are
Ric
i
=
0.4 c/w, Re
=
0.1°c/w and RsA
=
0.5° c/w. Calculate the
Maximum power dissipation
(6 marks)
Case temperature
(4 marks)
curve to
b
Draw a
c
List four
show the i - t characteristics of a fuse
(4) characteristic that are necessary
when
(6 marks
selectinga fuse
(4 marks)
QUESTION 7
a
Draw the symbols for the following
devices
Power diode
(1 mark)
i)
ii)
Schottky diode
(1 mark)
Fast recovery diode
(1 mark)
iv
S.C.R
v
G.T.O
vi
Triac
(1 mark)
(1 mark)
(1 mark)
vii
vii
Diac
(1 mark)
UJT
(1 mark)
ix)
PUT
(1 mark)
LASCR
(1 mark)
PAPER NO: 517/16/S02 POWER ELECTRONICS
(1 mark)
b)
xi)
I.G.B.T
xii)
MCT
(1 mark)
Draw the construction diagrams
i)
UJT
i)
PUT
iil
SCR
of the following
(3 marks)
(3 marks)
(2 marks)
QUESTION 8
a)
Draw the
where
b)
c
d)
steady-state
diagram showing
transient voltage
sharing is
turn
Draw BJT base drive for
achieved
on control
State any three (3) DC
drives
State three (3) types of
inverters
showinng
voltage and voltage
(8 marks)
(6 marks)
(3 marks)
(3 marks)
..../km