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| Electrical Machine |
1. Multiple inlet system of air cooling of turbo-alternators can be used for machines of rating up to_______.
(a) 10 MW
(b) 30 MW
(c) 60 MW
(d) 150 MW
2. 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
3. The density of hydrogen is _____ times the density of air.
(a) 0.07
(b) 1.5
(c) 2.5
(d) 3.5
4. Hydrogen has a heat transfer co-efficient _____ times that of air.
(a) 1.5
(b) 2.5
(c) 3.5
(d) 4.5
5. The thermal conductivity of hydrogen is ______ times that of air.
(a) 2
(b) 3
(c) 5
(d) 7
6. 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
7. The noise produced by a ______ cooled machine is less than the rotor moves in a medium of smaller density.
(a) air
(b) hydrogen
(c) either (a) or (b)
(d) none of the above
8. ______ 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
9. Machines cooled by direct cooling method may be called_______.
(a) “supercharged”
(b) “inner cooled”
(c) “conductor cooled”
(d) any of the above
10. In direct cooled system using hydrogen both stator and rotor conductors are made________.
(a) solid
(b) hollow
(c) perforated
(d) any of the above
11. With direct water cooling it is possible to have ratings of about_______.
(a) 200 MW
(b) 300 MW
(c) 400 MW
(d) 600 MW
12. 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
13. Direct water cooling of rotor winding presents_________.
(a) no mechanical difficulties
(b) lesser mechanical difficulties
(c) greater mechanical difficulties
(d) none of the above
14. 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
15. 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
16. 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
17. 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
18. Which of the following methods is most accurate ?
(a) Equivalent current method
(b) Equivalent power method
(c) Equivalent torque method
(d) Method of average losses
19. 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 the above
20. The slot leakage can be calculated by making which of the following assumptions ?
(a) The current in the slot conductors is uniformly distributed over their cross-section
(b) The leakage path is straight across the slot and around the iron at the bottom
(c) The permanence of air paths is only considered. The reluctance of iron paths is assumed as zero
(d) All of the above
21. The value of exciting or magnetizing current depends upon which of the following factors ?
(a) Total m.m.f. required
(b) The number of turns in the exciting winding
(c) The way in which the winding is distributed
(d) All of the above
22. Tractive magnets are operated from_____.
(a) a.c. supply
(b) d.c. supply
(c) either a.c. or d.c. supply
(d) none of the above
23. ___________electromagnets generally function as holding magnets.
(a) Tractive
(b) Portative
(c) Either of the above
(d) None of the above
24. Which of the following is the commonly used type of electromagnets ?
(a) Flat-faced armature type
(b) Horse shoe type
(c) Flat-faced plunger type
(d) All of the above
25. __________are used for construction of core of electromagnets.
(a) Soft magnetic materials
(b) Hard magnetic materials
(c) Either (a) or (b)
(d) None of the above
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