Andhra University 2007 B.E Electronics & Communication Engineering - Question Paper

II Year B.E./B.Tech Degree exam
ELEMENTS OF ELECTRICAL MACHINES
Electronics & Communication Engineering
Time : 3 hours Maximum : 75 Marks
ques. one is compulsory
ans any 4 from ques. two to 8
All ques. carry equal marks




1. a) What is Armature Reaction in a DC Machine.

b) discuss the term Synchronous Impedance of an Alternator.

c) describe ‘All Day Efficiency’ in a Transformer and discuss

how can it be improved.

d) define a starter suitable for a 3-phase slip-ring Induction

Motor.

e) What is a split-phase Induction Motor.

2. a) discuss the different kinds of excitations used in D.C.

generators.

b) A 230 volts, D.C. shunt motor has an armature resistance

of 275 ohms. It runs at 1000rpm while drawing an armature

current of 75 Amps. Cauculate the additional resistance to

be inserted in the field circuit to raise the speed to 1200 rpm

at an armature current of 125 Amps. presume linear

characteristics.

3. a) With usual notation, derive an expression for the torque

developed by D.C. motor and hence explain the

characteristics (torque Vs speed) of D.C.shunt and series

motors.

b) A four pole, wave wound, 750 rpm D.C. shunt generator has

an armature and field resistance of 0.04 ohm and 200

II Year B.E./B.Tech Degree exam

ELEMENTS OF ELECTRICAL MACHINES

Electronics & Communication Engineering

Time : 3 hours Maximum : 75 Marks

ques. one is compulsory

ans any 4 from ques. two to 8

All ques. carry equal marks

6

respectively. The armature has 720 conductors and the flux

per pole is 28.95 mwb. If the load resistance is 10 ohms,

determine the terminal voltage of the machine.

4. a) discuss the operation of 3-phase synchronous motor with

help of phasor diagram when the excitation is varied with

constant output power.

b) If a field excitation of 10 Amps in a certain 3-phase, star

connected alternator provide a current of 150 Amps on short

circuit and a terminal voltage of 900 volts on open circuit,

obtain the internal voltage drop with a load current of 60 Amps.

5. a) discuss the development of the equivalent circuit of singlephase

transformer.

b) The O.C.and S.C test outcomes on a single-phase, 50Hz, 250/

500 volts transformer are as:

O.C test (L.V.side): 250 volts, one Amp, 70 watts

S.C. test (H.V.side): 20 volts, 11 Amp, 120 watts.

compute the efficiency and approximate regulation while

supplying 10 Amps at 500 volts at 0.7 power factor lagging.

6. a) Derive an expression for the gross torque of a 3-phase

induction motor in terms of rotor standstill reactance and

resistance.

b) A 30 HP, 3-phase, 50 Hz, six pole induction motor has a full

load slip of 0.04, the Mechanical losses are 250 watts.

compute the rotor speed and the motor copper losses.

7. a) A 3-phase induction motor is self starting motor. Explain,

why it is necessary to have starter. Name the various kinds

of starters used on the stator side. provide the internal

connections of 1 kind of starter.

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b) A 3-phase induction motor has rotor resistance and stanstill

leakage reaqctance of one ohm and four ohms respectively per
phase. The torque developed is 50 N-m at a slip of 5% with

rated voltage. compute the maximum torque developed and

the slip at which it occurs assuming the rated voltage applied

to the stator.

8. a) Explain, how is rotating magnetic field produced in 1-phase

induction motor?

b) discuss the principle of operation, construction and provide the



applications of capacitor begin induction motor.

Earning:   Approval pending.