Jawaharlal Nehru Technological University Kakinada 2008 B.Tech Electrical and Electronics Engineering ELECTRICAL MACHINES-I - Question Paper

Code No: 07A30201 Set No. 1
II B.Tech I Semester Regular Examinations, November 2008
ELECTRICAL MACHINES-I
(Electrical & Electronic Engineering)
Time: three hours Max Marks: 80
ans any 5 ques.
All ques. carry equal marks
? ? ? ? ?
1. In a rectangular electro magnetic relay, he exciting coil has 1200 turns. Cross
sectional are of the core is A = six cm × six cm. neglect the reluctance of he magnetic
circuit and fringing e?ects. With coil current kept constant at 2A, derive expression
for force on armature as a function of air gap of length x. obtain the work done by
the magnetic ?eld when x reduces from one cm to 0.5 cm by integrating the force.
[16]
2. (a) Deduce an expression for the voltage induced in a DC generator.
(b) The lap wound armature of a four - pole generator armature has 51 slots. every
slot contains 20 conductors. What will be the e.m.f generated in machine
when driven at 1500 r.p.m. If useful ?ux per pole is 0.01Wb? [8+8]
3. (a) discuss the armature reaction in a D.C generator on no-load. Enumerate and
discuss the method to overcome the adverse e?ects of the armature reaction.
(b) A 4-pole 50 KW, 250v wave wound, shunt generator has 400 armature con-
ductors. Brushes are provided a lead of four commutator segments. compute the
demagnetizing ampere turns/pole if shunt ?eld resistance is 50 ohms. Also
compute extra shunt ?led turns/pole to neutralize the demagnetization.[8+8]
4. (a) How do you determine the magnetization characteristics of a DC generator.
(b) A separately excited generator when running at 1200 rpm supplies a current
of 200A at 125 V to circuit of constant resistance. What will be the current
when the speed drops to 1000 rpm if the ?eld current is unaltered? Armature
resistance is 0.04 ohm and the total voltage drop at the brushes is 2V. Ignore
the change in armature reaction. [8+8]
5. (a) discuss the drooping charectoristics of a D.C shunt generator
(b) A d.c series generator having an reternal charectristics which is a straight
line through zero to 50V at 200A is connected as a booster ranging from a station
busbar and a seeder of 0.3O resistance. compute the voltage di?erent ranging from
the station busbar and the far end O the seeder at a curent of
i. 1200A and
ii. 50A. [6+10]
6. A 20 KW,250 V dc shunt motor has a full-load armature current of 85 A at 1100
rpm.The armature resistance is 0.18 ohm.
Determine:
1 of 2Code No: 07A30201 Set No. 1
(a) the initial torque developed;
(b) the internal torque of the ?eld current is suddenly decreased to 80% of its original
value;
(c) The steady motor speed in part

Earning:   Approval pending.