Nepal Engineering College
Level: Bachelor
Assessment
Program me: BE
Course: Basic Electrical Engineering
Year
: 2015
Full Marks: 100
Time: 3 hrs.
Candidates are required to give their answers in their own words as far as practicable.
The figures in the margin indicate full marks.
Attempt all the questions.
1a.
Discuss briefly about Kirchhoff’s current and voltage law. 7
b.
Find the equivalent resistance between A& B? 8
OR
a.
b.
Define the term generation, transmission and distribution system for electrical
engineering? 7
Find the equivalent resistance between A& B? 8
OR
a.
Find equivalent resistance between points AB of the network shown. 7
b. Using KVL equation, find the current through 2 Ώ resistor of the network shown below.
8
2a. Calculate the power consumed by 8Ώ resistor, using superposition theorem? 7
b. State and derive the maximum power transfer theorem? Also find the efficiency for
maxm power condition? 8
OR
a. Using mesh analysis, determine current (IX) in the following network. 7
b. State “maximum power transfer theorem”. Obtain the Thevenin’s equivalent circuit at
terminal A& B of the following network. (2+6)
OR
a. Using KVL equation, find the current through 8Ώ resistor? 7
b. Using Thevenin’s theorem, find the current through 2Ώ resistor?
8
3a. Find the average value, effective value, form factor and peak factor of the sinusoidal
waveform? 7
b. An impedance of (5+j7) Ώ and (10-j5) Ώ are connected in series across 200 V, 50 Hz
supply. Find the current, active power, apparent power, power factor, resonance
frequency, quality factor and bandwidth? Also draw the phasor diagram.
8
OR
a. Explain the concept of single phases AC EMF generation? 7
b. Calculate the average value, effective value, form factor and peak factor of output
voltage wave of a half-wave rectifier shown below. 8
OR
a. Explain the concept of 3-phase AC EMF generation? 7
1
b. Derive resonant frequency, fr = 2𝜋√𝐿𝐶 in RLC series circuit and compare with parallel
resonant circuits with respect to current, impedance, power factor & resonant frequency.8
4a. Explain and describe with a neat diagram of the two wattmeter method for the
measurement of 3-phase power. 8
b. A balanced star connected load of (8+j6) Ώ per phase is connected to 3-phase 230
V,supply. Find the line current, power factor, volt-amperes and reactive power. Draw the
phasor diagram. 7
OR
a. Draw wave diagram and phasor diagram to illustrate clearly the relation between (V) and
(i) in case of:
[i]
R-L series circuit
[ii]
R-C series circuit
[iii] R-L-C series circuit
[iv]
R-L-C series resonance circuit
b. A coil of 1.5KΏ resistance and 0.2H inductance is connected in a parallel with a variable
capacitor across a 1.5V, 15KHz ac supply. Calculate the value of the capacitor when the
supply current is minimum.
OR
a.
Show that the active power in a 3-phase balanced system is measured by using single
wattmeter with the help of suitable phasor diagrams. 7
b. A balanced delta connected load of (8+j6) Ώ per phase is connected to 3-phase 230 V,
supply. Find the line current, power factor, volt-amperes and reactive power. Draw the
phasor diagram. 8
5a. Draw the exact equivalent circuit of single phase transformer? 7
b. The following test results were obtained for 100 KVA, 220/500 V, single phase
transformer.
O.C. Test
S.C. Test
Find: [i]
[ii]
[iii]
250 V
60V
6A
240A
800W
300W
LV Side
HV Side
Magnetizing component and core loss component of no load current
the voltage regulation at 0.8 power factor lagging
the efficiency at full load 0.8 power factor.
8
OR
a. Derive the emf equation of a single phase transformer. Draw the phasor diagram for
transformer on no-load. [5+2]
b. A 10 KVA, 4000/400 volt transformer has primary and secondary winding resistance
of 13 Ώ and 0.15 Ώ and leakage reactance of 20 Ώ and 0.25 Ώ respectively. The
magnetizing reactance is 6000 Ώ and the resistance representing core loss is 12000 Ώ.
Determine (i) equivalent resistance and reactance as referred to primary (ii) input
current with secondary terminals open circuited (iii) input current when the secondary
load current is 25 A at 0.8 power factor lagging.
8
a.
b.
6a.
b.
OR
Explain the working principle of an ideal transformer, when the secondary winding
delivers load currents. 7
A 50 KVA, 2500/250 V, single phase transformer has a primary winding resistance of
3 Ώ and a reactance of 5 Ώ. The secondary winding resistance and reactance are
0.02Ώ and 0.03Ώ respectively. Find (i) equivalent resistance, reactance and
impedance as referred to primary winding (ii) equivalent resistance, reactance and
impedance as referred to secondary winding (iii) total copper loss in the transformer.8
What is armature reaction? How does its effect can be neutralized in DC machines? 7
Derive the emf equation in DC machines? 8
OR
a.
b.
Draw the speed-load characteristics of DC shunt and series motors. 7
Explain the operating principle of 3-Φ induction motor. 8
OR
a.
b.
7.
Draw the circuit diagrams of the different types of DC motors. Also draw their speedload characteristics curves. 7
A 250 V shunt motor has an armature resistance of 0.5 Ώ and field resistance of 50 Ώ.
Find the value of resistance must be added to the field circuit to increase the speed from
1000 rpm to 1400 rpm, when the supply current is 150 A. 8
Write Short notes on any two? (5×2=10)
a) The role of electricity in modern society
b) Quality factor
c) Active and passive element
d) Advantages of 3-Φ system.
e) Bandwidth
f) Power factor
g) Ideal and Practical sources
h) Color coding of resistor