In this article, we shared the electrical machine design objective questions and answers for electrical and electronics engineers.
Electrical Machine Design Objective Questions
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 |
| D) | Aluminium |
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?
| A) | Windings |
| B) | slotting |
| C) | Irregularities in the air gap |
| D) | Saturation |
| E) | All of these answers |
9. The operation of a D.C. machine with large air gap lengths is comparatively
| A) | noisy |
| B) | quiet |
| 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 |
| C) | Portative |
| D) | Tractive |
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
| A) | perforated |
| B) | solid |
| C) | any of these answers |
| D) | hollow |
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.
| A) | hydrogen |
| B) | none of these answers |
| C) | air |
| 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
| A) | 2 MVA |
| B) | 5 MVA |
| 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 |
| B) | increase |
| C) | either increase or decrease |
| D) | decrease |
33. In ____ machines, the size of the shaft is decided by the critical speed which depends on the deflection of the shaft.
| A) | large |
| B) | medium |
| C) | any of these answers |
| D) | small |
34. The harmonic fields are produced due to which of the following?
| A) | Irregularities in the air gap |
| B) | Saturation |
| C) | All of these answers |
| D) | Slotting |
| E) | Windings |
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
| A) | steel |
| B) | tungsten |
| C) | copper |
| D) | aluminium |
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
| A) | Conventional |
| B) | Indirect |
| C) | Any of these answers |
| D) | Direct |
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 |
| D) | Electro-graphitic |
45. The cold rolled grain oriented steel has ____ permeability in the direction of the grain orientation.
| A) | minimum |
| B) | none of these answers |
| C) | nil |
| D) | maximum |
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.
| A) | 0.07 |
| B) | 3.5 |
| C) | 1.5 |
| D) | 2.5 |
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
| A) | supercharged |
| B) | conductor cooled |
| C) | inner cooled |
| D) | any of these answers |
51. The flash point of transformer oil should be higher than
| A) | 60°C |
| B) | 104°C |
| C) | 80°C |
| D) | 40°C |
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
| A) | low |
| B) | none of these answers |
| C) | zero |
| D) | high |
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.
| A) | high |
| B) | low |
| C) | any of these answers |
| D) | medium |
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.
| A) | circular |
| B) | rectangular |
| C) | trapezoidal |
| D) | triangular |
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
| A) | brass |
| 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 |
| C) | changes |
| 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 |
| B) | Cast-iron |
| C) | Aluminium |
| D) | Copper |
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 |
| C) | Saturation |
| D) | Power factor |
| E) | Efficiency |
74. ____ is universally used for windings of electrical machines because it is easily workable without any possibility of fracture.
| A) | Aluminium |
| B) | Silver |
| C) | Copper |
| D) | Steel |
75. The heated parts of an electrical machine dissipate heat into their surroundings by which of the following modes of heat dissipation ?
| A) | Conduction |
| B) | All of these answers |
| C) | Radiation |
| D) | Convection |
76. ___ is extensively used for those portions of magnetic circuit which carry steady flux and need superior mechanical properties ?
| A) | Cast-Steel |
| 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 |
| B) | less |
| C) | more |
| D) | same |
78. A short time intermittent rating of electric motors is considered while selecting motors for
| A) | sirens |
| B) | reciprocating compressors |
| C) | punching machines |
| D) | blowers |
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 |
| C) | multi-layered |
| 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 |
| B) | Fins |
| C) | Tubes |
| D) | All of these answers |
| E) | Corrugations |
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
| A) | same |
| B) | more |
| C) | any of these answers |
| D) | less |
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
| A) | crawling |
| B) | none of these answers |
| C) | cogging |
| 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 ____
| A) | insulators |
| B) | semiconductors |
| C) | conductors |
| D) | superconductors |
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
| A) | frame |
| B) | main poles |
| C) | all of these answers |
| D) | interpoles |
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 |
| C) | low |
| D) | high |
104. ___ is the common method of applying brushes to the commentator.
| A) | Reaction |
| B) | All of these answers |
| C) | Radial |
| D) | Trailing |
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 |
| C) | 2 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 |
| C) | large |
| D) | 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 |
| E) | Interpoles |
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.
| A) | high |
| B) | none of these answers |
| C) | low |
| 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
| A) | 1.5 |
| B) | 3.5 |
| C) | 2.5 |
| D) | 4.5 |
122. The increase in heat dissipation by air blasts is due to an increase in ____
| A) | conduction |
| B) | convection |
| C) | radiation |
| 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
| A) | 3 |
| B) | 5 |
| C) | 2 |
| D) | 7 |
132. In D.C. machines, the ____ in field mmf results in an increase in size and cost of machines.
| A) | decrease |
| B) | increase |
| 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 |
| E) | Cooling |
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 |
| B) | sometimes |
| C) | never |
| D) | always |
142. The action of electromagnetic machines can be related to which of the following basic principles ?
| A) | Interaction |
| B) | Alignment |
| C) | All of these answers |
| D) | Induction |
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 |
| D) | unaffected |
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?
| A) | Cooling |
| 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?
| A) | Cost |
| B) | Compliance with performance criteria as laid down in specifications |
| C) | All of these answers |
| D) | Durability |
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.
| A) | conduction |
| B) | convection |
| C) | any of these answers |
| D) | radiation |
153. Which of the following methods is used for the reduction/elimination of harmonic torques?
| A) | Chording |
| B) | Integral slot windings |
| C) | Increase in air gap length |
| D) | Skewing |
| 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?
| A) | Traction |
| 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?
| A) | Copper |
| B) | Aluminium |
| C) | Steel |
| D) | Cast-iron |
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