41. In self-cooled motors the cooling time constant is ……….. its heating time constant.
(a) equal to
(b) smaller than
(c) larger than
(d) unpredictable
Answer: (c) larger than
43. The system adopted for cooling of turbo-machines is
(a) open-circuit ventilation.
(b) closed-circuit air cooling.
(c) closed-circuit hydrogen cooling.
(d) none of the above
Answer: (c) closed-circuit hydrogen cooling.
44. Which one of the following statements regarding hydrogen and air is WRONG ?
(a) The density of hydrogen is one-fourteenth of that of air.
(b) The heat transfer coefficient of hydrogen is 1.5 times of that of air.
(c) Heat transfer capability of hydrogen is one seventh of that of air.
(d) The thermal conductivity of hydrogen is 7 times of that of air.
Answer: (c) Heat transfer capability of hydrogen is one seventh of that of air.
45. Hydrogen cooling has the advantage(s) of
(a) increased output rating, efficiency and operating life.
(b) elimination of fire hazard.
(c) reduced maintenance.
(d) significant reduction in windage noise and sound damping effect of heavy gas tight enclosure resulting in of gutter operation.
(e) small size of coolers.
(f) all of the above.
Answer: (f) all of the above.
46. Since hydrogen has density 1/14 th of that of air so it reduces
(a) windage loss.
(b) noise.
(c) efficiency.
(d) both (a) and (b).
Answer: (d) both (a) and (b).
47. The precaution(s) to be with hydrogen cooled machines is/are
(a) frame of hydrogen cooled machines must be gas-tight and explosion proof.
(b) hydrogen gas must be maintained above atmospheric pressure in the machine in order to avoid internal explosion.
(c) while filling and emptying the machine casing, an explosive hydrogen air mixture must be avoided.
(d) oil film gas seals must be used to prevent leakage of hydrogen.
(e) all of the above.
Answer: (e) all of the above.
48.………. cooling is the process of dissipating the armature and field losses to a cooling medium circulating within the winding insulation wall.
(a) Direct
(b) Indirect
(c) Conventional
(d) None of these.
Answer: (a) Direct
49. Machines cooled by direct cooling may be called the
(a) conductor cooled.
(b) inner cooled.
(c) supercharged.
(d) any of these.
Answer: (d) any of these.
50. In direct hydrogen gas cooling both stator and rotor conductors are made
(a) solid.
(b) hollow.
(c) perforated.
(d) any of these.
Answer: (b) hollow.
51. With conventional hydrogen cooling it is possible to increase the rating of a machine up to
(a) 50 M W
(b) 100 MW
(c) 200 MW
(d) 600 MW
Answer: (c) 200 MW
52. With direct water cooling it is possible to have rating up to
(a) 600 MW
(b) 400 MW
(c) 200 MW
(d) 100 MW
Answer: (a) 600 MW
53. Direct water cooling of rotor winding creates mechanical …….. difficulty(ies)
(a) no
(b) lesser
(c) greater
(d) none of these
Answer: (c) greater
54. The method (s) of determining the rating of motor for variable load drives is/are
(a) average loss method.
(b) equivalent current method.
(c) equivalent torque method.
(d) all of the above.
Answer: (d) all of the above.
55. Which one of the following methods of determination of motor rating for variable load drives is most accurate ?
(a) Equivalent current method.
(b) Equivalent torque method.
(c) Equivalent power method.
(d) Average loss method.
Answer: (a) Equivalent current method.
56. ………… method does not take into account the maximum temperature rise.
(a) Average loss
(b) Equivalent current
(c) Equivalent torque
(d) Equivalent power
Answer: (a) Average loss
57. Electrical machines having power output rating up to about 750 W may be called the ………. size machines.
(a) small
(b) medium
(c) large
Answer: (a) small
58. Electrical machines having power output rating ranging from a few kW up to about 250 kW may be called the ……….. size machines.
(a) small
(b) medium
(c) large
Answer: (b) medium
59. Commercially available medium size machines have the speeds ranging from
(a) 300 to 400 rpm.
(b) 600 to 800 rpm.
(c) 1,200 to 1,500 rpm.
(d) 2,000 to 2,500 rpm.
(e) above 3,000 rpm.
Answer: (d) 2,000 to 2,500 rpm.
60. In case of electrical machines, the intermittent rating is ……….. in comparison to its continuous rating.
(a) same
(b) less
(c) more
(d) any of these
Answer: (c) more