JNTU II B.Tech I Semester Supplimentary Examinations, November 2008
ELECTRICAL MACHINES – I
(Electrical & Electronic Engineering)
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
2. Explain the various pitch(es) related with armature winding. Derive the relation between each for lap and wave 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 calculate the critical speed in laboratory.
5. (a) Explain the external characteristic of DC series generator. Why does the
terminal voltage start decreasing after a certain value of load current? Also give its applications.
(b) A separately excited generator with constant excitation is connected to a constant load. When the speed is 1000 rpm, it delivers 120A at 500V. At what speed will the current be reduced to 60A ? Armature resistance is 0.1 ohm, Contact drop/brush is 1V. Armature reaction may be ignored.
6. (a) Define torque. Derive the expression for torque developed by a D.C. motor from fundamentals.
(b) Determine the torque developed when a current of 30A passes through the armature of a motor with the following particulars: lap winding, 310 conductors, 4-pole, poleshoes 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) Explain the various losses taking place in DC machines. With the help of these losses draw the power flow diagram for a DC Motor.
(b) A 440 V DC shunt motor takes a current of 3 A. at no-load. The armature resistance including brushes is 0.3 and the field current is 1 A. Calculate the output and efficiency when the input current is 20 A.