Deck 12: D.C Generators and Commutators

Question 1

The material for commutator brushes is generally

A)mica
B)copper
C)cast iron
D)carbon

Answer 1

carbon

Question 2

The insulating material used between the commutator segments is normally

A)graphite
B)paper
C)mica
D)insulating varnish

Answer 2

mica

Question 3

In D.C. generators, the brushes on commutator remain in contact with conductors which

A)lie under south pole
B)lie under north pole
C)lie under interpolar region
D)are farthest from the poles

Answer 3

lie under interpolar region

Question 4

If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be

A)demagnetisation only
B)cross magnetisation as well as magnetisation
C)crossmagnetisation as well as demagnetising
D)cross magnetisation only

Question 5

Armature reaction of an unsaturated D.C. machine is

A)crossmagnetising
B)demagnetising
C)magnetising
D)none of above

Question 6

D.C. generators are connected to the busbars or disconnected from them only under the floating condition

A)to avoid sudden loading of the primemover
B)to avoid mechanicaljerk to the shaft
C)to avoid burning of switch contacts
D)all above

Question 7

Eddy currents are induced in the pole shoes of a D.C. machine due to

A)oscillating magnetic field
B)pulsating magnetic flux
C)relative rotation between field and armature
D)all above

Question 8

Equilizer rings are required in case armature is

A)wave wound
B)lap wound
C)delta wound
D)duplex wound

Question 9

Welding generator will have

A)lap winding
B)wave winding
C)delta winding
D)duplex wave winding

Question 10

In case of D.C. machine winding, number of commutator segments is equal to

A)number of armature coils
B)number of armature coil sides
C)number of armature conductors
D)number of armature turns

Question 11

For a D.C. machines laboratory following type of D.C. supply will be suitable

A)rotary converter
B)mercury are rectifier
C)induction motor D.C. generator set
D)synchronous motor D.C. generator set

Question 12

The function of pole shoes in the case of D.C. machine is

A)to reduce the reluctance of the magnetic path
B)to spread out the flux to achieve uniform flux density
C)to support the field coil
D)to discharge all the above functions

Question 13

In the case of lap winding resultant pitch is

A)multiplication of front and back pitches
B)division of front pitch by back pitch
C)sum of front and back pitches
D)difference of front and back pitches

Question 14

A D.C. welding generator has

A)lap winding
B)wave moving
C)duplex winding
D)All of the above

Question 15

Which of the following statement about D.C. generators is false ?

A)Compensating winding in a D.C. machine helps in commutation
B)In a D. C. generator interpoles winding is connected in series with the armature winding
C)Back pitch and front pitch are both odd and approximately equal to the pole pitch
D)Equilizing bus bars are used with parallel running of D.C. shunt generators

Question 16

The demagnetising component of armature reaction in a D.C. generator

A)reduces generator e.m.f.
B)increases armature speed
C)reduces interpoles flux density
D)results in sparking trouble

Question 17

Magnetic field in a D.C. generator is produced by

A)electromagnets
B)permanent magnets
C)both (a) and (b)
D)none of the above

Question 18

The number of brushes in a commutator depends on

A)speed of armature
B)type of winding
C)voltage
D)amount of current to be collected

Question 19

Compensating windings are used in D.C. generators

A)mainly to reduce the eddy currents by providing local short-circuits
B)to provide path for the circulation of cooling air
C)to neutralise the cross-magnetising effect of the armature reaction
D)none of the above

Question 20

Which of the following components of a D.C, generator plays vital role for providing direct current of a D.C. generator ?

A)Dummy coils
B)Commutator
C)Eye bolt
D)Equilizer rings

Question 21

In a D.C. generator the ripples in the direct e.m.f. generated are reduced by

A)using conductor of annealed copper
B)using commutator with large number of segments
C)using carbon brushes of superior quality
D)using equiliser rings

Question 22

In D.C. generators, lap winding is used for

A)high voltage, high current
B)low voltage, high current
C)high voltage, low current
D)low voltage, low current

Question 23

Two generators A and B have 6-poles each. Generator A has wave wound armature while generator B has lap wound armature. The ratio of the induced e.m.f. is generatorA and B will be

A)2 : 3
B)3 : 1
C)3 : 2
D)1 :3

Question 24

The voltage drop for which of the following types of brush can be expected to be least?

A)Graphite brushes
B)Carbon brushes
C)Metal graphite brushes
D)None of the above

Question 25

The e.m.f. generated by a shunt wound D.C. generator isE. Now while pole flux remains constant, if the speed of the generator is doubled, the e.m.f. generated will be

A)E/2
B)2E
C)slightly less than E
D)E