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Posted Date: 09 Dec 2010 Posted By:: VEERAIYAN BOSE Member Level: Gold Points: 5 (Rs. 1)
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2010 Anna University Chennai B.E Computer Science and Engineering Ec2201 — electrical engineering Question paper
B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2010
Third Semester
Electronics and Communication Engineering
EC2201 — ELECTRICAL ENGINEERING
(Regulation 2008)
Time: Three hours Maximum: 100 Marks
Answer ALL Questions
PART A — (10 × 2 = 20 Marks)
1. What are the conditions to be fulfilled for the self-excitation of a dc shunt
generator?
2. What are the functions of inter poles and how are the inter-poles windings
connected?
3. The emf per turn of a single phase, 6.6 kV/440 V, 50 Hz Transformer is
approximately 12 V.
Calculate
(a) the number of turns in the HV and LV windings and
(b) the net cross-sectional area of the core for a maximum flux density of
1.5 T.
4. Define voltage regulation of a transformer.
5. Why cannot an induction motor run at synchronous speed?
6. Why are single phase induction motors not self-starting?
7. What are the causes of faulty starting of a synchronous motor?
8. What are the applications of stepper motors?
9. Why are insulators used with overhead lines?
10. Define skin effect.
PART B — (5 × 16 = 80 Marks)
11. (a) (i) Describe with a neat sketch, the construction of a d.c. machine.
(ii) A separately excited dc generator running at 1000 r.p.m. supplied
110 A at 220 V to a resistive load. If the load resistance remains
constant, what will the load current if the speed is reduced to
800 r.p.m? Armature resistance is 0.02 . Field current is
unaltered. Assume a voltage drop of 1 V per brush. Ignore the effect
of armature reaction. [Marks 16]
Or
(b) (i) Derive from the first principle, an expression for the torque
developed in d.c. motor.
(ii) In a brake test on a dc shunt motor, the load on one side of the
brake was 35 kg and on the other side 5 kg. The motor was running
at 1500 r.p.m. its input being 34 A at 400 V. The diameter of the
pulley is 50 c.m. Determine the torque and efficiency of the motor. [Marks 16]
12. (a) (i) From first principles, derive the emf equation of a transformer. Also
show that the voltage induced per turn is the same, whether it is
primary or secondary.
(ii) A single phase transformer with a ratio of 6.6. kV/415 V takes a
no-load current of 0.75 A at 0.22 p.f. If the secondary supplies a
current of 120 A at 0.8 p.f. calculate the total current taken by the
primary. [Marks 16]
Or
(b) (i) Develop an equivalent circuit for a single phase two winding
transformer.
(ii) Calculate the full-load efficiency at 0.8 p.f. and the voltage at the
secondary terminals when supplying full load secondary current at
unity power factor, for a 4 kVA, 200/400 V, 50 Hz, single phase
transformer of which the following are the test results :
OC test (on primary) : V = 200 V ; I = 0.8 A ; W = 50 W
SC test (on secondary) : V = 17.5 V ; I = 9 A; W = 50 W [Marks 16]
13. (a) (i) Explain with neat sketches, the principle of operation of a three phase
induction motor.
(ii) A 6 pole, 3 phase, 50 Hz induction motor develops a maximum
torque of 30 Nm at 960 r.p.m. Determine the torque exerted by the
motor at 5% slip. The rotor resistance per phase is 0.6 . [Marks 16]
Or
(b) Discuss briefly the various methods of speed control of three-phase
induction motors. [Marks 16]
14. (a) (i) Derive the emf equation of an alternator. Discuss the effect of
winding factor on the induced emf.
(ii) Explain the speed-torque characteristics of a reluctance motor. [Marks 16]
Or
(b) (i) A 500 kVA, 3.3. kV, 3-phase, star-connected alternator is found to
give a short circuit current of 290 A at normal field current. Its
effective winding resistance per phase is 0.7 . Estimate the full
load of voltage regulation by EMF method for 0.8 pf lagging.
(ii) Explain the speed-torque characteristics of a hysteresis motor. [Marks 16]
15. (a) (i) Draw the single diagram of a typical a.c. power supply scheme.
(ii) With a neat sketch, explain the inter sheath grading of cables. [Marks 16]
Or
(b) (i) Explain briefly the advantages of EHVAC transmission over
EHVDC transmission. [Marks 16]
(ii) Draw the layout of a substation with the main equipments. [Marks 16]
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