Download QUESTION 1

(BENE 4173)
SULIT
PART A
QUESTION 1
(a)
Three branches, possessing a resistance of 50 , and inductance of 0.15 H and a
capacitance of 100 F respectively are connected in parallel across a 240 V, 50 Hz
supply as shown in Figure Q1(a). Calculate:
(i)
the current in each branch.
[3 marks]
(ii)
the supply current.
[3 marks]
(iii)
the phase angle between the supply current and the supply
voltage.
[2 marks]
I
IR
IC
L
C
+
IL
V
R
Figure Q1(a): xxxx
(b)
A simple series resonant comprises an ac source with an inductor, a capacitor and
optionally a resistor as shown in Figure Q1(b). Prove that the resonance frequency,
fr 
1
2 LC
(Hz) .
[4 marks]
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SULIT
(BENE 4173)
SULIT
L
R
+
V
C
Figure Q1(b) : xxxx
(c)
A 200V, 50 Hz single-phase supply feeds the following loads as written in the
Table Q1(c).
(i)
Determine the total current taken from the supply and the overall
power factor.
[6 marks]
(ii)
Find also the value of a static capacitor connected in parallel with
the loads to improve power factor to 0.98 lagging.
[7 marks]
Table Q1(c):xxxx
Item
Load Current
Power factor
Fluorescent lamps
8A
0.9 leading
Incandescent lamps
6A
unity
Motor
12 A
0.65 lagging
[25 MARKS]
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(BENE 4173)
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QUESTION 2
(a)
A magnetic circuit consists of a cast steel yoke which has a cross-sectional area of 200
mm2 and a mean length of 120 mm. There are two air gaps, each 0.2 mm long. The
magnetization curve for cast steel is given by the following:
Table Q2(a):xxxx
B (Tesla)
0.1
0.2
0.3
0.4
H (A/m)
170
300
380
460
Calculate:
(i)
the m.m.f. required to produce a flux of 0.05 mWb in the air gaps.
[5 marks]
(ii)
the value of relative permeability of cast steel at this flux density.
[1 mark]
(b)
A certain magnetic circuit may be regarded as consisting of three parts, A, B and C in
series, each one of which has a uniform cross-sectional area. Part A has a length of
300 mm and a cross-sectional area of 450 mm2. Part B has a length of 120 mm and a
cross-sectional area of 300 mm2. Part C is an airgap 1.0 mm in length and of crosssectional area 350 mm2. Neglecting magnetic leakage and fringing, determine the
m.m.f. necessary to produce a flux of 0.35 mWb in the airgap. The magnetic
characteristic for parts A and B is given by:
Table Q2(b):xxxx
B (Tesla)
0.7
0.85
1.0
1.15
1.25
H (A/m)
400
560
800
1280
1800
[10 marks]
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(BENE 4173)
(c)
SULIT
Clearly explain a magnetic hysteresis whereby a change in magnetization lags the
application of magnetic field intensity, H. Illustrate and label the curve.
[9 marks]
[25 MARKS]
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(BENE 4173)
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QUESTION 3
(a)
A 60 kVA, 1600 V/100 V, 50 Hz, single-phase transformer has 50 secondary
windings. Calculate:
(i)
the primary and secondary current.
[2 marks]
(ii)
the number of primary turns.
[1 mark]
(iii)
the maximum value of flux.
[2 marks]
(b)
A single-phase transformer has 2400 turns on the primary and 600 turns on the
secondary. Its no-load current is 4 A at a power factor of 0.25 lagging. Assuming the
voltage drop in the windings is negligible, calculate the primary current and power
factor when the secondary current is 80 A at a power factor of 0.8 lagging.
[6 marks]
(c)
The following results were obtained on a 50 kVA transformer: open-circuit test –
primary voltage, 3300 V; secondary voltage, 400 V; primary power, 430 W. Shortcircuit
test – primary voltage, 124 V; primary current, 15.3 A; primary power,
525 W; secondary current, full-load value. Calculate:
(i)
the efficiencies at full load and at half load for 0.7 power factor.
[6 marks]
(ii)
the voltage regulations for power factor 0.8 lagging and 0.8
leading.
(iii)
[5 marks]
the secondary terminal voltages corresponding to (i) and (ii).
[3 marks]
[25 MARKS]
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(BENE 4173)
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PART B
QUESTION 4
(a)
For the circuit of Figure Q4(a), suppose ERN = 240 0 V, Zrn = Zyn = Zbn = 12 – j9
.
(i)
Determine the phase voltages at the load.
[3 marks]
(ii)
Determine the line voltages at the load.
[2 marks]
(iii)
Show all voltages on a phasor diagram.
[1 mark]
+
ERN
–
E
+
n
IY
+
Y
yn
V
n
Vb
YN
VnN = 0 +
+
–
yn
–
–
–
N
EB
+
B
Zrn Vrn
IN = 0
– Z b n
–
N
+
r
IR
Z
R
b
y
IB
Figure Q4(a):xxxx
(b)
An 11 kV, three-phase, 50 Hz line of resistance 3 /phase and reactance j7 /phase
supplies an 11 kV/400 V transformer having negligible resistance and reactance of
j2 /phase referred to 11kV as shown in Figure Q4(b). The transformer supplies a 400
V feeder of resistance 0.01 /phase and reactance j0.005 /phase. If VR, the
receiving-end voltage, is 400 V, calculate VS, the sending-end voltage, when the threephase load delivered is 250 kW at unity power factor.
[7 marks]
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(BENE 4173)
SULIT
IR
3+j7 
0+j2 
0.01+j0.005 
VR 
VS
11 kV line
11 kV/400 V
transformer
400
V
3
400 V feeder
Figure Q4(b):xxxx
(c)
A supply system is shown in Figure Q4(c). The receiving-end voltage at the load is 33
kV. Determine the sending-end, input voltage at the nominal 275 kV grid.
[12 marks]
A
275/132 kV
C
Overhead
line
D
132/33 kV
50 MW
load
20 MVA
0.1 pu
j4.0 /ph
100 MVA
0.06 pu
0.85
pf. lag
Figure Q4(c)
[25 MARKS]
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(BENE 4173)
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QUESTION 5
(a)
Briefly explain the fundamental operation of AC motor which has two pieces with a
permanent magnet place between them. Sketch the position of rotor.
[6 marks]
(b)
Self-excited DC motor has three subdivisions which consist of shunt-wound machines,
series-wound machines and compound-wound machines. Describe each connection of
the field winding and the armature winding with the proper schematic diagram in each
subdivision.
[6 marks]
(c)
List down TWO (2) limitations to the thyristor operation in power electronic circuit.
[4 marks]
(d)
Pulse Width Modulation is one of the methods to control output voltage in DC to DC
conversion. Describe the concept of Pulse Width Modulation.
[9 marks]
[25 MARKS]
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