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

for its ?eld winding. Under normal operating conditions, the I2
f rf loss in the
?led winding is 600 watts. calculate the energy stored in the magnetic ?eld
produced by the ?eld winding, under normal operating conditions. [8+8]
2. elaborate the ordinary kinds of armature winding for dc machine? discuss the
essential di?erence ranging from them and provide relative merits and the applications of
the 2 kinds windings. [16]
3. (a) How demagnetizing and cross magnetizing ampere turns per pole are calcu-
lated in a DC machine?
(b) Determine per pole, the number of
i. across magnetizing ampere turns
ii. demagnetising ampere turns
iii. series ?eld turns to balance the back ampere turns in the case of a dc
generator having the subsequent data
6000 conductors, total current 100A, six pole wave wound, angle of lead is
100
, leakage coe?cient = 1.3 [8+8]
4. (a) List the conditions for building up of a dc shunt generator.
(b) A d.c. shunt generator is supplying load connected to a bus - bar voltage of
220 V. It has an armature resistance of 0.025 O and ?eld resistance of 110 O.
compute the value of load current and load power when it generates an emf
of 230 V. Neglect the e?ect of armature reaction. Draw circuit diagram.[8+8]
5. (a) elaborate the reasons for failure of voltage build up in a self excited D.C
generator
(b) A shunt generator is to be converted into a level compounded generator by
the addition of a series ?eld winding. From a test on the machine with shunt
excitation only, it to provide 400V on no-load and 4.8A to provide the identical voltage
when the machine is supplying its full load of 200A. the shunt winding has
1200 turns/pole. ?nd the noos series turns needed per pole. [10+6]
6. (a) Draw and discuss the dc Series motor characterstics.
1 of 2Code No: 07A30201 Set No. 3
(b) The magnetization characteristic of a 4-pole dc series motor may be taken as
proportional to current over a part of the working range; on this basis the
?ux per pole is 4.5 mwb/A. The load requires a gross torque proportional to
the square of the speed equal to 30 Nm at 1000 rev/min. The armature is
wave-wound and has 492 active conductors. Determine the speed at which
the motor will run and the current it will draw when connected to a 220 V
supply, the total resistance of the motor being 2.0 ohm. [8+8]
7. A 250V dc series motor has armature and series ?eld resistance of 0.25 and 0.15ohms
respectively.
(a) compute the current for developing a torque of 80Nm at 1200 rpm.
(b) compute the percentage reduction in ?ux when the motor runs at 1800 rpm
at half the current found in part (a). [8+8]
8. A 50 Kw, 440 V shunt generator having an armature circuit resistance including
inter-pole winding of 0.15 ohm at normal working temperature was run as a shunt
motor on no-load at rated voltage and speed. The total current drawn by the motor
was five A including shunt ?eld current of 1.5 A. compute the e?ciency of the shunt
generator at 3/4th full-load. [16]
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2 of 2Code No: 07A30201 Set No. 4
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. (a) Prove that energy and coenergy in a linear magnetic system are provided by
identical expressions.
(b) A 10 KW, 1500rpm d.c shunt generator has a time constant Lf/Rf of 0.5 sec
for its ?eld winding. Under normal operating conditions, the I2
f rf loss in the
?led winding is 600 watts. calculate the energy stored in the magnetic ?eld
produced by the ?eld winding, under normal operating conditions. [8+8]
2. (a) With neat diagram provide the constructional features of dc machine.
(b) An six pole lap wound generator armature has 720 conductors, a ?ux of 30 mwb
and a speed of 600 r.p.m. compute the e.m.f generated on open circuit. If
the identical armature is wave wound, at what speed it be driven to generate 600
volts? [8+8]
3. A 4-pole generator supplies a current of 143A. It has 492 armature conductors
(a) wave wound
(b) lap wound connected. When delivering full load, the brushes are provided an
true lead of 100
.
compute the demagnetizing ampere turns/pole. This ?eld winding is shunt con-
nected and takes 10A. obtain the number of extra shunt ?led turns necessary to
neutralize this demagnetization. [8+8]
4. (a) Draw OCC of a dc shunt generator and de?ne critical speed and critical resis-
tance.
(b) A dc shunt generator has the subsequent open circuit magnetization curve at its
rated speed
Field current ( A ) : 0.5 1.0 1.5 two three 4
EMF ( V ) : 180 340 450 500 550 570
The resistance of the ?eld circuit is 200 O. The generator is driven at its rated
speed. obtain the terminal voltage on open circuit. (Use graph paper) [8+8]
5. (a) discuss how critical resistale and critical speed of a D.C. generator can be
obtained.
(b) Draw the load charectristics of all the D.C generators. [8+8]
6. A 250v four pole shunt motor has 2 circuit armature winding with 500 conduc-
tors.The armature circuit resistance is 0.25 ohms ?eld resstance is 125 ohm and the
?ux per pole is 0.02wb.neglect armature reaction.?nd the sped and torque devel-
oped if the motor draws 14 A from the mains. [16]
1 of 2Code No: 07A30201 Set No. 4
7. (a) elaborate the factors e?ecting the speed of dc motors.
(b) State 1 advantage and 1 disadvantage in the application of every of the
three basic kinds of dc motors. [8+8]
8. A 200V shunt motor has Ra=0.1O, Rf=240O and rotational loss=236W. On full
load the line current is 9.8A with the motor running at 1450 rpm. Determine:
(a) the mechanical power developed
(b) the power output
(c) the load torque
(d) the full load e?ciency. [4+4+4+4]
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Earning:   Approval pending.