UNIT 2

UNIT 2

UNIT 2BEE SET-1PART A

1. Derive the induced e.m.f equation of a D.C. Generator.2. Discuss about various types of D.C generators.3. Explain the construction and operating principle of DC machine4. Explain the load characteristics of DC series generator and mention its applications?5. Explain the Load characteristics of DC shunt generator.6. A 4 pole lap wound D. C. shunt generator has a useful flux per pole of 0.07 wb. The armature winding consists of 220-turns each of 0.004 resistance. Calculate the terminal voltage when running at 900 rpm if the armature current is 50A.

PART B

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

rivets

counter sunk screws

brazing

welding

ANSWER : : counter sunk screws Explanation :

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

speed

load

voltage

speed and voltage

ANSWER : : load Explanation :

3.In lap winding, the number of brushes is always

double the number of poles

same as the number of poles

half the number of poles

two

ANSWER : : same as the number of poles Explanation :

4.The armature of D.C. generator is laminated to

reduce the bulk

provide the bulk

insulate the core

reduce eddy current loss

ANSWER : : reduce eddy current loss Explanation :

5.The resistance of armature winding depends on

length of conductor

cross-sectional area of the conductor

number of conductors

all of the above

ANSWER : : all of the above Explanation :

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

0.005 mm

0.05 mm

0.5 m

5 m

ANSWER : : 0.5 m Explanation :

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

mica

copper

cast iron

carbon

ANSWER : : copper Explanation :

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

current

voltage

speed

none of above

ANSWER : : speed Explanation :

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

is amenable to better voltage con-trol

is more stable

has exciting current independent of load current

has all above features

ANSWER : : has all above features Explanation :

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

copper lugs

resistance wires

insulation pads

brazing

ANSWER : : copper lugs

11.Brushes of D.C. machines are made of

carbon

soft copper

hard copper

all of above

ANSWER : : carbon Explanation :

12.The bearings used to support the rotor shafts are generally

ball bearings

bush bearings

magnetic bearmgs

needle bearings

ANSWER : : ball bearings Explanation :

13.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. ?

Lap winding

Wave winding

Either of (a) and (b) above

Depends on other features of design

ANSWER : : Wave winding Explanation :

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

severe sparking

rough commutator surface

imperfect contact

any of the above

ANSWER : : any of the above Explanation :

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

4

8

16

32

ANSWER : : 8

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

commutator

solid connection

slip rings

none of above

ANSWER : : commutator Explanation :

17.In a commutator

copper is harder than mica

mica and copper are equally hard

mica is harder than copper

none of the above

ANSWER : : mica is harder than copper Explanation :

18.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

Blv

Blv2

Bl2v

Bl2v2

ANSWER : : Blv Explanation :

19.The material for commutator brushes is generally

mica

copper

cast iron

carbon


20.Copper brushes in D.C. machine are used

where low voltage and high currents are involved

where high voltage and small cur-rents are involved

in both of the above cases

in none of the above cases

ANSWER : : where low voltage and high currents are involved

21.In a four-pole D.C. machine

all the four poles are north poles

alternate poles are north and south

all the four poles are south poles

two north poles follow two south poles

ANSWER : : alternate poles are north and south Explanation :

22.According to Fleming's right-hand rule for finding the direction of induced e.m.f., when middle finger points in the direction of induced e.m.f., forefinger will point in the direction of

motion of conductor

lines of force

either of the above

none of the above

ANSWER : : lines of force Explanation :

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

direction of induced e.m.f.

direction of flux

direction of motion of the conductor if forefinger points in the direction of generated e.m.f.

direction of motion of conductor, if forefinger points along the lines of flux

ANSWER : : direction of motion of conductor, if forefinger points along the lines of flux Explanation :

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

magnetic flux, direction of current flow and resultant force

magnetic flux, direction of motion and the direction of e.m.f. induced

magnetic field strength, induced voltage and current

magnetic flux, direction of force and direction of motion of conductor

ANSWER : : magnetic flux, direction of motion and the direction of e.m.f. induced

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

demagnetisation only

cross magnetisation as well as magnetisation

crossmagnetisation as well as demagnetising

cross magnetisation only

ANSWER : : crossmagnetisation as well as demagnetising Explanation :

26.Armature reaction of an unsaturated D.C. machine is

crossmagnetising

demagnetising

magnetising

none of above

ANSWER : : crossmagnetising Explanation :

27.In the case of lap winding resultant pitch is

multiplication of front and back pitches

division of front pitch by back pitch

sum of front and back pitches

difference of front and back pitches

ANSWER : : difference of front and back pitches Explanation :

28.Welding generator will have

lap winding

wave winding

delta winding

duplex wave winding

ANSWER : : lap winding Explanation :

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

silicon steel

copper

non-ferrous material

cast-iron

ANSWER : : silicon steel

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

Graphite brushes

Carbon brushes

Metal graphite brushes

None of the above

ANSWER : : Metal graphite brushes Explanation :

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

rotary converter

mercury are rectifier

induction motor D.C. generator set

synchronous motor D.C. generator set

ANSWER : : induction motor D.C. generator set Explanation :

32The number of brushes in a commutator depends on

speed of armature

type of winding

voltage

amount of current to be collected

ANSWER : : amount of current to be collected Explanation :

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

Compensating winding in a D.C. machine helps in commutation

In a D. C. generator interpoles winding is connected in series with the armature winding

Back pitch and front pitch are both odd and approximately equal to the pole pitch

Equilizing bus bars are used with parallel running of D.C. shunt generators

ANSWER : : Equilizing bus bars are used with parallel running of D.C. shunt generators Explanation :

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

reduces generator e.m.f.

increases armature speed

reduces interpoles flux density

results in sparking trouble

ANSWER : : reduces generator e.m.f.

35.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

E/2

2E

slightly less than E

E

ANSWER : : 2E Explanation :

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

high voltage, high current

low voltage, high current

high voltage, low current

low voltage, low current

ANSWER : : low voltage, high current Explanation :

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

lie under south pole

lie under north pole

lie under interpolar region

are farthest from the poles

ANSWER : : lie under interpolar region Explanation :

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

to reduce the reluctance of the magnetic path

to spread out the flux to achieve uniform flux density

to support the field coil

to discharge all the above functions

ANSWER : : to discharge all the above functions Explanation :

39.Equilizer rings are required in case armature is

wave wound

lap wound

delta wound

duplex wound

ANSWER : : lap wound

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

to avoid sudden loading of the primemover

to avoid mechanicaljerk to the shaft

to avoid burning of switch contacts

all above

ANSWER : : all above

BEE SET-2PART A

1. Explain the Load characteristics of DC shunt and compound generators.2. Derive the e.m.f equation of a DC machine.3. Explain the function of commutator in a DC generator4. Derive the e.m.f equation of a D.C Generator and discuss the factors affecting the generated e.m.f?5. Draw a neat sketch of a DC generator and explain the function of each part.6. Explain why the external characteristic of a dc shunt generator is more drooping than that ofa separately excited generator.

PART B

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

electromagnets

permanent magnets

both (a) and (b)

none of the above

ANSWER : : electromagnets Explanation :

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

using conductor of annealed copper

using commutator with large number of segments

using carbon brushes of superior quality

using equiliser rings

ANSWER : : using carbon brushes of superior quality Explanation :

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

number of armature coils

number of armature coil sides

number of armature conductors

number of armature turns

ANSWER : : number of armature coils Explanation :

44.A D.C. welding generator has

lap winding

wave moving

duplex winding

any of the above

ANSWER : : lap winding

45.Compensating windings are used in D.C. generators

mainly to reduce the eddy currents by providing local short-circuits

to provide path for the circulation of cooling air

to neutralise the cross-magnetising effect of the armature reaction

none of the above

ANSWER : : to neutralise the cross-magnetising effect of the armature reaction Explanation :

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

Dummy coils

Commutator

Eye bolt

Equilizer rings

ANSWER : : Commutator Explanation :

47.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 generator A and B will be

2 : 3

3 : 1

3 : 2

1 : 3

ANSWER : : 3 : 1 Explanation :

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

oscillating magnetic field

pulsating magnetic flux

relative rotation between field and armature

all above

ANSWER : : relative rotation between field and armature

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

imperfect brush contact

field resistance less than the critical resistance

no residual magnetism in the generator

faulty shunt connections tending to reduce the residual magnetism

ANSWER : : field resistance less than the critical resistance Explanation :

50.Permeance is the reciprocal of

flux density

reluctance

ampere-turns

resistance

ANSWER : : reluctance Explanation :

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

r.p.m. is more than 300

r.p.m. is less than 300

number of poles is 4


ANSWER : : number of poles is 2 Explanation :

52.The purpose of providing dummy coils in a generator is

to reduce eddy current losses

to enhance flux density

to amplify voltage

to provide mechanical balance for the rotor

ANSWER : : to provide mechanical balance for the rotor Explanation :

60.In a D.C. generator the magnetic neutral axis coincides with the geometrical neutral axis, when

there is no load on the generator

the generator runs on full load

the generator runs on overload

the generator runs on designed speed

ANSWER : : there is no load on the generator 1.In D.C. generators the pole shoes are fastened to the pole core by

rivets

counter sunk screws

brazing

welding



speed

load

voltage

speed and voltage






two



reduce the bulk

provide the bulk

insulate the core




length of conductor



all of the above



0.005 mm

0.05 mm

0.5 m

5 m



mica

copper

cast iron

carbon



current

voltage

speed

none of above




is more stable





copper lugs

resistance wires

insulation pads

brazing



carbon

soft copper

hard copper

all of above



ball bearings

bush bearings

magnetic bearmgs

needle bearings



Lap winding

Wave winding





severe sparking


imperfect contact

any of the above



4

8

16

32

ANSWER : : 8


commutator

solid connection

slip rings

none of above


17.In a commutator




none of the above



Blv

Blv2

Bl2v

Bl2v2



mica

copper

cast iron

carbon








BEE SET-3PART A1. Explain external characteristics of DC series generator?2. Distinguish between self-excited and separately excited d.c. generators. How are self-excited d.c. generators classified? Give their circuit diagrams3. Explain the open circuit characteristics of a dc generator and explain its significance.4. Explain the load characteristics of DC series generator and mention its applications?5. Explain the Load characteristics of DC shunt generator.6. Explain why the external characteristic of a dc shunt generator is more drooping than that ofa separately excited generator.

PART B














motion of conductor

lines of force

either of the above

none of the above




direction of flux

















crossmagnetising

demagnetising

magnetising

none of above









lap winding

wave winding

delta winding

duplex wave winding



silicon steel

copper


cast-iron



Graphite brushes

Carbon brushes


None of the above


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

rotary converter

mercury are rectifier

induction motor D.C. generator set

synchronous motor D.C. generator set

ANSWER : : induction motor D.C. generator set Explanation :

32The number of brushes in a commutator depends on

speed of armature

type of winding

voltage

amount of current to be collected

ANSWER : : amount of current to be collected Explanation :

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

Compensating winding in a D.C. machine helps in commutation

In a D. C. generator interpoles winding is connected in series with the armature winding

Back pitch and front pitch are both odd and approximately equal to the pole pitch

Equilizing bus bars are used with parallel running of D.C. shunt generators

ANSWER : : Equilizing bus bars are used with parallel running of D.C. shunt generators Explanation :

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

reduces generator e.m.f.

increases armature speed

reduces interpoles flux density

results in sparking trouble

ANSWER : : reduces generator e.m.f.

35.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

E/2

2E

slightly less than E

E

ANSWER : : 2E Explanation :

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

high voltage, high current

low voltage, high current

high voltage, low current

low voltage, low current

ANSWER : : low voltage, high current Explanation :

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

lie under south pole

lie under north pole

lie under interpolar region

are farthest from the poles

ANSWER : : lie under interpolar region Explanation :

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

to reduce the reluctance of the magnetic path

to spread out the flux to achieve uniform flux density

to support the field coil

to discharge all the above functions

ANSWER : : to discharge all the above functions Explanation :

39.Equilizer rings are required in case armature is

wave wound

lap wound

delta wound

duplex wound

ANSWER : : lap wound

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

to avoid sudden loading of the primemover

to avoid mechanicaljerk to the shaft

to avoid burning of switch contacts

all above

ANSWER : : all above 11.Brushes of D.C. machines are made of

carbon

soft copper

hard copper

all of above



ball bearings

bush bearings

magnetic bearmgs

needle bearings



Lap winding

Wave winding





severe sparking


imperfect contact

any of the above



4

8

16

32

ANSWER : : 8


commutator

solid connection

slip rings

none of above


17.In a commutator




none of the above



Blv

Blv2

Bl2v

Bl2v2



mica

copper

cast iron

carbon


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

electromagnets

permanent magnets

both (a) and (b)

none of the above

ANSWER : : electromagnets Explanation :

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

using conductor of annealed copper

using commutator with large number of segments

using carbon brushes of superior quality

using equiliser rings

ANSWER : : using carbon brushes of superior quality Explanation :

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

number of armature coils

number of armature coil sides

number of armature conductors

number of armature turns

ANSWER : : number of armature coils Explanation :

44.A D.C. welding generator has

lap winding

wave moving

duplex winding

any of the above

ANSWER : : lap winding

45.Compensating windings are used in D.C. generators

mainly to reduce the eddy currents by providing local short-circuits

to provide path for the circulation of cooling air

to neutralise the cross-magnetising effect of the armature reaction

none of the above

ANSWER : : to neutralise the cross-magnetising effect of the armature reaction Explanation :

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

Dummy coils

Commutator

Eye bolt

Equilizer rings

ANSWER : : Commutator Explanation :

47.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 generator A and B will be

2 : 3

3 : 1

3 : 2

1 : 3

ANSWER : : 3 : 1 Explanation :

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

oscillating magnetic field

pulsating magnetic flux

relative rotation between field and armature

all above

ANSWER : : relative rotation between field and armature

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

imperfect brush contact

field resistance less than the critical resistance

no residual magnetism in the generator

faulty shunt connections tending to reduce the residual magnetism

ANSWER : : field resistance less than the critical resistance Explanation :

50.Permeance is the reciprocal of

flux density

reluctance

ampere-turns

resistance

ANSWER : : reluctance

BEE SET-4PART A

1. Discuss about various types of D.C generators.2. A 4 pole lap wound D. C. shunt generator has a useful flux per pole of 0.07 wb. The armature winding consists of 220-turns each of 0.004 resistance. Calculate the terminal voltage when running at 900 rpm if the armature current is 50A. 3. Derive the e.m.f equation of a DC machine.4. Explain the function of commutator in a DC generator5. Explain external characteristics of DC series generator?6. Explain the open circuit characteristics of a dc generator and explain its significance.

PART B

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

rivets

counter sunk screws

brazing

welding



speed

load

voltage

speed and voltage






two



reduce the bulk

provide the bulk

insulate the core




length of conductor



all of the above



0.005 mm

0.05 mm

0.5 m

5 m



mica

copper

cast iron

carbon



current

voltage

speed

none of above




is more stable





copper lugs

resistance wires

insulation pads

brazing



carbon

soft copper

hard copper

all of above



ball bearings

bush bearings

magnetic bearmgs

needle bearings



Lap winding

Wave winding





severe sparking


imperfect contact

any of the above



4

8

16

32

ANSWER : : 8


commutator

solid connection

slip rings

none of above


17.In a commutator




none of the above



Blv

Blv2

Bl2v

Bl2v2



mica

copper

cast iron

carbon















motion of conductor

lines of force

either of the above

none of the above




direction of flux

















crossmagnetising

demagnetising

magnetising

none of above









lap winding

wave winding

delta winding

duplex wave winding



silicon steel

copper


cast-iron



Graphite brushes

Carbon brushes


None of the above


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

r.p.m. is more than 300

r.p.m. is less than 300



ANSWER : : number of poles is 2 Explanation :

52.The purpose of providing dummy coils in a generator is

to reduce eddy current losses

to enhance flux density

to amplify voltage

to provide mechanical balance for the rotor

ANSWER : : to provide mechanical balance for the rotor Explanation :

60.In a D.C. generator the magnetic neutral axis coincides with the geometrical neutral axis, when

there is no load on the generator

the generator runs on full load

the generator runs on overload

the generator runs on designed speedANSWER : : there is no load on the generator

UNIT 2

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