# Frequently asked Design of Electrical Machines mcq questions

Q. 1. Thickness of core lamination of transformer is …….
A : 18cm
B : 25cm
C : 20cm
D : 15cm

20cm

Q. 2. Which type of slots are generally used in induction motors ?
A : Open type
B : Semi-closed type
C : Closed type
D : None of the above.

Semi-closed type

Q. 3. What is the formula for the magnetizing current per phase?
A : magnetizing current per phase =( 0.427 * no. of poles * total magnetizing mmf per
pole ) / stator winding factor * no of turns of stator slots
B : magnetizing current per phase =( 0.427 / no. of poles * total magnetizing mmf per
pole ) / stator winding factor * no of turns of stator slots
C : magnetizing current per phase =( 0.427 * no. of poles / total magnetizing mmf per
pole ) / stator winding factor * no of turns of stator slots
D : magnetizing current per phase =( 0.427 * no. of poles * total magnetizing mmf per

magnetizing current per phase =( 0.427 * no. of poles * total magnetizing mmf per pole ) / stator winding factor * no of turns of stator slots

Q. 4. What is the relation between rotor mmf and stator mmf?
A : rotor mmf = 0.85 * stator mmf
B : rotor mmf = 0.80 * stator mmf
C : rotor mmf = 0.75 * stator mmf
D : rotor mmf = 0.70 * stator mmf

rotor mmf = 0.85 * stator mmf

Q. 5. Carter’s coefficient is applicable for estimating
A : requirement of air-gap mmf
B : flux distribution in air-gap
C : length of air gap

requirement of air-gap mmf

Q. 6. The heat generated in transformer is dissipated to surroundings mainly
by………………

A : Conduction
B : convection
D : conduction and convection

convection

Q. 7. Which winding will have winding factor equal to one
A : Lap
B : Wave
C : Full pitched concentric
D : Full pitched distributed

Full pitched concentric

Q. 8. The range of specific magnetic loading in weber per meter square is
A : 0.35 to 0.6
B : 0.3 to 0.7
C : 0.3 to 0.65
D : 0.4 to 0.8

0.4 to 0.8

Q. 9. For avoiding cogging in induction motor the difference between the
number of stator and rotor slots should not be

A : P
B : 2P
C : 3P
D : 4P

3P

Q. 10. What is the range of current density in rotor bars in Ampere per meter
square?

A : 4 to 9
B : 4 to 6
C : 4 to 7
D : 5 to 6

4 to 7

Q. 11. What type of winding is used for the small rating induction motors?
A : mush windings
B : cross windings
C : interconnected windings
D : rounded windings

mush windings

Q. 12. Transformer core is laminated to reduce
A : Hysteresis loss
B : Copper loss
C : Eddy current loss
D : Stray loss

Eddy current loss

Q. 13. Aluminum when used as conductor in large size of transformer it……….
A : Gives reduced overall cost
B : Gives increased size and cost
C : Does not affect the size and cost
D : None of these

Gives increased size and cost

Q. 14. The output of the transformer for the same transformation ratio can be
increased by

A : Increasing rate of oil flow
B : Using tappings
C : Output current
D : All of the above

Increasing rate of oil flow

Q. 15. The slip rings of wound rotor machines are made from
A : electrolytic copper
B : brass
C : cast iron
D : mild steel.

brass

Q. 16. Thermal Resistivity of Air in Ohm meter
A : 10
B : 15
C : 20
D : 25

20

Q. 17. Stator frame of three phase induction motor can be made of
A : Aluminum alloy
B : Laminated silicon steel
D : None of the above

None of the above

Q. 18. For calculating magnetizing current in transformer, which MMF is
considered

A : MMF for limb
B : MMF for yoke
C : MMF for Limb & Yoke both
D : None of above

MMF for Limb & Yoke both

Q. 19. What is the rotor windings for wound rotor motors?
A : single phase windings
B : double phase windings
C : three phase windings
D : concentrated phase windings

three phase windings

Q. 20. In case of air natural cooling, the cooling surface increases as the
…………..of the linear dimensions

A : square
B : inverse
C : cube
D : inverse cube

square

Q. 21. With increase in voltage, the window space factor of a
transformer……………

A : decreases
B : increases
C : remains constant
D : Decreases or increases depending upon the whether it is a distribution or power transformer.

decreases

Q. 22. In the design of induction motors, normally the number of slots per pole
per phase is taken as

A : two
B : three
C : three or more
D : one

three or more

Q. 23. In a single phase transformer, iron loss is 660 W and voltage supply is
6600 Volt, core loss component of the no load current is given by

A : 0.3
B : 0.2
C : 10
D : 0.1

0.1

Q. 24. The core of the transformer is stepped to reduce
A : Hysteresis Loss
B : Eddy current Loss
C : Iron Loss
D : Copper Loss

Copper Loss

Q. 25. Which among the following is the example of Class Y
A : Varnish
B : Transformer Oil
C : Paper
D : Resin

Paper

Q. 26. What is the temperature of Class B in degree celcius
A : 120
B : 130
C : 155
D : 180

130

Q. 27. To minimize the effect of radial force we should use
A : Circular coil
B : Rectangular conductor
C : Shell type core
D : Core type core

Circular coil

Q. 28. Which property of aluminum makes it the most preferred element
A : most abundant element
B : good conductivity
C : highly malleable, highly ductile
D : good corrosion resistant

most abundant element

Q. 29. If total MMF for the core is 750 AT, amplitude factor is 1.5, and no.of
Primary turns 1100, the magnetizing current in ampere, is given by

A : 0.45
B : 0.54
C : 0.62
D : 0.75

0.45

Q. 30. When the current in the bars is maximum , the current in the end ring is

A : also maximum
B : zero
C : negative
D : minimum

zero

Q. 31. What is the formula for mmf for air gap?
A : mmf for air gap = 800000 * air gap flux density * air gap factor * length of air gap
B : mmf for air gap = 800000 / air gap flux density * air gap factor * length of air gap
C : mmf for air gap = 800000 * air gap flux density / air gap factor * length of air gap
D : mmf for air gap = 800000 * air gap flux density * air gap factor / length of air gap

mmf for air gap = 800000 * air gap flux density * air gap factor * length of air gap

Q. 32. The iron loss in the rotor of three phase induction motor is negligible due
to:

A : very low frequency of emf induced in the rotor
B : very low emf induced in the rotor
C : very low flux density in rotor parts
D : laminated core

very low frequency of emf induced in the rotor

Q. 33. In induction motor if the number of rotor slots is equal to the stator slots
then

A : the motor will run smoothly
B : the motor will run at zero slip
C : the noise level will be low
D : the motor may refuse to start.

the motor may refuse to start.

Q. 34. What is the range of the average flux density in weber per meter square
used in the output equation?

A : 0.30 to 0.55
B : 0.30 to 0.50
C : 0.35 to 0.45
D : 0.35 to 0.55

0.35 to 0.55

Q. 35. The magnetizing current in induction is mainly depend on:
A : air-gap mmf
B : stator mmf
C : rotor mmf
D : none of the above

air-gap mmf

Q. 36. Which class has the lowest and the highest temperature
A : Class H, Class C
B : Class Y, Class C
C : Class B, Class H
D : Class Y, Class H

Class Y, Class C

Q. 37. What are the characteristics of high conductivity materials based on cost
and flexibility

A : Low cost, low flexibility
B : Low cost, high flexibility
C : High cost, low flexibility
D : High cost, high flexibility

Low cost, high flexibility

Q. 38. Total iron loss during the design of transformer core is worked out as 400
W. The total iron loss at 1.5 times full load will be

A : 400 W
B : 600 W
C : 800 W
D : 200W

400 W

Q. 39. What is the usage of the tanks with tubes?
A : if the temperature rise with plain tank is very low
B : if the temperature rise with plain tank is very high
C : if the temperature rise is zero
D : if the temperature rise with plain tank exceeds the specific limits

if the temperature rise with plain tank exceeds the specific limits

Q. 40. What is the formula for stator turns per phase?
A : stator turns per phase = Stator voltage per phase / 4.44 * f * maximum flux / stator
winding factor
B : stator turns per phase = Stator voltage per phase * 4.44 * f * maximum flux * stator
winding factor
C : stator turns per phase = Stator voltage per phase / 4.44 * f * maximum flux * stator
winding factor
D : stator turns per phase = Stator voltage per phase * 4.44 * f * maximum flux / stator
winding factor

stator turns per phase = Stator voltage per phase / 4.44 * f * maximum flux / stator winding factor

Q. 41. What is the range of the core length for which the stator is provided with
the ventilating ducts?

A : 105-120 mm
B : 100-120 mm
C : 100-150 mm
D : 100-125 mm

100-125 mm

Q. 42. Transformer with a capacity of up to _______ have a cooling radiator
system with natural cooling

A : 2MVA
B : 5MVA
C : 10MVA
D : 25MVA

10MVA

Q. 43. Typical value of per unit impedance for transformer is
A : 0.15
B : 01
C : 06
D : 0.2

06

Q. 44. The width of the largest stamping of a three stepped core having net core
area of 0.005 meter square is

A : 0.0821 m
B : 0.0411 m
C : 0.0205 m
D : 0.164 m

0.0821 m

Q. 45. What is the relation between the overload capacity and dispersion
coefficient

A : overload capacity is directly proportional to the dispersion coefficient
B : overload capacity is indirectly proportional to the dispersion coefficient
C : overload capacity is directly proportional to the square of the dispersion coefficient
D : overload capacity is indirectly proportional to the square of the dispersion

overload capacity is indirectly proportional to the dispersion coefficient

coefficient
Q. 46. What is the range of the rotor voltage for the large machines?
A : 1000 to 1500 V
B : 1000 to 1750 V
C : 500 to 1500 V
D : 1000 to 2000 V

1000 to 2000 V

Q. 47. What is the value of ratio of gross core area to the area of circumscribing
circle in stepped cores?

A : 0.71
B : 0.79
C : 0.89
D : 0.91

0.79

Q. 48. an induction motor will draw ——magnetizing current for a —–air gap
length

A : small, large
B : large, small
C : large, large
D : small, small

large, large

Q. 49. For a machine having final steady temperature rise of 29.4 degree
celcius, dissipating a loss of 150W over a surface area of 0.15 meter square the
emissivity will be

A : 0294
B : 661.5
C : 1323
D : 29400

661.5

Q. 50. The size and cost of machine will reduce if
A : Higher Bav and lower ac
B : Higher ac and lower Bav
C : Higher Bav and higher ac
D : lower Bav and lower ac

Higher Bav and higher ac

Q. 51. What is the range of current density in ampaer per milimeter square for
small and medium power transformers?

A : 1 to 2
B : 1 to 2.5
C : 1.1 to 2.2
D : 1.1 to 2.3

1.1 to 2.3

Q. 52. The range of ratio of stator core length to pole pitch is
A : 0 to 1
B : 1 to 2
C : 0.6 to 2
D : 0.6 to 1.5

1 to 2

Q. 53. A clearance of ———-can be left between rotor bars and the core
depending on whether the slots are skewed or not .

A : 0.15 to 0.4 mm
B : 0.6 to 1 mm
C : 2.0 to 2.2 mm
D : 0.75 to 0.82mm

0.15 to 0.4 mm

Q. 54. What is the range of the permissible peripheral speeds in meter per
second for three phase induction machine

A : 60 to70
B : 60 to 75
C : 40 to 70
D : 50 to 70

60 to 75

Q. 55. What is the Relation between pole pitch and the core length in terms of
the best power factor?

A : pole pitch = (0.18 * core length)3
B : pole pitch = (0.18 * core length)2
C : pole pitch = (0.18 * core length)1/2
D : pole pitch = (0.18 * core length)1/3

pole pitch = (0.18 * core length)1/2

Q. 56. In an induction motor ——– magnetizing current will lead to —–power
factor

A : large, poor
B : large, good
C : small, poor
D : small, good

large, poor

Q. 57. What is the ratio of the net core area to the area of the circumscribing
circle in square cores?

A : 0.58
B : 0.64
C : 0.7
D : 0.8

0.58

Q. 58. The specific loss of laminations is dependent on
A : Weight of the core
B : Iron loss
C : Stacking factor
D : Thickness of lamination

Thickness of lamination

Q. 59. What should be the range of current density in the stator windings in
ampere per meter square?

A : 2 to 5
B : 4 to 5
C : 3 to 5
D : 2 to 3