Code No: R05220401
Set No. 1
II B.Tech II Semester Regular Examinations, Apr/May 2008
ELECTRICAL TECHNOLOGY
( Common to Electronics & Communication Engineering and Electronics &
Telematics)
Time: 3 hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1.
(a) Explain with suitable characteristics, the concept of critical resistance and
critical speed in a DC machine.
(b) A Series generator having combined armature and field resistance of 0.4Ω is
running at 1000 rpm and delivering 5.5kW at a terminal voltage of 110V. If
the speed is raised to 1500 rpm and load is adjusted to 10kW, find the new
current and terminal voltage. Assume the machine is working on the straight
line portion of the magnetization characteristic.
[8+8]
2.
(a) Derive an expression for the torque of a DC motor.
(b) A 240V, 50A, 800rpm DC shunt motor has armature resistance of 0.2Ω. If
the load torque is reduced to 60% of its full load value and a resistance of 2Ω.
is inserted in series with armature circuit, find the motor speed. Armature
reaction weakens the field flux by 4% at full load and by 2% at 60% of full
load.
[6+10]
3. What is an ideal transformer. Derive an expression for induced emf in a transformer
in terms of frequency the maximum value of flux, and the number of turns on the
winding.
[16]
4.
(a) Draw the equivalent circuit of a transformer and explain the procedure to
obtain the same from opencircuit and short circuit tests .
(b) A 10 KVA 2000/250 Volts single phase transformer gave the following test
results
Open circuit test : 250 V, 0.8 A, 50 W
Short circuit test: 60V 3A 45 W
Calculate the efficiency and voltage regulation at full load 0.8 power factor
lag.
[8+8]
5.
(a) Explain DOL starter with a neat sketch and obtain the expression for starting
toque in terms of Full-load torque.
(b) A 11KW, 3-phase, 6-pole, 50Hz, 400V Induction Motor runs at 960 r.p.m. on
full-load. If it takes 80A on Direct On Line switching, find the ratio of starting
torque to full-load torque when started by
i. DOL starte