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-2

 

1. (a) Derive an expression for the force on a current carrying conductor placed in a

magnetic field.

(b) An air cored toroidal coil has 450 turns and a mean diameter of 30 cm and

a cross sectional area of 5 cm2. Calculate the inductance of the coil and the

average induced e.m.f if a current of 4 Amperes is reversed in 60 milliseconds.

 

2. The readings of the two wattmeters used to measure the power in a balanced 3 phase circuit are 2000 watts and 600 watts respectively. Find the power factor of

the circuit

(a) when both the readings are positive and

(b) when the readings of second wattmeter is obtained after reversing the connections to the current coil.

Draw the connection diagram and phasor diagram of the load.

 

3. (a) Explain the difference between the excitation system of a DC shunt generator and that of a DC series generator?

(b) What do you mean by self excitation mode of DC machine? Discuss the merits and demerits of self excitation over separately excitation.

 

4. Explain why a starter is required for starting a DC motor. Describe a 3-point

starter having no volt coil and over-load release protections for starting a DC shunt motor with neat diagram.

 

5. (a) A transformer takes a current of 0.6A and absorbs 64W when primary is

connected to a supply of 200V, 50Hz, the secondary being on open circuit. Find the magnetizing and iron loss currents

(b) Explain the basic types of transformer construction.

 

6. (a) Discuss the slip-torque characteristics of a 3-phase induction motor.

(b) A 50 Hz, 8-pole induction motor has full load slip of 4%. The rotor resistance and stand still reactance are 0.01  and 0.1  per phase respectively. Find the speed at which maximum torque occurs.

 

7. (a) Describe the two types of alternators based on rotor construction.

(b) Explain the winding factor and derive an expression for it.

 

8. What are moving iron instruments? List out their advantages and disadvantages.

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