Deck 11: D.C Machine and Generator Laminations, Commutation, and Components

Question 1

In the D.C. motor the iron losses occur in

A)the field
B)the armature
C)the brushes
D)the commutator

Answer 1

the armature

Question 2

Laminations of core are generally made of

A)case iron
B)carbon
C)silicon steel
D)stainless steel

Answer 2

silicon steel

Question 3

Which of the following could be lamina-proximately the thickness of laminations of a D.C. machine ?

A)0.005 mm
B)0.05 mm
C)0.5 m
D)5 m

Answer 3

0.5 m

Question 4

The armature of D.C. generator is laminated to

A)reduce the bulk
B)provide the bulk
C)insulate the core
D)reduce eddy current loss

Question 5

The resistance of armature winding depends on

A)length of conductor
B)cross-sectional area of the conductor
C)number of conductors
D)all of the above

Question 6

The field coils of D.C. generator are usually made of

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

Question 7

The commutator segments are connected to the armature conductors by means of

A)copper lugs
B)resistance wires
C)insulation pads
D)brazing

Question 8

In a commutator

A)copper is harder than mica
B)mica and copper are equally hard
C)mica is harder than copper
D)none of the above

Question 9

In D.C. generators the pole shoes are fastened to the pole core by

A)rivets
B)counter sunk screws
C)brazing
D)welding

Question 10

Satisfactory commutation of D.C. machines requires

A)brushes should be of proper grade and size
B)brushes should smoothly run in the holders
C)smooth, concentric commutator properly undercut
D)all of the above

Question 11

Fleming's right-hand rule regarding direction of induced e.m.f., correlates

A)magnetic flux, direction of current flow and resultant force
B)magnetic flux, direction of motion and the direction of e.m.f. induced
C)magnetic field strength, induced voltage and current
D)magnetic flux, direction of force and direction of motion of conductor

Question 12

While applying Fleming's right-hand rule to And the direction of induced e.m.f., the thumb points towards

A)direction of induced e.m.f.
B)direction of flux
C)direction of motion of the conductor if forefinger points in the direction of generated e.m.f.
D)direction of motion of conductor, if forefinger points along the lines of flux

Question 13

The bearings used to support the rotor shafts are generally

A)ball bearings
B)bush bearings
C)magnetic bearmgs
D)needle bearings

Question 14

In D.C. generators, the cause of rapid brush wear may be

A)severe sparking
B)rough commutator surface
C)imperfect contact
D)All of the above

Question 15

In lap winding, the number of brushes is always

A)double the number of poles
B)same as the number of poles
C)half the number of poles
D)two

Question 16

For a D.C. generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher e.m.f. ?

A)Lap winding
B)Wave winding
C)Either of (a) and (b) above
D)Depends on other features of design

Question 17

In a four-pole D.C. machine

A)all the four poles are north poles
B)alternate poles are north and south
C)all the four poles are south poles
D)two north poles follow two south poles

Question 18

Copper brushes in D.C. machine are used

A)where low voltage and high currents are involved
B)where high voltage and small cur-rents are involved
C)in both of the above cases
D)in none of the above cases

Question 19

A separately excited generator as compared to a self-excited generator

A)is amenable to better voltage con-trol
B)is more stable
C)has exciting current independent of load current
D)has all above features

Question 20

In case of D.C. machines, mechanical losses are primary function of

A)current
B)voltage
C)speed
D)none of above

Question 21

Iron losses in a D.C. machine are independent of variations in

A)speed
B)load
C)voltage
D)speed and voltage

Question 22

In D.C. generators, current to the external circuit from armature is given through

A)commutator
B)solid connection
C)slip rings
D)none of above

Question 23

Brushes of D.C. machines are made of

A)carbon
B)soft copper
C)hard copper
D)all of above

Question 24

If B is the flux density, I the length of conductor and v the velocity of conductor, then induced e.m.f. is given by

A)Blv
B)Blv²
C)Bl²v
D)Bl²v²

Question 25

In case of a 4-pole D.C. generator provided with a two layer lap winding with sixteen coils, the pole pitch will be

A)4
B)8
C)16
D)32