JNTU II B.Tech I Semester Regular Examinations, November 2008

JNTU II B.Tech I Semester Regular Examinations, November 2008

ELECTRICAL ENGINEERING

( Common to Chemical Engineering and Metallurgy & Material Technology)

SET-4

 

 

1. (a) State the logic supporting the Kirchhoff‘s Laws of Electrical Circuit?

(b) A coil has a resistance of 22 ohms when its mean temperature is 20 degree centigrade and 26 ohms when its mean temperature is 70 degree centigrade. Find its mean temperature rise, when its resistance is 28 ohms and the ambient temperature is 15 degree centigrade.

2. (a) A circuit of resistance 20 ohms and inductive reactance 15 ohms is connected

in parallel with another circuit which contains resistance of 20 ohms in series with a capacitive reactance of 20 ohms. This combination is energized from a 200V, 40 Hz supply. Determine the value of capacitance to be connected in parallel to improve the power factor of this circuit to unity.

(b) An inductive coil having resistance of 20 ohms and inductive reactance of 40

ohms is connected in parallel with a capacitive reactance of 48 ohms. This combination is energized from a 200 V, 30 Hz AC supply. Find the total current drawn by the circuit and its power factor.

 

3. (a) An 8 – pole DC shunt generator with 788 wave connected conductors and run-

ning at 500 r.p.m, supplies a load of 12.5 ohms resistance at a terminal voltage of 250 volts. The armature resistance is 0.24 ohms and the field resistance is 250 ohms. Find:

i. Armature current

ii. Induced e.m.f and

iii. Flux/pole.

(b) A short shunt compound generator has armature, series field and shunt field

resistance of 0.08 ohm, 0.04 ohm and 100 ohm respectively. It supplies 125 lamps each rated at 250 volts, 40 watts. Find the generated e.m.f and the armature current. Drop under each brush may be taken as 1 volt.

 

4. Explain the speed control methods of a DC shunt and series motors with the help of neat circuits and also mention the limitations of each method.

 

5. Derive the expression for KVA output corresponding to the maximum efficiency of a 1- ? transformer and explain voltage regulation.

 

6. (a) Obtain an expression for torque produced in 3- ? Induction motor.

(b) Discuss the slip-torque characteristics of an Induction motor with neat sketch.

 

7. A 500V, 50 KVA, 3- ? alternator has an effective resistance of 0.2 . A field current

of 10A produces an armature current of 200A on short circuit and an emf of 450V on open circuit. Calculate the full load regulation at 0.8 lagging and leading power factors.

 

8. Describe the construction and working of PMMC instrument. Derive the equation

for the deflection of the instrument when it is spring controlled. Describe the method of damping used in these instruments.

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