Electrical Machines Objective Type Questions and Answer-PDF

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Electrical Machines Objective Type Questions and Answer-PDF  for Competitive Exams


1.Direct water cooling of rotor winding presents
(a) no mechanical difficulties
(b) lesser mechanical difficulties
(c) greater mechanical difficulties
(d) none of the above
Ans: c

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

(a) heating time constant
(b) cooling time constant
(c) either (a) or (b)
(d) none of the above
Ans: a

3.In self cooled motors the cooling time constant is about ______ than the heating time constant because cooling conditions are worse at standstill.
(a) 2 to 3 times greater
(b) 3 to 4 times greater
(c) 4 to 5 times greater
(d) none of the above
Ans: a

4.By which of the following methods motor rating for variable load drives can be determined ?
(a) Method of average losses
(b) Equivalent current method
(c) Equivalent torque method
(d) Equivalent power method
(e) All of the above.
Ans: e

5.Which of the following methods does not take into account the maximum temperature rise under variable load
conditions ?
(a) Equivalent power method
(b) Equivalent current method
(c) Method of average losses
(d) Equivalent torque method
Ans: c
6.Machines cooled by direct cooling method may be called
(a) “supercharged”
(b) “inner cooled”
(c) “conductor cooled”
(d) any of the above
Ans: d

7.In direct cooled system using hydrogen both stator and rotor conductors are made
(a) solid
(b) hollow
(c) perforated
(d) any ofthe above
Ans: b

8.With direct water cooling it is possible to have ratings of about
(a) 200 MW
(b) 300 MW
(c) 400 MW
(d) 600 MW
Ans: d

9.The resistivity of water should not be less than
(a) 10 Q m
(b) 100 Q m
(c) 1000 Q m
(d) 2000 Q m
Ans: d

10.Hydrogen has a heat transfer co-efficient _____ times that of air
(a) 1.5
(b) 2.5
(c) 3.5
(d) 4.5
Ans: a

11.The thermal conductivity of hydrogen is ______ times that of air
(a) 2
(b) 3
(c) 5
(d) 7
Ans: d

12.With conventional hydrogen cooling it is possible to increase the rating of a single unit to
(a) 50 MW
(b) 100 MW
(c) 200 MW
(d) none of the above
Ans: c

13.The noise produced by a ______ cooled machine is less as the rotor moves in a medium of smaller density.
(a) air
(b) hydrogen
(c) either (a) or (b)
(d) none of the above
Ans: b

14.cooling is the process of dissipating the armature and field winding losses to a cooling medium circulating within the winding insulation wall
(a) Direct
(b) Indirect
(c) Conventional
(d) Any of the above
Ans: a

15.The mcrease in heat dissipation by air blasts is due to increase in
(a) conduction
(b) convection
(e) radiation
(d) any ofthe above
Ans: b

16.On which of the following variables heat convected depends ?
(a) Power density
(b) Temperature difference between heated surface and coolant
(c) Thermal resistivity, density, specific heat
(d) Gravitational constant
(e) All of the above
Ans: e

17.Which of the following methods is used for air cooling of turbo-alternators ?
(a) One sided axial ventilation
(b) Two sided axial ventilation
(c) Multiple inlet system
(d) All of the above
Ans: d

18.Multiple inlet system of air cooling of turbo-alternators can be used for machines of rating upto
(a) 10 MW
(b) 30 MW
(c) 60 MW
(d) 150 MW
Ans: c

19.Which of the following is an advantage of hydrogen cooling ?
(a) Increase in efficiency
(b) Increase in ratings
(c) Increase in life
(d) Smaller size of coolers
(e) All of the above
Ans: e

20.The density of hydrogen is _____ times the density of air.
(a) 0.07
(b) 1.5
(c) 2.5
(d) 3.5
Ans: a

21.Aluminium when adopted as a conductor material in ______ transformers, decreases the overall cost of the transformer
(a) small size
(b) medium size
(c) large size
(d) any of the above size
Ans: a

22.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
Ans: d

23.Materials exhibiting zero value of resistivity are known as ______ .
(a) conductors
(b) semiconductors
(c) insulators
(d) superconductors
Ans: d

24.________ has a lowrelative permeability and is used principally in field frames when cost is of primary importance and extra weight is not objectionable.
(a) Cast steel
(b) Aluminium
(c) Soft steel
(d) Cast iron
Ans: d

25.______is extensively used for those portions of magnetic circuit which carry steady flux and need superior mechanical properties ?
(a) Grey cast-iron
(b) Cast steel
(c) High carbon steel
(d) Stainless steel
Ans: b

26.Hot rolled sheets have ______ value of permeability
(a) zero
(b) low
(c) high
(d) none of the above
Ans: b

27.The heated parts of an electrical machine dissipate heat into their surroundings by which of the following
modes of heat dissipation ?
(a) Conduction
(b) Convection
(c) Radiation
(d) All of the above
Ans: d

28.The heat dissipated by from a surface depends upon its temperature and its characteristics like colour, roughness etc.
(a) conduction
(b) convection
(c) radiation
(d) any of the above
Ans: c

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