Download BBA IInd SEMESTER EXAMINATION 2008-09

B.Tech IV (Fourth) Semester Examination 2012-13
Course Code: EEE403
Paper ID: 0964416
A C Machines
Time: 3 Hours
Max. Marks: 70
Note: Attempt six questions in all. Q. No. 1 is compulsory.
1.
a)
b)
c)
d)
e)
f)
g)
h)
Answer any five of the following (limit your answer to 50
words).
(4x5=20)
Compare capacitor start induction motor with resistance split
phase motor. What are the advantages of capacitor start
motor over resistance split phase motor?
Explain briefly the process of synchronising a synchronous
motor to the bus bar. What conditions determine the instant
of synchronising?
Compare squirrel cage induction motor with slip ring
induction motor.
Distinguish between unsaturated synchronous reactance and
saturated synchronous reactance. Which reactance is used to
predict the voltage regulation of a synchronous generator?
Show that at super synchronous speed the induction machine
acts as a generator. Write the expression for power generated
in the air gap. What is the net mechanical power input and
output?
Why are distributed windings preferred over concentrated
windings in the armature of a synchronous machine?
A 3-phase, 16 pole synchronous generator has a star
connected winding with 144 slots and 10 conductors per slot.
The flux per pole is 0.03Wb, sinusoidally distributed and
speed is 375 revolutions per minute. Calculate the frequency
and line induced emf assuming full pitched coil.
A 3-phase squirrel cage induction motor takes a starting
current of 6 times the full load current. Find the starting
torque as a percentage of full load torque if the motor is
started using star delta starter; the full load slip of the motor
being 4%.
2.
What methods are used in starting squirrel cage induction
motors? Discuss any one method of starting induction
motors.
(10)
3.
A 400V, 40hp, 3 phase induction motor gave the following
test data:
No load test:
400V, 20A, 1200W
Blocked rotor test:
100V, 45A, 2750W
Stator dc resistance per phase is 0.01Ω. The ratio of dc to ac
resistance per phase is 1.5. Friction and windage loss is
300W. Calculate the circuit elements of approximate
equivalent circuit of the motor.
(10)
4.
Explain the principle of split phasing used in a single-phase
induction motor. What are the different methods employed?
Explain each method.
(10)
5.
Explain the principle, working and characteristics of a
variable reluctance stepper motor.
(10)
6.
With relevant phasors, discuss the effect of change of
excitation of a synchronous motor on armature current and
power factor.
(10)
7.
A 6MVA, 11KV, 8 pole synchronous generator having
saturated synchronous reactance of 0.5pu is synchronised to
an 11KV bus. Calculate its synchronising power and torque
coefficient per degree mechanical shift of rotor angle at full
load and 0.8 pf lagging.
(10)
8.
Explain the term armature reaction. What is the effect of
armature reaction at lagging power factor load in a
synchronous generator?
(10)
.