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Posted By: gopal Member Level: Diamond Posted Date: 01 Sep 2008
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2005 Jawaharlal Nehru Technological University Electrical and Electronics Engineering II B.Tech. I Semester Supplementary Examinations, May -2005 ELECTRO MECHANICS-I Question paper
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Code No: RR210205 Set No.3
II B.Tech. I Semester Supplementary Examinations, May -2005
ELECTRO MECHANICS-I
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
? ? ? ? ?
1. (a) Explain the principle of energy conversion of electromechanical system.
(b) Derive an expression for the energy stored in a magnetic field system.
2. A 4-pole wave connected armature has 51 slots. Draw a developed winding diagram
and show the brush positions. Assume any other data required.
3. (a) Explain the effects of armature reaction in a d. c. generator and discuss briefly
the methods to minimize these effects.
(b) With a neat sketch explain the function of commutator in a d.c. mechine.
4. Two d.c. compound generators, A and B with an equilising bar, supply a total load
of 500A. The data relating to the machine are as follows.
Armature resistance, RA = 0.05 ohm, RB = 0.03 ohm
Series field winding RSA = 0.02 ohm, RSB = 0.01 ohm
Generated emf EA = 463V, EB = 470X.
Calculate
(a) The current in each armature.
(b) The current in each series winding.
(c) The current in the equilising bar and
(d) The bus-bar voltage.
Neglect the shunt currents and state the necessary assumptions made, if any.
5. (a) Distinguish between generator and motor action. Derive the equation for the
back emf of a DC motor.
(b) What are the different types of DC motors and give their applications.
6. (a) What is the power flow diagram of DC motor? And explain about losses
involved in each stage?
(b) A 4-pole 120KW, 240V, 800rpm wave wound generator has shunt field current
of 4A at rated voltage. The generator has the following data.
Armature winding single turn coils
Length of conductors (including over hang) = 0.48 m
Number of conductors = 480 : Voltage drop/brush = 1 volt
Cross sectional area of conductors = 25 mm2
Full load temperature = 600C : Commutator diameter = 0.6 m
Specific resistance of copper at 200C = 1.725 × 10-2
/m/mm2 Find
i. Full - load armature copper loss
ii. Shunt field copper loss, and
iii. Brush contact loss
7. (a) Explain why Swinburne’s test cannot be used to determine the efficiency of
d.c. series machines.
(b) Explain how a swinburne’s test can be used to predetermine the efficiency of
a d.c. machine, when used as
i. a generator &
ii. as a motor
(c) Write a note on Constant and Variable losses.
8. (a) Explain how the efficiency of d.c. series motors can be determined, by con-
ducting field’s test, with a neat circuit diagram.
(b) A test on two coupled similar tramway motors, with their fields connected in
series, gave the following results when one machine acted as a motor and the
other as a generator. Calculate the efficiency of motor and generator.
Motor : Armature Current : 56 A
Armature Voltage : 590 V
Voltage drop a cross field winding : 40 V
Generator:
Armature Current : 44 A
Armature Voltage : 400 V
Field Voltage drop : 40 V
Resistance of each armature : 0.3
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