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

1. With the help of neat diagram obtain the expression for the energy stored in a magnetic system for a simple attracted armature type relay. Explain the operation of system.

 

2. (a) Explain the construction of armature core. What type of material is used for armature core? Explain the reasons.

(b) The armature of a DC generator is Wave wound with 2 poles. There are 40 slots on the armature surface and 12 turns per coil. The armature winding is double layer winding. The resistance of each conductor is 10 milliohm, find the resistance of armature and EMF generated, if flux per pole is 45mwb and generator is rotated at 350 rpm. Repeat calculation for Lap winding.

 

3. (a) What are the causes of sparking in a d. c. machine? Explain how commutation

is improved by use of inter poles.

(b) A 22.38 kW, 400-V, 2-pole wave-wound d.c., shunt motor has 840 armature

conductors and 140 commutator segments. Its full-load efficiency is 88% and

the shunt field current is 1.57A. If brushes are shifted backward through 1.5 segments from the geometrical neutral axis, find the demagnetizing and distorting amp-turn /pole.

 

4. What is critical speed? How do you draw magnetization characteristics in laboratory.

 

5. (a) Distinguish between:

i. DC shunt generator and

ii. DC compound generator.

Show and explain the nature of external characteristic of each of them. Also give applications of each of them.

(b) Eight DC Shunt Generators are running in parallel. Each generator supplies a load current o f 400A at 210V. The shunt field current of each generator is 4A and the armature resistance of each generator is 0.04 ohm. If one generator is suddenly switched off, determine the % change in terminal voltage, the total load current being kept unchanged.

 

6. (a) Define torque. Derive from first principles, the torque equation of a DC motor.

 (b) Determine the torque developed when a current of 30A passes through the armature of a motor with the following particulars: wave winding, 310 conductors, 4-pole, pole-shoes 16.2 cm long subtending an angle of 600 at the centre, bore radius 16.2 cm, flux density in air gap 0.7 tesla.

 

7. (a) Explain the functions of NVC and OLC in 4 Point starter. What are the advantages and limitations of this starter on 3 point starter?

(b) The EMF developed in the armature of a shunt generator at 1155 RPM is 240 Volts for a field current of 4.5 Amps and 255 V for a field current of 5 Amps. The generator is now used as a motor on a 260 V supply and takes an armature current of 75 Amps. Find the motor speed, when the field current is adjusted to 4.8 Amps, total armature resistance is 0.12 .

 

8. (a) The following readings are obtained when performing a brake test on DC shunt

motor. Spring Balances, 08 kgs and 30 kgs. Diameter of drum = 42 cm. Speed of the motor = 1000 rpm. Applied voltage = 220 V. Line current = 50 A.

 out put power and efficiency.

(b) A 500 V DC shunt motor takes a current of 5 A on no-load. The resistances

of the armature and field circuit are 0.2  and 300  respectively. Find, the efficiency when loaded and taking a current of 125 A and the percentage change of speed.

 

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