EE2355 DESIGN OF ELECTRICAL MACHINES
UNIT – I
1. A laminated steel tooth of armature of a d.c. machine is 30mm
long' and has a.taper such the maximum width is 1.4. times the minimum.
Estimate the mmf required for a mean flux density of 1.9 wb/m2 in the tooth.
B-H characteristics of steel is given below:
B wb/m2 1.6 1.8
1.9 2.0 2.1
2.2 2.3
H A/m 8700 10000
17000 27000 41000
70000 10900
2. Determine the apparent flux density in the teeth of a d.c.
machine when the real flux density is 2.15wb/m2. Slot pitch is 28 mm, slot
width is 10 mm and the gross core length 0.35 metre. The number of ventilating
ducts is 4. Each duct is 10 mm wide. The magnetizing force for a flux density
of 2.15 wb/m2,is 55000 H/m. The iron stacking factor is 0.9.
3. Compute the main dimensions of a 2500 KVA, 187.5 rpm, 50 Hz.
Three phase, 3 KV salient pole synchronous generator. The specific magnetic
loading is 0.6 wb/m2 and the specific electric loading is 3400 ac/m. The ratio
of core length to pole pitch= 0.65.
4. State and explain the main factors which influence the choice
of specific magnetic loading and specific electric loading in a synchronous
machine. Explain the role of digital computes in the design of electrical
machines.
5. A 15 kW, 230 V, 4 pole d.c machine has the following data:
Armature diameter = 0.25
m ; armature core length = 0.125 m ; length of air gap at pole centre =
2.5 mm ; flux per pole =
11.7 x 10-3 Wb, ratio Polearc/ pole pitch=0.66. Calculate the mmf
required for air gap.
6. Derive the expression for the specific permeance of slots with
double layer windings and for
special purpose
induction motors.
7. Compute apparent magnetic flux density in the teeth of a dc
machine when the real flux density is
2.15 Wb/m2. Slot pitch
is 28 mm. Slot width is 10mm and the gross core length is 0.35 metre. The
number of ventilating
ducts is 4, each 10 mm wide. The magnetizing force for a flux density of
2.15 Wb/m2 is 55000 A/m.
The iron stacking factor is 0.9.
UNIT –II
1. Derive output equation of a d.c. machine and point out its
salient features.
2. State and explain the factors which governs the choice of spe
ific magnetic loading in a
d.c. machine.
3. A 5 KW, 250 volts and 4 pole, 1500 rpm d.c. shunt generator is
designed to have a square pole
face. The average
magnetic flux density in the air gap is 0.42 wb/m2 and ampere conductors per
metre = 15000. Compute
the main dimensions of the machine. Assyme full load efficiency = 87%.
The ratio of pole arc to
pole pitch = 0.06.
4. Find the main dimensions of .a 200 kW, 250 volts, 6 pole, 1000,
rpm DC generator. The maximum
value of flux density in
the air gap is 0.87 wb/m2 and the ampere conductors per metre length of
armature periphery are
31000; The ratio of pole arc to pole pitch is 0.67 and the efficiency is 91
percent. Assume that the
ratio of length of core to pole pitch = 0.75.
5. A rectangular field coil of a dc machine is to produce an mmf
of 7500 ampere turns when
dissipating 220 watts at
a, temperature of 60°C The inner dimensions of the coil are: length = 0.24
metre. Width == 0.1
metre. Height of the coil = 0.15 metre. The heat dissipation is 30 w/m2/oC
from the outer surface
neglecting the top and bottom surfaces of the coil. The temperature of the
ambient air is 20°C.
Compute the thickness of the coil. Resistivity of copper is 0.02 Ω/m and mm2 .
6. A 250 kW, 500 volt, 600 rpm. d.c. generator is built with an
armature diameter of 0.75 m and a
core length of 0.3 m.
The lap connected armature has 720 conductors. Using the data obtained
from this machine,
determine the armature diameter core length, number of armature slots,
armature conductors and
commutator segments for 350 kW, 440 volt, 720 rpm. 6 pole d.c.
generator. Assume a
square pole face with ratio of pole are to pole pitch equal to 0.66.The 'full
load efficiency is 0.91
and the internal voltage drop is 4 percent of rated voltage.The diameter of
commutator is 0.7 of
armature diameter. The pitch of commutator segments should not be less than
4mm. The voltage between
adjacent segments should not exceed 15 V at no load.
7. Explain the design procedure for the shunt field winding of DC
machine.
8. Compute suitable dimensions of armature core of a d.c.
generator which is rated 50 kW. P = 4, N =
600 rpm. Full load
terminal voltage is 220 volts. Maximum gap flux density is 0.83 Wb/ m2 and
specific electric
loading is 30,000. ampere conductors/metre. Full load armature voltage drop is
3
percent of rated
terminal voltage. Field current is 1 percent of full load current Ratio of pole
arc to
pole pitch is 0.,67 pole
face is a square.
9. A 4 pole 50 HP de shunt motor operates with rated voltages of
480 volts at rated speed of 600 rpm.
It has wave wound
armature with 770 conductors. The leakage factor of the poles is 1.2 . The
poles
are of circular cross
section. The flux density in the poles is 1.5 Wb/ m2. Compute diameter of
each pole.
UNIT –III
1. Determine the dimensions of core and yoke for a 200 KVA, 50 Hz
single phase core type
transformer. A cruciform
core is used with distance between adjacent limbs equal to 1.6 times the
width of core
laminations. Assume voltage per turn of 14 volts, maximum flux density of 1.1
wb/m2, window space face
of 0.32, current density of 3 A/mm2 and stacking factor equal to 0.9. the
net iron area is 0.56 d2
wher d is diameter of circumscribing circle. Width of the large-stamping is
0.85d.
2. A 250 KVA, 6600/400 volts, 3 phase core type transformer has a
total loss of 4800 watts at full
load. The transformer tank
is 1.25 metre in height and 1 m x 0.5 m in plan. Design a suitable
scheme of tubes if the
average temperature rise is to be limited to 35 oC. The diameter of each tube
is 50 mm 'and the tubes
are spaced 75 mm from each other due to radiation and convection is
respectively 6 and. 6.5
W/m2 oC. Assume that convection is improved by 35 percent due to
provision of tubes.
3. A three A three phase 50 Hz core type transformer has the
following data:
Width of H.V. winding =
25mm
Width of L.V. winding =
16 mm
Height of the coils =0.5
metre
H.V. winding turns =
830.
Width of duct between HV
and LV windings. = 15mm, Complete leakage reactance of the
transformer referred to
H.V. side. The transformer ratings are 300KVA, 6600/400 volts,
Delta/Star.
4. Determine the main dimensions of the core of a 5 KVA,
11000/1400 volts, 50 Hz, single phase
core type distribution
transformer having the following data: The net conductor area in the window
is 0.6 times the net
cross sectional area of iron in the core. The core is of square cross section,
maximum flux density is
1 wb/m2. Current density is 1.4 A/mm2. Window space' factor is 0.2.
Height of the window is
3 times its width.
5. Derive output equation of a three phase transformer.
6. State different methods of cooling the transformers and explain
each method with relevant
diagrams. State merits
and limitations of each method.
7. Calculate approximate overall dimensions for a 200 KVA,
6600/440 V, 50 Hz, 3 phase core type
transformer. The
following data may be assumed: emf per turn = 10 V, Maximum-flux density =
1/3 Wb/m2 ,
currentdensity = 2.5 A/mm2, window space factor = 0.3, overall height = overall
width, stacking factor =
0.9. Use a 3 stepped core. For a three stepped core, width of largest
stamping = 0.9 d and net
iron area == 0.6 d2 where d is a diameter of circumscribing circle.
8. A 250 KVA 6600/400, 3 phase core type transformer has a total
loss of 4800 W at full load. The
transformer tank is l.25
m in height and 1m x 0.5m in plan. Design a suitable scheme for tubes if
the average temperature
rise is to be limited to 35 oC. The diameter of tubes is 50mm and are
spaced 75 mm from each
other. The average height of tubes is1.05m.
9. Calculate the main dimensions of core of 100 KVA, 2000/400
Volts, 50 Hz, single phase shell
type transformer.
Voltage per turn = 10 volts. Peak flux density in the core is 1.1 Wb/ m2.
Window
space factor is 0.33.
Ratio of core depth to width of central limb = 2.5. Ratio of window height to
window width = 3.0
current density in the winding is 2 A/mm2, Stacking factor = 0.9.
10. A 250 KVA, 6600/400 volts, three phase core type transformer
has a total loss of 4800 watts at
full load. The
transformer tank is 1.25 m in height and 1m x 0.5m in plan. Design a suitable
scheme for tubes if the
average temperature rise is to be limited to 35°C. The diameter of tubes is
50 mm and are spaced 75
mm from each other. The average, height of tubes is 1.05 m. Specific
heat dissipation due to
radiation and convection is respectively 6 and 6.5 W/ m2 / oC. Assume that
convection is improved
by 35 percent due to provision of tubes.
UNIT – 4
1. Derive the output equation of a three phase induction motor.
2. State and explain factors governing the choice of ampere
conductors per metre in the design of a
three phase induction
motor.
3. Compute the' main dimensions of a 15 KW, three phase, 400
volts, 50 Hz, 2810 rpm squirrel cage
Im having efficiency of
88 percent and full load power factor of 0.9.Assume specific magnetic
loading equal to 0.5
Wb/m2 and specific electric loading equal to 25,000 A/m. The rotor peripheral
speed' may be approximately
20 m/sec at' synchronous speed.
4. Determined diameter and length of the stator core for a 11 kW,
400 V, 3 phase, 4- pole,1425 rpm
induction motor.
Specific magnetic loading is 0.45 wb/m2 and specific electric loading is 23000
ac/m. Full load.
efficiency is 0.85 and full load power factor is 0.88. The ratio of core length
to
pole pitch = 1
5. A 90 kW, 500 volts, 50 Hz; three phase, 8 pole slip-ring
induction motor has star connected stator
accommodating 6
conductor per slot. The number of stator slots = 63. If the slip ring voltage
on
open circuit is to be
about 400 volts, find the number of rotor slots and the number of conductors
in each rotor slot.
6. Write notes on:
i. Design of rotor
bars and slots.
ii. Design of end rings.
7. Find the values of diameter and length of stator core of a. 7.5
kW. 220 V, 50 Hz. 4 pole. 3 phase
induction motor for best
power factor.
8. Find the main dimensions of a 15 kW, three phase, 400 volts, 50
Hz, 2810 rpm squirrel cage
induction. motor having
all efficiency of 88 percent and full load power factor of 0.9. Specific
magnetic loading is 0.5
Wb/ m2. Specific electric loading = 25000 A/m. Take rotor peripheral
speed 'as approximately
20 m/sec synchronous speed.
9. A 11 kW, three phase 6 pole, 50 Hz; 220 volts star connected
induction motor has 54 stator slots,
each containing 9
conductors. Calculate the value of bar and end ring currents. The number of
rotor bars is 64. The
machine has an efficiency of 8.6 percent and a powerfactor of 0.85. The rotor
MMF may be assumed to be
85 percent of stator MM F. Also find the bar 'and the end ring
sections if the current
density is 5 A/mm2
UNIT – V
1. Compute the main dimensions of a 1000 KVA, 50 Hz, three phase,
375 rpm alternator. The
average air gap flux
density is 0.55 Wb/m2 and ampere conductors per metre are 28000. Use
rectangular poles.
'Assume the ratio of arc length to pole pitch equal to 2. Maximum permissible
peripheral speed is 50 m/sec. The runaway
speed is 1.8 times the synchronous speed.
2. The field coils of a salient pole alternator are wound with a
single layer winding of bare copper
strip 30 mm drop, with
separating insulation 0.15 mm thick. Compute thickness of the conductor,
number of turns and
height of the winding to develop an mmf of 12000 ampere turns with a
potential difference of
5 volts per coil and a loss of 1200 watts/m2 of coil surface area. Mean
length of turn is 1.2 metre.
Resistivity of copper is 0.021.Ω/m/mm2.
3. Calculate the .mmf required for the air gap of a salient pole
synchronous machine having core
length of 0.32 metre
including 4 ducts of 10 mm each; pole arc = 0.19 metre. Slot pitch = 65.4
mm, Slot opening = 5 mm.
Air gap length = 5 mm. Flux per pole = 52 mwb.
4. Explain the role of digital computers in the design of
electrical machine.
5. Describe the construction of Turbo – alternators with sketch.
6. Explain the design of field winding of alternator.
7. State and explain advantages of hydrogen cooling as applied to
turbo alternator.
8. Draw cross sectional view of rotor slot and explain the method
involved in direct hydrogen cooling
of rotor of turbo
alternator.
9. Derive the output equation of a synchronous machine.
10. State and explain the salint features of Computer Aided design
of electrical apparatus.
11. Determine the main dimensions of a 75000 KVA, 13.8 kV, 50 Hz,
62.5 rpm, three phase star
connected alternator.
The peripheral speed of rotor should be about 40 m/sec. Assume average
gap density equal to
0.65 Wb/ m2, ampere conductors per metre equal to 40,000 and current
density =4 A/ mm2.
Assume Kw = 0.955.
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