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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
Code No: RR210205 Set No.2
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) What are the significances of energy and co-energy of Energy conversion sys-
tem?
(b) Derive expression for the magnetic force developed in linear electromagnetic
system.
2. Design a lap winding for 32 conductor, 4 pole d. c. machine. Show also the brush
positions.
3. Explain the importance of series field, interpole and compensating windings in d.
c. compound machine.
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) Why is a starter necessary for a DC motor? Explain the working of a 3-point
starter with the help of a neat diagram.
(b) Develop the general expression for the speed of a motor in terms of supply
voltage, armature resistance and flux per pole.
8. (a) Describe a suitable method for determining the efficiency of series motor?
(b) A test on two coupled similar tramway motors, with their fields connected in
series, gave the following results when one machine acted as motor and the
other as a generator.
Motor : armature current = 56 A, armature voltage = 590 V, voltage drop
across field winding = 40 V.
Generator : armature current = 44 A, armature voltage = 400 V, field voltage
drop = 40 V, resistance of each armature = 0.3 .
Calculate the efficiency of the motor and generator at this load.
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