Realtime Electrical Machine Design Multiple choice Questions and Answers pdf

21.    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 ofthe above
Ans: d

22.    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 ofthe above
Ans: c

23.    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

24.    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 ofthe above
Ans: e

25.    Which ofthe 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 ofthe above
Ans: d

26.    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

27.    Which ofthe 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 ofthe above
Ans: e

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

29.    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

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

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