Jadavpur University 2007 B.E Mechanical Engineering ELECTRICAL TECHNOLOGY-I - exam paper

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FIRST ENGINEERING (Mechanical) EXAMINATION, 2007
(1st Semester, Supplementary)
ELECTRICAL TECHNOLOGY–I
Time : 3 hours Full Marks : 100
(50 marks for every part)
Use a separate Answer-Script for every part.
PART–I
Attempt ques. No. one and 2 others from the rest.
1. a) State Thevenin's Theorem and discuss its limitation. 4
b) obtain IO in the circuit provided beneath : 7
c) obtain Rin 7
Ex/E.TECH./EE/T/I/70/07(SUPPLE)
2. a) A D.C. series motor, when giving 50 hp at 500 rpm the
copper losses are 7% of input. and the other losses are
4% of input. obtain the output and the speed when the
current is halved. The flux is decreased by 30% due to
reduction in current. 7
b) i) A D.C. shunt motor is driving a load at rated voltage
and rated excitation what will be the effect if a diverter
is put across the armature. 3
ii) A D.C. shunt generator is supplying a variable load.
discuss its behaviour graphically with increase in loading.
3
iii) How the external characteristic of a shunt generator
gets replaced from its open circuit characteristic. 3
3. a) What is the importance of damping torque in electrical
measuring instruments. How does it work. 4
b) Deduce the working relation of a spring controlled
permanent magnet moving coil ammeter with a sketch.
Can it be used for A.C. and D.C ? 4
c) A 250 V d.c. shunt motor with Rf = 150 O and
Ra = 0.6O. The motor operates on no load with full field
flux at its base speed of 1000 rpm with Ia = five amp. If
the machine drives a load requiring a torque of 100 Nm.
compute armature current and speed of motor. If the
motor is needed to develop 10 kW at 1200 rpm what
is the needed value of external resistance in the field
circuit ? Neglect saturation and armature reaction. 8
( two )
8. a) A coil of resistance 10.0 Ohms and inductance 0.1 H is
connected in series with an 80 microFarad capacitor. At
what frequency does resonance occur ? If the applied
voltage is 200 volts, obtain the current at this frequency.
obtain also the voltage across the coil and the voltage
across the capacitor. 8
b) 3 inductive coils every with a resistance 15.0 Ohm
and an inductance of 0.03 H are connected (i) in star
and (ii) in delta, to three phase, 400 V, 50 Hz balanced
supply. compute for every case (i) phase current and
(ii) total power absorbed. 8
9. a) discuss clearly why the rotor of a 3-phase induction
motor rotates in the direction of air gap rotating
magnetic field. 6
b) Prove that torque-slip characteristic of 3 phase
indirection motor at small slips is very nearly a straight
line. 4
c) An 8-pole alternator runs at 750 rpm. It supplies power
to a 6-pole, 3-phase induction motor, which has a full
load slip of three per cent. obtain the full load speed of the
induction motor and the frequency of its rotor emf. 6
––––––––×–––––––
( five )
c) An iron ring 15 cm in diameter and 10 cm2 in cross
part in wound with 200 turns of wire. For a flux
density of 1.0 wb/m2 and a permeability of 500, obtain the
exciting current. 6
6. a) define the open circuit test of a 1-phase transformer
with the help of a circuit diagram, showing the
connections of different meters used for this test. Draw
the equivalent circuit. What is the value of applied
voltage ? What info do we find from the
readings of these meters ? 10
b) A 40 KVA, 3000/240 V, 50 Hz 1-phase transformer has
600 turns on the primary. Determine :
(i) The number of turns on the secondary.
(ii) The maximum flux in the core.
(iii) The approximate values of primary and secondary
currents at full load. 6
7. a) Develop and discuss the phasor diagram of a 1-phase
real transformer on load under lagging power factor
condition. 8
b) A 100 KVA, two winding transformer has an iron loss of 1
KW and a copper loss on normal output current is 1.5
KW. obtain out the KVA rating at which the efficiency is
maximum and the efficiency of the transformer at this
loading at 0.8 p.f. lagging. 8
( four )
4.a) i) discuss how load Power Factor of a synchronous
Alternator influences the excitation voltage with the help
of phasor diagram. 3
ii) explain the effect of armature current with its excitation
in a synchronous motor. 3
b) A 22 KV three phase ?-connected turbo alternator with a
synchronous impedance of three + j4 O/phase is delivering
230 MW at unity power factor to 22 KV grid. With the
turbine power remaining constant the alternator is
excited 30% more. Determine the machine current and
Power factor based on linearity assumption.
At the new excitation ; the turbine power is now
increased till the machine delivers 275 MW. compute
the new current and Power Factor. 10
PART–II
ans any 3 ques..
All ques. carry equal marks.
Two marks allotted for neatness.
5. a) Bring out the analogy ranging from magnetic and electric
circuit. 6
(b) Show that f = where
f = magnetic flux
m.m.f. = magnetomotive force
d = reluctance 4
( three )
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m.m.f.
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Earning:   Approval pending.