SNS COLLEGE OF ENGINEERING
Coimbatore – 641 107
DEPARTMENT OF ELECTRICAL & ELECTRONICS
ENGINEERING
Internal Assessment Exam – I, JAN 2013
VI SEMESTER
should be about 70% of pole pitch and length of core about
1.1 times the pole arc. Allow 10 ampere for the field current
and assume a voltage drop of 4 volts for the armature circuit.
Specify the winding to be used and also determine suitable
values for the number of armature conductors and slots. (16)
(OR)
ELECTRICAL MACHINE DESIGN
(b) (8)
Time: 9:30am-12:30pm
Date: 02/02/2013
Max Marks: 100
Part A
Answer all questions: (102=20Marks)
1. Define specific electric and magnetic loadings.
2. What is leakage flux and leakage co-efficient?
3. Define gap contraction factor for slots.
4. List the methods used for estimating the mmf for teeth.
5. Define field form factor.
6. Define real and apparent flux density.
12. (a)(i)Calculate the mmf required for air-gap of a dc
machine with an axial length of 20cm(no ducts) and a pole
arc of 18cm. The slot pitch=27mm, slot opening=12mm, air
gap=6mm and the useful flux per pole=25mWb. Take carter’s
coefficient for slot as 0.3.
(8)
(ii) A 15kW, 230V, 4 pole DC machine has the
following data: armature diameter=0.25m; armature core
length=0.125m, length of airgap at pole centre=2.5mm, flux
per pole=11.7×10-3 Wb, pole arc/pole pitch=0.66.
Calculate the mmf required for airgap (i) if the
armature surface is treated as smooth (ii) if the armature is
slotted and the gap contraction factor is 1.18.
(8)
7. What is meant by fringing effect?
8. What are the main dimensions of a rotating machine?
9. What are the factors affecting limitations on design of
electrical machines?
10. List the factors that are affected by the leakage flux.
Part-B
Answer all questions (5x16=80)
11. (a) Determine the diameter and length of armature core
for a 55kW, 110V, 1000 rpm, 4 pole shunt generator,
assuming specific electric and magnetic loadings of 26000
amp.cond. /m and 0.5 Wb / m2 respectively. The pole arc
(OR)
(b) A 350kW, 500V, 450 rpm, 6 pole DC generator is
built with an armature diameter of 0.87m and core length of
0.32m. The lap wound armature has 660 conductors.
Calculate the specific electric and magnetic loadings.
(8)
(c) Derive the output equation of DC machines.
(8)
13. (a) (i) Calculate the apparent flux density at a particular
section of a tooth from the following data:
Tooth width=12mm; slot width=10mm; gross core
length=0.32m, no. of ducts=4 each 10mm wide; real flux
density=2.2wb/m2, permeability of teeth corresponding to
real flux density=31.4×10-6 H/m; stacking factor=0.9.
(8)
(ii) Find the main dimensions and the no. of poles of a
37kW, 230V; 1400 rpm shunt motor so that a square pole
face is obtained. The average gap density is 0.5wb/m2 and the
ampere conductors per metre are 22000. The ratio of pole arc
to pole pitch is 0.7 and the full load efficiency is 90%.
(8)
(OR)
(b) (i) Find the main dimensions of a 200kW, 250V, 6
pole, and 1000rpm generator. The maximum value of flux
density in the gap is 0.87wb/m2 and the ampere conductor
per metre of armature periphery is 31000. The ratio of pole
arc to pole pitch is 0.67 and the efficiency is 91%. Assume
the ratio of length of core to pole pitch =0.75.
(8)
(ii) Calculate the main dimensions of a 20HP, 1000
rpm, 400V, DC motor. Given that Bav=0.37wb/ m2 and
ac=16000
amp.cond./m. Assume
an
efficiency
of
90%(square pole face,L/t=0.7).
(8)
The efficiency is 91%. Assume that the mmf required for air
gap is 55% of armature mmf and the gap contraction factor is
1.15.
(16)
15.(a). Determine the main dimensions, number of poles and
the length of airgap of a 600kW, 500V, 900 rpm generator.
Assume average gap density as 0.6 Wb/ m2 and ampere
conductors per metre as 35000. The ratio of pole arc to pole
pitch is 0.75 and the efficiency is 91%.
The following are the design constraints: peripheral
speed
40m/s, frequency of flux reversal 50Hz, current
per brush arm
400A and armature mmf per pole
7500A. The mmf required for air gap is 50 % of armature
mmf and gap contraction factor is 1.15
(16)
(b)(i) A 5 kW, 250V, 4 pole, 1500 rpm, shunt generator is
designed to have a square pole face. The loadings are:
14.(a)(i) Derive the relation between real and apparent flux
densities in a DC machine.
(8)
Average flux density in the gap=0.42Wb/m2 and
ampere conductors per metre=15,000. Find the main
dimensions of the machine. Assume full load efficiency=0.87
and ratio of pole arc to pole pitch=0.66.
(8)
(ii) Determine the apparent flux densitiy in teeth of a
DC machine if the real flux density in teeth is 2.15 Wb/sq.m,
slot pitch is 28mm, slot width is 10 mm, gross core length is
0.35m, no. of ventilating ducts is 4, each 10mm wide. The
magnetizing force corresponding to flux density of 2.15
Wb/sq.m is 55000 AT/m and iron stacking factor is 0.9. (8)
(ii) A design is required for a 50kW, 4 pole, 600 rpm,
DC shunt generator, the full load terminal voltage being
220V. If the maximum gap density is 0.83 Wb/ m2 and the
armature ampere conductors per metre are 30,000. Calculate
suitable dimensions of armature core to give a square pole
face.
(OR)
Assume that the full load armature voltage drop is 3%
of the rated terminal voltage, and that the field current is 1%
of rated full load current. Ratio of pole arc to pole pitch is
0.67.
(8)
(b). A 150 kW, 230V, 500 rpm, DC shunt motor has a
square field coil. Find its no. of poles and the main
dimensions and air gap length. Assume the average gap
density over the pole arc as 0.85 Wb/m2 and the
amp.cond./m as 29000. The ratio of width of pole body to
pole pitch is 0.55 and the ratio of pole arc to pole pitch is 0.7.
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