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Third Semester

Electronics and Communication Engineering


(Regulation 2008)

Time: Three hours

Answer ALL Questions

PART A — (10 × 2 = 20 Marks)

  1. What are the conditions to be fulfilled for the self-excitation of a dc shuntgenerator?
  2. What are the functionS of interpoles and how are the interpoles windingsconnected?
  3. The emf per turn of a single phase, 6.6 kV/440 V, 50 Hz Transformer isapproximately 12V Calculatethe number of turns in the HV and LV windings andthe 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)

(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 Ω . Field current is
unaltered. Assume a voltage drop of 1 V per brush. Ignore the effect
of armature reaction.

(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.


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.

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



Develop an equivalent circuit for a single phase two winding
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) :

SC test (on secondary) : V = 17.5 V ; I = 9 A; W = 50 W

(i)Explain with neat sketches, the principle of operation of a three-
phase induction motor.
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 Ω .

(ii) V = 200 V ; I = 0.8 A ; W = 50 WDiscuss briefly the various methods of speed control of three-phase
induction motors.


(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.


(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 Ω . 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.

(i)Draw the single diagram of a typical a.c. power supply scheme.

(ii)With a neat sketch, explain the intersheath grading of cables.


(i)Explain briefly the advantages of EHVAC transmission over
EHVDC transmission.

(ii)Draw the layout of a substation with the main equipments.


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