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Code No: W0221/R07
Set No. 1
II B.Tech I Semester Regular Examinations, November 2009
ELECTRICAL MACHINES-I
(Electrical & Electronic Engineering)
Time: 3 hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
? ? ? ? ?
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.
[16]
2.
(a) Explain the role of brushes in DC machines. What is the material used for
brushes? What is importance of brush pressure on performance of machine?
(b) A armature is wound with lap winding and 8 poles. There are 72 slots on the
armature surface and 8 conductors per slot. If the resistance of one conductor
is 2 mΩ, find the resistance of armature. Neglect the resistance of overhang.
Find the value of EMF induced if flux per pole is 60mwb and generator is
rotated at 300 rpm.
[6+10]
3.
(a) With a neat sketch explain the function of commutator in a d.c. machine.
(b) A 2-pole, 25KW, 220V wave-wound shunt generator has 400 armature con-
ductors. Brushes are given a lead of 4 commutator segments. Calculate the
demagnetization amp-turns/pole if shunt field resistance is 110Ω. Also calcu-
late extra shunt field turns/pole to neutralize the demagnetization.
[8+8]
4. What is critical speed? How do you calculate the critical speed in laboratory. [16]
5.
(a) What are the reasons for the parallel operation of DC generators?
(b) What are the conditions necessary for parallel operation of DC shunt genera-
tors?
(c) Explain the procedure of parallel operation of DC shunt generators. [4+4+8]
6.
(a) Differentiate between generator action and motor action of a DC machin