Deck 13: D.C Generator Operation and Construction

Question 1

The armature core of a D.C. generator is usually made of

A)silicon steel
B)copper
C)non-ferrous material
D)cast-iron

Answer 1

silicon steel

Question 2

Open circuited armature coil of a D.C. machine is

A)identified by the scarring of the commutator segment to which open circuited coil is connected
B)indicated by a spark completely around the commutator
C)both (a) and (b)
D)none of the above

Answer 2

both (a) and (b)

Question 3

For the parallel operation of two or more D.C. compound generators, we should ensure that

A)voltage of the incoming generator should be same as that of bus bar
B)polarity of incoming generator should be same as that of bus bar
C)all the series fields should be run in parallel by means of equilizer connection
D)series fields of all generators should be either on positive side or negative side of the armature

Answer 3

series fields of all generators should be either on positive side or negative side of the armature

Question 4

D.C. series generator is used

A)to supply traction load
B)to supply industrial load at constant voltage
C)voltage at the toad end of the feeder
D)for none of the above purpose

Question 5

Following D.C. generator will be in a position to build up without any residual magnetism in the poles

A)series generator
B)shunt generator
C)compound generator
D)self-excited generator

Question 6

Interpole flux should be sufficient to

A)neutralise the commutating self induced e.m.f.
B)neutralise the armature reaction flux
C)neutralise both the armature reaction flux as well as commutating e.m.f. induced in the coil
D)perform none of the above functions

Question 7

D.C. generator generally preferred for charging automobile batteries is

A)series generator
B)shunt generator
C)long shunt compound generator
D)any of'the above

Question 8

In a D.C. generator the number of mechanical degrees and electrical degrees will be the same when

A)r.p.m. is more than 300
B)r.p.m. is less than 300
C)number of poles is 4
D)number of poles is 2

Question 9

Permeance is the reciprocal of

A)flux density
B)reluctance
C)ampere-turns
D)resis tance

Question 10

In D.C. generators the polarity of the interpoles

A)is the same as that of the main pole ahead
B)is the same as that of the immediately preceding pole
C)is opposite to that of the main pole ahead
D)is neutral as these poles do not play part in generating e.m.f.

Question 11

The e.m.f. generated in a D.C. generator is directly proportional to

A)flux/pole
B)speed of armature
C)number of poles
D)all of the above

Question 12

In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the coil is neutralised by all of the following except

A)interpoles
B)dummy coils
C)compensating winding
D)shifting of axis of brushes

Question 13

In D.C. generators on no-load, the air gap flux distribution in space is

A)sinusoidal
B)triangular
C)pulsating
D)flat topped

Question 14

A shunt generator running at 1000 r.p.m. has generated e.m.f. as 200 V. If thespeed increases to 1200 r.p.m., the generated e.m.f. will be nearly

A)150 V
B)175 V
C)240 V
D)290V

Question 15

The purpose of providing dummy coils in a generator is

A)to reduce eddy current losses
B)to enhance flux density
C)to amplify voltage
D)to provide mechanical balance for the rotor

Question 16

If a D.C. generator fails to build up the probable cause could not be

A)imperfect brush contact
B)field resistance less than the critical resistance
C)no residual magnetism in the generator
D)faulty shunt connections tending to reduce the residual magnetism

Question 17

Flashing the field of D.C. generator means

A)neutralising residual magnetism
B)creating residual magnetism by a D.C. source
C)making the magnetic losses of forces parallel
D)increasing flux density by adding extra turns of windings on poles

Question 18

The e.m.f. induced in the armature of a shunt generator is 600 V. The armature resistance is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be

A)640 V
B)620 V
C)600 V
D)580V

Question 19

In a D.C. generator the critical resistance refers to the resistance of

A)brushes
B)field
C)armature
D)load

Question 20

To achieve sparkless commutation brushes of a D.C. generator are rockedm ahead soas to bring them

A)just ahead of magnetic neutral axis
B)in magnetic neutral axis
C)just behind the magnetic neutral axis
D)none

Question 21

Armature coil is short circuited by brushes when it lies

A)along neutral axis
B)along field axis
C)in All of the above positions
D)in none of the above positions

Question 22

A cumulatively compounded long shunt generator when operating as a motor would be

A)cumulatively compounded long shunt
B)differentially compounded long shunt
C)cumulatively compounded short shunt
D)differentially compounded short shunt

Question 23

To avoid formation of grooves in the commutator of a D.C. machine

A)the brushes of opposite polarity should track each other
B)the brushes of same polarity should track each other
C)brush position has no effect on the commutator grooving

Question 24

The following constitute short-circuit in the armature winding.

A)Insulation failure between two com-mutator bars
B)Insulation failure between two turns of a coil
C)Two of more turns of the same coil getting grounded
D)All of the above

Question 25

The rapid wear of brushes takes place due to

A)abrasion from dust
B)excessive spring pressure
C)rough commutator bars
D)all of the above factors