Download File - ELECTRICAL ENGINEERING DEPT, DCE

DHANALAKSHMI COLLEGE OF ENGINEERING, CHENNAI
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
EE-2355 -DESIGN OF ELECTRICAL MACHINES
UNIT-1 INTRODUCTION
PART-A
1. What are the major design considerations?
2. How are materials classified according to their degree of magnetism?
3. What is meant by specific electric loading?
4. Write the expression for temperature rise.
5. Define – Heating Time Constant and Cooling Time Constant
6. Distinguish between continuous rating and short time rating of an electrical machine.
7. Write the expression for cooling.
8. What are the limitations in design a machine?
9. Write the expression for temperature difference between centre and overhang of a conductor.
10. What are the major ratings of electrical machines?
PART- B
1. What are the main groups of electrical conducting materials? Describe the properties and
application of those materials.
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2. Explain in detail, the various duties and ratings of rotating machines and give their respective
temperature - rise curves.
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3. A field coil has a heat dissipating surface of 0.15 m2 and length of mean turn 1 m. It
dissipates loss of 150 W, the emissivity being 34 W/m2-°C. Estimate the final temperature
rise of the coil and its time constant, if the cross section of the coil and its time constant, if
the cross section of the coil is 100*50 mm2. Specific heat of copper is 390 J/kg°C. The space
factor is 0.56. Copper weight 8900 kg/m2.
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4. Explain in detail, the methods used for the determination of motor rating for variable load
drives.
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5. Explain in detail, the methods of measurement of temperature rise in various parts of an
electrical machine.
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6. Derive an expression for effective thermal resistivity of the winding of an electrical motor.
Also write importance of these parameters.
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7. A 400 kVA, 11000/400V, 3-phase transformer is working in an ambient temperature of 35
°C. On switching the full load, its oil temperature is recorded as follows: 59 °C after
1.5 hours 71°C after 3.0 hours. Its full load copper loss is 2 times the iron loss. Calculate its,
heating time constant, final steady temperature and one hour rating.
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8. Explain in detail, the various electrical engineering materials.
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9. Explain in detail, the factors affecting the size of rotating machine.
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10. Derive an expression for temperature rise -time curve for an electrical machine.
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11. Explain in detail, the factors which govern the choice of specific magnetic loading.
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UNIT 2
DC MACHINES
PART-A
1. Write the output equation of a DC Machines.
2. Show how the specific magnetic and electric loadings are interdependent.
3. What is meant by magnetic loading?
4. What are the main factors to be considered for the selection of number of poles in a D.C
machine?
5. What is meant by pheripheral speed?
6. State the relation between the number of commutator segments and number of armature coils
in DC generator
7. Define – Field Form Factor
8. What is meant unbalanced magnetic pull?
9. What is meant by slot loading?
10. What are the methods available to reduce armature reaction?
PART- B
1. Derive an expression for apparent and real flux density.
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2. A 5 kW, 250 V, 4 pole, 1500 rpm DC shunt generator is designed to have a square pole face.
The specific magnetic loading and specific electric loading are 0.42 wb / m2 and
15000 AC / m respectively. Find the main dimensions of the machine. Assume full load
efficiency = 0.87 and pole arc to pole pitch ratio is 0.66.
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3. Explain in detail, the various methods to determine mmf required for teeth of an electric
machine.
4.
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Explain in detail, the various steps involved in the design of armature winding of DC
machine.
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5. A design is required for a 50 kW, 4 pole, and 600 rpm DC shunt generator. The full load
terminal voltage being 220V. If the maximum gap density is 0.83 wb / m 2 and the armature
AC per meter are 30,000, calculate the suitable dimensions of the armature core to give a
square pole face. Assume the full load armature drop is 3% of the rated terminal voltage and
the field current is 1% of the full load current. Ratio of pole arc to pole pitch is 0.67.
6.
Explain the choice of specific magnetic loading.
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7. Determine the diameter and length of armature core for a 55kW, 110v, 1000rpm, 4 pole
shunt generator, assuming specific electric and magnetic loadings of 26000 AC / m and
0.5 wb / m2 respectively. The pole arc should be about 70% of pole pitch and length of core
about 1.1 times the pole arc. Allow10 A for the field current and assume a voltage of 4V for
the armature circuit. Specify the winding used and also determine the suitable
values for the number of armatuer conductors &number of slots.
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8.
Explain the various steps involved in the design of commutator & brushes of a DC Machine.
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9. Calculate the diameter and Length of armature for a 75 kW, 4 pole, and 1000 rpm DC shunt
motor. The full load terminal voltage being 220V. If the maximum gap density is 0.9 wb/m2
and the armature AC per meter are 30,000, Assume square pole face. Assume that the
maximum efficiency occurs at full load and the field current is 2.5 percent of rated current.
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10. Find the main dimensions of a 200 kW, 250 V, 6 pole, and 1000 rpm generator. The
maximum value of flux density in the gap is 0.87 Wb/m2 and the ampere conductors per
metre of armature periphery are 31000. The ratio of pole arc to pole pitch is 0.67 and the
efficiency is 91 percent. Assume the ratio of length of core to pole pitch = 0.75
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11. Calculate the main dimensions of a 20 HP, 1000 rpm, 400 V, DC motor. Given that
bav = 0.37 wb / m2 and ac = 16000 AC / m. Assume an efficiency of 90%.
UNIT III
PART A
1. Write the relationship between emf per turn and kVA rating in a transformer.
2. What are the factors affecting the choice of flux density of core in a transformer?
3. Define ‒ Voltage Regulation
4. What are the methods by which heat dissipation occurs in a transformer?
5. Write the advantages of shell type transformer over core type transformer?
6. What is meant by stacking factor?
7. What are the cooling methods used for dry type transformers?
8. Define ‒ Window Space Factor
9. What are the advantages of using higher flux density in core?
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10. What is meant by inter leaved winding?
PART- B
1. A 1 Φ, 400 V, 50 Hz transformer is built from stamping having a relative permeability of
1000. The length of flux path is 2.5 m. Area of cross section of core is 2.5* 10-3 m2. The
primary winding has 800 turns. Estimate the maximum flux and no load current of the
transformer. The iron loss at working flux density is 2.6 watts/kg. Density of iron is
7.8*103 kg/m3. Stacking factor is 0.9.
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2. Derive the output equation of a three phase transformer. And also calculate the net iron area
and the window area of a single phase transformer if the ratio of flux to full load mmf in a
400 kVA, 50 Hz single phase transformer is 2.4*10-6, maximum flux density is 1.3 wb/m2,
current density is 2.7A/mm2 and window space factor is 0.26. Also calculate full load mmf.
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3
A 250 kVA, 6600/400 V, 3-phase core type transformer has a total loss of 4800 W on full
load. The transformer tank is 1.25 m in height and 1 m x 0.5 m in plan. Design a suitable
scheme for cooling tubes if the average temperature rise is to be limited to 350 C. The
diameter of the tube is 50 mm and is spaced 75 mm from each other. The average height of
the tube is 1.05 m. Specific heat dissipation due to radiation and convection is 6 and
6.5 W/m2-°C. Assume that convection is improved by 35% due to provision of tubes.
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4. A 6600 V, 60 Hz single phase transformer has a core of sheet steel. The net iron crosssectional area is 22.6X10-3 m2. The mean length is 2.23 m and there are four lap joints. Each
lap joints takes1/4 times as much reactive mmf as if required per meter of core. If
Bm = 1.1 wb/m2, determine the following:
(i) The number of turns on the 6600V winding
(ii) The no load current. Assume an amplitude factor of 1.52 and that for given flux density
mmf per meter = 232ac/m, specific loss 1.76W/kg. Specific gravity of plates =7.5 (16)
5.
Derive the output equation of single phase transformer in terms of core and window area.
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6. Determine the dimensions of core and yoke for a 200kVA, 50Hz, single phase core type
transformer. A cruciform core is used with a distance between adjacent limbs= 1.65 times the
width of core laminations. Assume voltage per turn 14V, maximum flux density 1.1Wb/m 2,
window space factor 0.32, current density 3A/m2 and stacking factor = 0.9. The net iron area
is 0.56d2 in a cruciform core where d is a diameter of circumscibing circle. Also the width of
largest stamping is 0.85d.
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7. Explain the temperature rise and methods of cooling in transformer.
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8. A-3phase, 50 Hz oil cooled core type transformer has the following dimensions: Distance
between core centers = 0.2 m. Height of window = 0.24 m Diameter of circumscribing circle
= 0.14 m. The flux density in the core = 1.25 Wb/ m2. The current density in the conductor
= 2.5 A/mm2. Assume a window space factor of 0.2 and the core area factor = 0.56. The core
is 2-stepped. Estimate kVA rating of the transformer.
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9. Calculate the dimension of the core, the number of turns and cross-sectional area of
conductors in the primary and secondary windings of a 100 kVA, 2300/400V, 50 Hz, 1-phase
shell type transformer. Ratio of magnetic and electric loadings equal to 480x10 -8 (i.e. Flux
and secondary mmf at full load).
Bm = 1.1 Wb/m2,
δ = 2.2 A/mm2,
kw= 0.3,
Depth of stacked core = 2.6
Height of window = 2.5
Width of central limb
Width of window
Stacking factor = 0.9
UNIT IV DESIGN OF INDUCTION MOTOR
PART A
1. Write the expression for O/P equation and output co-efficient of induction motor.
2. Define ‒ Dispersion Coefficient of an Induction motor
3. How crawling can be prevented by design in an Induction motor?
4. Define ‒ Stator Slot Pitch
5. Write the equation for output co-efficient in an induction motor.
6. What are the advantages and disadvantages of larger air gap length in induction motor?
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7. What are the factors to be considered for selecting the number of slots in induction machine
stator?
(May 2012)
8. Why rotor slots are skewed?
9. What are the various leakage fluxes in IM?
10. What are the methods to improve Pf?
PART- B
1. Determine the approximate diameter and length of stator core, the number of stator slots and
the number of conductors for a 11 kW, 400V, 3Ф, 4-pole, 1425 rpm, delta connected
induction motor. Bav = 0.45 Wb/m2, ac=23000 amp. Cond/m, full load efficiency = 0.85, pf
= 0.88, L/τ = 1. The stator employs a double layer winding.
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2. Estimate the stator core dimensions, number of stator slots and number of stator conductors
per slot for a 100 KW, 300 V, 50 Hz, 12 pole, star connected slip ring induction motor.
Bav= 0.4 Wb/m2, ac = 25000 amp.cond/m, η = 0.9, pf = 0.9. Choose main dimensions to give
best power factor. The slot loading should not exceed 500 amp. Conductors.
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3. Estimate the main dimensions, air-gap length, stator slots, stator turns per phase and cross
sectional area of stator and rotor conductors for a 3-phase, 15 HP, 400 V, 6 pole, 50 Hz, 975
rpm, induction motor. The motor is suitable for star delta starting. Bav = 0.45 Wb/m2, ac =
20000 amp.cond / m, L\ τ = 0.85, η = 0.9, pf = 0.85
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4. Determine the leakage permeance per meter length of a rectangular semi enclosed slot having
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the following dimensions in mm:
Slot width =10
Slot opening = 4
Wedge height = 3 lip height = 1.5
Height of conductor portion = 25
height above conductor but below wedge = 1
5. Find the main dimensions, net length of iron of a 3.7 kW,400V, 3Φ, 4 pole, 50Hz, Squirrel
cage Induction Motor which is to be started by a star delta starter. Specific magnetic loading
0.45wb/m2, Specific electric loading 23000 AC/m, efficiency= 0.85. Power factor=0.84,
winding factor=0.955, L/τ ratio is 1.5.
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6. Explain in detail, the factors to be considered in estimating the length of air gap of an
induction motor. Also discuss the step by step procedure to design the rotor of a squirrel cage
induction motor.
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7. Describe the steps involved in the design of magnetizing current for an induction motor from
the design data.
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8. Estimate the stator core dimensions and the total number of stator conductors for a three
phase, 100 kW, 3300 V, 50 Hz, 12 pole star connected slip ring induction motor. Assume
average gap density= 0.4wb/m2, conductors per meter=25,000ac/m, efficiency=0.9, power
factor=0.9 and winding factor =0.96.
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Diameter of armature, D =0.78 m, length of armature, L = 0.23 m, Stator conductors=2922
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9. Derive the output equation of AC machines in terms of its main dimensions.
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10. Describe the effect of dispersion co-efficient due to the following factors in an induction
motor:
(i) Overload capacity, (ii) Airgap Length, (iii) Number of poles and (iv) Frequency
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UNIT V DESIGN OF SYNCHRONOUS MACHINES
PART A
1. What is runaway speed of synchronous machine?
2. Write the need for damper winding in synchronous machine.
3. How does the value of SCR affect the design of alternator?
4. Define ‒ SCR
5. How is cylindrical pole different from salient pole in a synchronous machine?
6. What is meant by critical speed of alternator?
7. Why is salient pole construction rejected for high speed alternators?
8. Write the expression for armature ampere turns/ pole.
9. What are the factors to be considered for the choice of specific magnetic loading in
synchronous machine?
10. List out the factors to be considered for the choice of number of slots in synchronous
machines.
PART- B
1. Explain how SCR is determined for an alternator. Also discuss its effect on the performance
of alternator.
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2. Derive the output equation of an alternator. And also determine the main dimensions for a
1000 kVA, 50 Hz, 3 phase, 375 rpm alternator. The average air gap flux density is
0.55 wb/m2 and the ampere conductors/meter are 28000. Use rectangular poles and assume a
suitable value for ratio of core length to pole pitch in order that bolted on pole construction is
used for which maximum permissible peripheral speed is 50m/s. The runaway speed is 1.8
times synchronous speed.
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3. Explain the step by step procedure for the design of field winding of synchronous machine.
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4. A 1000 kVA, 3300 V, 50 Hz, 300 rpm, 3-phase alternator has 180 slots with 5 conductors per
slot. Single layer winding with full pitch coils is used. The winding is star connected with
one circuit per phase. Determine the specific electric and specific magnetic loadings, if the
stator bore is 2.0 m and the core length is 0.4 m. The Machine has 60˚ phase spread.
5. Derive the expression for length of air gap of a synchronous machine.
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6. Determine the main dimensions of a 75000 kVA, 13.8 kV, 50 Hz, 62.5 rpm, 3 phase, star
connected alternator. Also find the number of stator slots, conductors/slots, conductor area
and workout winding details. The peripheral speed shouild be about 40 m/s. Assume, average
gap density = 0.65 Wb/m2, amp conductors per meter = 40000 and current density 4 A/mm2.
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7. Determine the suiotable number of slots and conductors per slot, for the stator winding of a
3 phase 3300 V, 50 Hz, 300 rpm alternator. The diameter is 2.3 m anr the axial length of core
is 0.35 m. The maximum flux density in the air gap should be approximately 0.9 Wb/m2.
Assume sinusoidal flux distribution. Use single layer winding and star connection for stator.
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