Lingaya’s
University
M.Tech. 1st Year (Term – II) (Part Time)
Examination – Feb 2011
Design
of Electrical Machines (EL - 502)
[Time: 3 Hours] [Max.
Marks: 100]
 |
Before answering the question,
candidate should ensure that they have been supplied the correct and complete
question paper. No complaint in this regard, will be entertained after
examination.
 |
Note: – Attempt five
questions in all. All questions carry equal marks. Question no. 1 is
compulsory. Select two questions from Section B and two questions from Section
C.
Section – A
Q-1. Part – A Select the answer from the multiple choice questions. (10x1=10)
(i)
What are the properties that a good conducting material must posses when used in
electrical machines?
(a) High
Conductivity (b) High
Reluctance
(c) High Permeability (d) Low Resistivity
(c) High Permeability (d) Low Resistivity
(ii)
How does the design change with a higher value of current density?
(a) Low
magnetic losses
(b) Reduction in volume & size of machine
(c) Increase in overall efficiency (d) all the above
(b) Reduction in volume & size of machine
(c) Increase in overall efficiency (d) all the above
(iii)
Insulating material which can withstand temperature higher than 180oC.
(a)
Class C (b) Class H (c) Class F (d) Class B
(iv)
Basis for classifying the insulating materials used in electrical machines
(a)
Weight & Size (b)
Cost
(c) Overall life (d)
Temp. rise permissible
(v)
In case of electrical machine, the temperature rise depends on
(a)
Method of cooling (b) Total losses
(c)
Size of the machines (d)
a, b & c
(vi)
Non magnetic materials consists of
(a)
Ferromagnetic (b) diamagnetic (c) Paramagnetic (d) b & c
(vii)
For a three phase shell type value of K is
(a)
0.45 (b) 0.6 to 0.7 (c) 1.3 (d) 1.0 to 1.2
(viii)
For a cruciform core section the value of net core area Ai = Kd2
(a)
0.45 (b) 0.56 (c) 0.6 (d) 0.62
(ix)
ln a Power transformer, for hot rolled Silicon Steel usual value of maximum
flux density in
webers
per meter square is
(a)
1.12 to 1.35 (b) 1.25 to 1.45 (c) 1.55 (d) 1.7
(x)
The value of current density in a distribution transformer self oil cooled in
amperes per mm2
(a)
1.1 to 2.3 (b) 2.2 to 3.2 (c) 5.4 to 6.2 (d) 8.5
Q-1. Part-B Briefly discuss.
(i)
conditions which impose limitation on the design of electrical machines
(ii)
Causes of failure of insulator
(iii)
Dielectric loss angle and ionization curve.
(iv)
"Short circuit ratio" SCR as applied to synchronous machines.
(v)
'Crawling & Cogging' in 3-phase induction motor. (5x2=10)
Section - B
Q.-2
(a) What are superconducting materials? What are the advantages of using these
superconducting materials in electrical machines? Give the various materials
which exhibit the superconducting properties along with their applications?
(10)
(b)
Briefly discuss the various conducting magnetic and insulating materials
required for the construction of large electrical machine. What are the
requirements of each of these materials? Discuss directional properties of cold
rolled gain oriented silicon sheet steel. (10)
Q-3
(a) The problem of ventilation in rotating electrical machines is closely
linked with the type of enclosures used. What are the various types of enclosures
used for electrical machines? Explain giving examples of application for each. (10)
(b)
Discuss the various types of duties and ratings for rotating electrical
machinery. Explain with neat diagram. (10)
Q-4.
(a) Explain the term “B60" as used in the calculation of the magnetization
current of art induction motor and hence determine the expression for
magnetizing current. (10)
(b)
A 12.0 HP, 4-pole, 3 phase, 50 Hz, 400V delta connected induction motor has the
following data.
(10)
Effective
turns per phase = 214
Diameter
of stator = 19cm
Length
of stator = 12 cm
Radial
air-gap on one side = 0.05
cm
Gap
coefficient = 1.35
Find
the airgap ampere turns required
Section – C
Q-5.
(a) Drive an output equation of a 3-phase transformer in terms of design
constants and hence deduce the equation for the case of a single phase
transformer. (10)
(b)
Calculate the main dimensions for a 250KVA, 6600 / 400Volts, 50Hz, 3-Phase,
Delta / Star, core type, oil immersed self cooled, outdoor type distribution transformer'
Assume suitable values for various design constants and specific magnetic
loading' (10)
Q-6 (a) Define the terms "specific
Electric loading" and "specific magnetic loading" as applied to
electrical machines. Discuss the various factors which influence the values of
these quantities
to be adopted in the design of d.c.
(b)
Calculate the output coefficient, the diameter and frame length, and the number
of armature conductors the number of armature slots and the number of
conductors per slot, size of armature conductors for a 4-pole, 10Hp, 230Volts,
1000rpm, d.c. shunt motor to work at an efficiency of 85%, if the ampere
conductors per cm of armature periphery is 280 and the Bg = 0.7 wbs/m2.
Assume proper values between pole are, pole-pitch and axial length. (10)
Q-7.
(a) Deduce for a 3-phase induction motor an expression showing the relationship
between HP output, its main dimension, speed, the specific electric and
magnetic loadings, efficiency and power factor. (10)
(b)
Apply the above expression to obtain suitable values for the main dimensions of
the stator of a 3-phase, 50HP, 2000 V, 50Hz, induction motor with a wound rotor
and to run at about 600 rpm. (10)
Q-8
(a) Explain the term “short circuit ratio" as applied to synchronous
machines. How does the value of S.C.R effect the design of alternators? (10)
(b)
Determine for a 400 KVA, 6600 V, 12-pole, 500rpm, star connected 3-phase salient
pole alternator, suitable values for:
(a)
Diameter at the airgap (b) core
length
(c)
The number of stator conductors (d) number of stator slots
Specific
magnetic loading and specific electric loading may be taken as 0.6 tesla and
300 ampere conductors per cm of the periphery. (10)
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