emd qb
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Q.BANKTRANSCRIPT
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PRIST UNIVERSITY - EEE DEPARTMENT
UEEE71-ELECTRICAL MACHINE DESIGN
QUESTION BANK
UNIT-1: INTRODUCTION TO MACHINE DESIGN
2 MARKS:
1. Define space factor.
2. Mention any four application of insulating material.
3. What is an eddy current loss?
4. List the types of iron loss.
5. Mention any four insulating materials used for machines.
6. Define high conductivity material.
7. Which insulating material is used in insulation of lamination?
8. Write down the formula for thermal resistivity.
9. Define rating of machines.
10. What is standard specification?
16 MARKS:
1. Describe the factors that limit the design of a machine. (16)
2. i) Explain in detail about the heat flow in two dimensions with expression (14)
ii) Calculate the temperature difference between the centre of the embedded
portion of the conductor and the overhang. The length of the machine is 0.5m and the
current density in the conductors is 4A/mm^2.The thermal resistivity of copper is 0.0025
ohmmeter. Assume that the total heat produced is conducted along the length of the
conductor and the electrical resistivity of conductor is 0.021*10^-6 ohm meter. (2)
3. i) The copper loss in a winding 25mm thick radially is 20w/kg. The thermal resistivity
of paper insulation used in this coil is 8 ohm meter and the copper space factor is 0.7. The
coil is mounted on a former of infinite thermal resistivity. Calculate the maximum
temperature difference between the coil surface and the winding. One cubic meter of
copper weighs 8900kg. (10)
ii) Derive an expression for thermal resistivity of winding. (6)
4. i)Explain in detail about temperature gradients in core. (4)
ii) The thermal resistivity of a stack of laminations measured along the laminations is
0.02 ohm the iron loss is 40 kw/m^3. Calculate the value of temperature difference
between the hot spot and the outside surface for the following cases
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Case 1:
(a) Length of stack measured along the laminations =0.2m
(b) Length of stack measured across the laminations = 0.1m.
Case 2:
(c) Length of stack measured along the laminations =1m
(d) Length of stack measured across the laminations = 0.1m.
The thermal resistivity across the laminations is 40 times that along the laminations.
Comment upon the results. (12)
5. Explain in detail about the applications of insulating materials. (16)
6. A 6600 v alternator has open slots each containing 4 conductors. Each conductor has a
cross section of 6*8 mm^2 and carries a current of 200A. The insulation between the
conductors and slot walls is 3mm thick and has a thermal resistivity of 3 ohm meter. The
length of slot portion of conductor is 0.3m.Calculate the temperature difference between
the conductor and the slot walls if
(i) the coils fit tightly in the slots.
(ii) there is an airspace 0.5mm thick between the coils and the slot walls.
Air has a thermal resistivity of 20 ohmmeter.
Take the resistivity of copper as 0.021*10^-6 ohmmeter. (16)
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UNIT:2 DC MACHINES
2 MARKS:
1. What is the range of specific magnetic loading in D.C. machine?
2. What are the factors to be considered for the selection of number of poles in D.C.
machine?
3. Why square pole is preferred?
4. What is front pitch?
5. What is the need for brushes in D.C. machine?
6. Define winding pitch.
7. What are the effects of armature reaction?
8. State different losses in a D.C. generator.
9. State the relationship between number of armature coils and number of commutator
segments in a D.C. machine.
10. Write the expressions for output coefficients of D.C. machine.
16 MARKS:
1. a.Derive an output equation of D.C. machine. (8)
b. Explain in detail about the guiding factors for selection of poles.(8)
2. Discuss the specific magnetic loading and specific electric loading of D.C.
machine.
3. Determine suitable values for the number of poles, D & L for a 1000kw, 500v,
300rpm dc shunt generator. Assume Bav= 1 Tesla , ac= 400Ac/cm.
4. A 250kw,500v , 600rpm dc generator is built with an Da= 0.75m , L= 0.3m. The
lap connected armature has 720 conductors. Using the data obtained from this
machine , Find the Da , L, no of armature slots, armature conductors and
commutator segments for a 350 kw , 440v , 720 rpm , 6pole dc generator. Assume
a square pole face with ratio of 0.66. The full load efficiency is 0.91 and the
internal voltage drop is 4% of rated voltage. The diameter of commutator is
0.7times of diameter of armature. The pitch of commutator segments should not
be less than 4mm. The voltage between adjacent segments should not be less than
4mm. The voltage between adjacent segments should not exceed 15v at no load.
5. Design a suitable commutator for a 350kw, 600 rpm, 440v, 6 pole dc generator
having Da= 0.75m c = 288. Assume suitable values whenever necessary.
6. The following particulars refer to the shunt field coil for a 440v, 6 pole dc
generator mmf/pole = 7000AT , df = 50mm ,li = 1.1mm , lo= 1.4mm, loss radiated
from outer surface excluding ends is 1400w/m2 , sf = 0.62 , thermal resistivity is
0.02 Ω/m.Find Df,lf,Tf,If Assume a voltage drop of 20% of V terminal across the
field regulator.
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UNIT:3 TRANSFORMERS
2 MARKS:
1. State merits of 3 phase transformers over 1 phase transformer.
2. What is the range of efficiency of a transformer?
3. What is transformer bank?
4. Why stepped core is generally used for transformer?
5. What is the cause of noise in transformer?
6. Define stacking factor.
7. What are the types of winding, commonly used for LV winding?
8. What is leg spacing?
9. What are the factors to be considered for selecting the cooling method of a
transformer?
10. Mention the various types of transformers.
16 MARKS:
1. Derive an output equation for a single phase and three phase transformer.
2. The ratio of flux to full load mmf in a 400KVA, 50Hz , single phase power
transformer is 2.4*10^-6. Find the net iron area,window area of the transformer.
Maximum B in the core is 1.3 wb/m^2, current density is 2.7 A/mm^2 and
winding factor is 0.26. Also calculate the full load mmf.
3. Determine the dimensions of the core and yoke for a 200KVA, 50Hz single phase
core type transformer. A cruciform core is used with distance between adjacent
limbs= 1.6 width of core laminations. Et=14v Bm= 1.1wb/m^2 kw = 0.32 ,
current density is 3 A/mm2
, stacking factor is 0.9, Ai= 0.56 d2 in a cruciform
core where d- diameter of circumscribing circle. Also the width of the largest
stamping is 0.85d.
4. Calculate the approximate overall dimensions for a 200KVA 6600/440v , 50Hz ,
3 phase core type transformer. The following data may be assumed. Et = 10v
Bmax = 1.3wb/m2 , current density is 2.5 A/mm
2 , window space factor = 0.3 ,
stacking factor = 0.9 , overall height = overall width. Use a 3 stepped core. For a 3
stepped core width of the largest stamping = 0.9d and net iron area = 0.6 d2
where
d is diameter of circumcircle.
5. A 250KVA, 6600/ 400v, 3 phase core type transformer has a total loss of 4800 w
at full load. Ht = 1.25m , wt *lt = 1m* 0.5m in plan. Design a suitable scheme for
tubes if the average temp rise is to be limited to 35 degree celcius. The diameter
of tube is 50mm and are spaced 75mm from each other. The average height of the
tube is 1.05m. Specific heat dissipation due to radiation and convection is 6 and
6.5 wb/m2-0c. Assume that convection is improved by 35% due to provision of
tubes.
6. The tank of a 500KVA , 1phase , 50Hz, 6600/400v transformer is 110cm * 65cm*
155cm if the load loss is 6.2kw and θ = 350c , diameter of cooling tube = 5cm ,
average length of cooling tube is 110cm.
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UNIT:4 INDUCTION MOTORS
2 MARKS:
1. What are the main dimensions of induction motor?
2. What is rotating transformer?
3. What are the materials used for slip-rings and brushes in induction motor.
4. What is slot space factor?
5. Why short chorded windings are employed in induction motor?
6. What type of slots are preferred in induction motor?
7. Write the expression for length of mean turn of stator winding.
8. List the undesirable effects produced by certain combination of rotor and stator slots.
9. What is cogging and crawling?
10. Explain the phenomenon of cogging.
16 MARKS:
1. Determine the approximate D and L of stator core, number of slots, number of
conductors for a 11kw, 400v , 3 phase 4 pole 1425rpm delta connected
induction motor. Bav = 0.45 wb/m2 and 23000 ac/m and efficiency is 0.85,
power factor is 0.88 and (L/pole pitch) = 1. The stator employs double layer
winding.
2. Estimate the main dimensions, air gap length , Ss, stator turns/phase, cross
sectional area of stator and rotor conductors for a 3phase 15Hp, 400v, 6 pole
,50Hz,975 rpm induction motor. The motor is suitable for star delta starting
Bav =0.45 wb/m2 ,ac = 20000 ac/m (L/ pole pitch) = 0.85, efficiency is 0.9
and the power factor is 0.85.
3. i)Find the main dimensions of 15kw, 3 phase, 400v, 50 Hz, 2810rpm squirrel
cage induction motor with efficiency 88% power factor is 0.9 Bav= 0.5 tesla,
ac = 25000 ac/m. The rotor peripheral speed should be approximately 20m/s
at synchronous speed.(8)
ii)Compute D and L of a 3.7kw,400v, 3 phase , 4 pole 50hz squirrel cage
induction motor Bav= 0.45tesla , ac = 23000 ac/m, efficiency is 0.85 , power
factor is 0.84 and kw = 0.955.(8)
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4. In the design of 30HP , 3 phase, 440v, 960 rpm, 50Hz, Delta connected
induction motor ac= 25000ac/m , Bav= 0.46wb/m2, efficiency is 0.86, power
factor is 0.87 , kw = 0.955, (L/pole pitch)= 1. Estimate core dimensions, number
of slots, winding turns
5. Estimate D and L , number of stator slots, number of stator conductors/slot for
a 100kw, 3300v, 50 Hz, 12 pole, star connected slip ring induction motor. Bav=
0.4wb/m2
ac = 25000 ac/m, efficiency is 0.9, power factor is 0.9 , kw is 0.96.
Choose the main dimensions to give best power factor. The slot loading should
not exceed 500 ac.
6.. A 90kw, 500v, 50 Hz, 3 phase, 8 pole induction motor has a stator connected
stator winding accommodated in 63 slots with 6 conductors/slots if the slipring
voltage on open circuit is to be about 400v, find suitable rotor winding:
a. number of slots.
b. number of conductor/slot.
c. coil span.
d. slipring voltage on open circuit.
e. applied full load current per phase in rotor
Efficiency is 0.9 and the power factor is 0.86.
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UNIT:5 SYNCHRONOUS MOTORS
2 MARKS:
1. Name the two types of synchronous machines.
2. What is runaway speed?
3. What are the two types of poles used in salient pole machines?
4. What is skewing?
5. What are the advantages and disadvantages of large air-gap length in induction
motor?
6. How the slip ring motor is started?
7. Why wound rotor construction is adopted?
8. What is leakage flux?
9. What are the differences between leakage flux and fringing flux?
10. How the tertiary winding is connected? Why?
16 MARKS:
1. Determine the main dimensions for a 1000 KVA, 50 Hz, 3phase, 375 rpm
alternator, Bav = 0.55wb/m2, ac = 28000 ac/m. Use rectangular poles and
assume suitable value for ratio of core length to pole pitch in order that bolted
on pole construction is used for which the maximum permissible peripheral
speed is 50m/s. The runaway speed is 1.8 times the synchronous speed.
2. Find the main dimensions of a 100 MVA , 50Hz, 11kv, 150 rpm 3phase water
wheel generator, Bav= 0.65wb/m2, ac = 40000 ac/m. The peripheral speed
should not exceed 65m/s at normal running speed in order to limit the
runaway peripheral speed.
3. Determine a suitable number of slots and conductors/slot for the stator
winding of a 3phase, 3300v, 50Hz, 300rpm alternator. The diameter is 2.3m
and L= 0.35m. The Bmax in the airgap should be approximately 0.9wb/m2.
Assume sinusoidal flux distribution. Use single layer winding and star
connection for stator.
4. Determine for a 250KVA,1100v,12 pole , 500rpm 3 phase alternator. Find out
D,L,Zs,Ss,as . Assume Bav = 0.6wb/m2 , ac= 28500 , current density is 3.5
A/mm2 take the ratio of core length to pole pitch is 1.5.
5. A 500KVA, 3.3kv, 600rpm, 3 phase salient pole alternator has 180
turns/phase. Estimate lg if Bav= 0.54wb/m2, the ratio of breadth to polepitch is
0.65, SCR = 1.2 , gap contraction factor is 1.15, kw = 0.955. The mmf required
for gap is 80% of no-load field mmf .
6. Explain in detail the design procedure for rotor and field winding of a
synchronous motor.
WISH U ALL THE BEST.
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