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EE2355 Design of Electrical Machines
UNIT - I
1. 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.
2. 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.
3. 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.
4. 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.
5. Derive the expression for the specific permeance of slots with double layer windings
and for special purpose induction motors.
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
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pole face. The average magnetic flux density in the air gap is 0.42 wb/m 2 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/m 2 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. Explain the design procedure for the shunt field winding of DC machine.
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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. 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.
4. Derive output equation of a three phase transformer.
5. State different methods of cooling the transformers and explain each
method with relevant diagrams. State merits and limitations of each method.
UNIT – IV
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
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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, 4pole,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.
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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.
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