SULIT

Seat No:

MOCK TEST SEMESTER 2

SESSION 2019/2020
FAKULTI KEJURUTERAAN ELEKTRIK

KOD MATAPELAJARAN : BEKG 2433

SUBJECT CODE

MATAPELAJARAN : SYSTEM ELEKTRIK

SUBJECT ELECTRICAL SYSTEMS

PENYELARAS/PENSYARAH : DR AIDA FAZLIANA

COORDINATOR/LECTURER

KURSUS : 2 BEKG
COURSE

MASA : 1 HOUR 30 MINUTES

TIME

TARIKH : 18 MAY 2020

DATE

ARAHAN KEPADA CALON

INSTRUCTION TO CANDIDATES

1. Answer all questions in this question sheets.

2. Total marks for this examination is __50_ marks.

THIS QUESTION PAPER CONTAINS ( 8 ) PAGES INCLUSIVE OF FRONT PAGE

CANDIDATE NAME:___________________________________

MATRIX NO:_________________________________________

PROBLEM 1:_______________________________ /25

PROBLEM 2:_______________________________ /25

SULIT

1
Question 1

a) Using relevant example and diagram, define period, frequency, amplitude and peak to peak
values of sine wave signal.
(4 marks)

Amplitude Frequency = 1/ Period(s) 1M

1M

Period 1M
1M

Or if only explanation of period, frequency, amplitude and peak to peak are given
(without diagram), only half value of mark will be given to the student.

b) A simple generator has a coil of 500 turns and 2 pair of poles. The coil is rotated at 1500
rpm in a magnetic field having a uniform density of 500 mT. The mean area per turn is 50
mm2. Determine the general expression of the generated e.m.f in trigonometric form (time
domain). (6 marks)
1500
n = speed of rotation = = 25 revolution per sec ond 1M
60
n  no of pole
f = frequency = n  pair of pole = = 25  2 = 50 Hz 1M
2
e = 2NnBAsin (2ft )

 sin (2  50t )

50
= 2  500  25  0.5  3M
1000  1000
= 1.96 sin (314.2t )V 1M

c) Two circuits connected in parallel take alternating currents which can be expressed
trigonometrically as i1(t)=14sin (314t -2/5) A and i2(t)=12sin (314t) A.
i. Sketch phasor diagram for i1 and i2 in one diagram/graph.
(4 marks)

2
 12 i2 2M

72o

14
i1
2M

ii. Determine the supply currents for the two circuits by using method of addition vector,
and express it in trigonometric form.
(8 marks)

o 12 i2 b
72o
i1+i2
14
i1
c
i2

bc = 14 sin 72 = 13.31 2M

ob = 12 + 14 cos 72 = 16.33

1M i1 + i2 = oc = 13.312 + 16.33 2 = 21.07 A
 13.31 
 = − tan −1   = −39.18
 16.33 
i1 + i2 = 21.07  − 39.18 A = 21.07 sin (314t − 39.18)A

1M

iii. Find the ammeter reading and the frequency of the resultant current.
(3 marks)
21.07 314
Irms = = 14.9 A, f = = 50 Hz
2 2
2M 1M

3
Question 2

a) With proper label, sketch the phasor diagram of voltage and current (take the voltage as
reference) in purely

i.Resistive circuit (2 marks)

ii.Inductive circuit (2 marks)

iii.Capacitive circuit (2 marks)

4
b) What is a power factor? How does the power factor can be improved? (4 marks)
The power factor of an AC electric power system is defined as the ratio of the real
power (P) flowing to the load to the apparent power (S) .
2M
Power Factor, PF = P/S = cos θ , where θ is the angle between V & I
PF can be improve or corrected by adding the Capacitive or Inductive loads. 2M

c) A 12  resistance is connected in series with a coil of inductance 0.2H and 90uF capacitor.
The circuit is energized by 50 Hz voltage supply. If the voltage across the resistor is
48 − 66.39 V . Determine:
i.The total impedance of the circuit
1M (4 marks)
1M
 
Z = 12 + j (2  50  0.2 ) − j 
1
 = 12 + j 62.83 − j 35.37 = 12 + j 27.46
 2  50  90u 
= 3066.39 2M

ii.The current
(3 marks)
48 − 66.39
I= = 4 − 66.39 = 1.6 − j3.7 3M
12

iii.The supply voltage

(3 marks)
Vsup ply = IZ = (4 − 66.39)  (3066.39) = 1200V 3M

iv.The phase difference and phasor relationship between the current and the supply
voltage
(2 marks)
phase difference = 66.39, voltage lead current by 66.39 2M

v.the total active power, total reactive power and total apparent power of the supply
(3 marks)

5
S = VI  = 1200  466.39 = 192.24 + j 439.8 = 48066.39 VA
P = 192.24 W, Q = 439.8 VAR, S = 480 VA 1M
1M 1M

END OF TEST 1