Jadavpur University 2007 B.E Mechanical Engineering ELECTRICAL TECHNOLOGY-I - Question Paper

Ex/ML 11 H/30/2006
FIRST ENGG. (MECH.) EXAMINATION, 2006
( first Semester )
ELECTRICAL TECHNOLOGY-I
Time : 3 hours Full Marks : 100
( 50 marks for every part )
Use a separate Answer-Script for every Part
PART – I
Q.1 is compulsory; other 2 from the Rest.
1 a)
compute the currents through the galvanometer by Thevenin's
Theorem. 6
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obtain the loop currents of the circuit shown. 6
c) What is the condition of maximum power transferred to
ZL state and derive it. 6
2. a) A 100HP 400V 3ph. star connected synchronous motor
has resistance and leakage reactance ranging from the lines as
0.08 ohm and 0.8 ohm respectively.
compute for full load 0.8 P.F. leading the open circuit
emf per phase and mechanical power developed when
efficiency of motor is 92%. 8
[2]
[3]
b) What is the synchronous reactance of a synchonous
machine how it can be measured in the laboratory. 5
c) How the regulation of a synchronous generator depends
on the P.F. of the machine - discuss 3
3. a) A 250V shunt motor has an armature resistance of 0.6
ohm and a field resistance of 250Q. When driving at 650
rpm, a constant torque load, the armature takes 20A. If
it is needed to raise the speed from 650 rpm to 850 rpm.
What resistance must be inserted in the shunt field
assuming linear magnetisation characteristic. 8
b) i) Justify the use of carbon as brushes in D.C. meachine.
ii) elaborate the reasons for a D.C self excited shunt
generater for failure to build up.
iii) Why interpoles are needed for D.C. machine? Draw
the connection diagram
iv) Why series motor is used to drive high inertia load?
4x2=8
4. a) Draw neatly and discuss the working principle of a spring
controlled moving coil ammeter. 6
Can it be used for AC/DC or both ? cause out. 2
b) A shunt to be designed for an ammeter in case of variable
frequency input. Derive the condition for which division
of currents should be independent of frequency. 5
c) compute the resistance of shunt needed to make a
milliammeter which provide maximum deflection for a
current of 15mA. Which has a resistance of five ohm learn
upto 10 amp. 3
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[4]
PART - II
ques. No. five is compulsory.
ans any 2 from the Rest.
5. a) What is leakage flux and fringing effect in a magnetic
circuit ? 4
b) Prove that unit of reluctance is ampere per weber. 4
c) A cast steel ring has a circular cross part 3cm in
diameter and a mean circumference of 80cm. The ring
is wound with a coil of 100 turns
i) compute the current needed to produce a flux of
0.5 mWb in the ring.
ii) If a saw cut 2mm wide is made in the ring, obtain the
value of current if the usefull flux needed in the air
gap is 0.5 mWb.
presume 25% of the useful flux as the leakage flux and
constant permeability of cast steel equal 1.1x10–3 H/m.
10
6 a) Prove that an ideal inductance does not consume any
power over the full cycle of a.c. sinusoidal wave. 4
b) A voltage v=100 sin 314t is applied to a circuit consisting
of a 25 ohm resistor and a 80 microFarad capacitor in
series. obtain
i) an expression for the value of current flowing at any
instant.
ii) the power consumed. 6
[5]
c) A resistor is connected in parallel with a 40 microFarad
capacitor to an a.c. supply at 110V, 50C/S. The supply
current and voltage are obtained to differ in phase by 30°.
Determine the value of the resistance and supply current.
Draw the relevant phasor diagram. 6
7. a) In a series R-L-C circuit prove that the voltage across
the capacitor is Q times the applied voltage under
resonance condition, where Q is the quality factor of the
circuit. 4
b) A 10 mH coil is connected in series with a loss-free
capacitor to a variable frequency source which supplies
a constant voltage of 10V. The circuit current has a
maximum value of 0.1 A at a frequency of 80 KHz.
Calculate
i) the capacitaree of the capacitor and
ii) the Q factor of the coil. 6
c) A star connected load has impedance of (3+j4) ohm in
every phase and connected across a balanced 3 phase
delta connected alternator having the line voltage of 120V.
find the line currents of both the load and alternator.
6
8. a) Justify with cause - in which side the open circuit test
of a 33 KVA, 2200/220V, 50Hz single phase transformer
is to be performed. 4
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[6]
b) What info we can get from the short circuit test
of a transformer ? 2
c) A 30 KVA, 2400/240V, 50 Hz single phase transformer
gave the subsequent test outcomes :
O.C. test : 240V, 3.75A, 180 Watts
S.C. test : 120V, 12.50A, 550 Watts
i) obtain the parameters of the approximate equivalent
circuit referred to L.V. side.
ii) obtain the full load efficiency at 0.8 p.f. lagging and
leading.
iii) obtain the maximum efficiency at unity power factor.
10
9. a) discuss clearly why the rotor of 3 phase induction
motor can never attain synchronous speed. 6
b) Prove that in a 3 phase induction motor,
where a is the ratio of rotor resistance to rotor stand still
reactance per phase. 5
c) A properly shunted centre - zero galvanometer is
connected in the rotor circuit of a 6-pole, 50Hz wound
rotor in duction motor. If the galvanometer makes 90
complete oscillations in 1 minute, compute the rotor
speed. 5

Earning:   Approval pending.