Deck 2: Cooling and Electromagnets for Machines

Question 1

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

Answer 1

60 MW

Question 2

Which ofthe following is an advantage of hydrogen cooling ?

A)Increase in efficiency
B)Increase in ratings
C)Increase in life
D)All ofthe above

Answer 2

All ofthe above

Question 3

The density of hydrogen is _____ times the density of air.

A)0.07
B)1.5
C)2.5
D)3.5

Answer 3

0.07

Question 4

Hydrogen has a heat transfer co-efficient _____ times that of air

A)1.5
B)2.5
C)3.5
D)4.5

Question 5

The thermal conductivity of hydrogen is ______ times that of air

A)2
B)3
C)5
D)7

Question 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 ofthe above

Question 7

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

Question 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 ofthe above

Question 9

Machines cooled by direct cooling method may be called

A)"supercharged"
B)"inner cooled"
C)"conductor cooled"
D)any of the above

Question 10

In direct cooled system using hydrogen both stator and rotor conductors are made

A)solid
B)hollow
C)perforated
D)any ofthe above

Question 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

Question 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

Question 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

Question 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

Question 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

Question 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)All of the above.

Question 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

Question 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

Question 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

Question 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

Question 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

Question 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

Question 23

electromagnets generally function as holding magnets.

A)Tractive
B)Portative
C)Either of the above
D)None of the above

Question 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

Question 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