150+ Electrical Machine Design Objective Questions and Answers

In this article, we shared the electrical machine design objective questions and answers for electrical and electronics engineers.

Electrical Machine Design Objective Questions

Electrical Machine Design Objective Questions and Answers

The below list provides all the multiple-choice questions related to electrical machine design.

Answers are available at the bottom of the article.

1. With direct water cooling it is possible to have ratings of about

A)600 MW
B)400 MW
C)200 MW
D)300 MW

2. The winding where dummy coils are used is sometimes called

A)none of these answers
B)duplex winding
C)triplex winding
D)forced winding

3. ___ has low relative permeability and is used principally in field frames when the cost is of primary importance and extra weight is not objectionable.

A)Cast steel
B)Soft steel
C)Cast iron

4. A 3-phase winding carrying sinusoidal currents produces harmonics of the order (where N is an integer)

A)n = 8 N ± 4
B)n = 6 N ± 4
C)none of these answers
D)n = 6 N ± 1
E)n = 8 N ± 1

5. The design of electromagnets is based upon which of the following fundamental equations?

A)All of these answers
B)Heating equation
C)Magnetic circuit equation
D)Force equation
E)Voltage equation

6. A large value of short-circuit current indicates

A)zero power factor
B)none of these answers
C)a good power factor
D)a poor power factor

7. In D.C. machines the usual limit of slot pitch is

A)between 15 to 20 mm
B)between 10 to 15 mm
C)between 5 to 10 mm
D)between 25 to 35 mm

8. Due to which of the following, harmonic fields are produced?

C)Irregularities in the air gap
E)All of these answers

9. The operation of a D.C. machine with large air gap lengths is comparatively

C)either quiet or noisy
D)none of these answers

10. The change in flux linkages can be caused in which of the following ways?

A)The coil is stationary with respect to flux and the flux varies in magnitude with respect to time
B)The flux is constant with respect to time and is stationary and the coil moves through it
C)All of these answers
D)Both the changes mentioned above occur together, i.e., the coil moves through a time-varying field

11. Which of the following problem arises in D.C. motors that are fed from thyristor bridge circuits?

A)Poor commutation
B)Increased IR ​​losses
C)Increased core losses
D)All of these answers
E)Change in motor parameters

12. The transformer ID noise is mainly due to

A)cooling coil
B)magnetic flux
C)all of these answers
D)sinusoidal current

13. The resistivity of water should not be less than

A)2000 Ω m
B)1000 Ω m
C)10 Ω m
D)100 Ω m

14. On which of the following variables heat convected depends?

A)All of these answers
B)Thermal resistivity, density, specific heat
C)Gravitational constant
D)Power density
E)The temperature difference between heated surface and coolant

15. The thickness of the brush should be so selected that it covers ____ commutator segments.

A)4 to 6
B)6 to 8
C)2 to 3
D)8 to 10

16. ____ electromagnets generally function as holding magnets.

A)None of these answers
B)Either Tractive or Portative

17. Large values of air-gap flux density can be adopted while designing induction motors of

A)large output
B)both large output and large diameter of rotor
C)large diameter of rotor
D)none of these answers

18. In direct cooled system using hydrogen both stator and rotor conductors are made

C)any of these answers

19. Power transformers have a rating

A)equal to 50 kVA
B)equal to 100 kVA
C)none of these answers
D)above 200 kVA

20. The noise produced by a ____ cooled machine is less as the rotor moves in a medium of smaller density.

B)none of these answers
D)either air or hydrogen

21. Tractive magnets are operated from

A)none of these answers
B)a.c. supply
C)d.c. supply
D)either a.c or d.c. supply

22. Short time rating machines have usually

A)longer operating times and longer shut down times
B)longer operating times and shorter shut down times
C)low operating times and longer shut down times
D)low operating times and shorter shut down times

23. Transformer oil should have

A)high flash point
B)high sludging tendencies
C)high viscosity
D)none of these answers

24. Turbo-alternators have ____

A)short diameters and narrow axial lengths
B)short diameters and great axial lengths
C)large diameters and great axial lengths
D)large diameters and narrow axial lengths

25. In self-cooled motors the cooling time constant is about ____ than the heating time constant because cooling conditions are worse at standstill.

A)4 to 5 times greater
B)none of these answers
C)2 to 3 times greater
D)3 to 4 times greater

26. The length of air gap to be provided in ____ is primarily determined by power factor consideration.

A)d.c. series motor
B)synchronous motor
C)induction motor
D)d.c. shunt motor

27. Multi-layer helical windings are commonly used in the transformers as high voltage windings

A)for 110 kV and above
B)upto 50 kV
C)upto 80 kV
D)upto 20 kV

28. Which of the following is the commonly used type of electromagnets?

A)Flat-faced plunger type
B)Flat-faced armature type
C)All of these answers
D)Horse shoe type

29. In D.C. machines the width of the duct is usually

A)4 mm
B)8 mm
C)10 mm
D)6 mm

30. Transformers with a capacity of up to ____ have a cooling radiator system with natural cooling

C)10 MVA
D)7.5 MVA

31. For induction motors requiring speed control or extremely high values of starting torque which of the following rotor construction is employed?

A)Squirrel cage rotor construction
B)none of these answers
C)wound rotor construction
D)either Squirrel cage rotor construction or wound rotor construction

32. The weight of copper of both armature and field windings decreases within the number of poles.

A)none of these answers
C)either increase or decrease

33. In ____ machines, the size of the shaft is decided by the critical speed which depends on the deflection of the shaft.

C)any of these answers

34. The harmonic fields are produced due to which of the following?

A)Irregularities in the air gap
C)All of these answers

35. For large power transformers, self oil cooled type or air blast type which of the following values af current density may be used?

A)3.2 to 4.2 A/mm2
B)2.2 to 3.2 A/mm2
C)1.0 to 1.2 A/mm2
D)1.5 to 2.0 A/mm2

36. The yoke of a D.C. motor is generally made of


37. In a D.C. machine the number of brush arms ____ is the number of poles.

A)greater than
B)less than
C)equal to
D)none of these answers

38. In transformers using hot rolled steel, the cross-section of the yoke is made about ___ greater than that of the core

A)none of these answers
B)15 percent
C)10 percent
D)5 percent
E)30 percent

39. The great majority of present-day induction motors are manufactured with

A)squirrel-cage rotors
B)both squirrel-cage rotors ​ and wound rotors
C)wound rotors
D)either squirrel cage rotors or wound rotors

40. Power transformers should be designed to have maximum efficiency

A)at or near full load
B)at one-half load
C)at one-fourth load
D)any of these answers

41. ____ cooling is the process of dissipating the armature and field winding losses to a cooling medium circulating within the winding insulation wall

C)Any of these answers

42. In a D.C. machine, the value of peripheral speed should not, normally, exceed

A)10 m/s
B)20 m/s
C)40 m/s
D)30 m/s

43. A practical formula for determining the thickness of insulation between low voltage and high voltage windings is

A)1 + 0.2 kV mm
B)5 + 0.9 kV ram
C)4 + 0.7 kV aim
D)2 + 0.5 kV ram

44. ____ brushes are fragile and cause excessive wear of commutator.

A)Natural graphite
B)Hard carbon
C)Metal graphite

45. The cold rolled grain oriented steel has ____ permeability in the direction of the grain orientation.

B)none of these answers

46. ____ of lots are generally used in induction motors.

A)Any of these answers
B)Open type
C)Semi-closed type
D)Closed type

47. The density of hydrogen is ____ times the density of air.


48. For 275 KV transformers, using cold rolled grain oriented steel, which of the following values of flux density may be used?

A)1.0 Wb/m2
B)1.6 Wb/m2
C)1.1 Wb/m2
D)None of these answers
E)1.3 Wb/m2

49. ____ are always double layer types.

A)Closed windings
B)Either Closed windings ​ or Open windings
C)Open windings
D)None of these answers

50. Machines cooled by the direct cooling method may be called

B)conductor cooled
C)inner cooled
D)any of these answers

51. The flash point of transformer oil should be higher than


52. Tap changing facility is generally provided on

A)distribution transformers
B)high voltage transformers
C)none of these answers
D)current transformers

53. Hot rolled sheets have ____ value of permeability

B)none of these answers

54. The time taken by the machine to attain 0.632 of its final steady temperature rise is called

A)either heating time constant or cooling time constant
B)none of these answers
C)cooling time constant
D)heating time constant

55. The design of mechanical parts is particularly important in case of ____ speed machines.

C)any of these answers

56. Which of the following methods is most accurate?

A)Equivalent power method
B)Equivalent torque method
C)Equivalent current method
D)Method of average losses

57. The conductors for large sizes of D.C. machines are ____ in cross-section.


58. In D.C. machines, in order to prevent excessive distortion of field form by the armature reaction, the field mmf must be made

A)large in comparison with the armature mmf
B)none of these answers
C)equal to that of armature mmf
D)less in comparison with the armature mmf

59. Frames of electrical machines are used to serve which of the following purposes ?

A)All of these answers
B)To enclose the core and windings
C)To transmit the torque to the machine supports
D)To serve as ventilating housing
E)To shield the live and moving machine parts from human contact and from injury caused by intruding objects or weather exposure

60. By which of the following methods motor rating for variable load drives can be determined ?

A)Equivalent torque method
B)Method of average losses
C)All of these answers
D)Equivalent power method
E)Equivalent current method

61. Electrical machines having a power output upto about 750 W may be called ____ machines.

A)large size
B)medium size
C)small size
D)any of these answers

62. For a normal design of an induction motor, the diameter should be so chosen that the peripheral speed does not exceed about

A)20 m/s
B)30 m/s
C)50 m/s
D)10 m/s

63. The area of each individual brush should be taken so that it does not carry

A)more than 20 A
B)more than 40 A
C)more than 70 A
D)more than 50 A

64. Which of the following methods is used for air cooling of turbo-alternators ?

A)Two sided axial ventilation
B)All of these answers
C)One sided axial ventilation
D)Multiple inlet system

65. The slip rings of wound rotor machines are made of

B)cast iron
C)electrolytic copper
D)mild steel

66. Dummy coil should not be used in

A)small machines
B)either small machines or large machines
C)none of these answers
D)large machines

67. ____ are used for construction of core of electromagnets.

A)Either Soft magnetic materials or Hard magnetic materials
B)Hard magnetic materials
C)Soft magnetic materials
D)None of these answers

68. If the frequency is changed without altering the number of poles the dispersion co-efficient

A)none of these answers
B)does not change
D)becomes zero

69. In case of phase wound induction motors the full load rotor m.m.f. is taken as ____ of stator m.m.f.

A)60 percent
B)95 percent
C)85 percent
D)40 percent

70. On which of the following factors, does the brush friction loss depend ?

A)Co-efficient of friction between brush and the commutator
B)All of these answers
C)Brush pressure
D)Peripheral speed of the commutator

71. ____ is usually used for brush rockers

A)Mild steel

72. In a D.C. machine the current per brush arm should not be more than

A)100 A
B)200 A
C)400 A
D)300 A

73. ___ impose limitation on design.

A)All of these answers
B)Temperature rise
D)Power factor

74. ____ is universally used for windings of electrical machines because it is easily workable without any possibility of fracture.


75. The heated parts of an electrical machine dissipate heat into their surroundings by which of the following modes of heat dissipation ?

B)All of these answers

76. ___ is extensively used for those portions of magnetic circuit which carry steady flux and need superior mechanical properties ?

B)Stainless steel
C)Grey cast-iron
D)High carbon steel

77. The cost of an induction motor as compared to that of a D.C. motor, of the same rating, is

A)none of these answers

78. A short time intermittent rating of electric motors is considered while selecting motors for

B)reciprocating compressors
C)punching machines

79. Which loss occurs in the yoke of a D.C. machine ?

A)No loss
B)Copper loss
C)Heat loss
D)Iron loss

80. Electrical machines having power outputs ranging from a few KW upto approximately 250 KW may be classified as ___

A)any of these answers
B)medium size machines
C)small size machines
D)large size machines

81. Cylindrical windings using circular conductors, employed in transformers, are

A)single layered
B)none of these answers
D)double layered

82. A synchronous machine with large length of air gap will have

A)smaller unbalanced magnetic pull
B)lower limit of stability
C)higher value of inherent regulation
D)higher noise level

83. The heat dissipating capability of transformers of ratings higher than 30 kVA in increased by providing which of the following ?

A)Radiator tanks
D)All of these answers

84. The insulation between windings and grounded core and the insulation between the windings of the same phase is called

A)minor insulation
B)major insulation
C)either minor insulation or major insulation
D)none of these answers

85. The flux density in the stator core of an induction motor should not exceed about

A)0.9 Wb/m2
B)1.1 Wb/m2
C)1.5 Wb/m2
D)0.6 Wb/m2

86. In case of electrical machines, the intermittent rating as compared to its continuous rating is

C)any of these answers

87. Which of the following is the basic consideration in the design of insulation ?

A)Mechanical considerations
B)Electrical considerations
C)All of these answers
D)Thermal considerations

88. Aluminium when adopted as a conductor material in ____ transformers, decreases the overall cost of the transformer

A)medium size
B)small size
C)any of these answers
D)large size

89. With conventional hydrogen cooling it is possible to increase the rating of a single unit to

A)50 MW
B)200 MW
C)none of these answers
D)100 MW

90. If the number of rotor slots is equal to the number of stator slots, the machine would refuse to start, this is known as

B)none of these answers
D)either cogging or crawling

91. Commercial available medium size machines have a speed range of ____

A)2000 to 2500 r.p.m.
B)1000 to 1500 r.p.m.
C)200 to 400 r.p.m.
D)600 to 1000 r.p.m.

92. Yokes with rectangular cross-section are used for

A)medium capacity transformers
B)large capacity transformers
C)any of these answers
D)small capacity transformers

93. The laminations of the armature of a D.C. machine are usually ____ thick.

A)0.3 to 0.4 mm
B)0.1 to 0.2 mm
C)0.2 to 0.3 mm
D)0.4 to 0.5 mm

94. Materials exhibiting zero value of resistivity are known as ____


95. The voltage control in electric supply networks in required on account of which of the following reasons?

A)Control of active and reactive power
B)Adjustment of short period daily and seasonal voltage variations in accordance with variations of load
C)All of these answers
D)Adjustment of voltage at consumers premises within statutory limits

96. Helical windings are used in

A)distribution transformers
B)power transformers
C)shell type transformers
D)none of these answers

97. The high voltage winding is usually which of the following type ?

A)Cross-over winding with either circular or small rectangular conductors
B)All of these answers
C)Cylindrical winding with circular conductors
D)Continuous disc type winding with rectangular conductors

98. The forced oil and air circulation method is usual one for transformers of capacities

A)upto 10 MVA
B)30 MVA upwards
C)upto 5 MVA
D)upto 20 MVA

99. The usual values of maximum flux densities for distribution transformers using hot rolled silicon steel are

A)1.1 to 1.35 Wb/m2
B)1.4 to 1.8 Wb/m2
C)0.5 to 0.8 Wb/m2
D)0.8 to 1.0 Wb/m2

100. D.C. machines designed with a large value of air gap length have

A)poor ventilation
B)better ventilation
C)worst ventilation
D)none of these answers

101. The stator of a D.C. machine comprises of

B)main poles
C)all of these answers

102. In induction motors, the length of air gap in kept as small as mechanically possible is order to have

A)high power factor
B)high over load capacity
C)low power factor
D)any of these answers

103. It is a common practice to use deep bar rotors when ____ starting torque is required.

A)either low or high
B)none of these answers

104. ___ is the common method of applying brushes to the commentator.

B)All of these answers

105. The diameter of commutator generally lies

A)between 0.4 to 0.6 of armature diameter
B)between 0.2 to 0.4 of armature diameter
C)none of these answers
D)between 0.6 to 0.8 of armature diameter

106. A wire of ____ will have least diameter.

A)30 SWG
B)20 SWG
D)10 SWG

107. The air gap of an induction motor is made as ____ as possible.

A)none of these answers
B)either large or small

108. Which of the following methods may be adopted to reduce the effects of armature reaction ?

A)Compensating windings
B)All of these answers
C)Increase in length of air gap at pole tips
D)Increasing reluctance of pole tips

109. Which of the following methods does not take into account the maximum temperature rise under variable load conditions?

A)Equivalent current method
B)Method of average losses
C)Equivalent torque method
D)Equivalent power method

110. A current density of ____ is used for large power transformers with forced circulation of oil or with water cooling coils

A)3.5 to 4.5​​ A/mm²
B)1.5 to 2.5​​ A/mm²
C)4.0 to 5.0​​ A/mm²
D)5.4 to 6.2​​ A/mm²

111. The shafts of electric motors is generally supported in

A)cast-iron bearings
B)magnetic bearings
C)ball or roller bearings
D)bush bearings

112. The efficiency of a machine should be as ____ possible to reduce the operating cost.

B)none of these answers
D)either high or low

113. Disc windings are primarily used in

A)high capacity transformers
B)any of these answers
C)short capacity transformers
D)medium capacity transformers

114. In a D.C. generator, the number of commutator segments is

A)less than the number of coils
B)equal to the number of coils
C)none of these answers
D)greater than the number of coils

115. The use of salient poles on high speed alternators will cause

A)reduced windage loss with low noise
B)reduced windage loss but excessive noise
C)excessive windage loss and excessive noise
D)excessive windage less but reduced noise

116. D.C. windings are

A)always 2-layer type
B)none of these answers
C)never 2-layer type
D)sometimes 2-layer type

117. The space factor depends on all of the following except

A)density of the conductor
B)insulation thickness
C)size of the conductor
D)arrangement of the conductor

118. By which of the following methods the temperature rise of windings and other parts may be determined?

A)Thermometer method
B)Resistance method
C)Embedded temperature detector method
D)Any of these answers

119. Direct water cooling of rotor winding presents

A)greater mechanical difficulties
B)none of these answers
C)no mechanical difficulties
D)lesser mechanical difficulties

120. The distance between the starts of two consecutive coils measured in terms of coil sides is called

A)back pitch
B)commutator pitch
C)winding pitch
D)front pitch

121. Hydrogen has a heat transfer co-efficient ____ times that of air


122. The increase in heat dissipation by air blasts is due to an increase in ____

D)any of these answers

123. Current density in the rotor bars of an induction motor may be taken between

A)3 to 4 A/mm2
B)1.5 to 2.5 A/mm2
C)4 to 7 A/mm2
D)8 to 10 A/mm2

124. The ratio of magnetizing current to ideal short circuit current is called

A)leakage co-efficient
B)either leakage co-efficient or dispersion co-efficient
C)none of these answers
D)dispersion co-efficient

125. The regulation of a transformer is least affected by changes in frequency at

A)lagging power factor
B)leading power factor
C)all of these answers
D)unity power factor

126. In an induction motor, closed slots are preferred for

A)small size machines
B)large size machines
C)medium size machines
D)none of these answers

127. The slot leakage can be calculated by making which of the following assumptions?

A)The leakage path is straight across the slot and around the iron at the bottom
B)All of these answers
C)The permanence of air paths is only considered.
D)The current in the slot conductors is uniformly distributed over their cross-section

128. Arc welding transformer is basically a

A)frequency changing device
B)step down transformer
C)current transformer
D)step up transformer

129. A semi-enclosed slot gives

A)a better overload capacity
B)less noise
C)none of these answers
D)low power factor

130. The number of poles in D.C. machines is generally decided by

A)weight of iron parts
B)weight of copper
C)all of these answers
D)frequency of flux reversals

131. The thermal conductivity of hydrogen is ____ times that of air


132. In D.C. machines, the ____ in field mmf results in an increase in size and cost of machines.

C)none of these answers
D)either increase or decrease

133. The surge phenomenon is particularly important in the case of

A)low voltage transformers
B)any of these answers
C)medium voltage transformers
D)high voltage transformers

134. The value of exciting or magnetizing current depends upon which of the following factors?

A)The way in which the winding is distributed
B)Total m.m.f. required
C)All of these answers
D)The number of turns in the exciting winding

135. Which of the following, primarily, determines the magnetizing current drawn by three phase induction motor?

A)Unbalanced magnetic pull
B)Length of air gap
C)Any of these answers
D)Over-load capacity

136. The higher commutator peripheral speeds

A)generally lead to commutation difficulties
B)always lead to commutation difficulties
C)any of these answers
D)never lead to commutation difficulties

137. Multiple inlet system of air cooling of turbo-alternators can be used for machines of rating upto

A)60 MW
B)10 MW
C)150 MW
D)30 MW

138. The cylindrical windings using circular conductors are used for the current rating of

A)upto 80 A
B)upto 20 A
C)upto 40 A
D)upto 60 A

139. D.C. servo motors are used in

A)purely A.C. control systems
B)purely D.C. control systems
C)both D.C. and A.C. control systems
D)none of these answers

140. Flame proof motors are used in

A)moisture free atmosphere
B)moisture and dust free atmosphere
C)explosive atmosphere
D)dust free atmosphere

141. The number of stator slots should ____ be equal to the number of rotor slots

A)none of these answers

142. The action of electromagnetic machines can be related to which of the following basic principles ?

C)All of these answers

143. When the two coil sides forming a coil are spaced exactly one pole pitch apart they are said to be of

A)full pitch
B)short pitch
C)either short pitch or full pitch
D)none of these answers

144. Which of the following limits the output of a rotating electrical machine?

A)Peripheral velocity
B)Size of the machine
C)Temperature rise
D)None of these answers

145. Pitch of commutator segment should not be less than

A)4 mm
B)3 mm
C)1 mm
D)2 mm

146. If an insulating material is operated beyond the maximum allowable temperature, its life is

A)drastically increased
B)none of these answers
C)drastically reduced

147. Which of the following brushes can be used for high values of current density?

A)Metal graphite brushes
B)Hard carbon brushes
C)Natural graphite brushes
D)Electro-graphitic brushes

148. Which of the following factors should be considered when estimating the length of air gap?

B)Pulsation loss
C)Unbalanced magnetic pull and noise
D)All of these answers
E)Power factor & Over-load capacity

149. In D.C. machines the number of slots per pole usually lies

A)between 2 to 4
B)between 6 to 8
C)between 9 to 16
D)between 20 to 30

150. Which of the following is the major consideration to evolve a good design?

B)Compliance with performance criteria as laid down in specifications
C)All of these answers

151. In general, the ratio of core length to pole pitch, in an induction motor, lies between ____ depending upon the size of the machine and the characteristics desired

A)0.1 and 0.3
B)0.6 and 2.0
C)0.4 and 1.0
D)0.7 and 3.0

152. The heat dissipated by from a surface depends upon its temperature and its characteristics like colour, roughness etc.

C)any of these answers

153. Which of the following methods is used for the reduction/elimination of harmonic torques?

B)Integral slot windings
C)Increase in air gap length
E)All of these answers

154. Which of the following is an advantage of hydrogen cooling?

A)Increase in efficiency
B)Increase in life
C)Increase in rating
D)Smaller size of coolers

155. Which of the following in an application of DC motors?

B)All of these answers
C)Drives for process industry
D)Battery driven vehicles
E)Automatic control

156. The slip rings for wound rotor machines are made of

A)chromium steel
B)cast iron
C)either brass or phosphor bronze
D)mild steel

157. Which of the following materials is used in the manufacture of resistance grids to be used in the starters of large motors?


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