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ELECTRICAL MACHINE 1 (DC MACHINE & MOTOR)

Total questions: 61

Worksheet time: 31mins

Name
Class
Date
1.

by looking at which part of the motor, it can be easily confirmed that a particular motor is DC motor?

a)

Frame

b)

Commutator

c)

Shaft

d)

Stator

2.

if a speed of a DC shunt motor is increased, the back emf of the motor will

a)

increase

b)

remain same

c)

decrease

d)

become zero

3.

as the load is increased the speed of DC shunt motor will

a)

reduce slightly

b)

increase slightly

c)

increase proportionately

d)

remain unchanged

4.

it is a DC motor that tends to over-speed when the field opened while running.

a)

shunt motor

b)

series motor

c)

split phase motor

d)

over-compounded motor

5.

A DC shunt motor is rotating in clockwise directions as viewed from one end. The polarity of connection of the armature and field winding are reversed simultaneously. The motor will

a)

rotate in clockwise direction

b)

rotate in anti-clockwise direction

c)

will not rotate at all

d)

gain excessively high speed

6.

DC shunt motors are ed for driving

a)

trains

b)

hoists

c)

cranes

d)

machine tools

7.

which DC motor will be preferred for elevators?

a)

shunt motor

b)

series motor

c)

differential compound motor

d)

cumulative compound motor

8.

which of the following statement is correct. in case of a DC series motor?

a)

it should be directly connected to the load

b)

it may run away if its field becomes open

c)

it is a constant speed motor

d)

it cannot be used where high starting torque is required.

9.

for both lap and wave windings, there are as many commutator bars as the number of

a)

poles

b)

slots

c)

armature conductors

d)

winding elements

10.

for a DC generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher emf?

a)

lap winding

b)

depends on other features of design

c)

wave winding

d)

either of (A) and (C) above

11.

which of the following components of a DC generator play vital role for providing direct current of a DC generator

a)

dummy coils

b)

eye bolt

c)

commutator

d)

equalizer rings

12.

In a DC generator the critical resistance refers to the resistance of 

a)

brushes

b)

field

c)

armature

d)

load

13.

in DC 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

14.

the number of armature parallel paths in two-pole DC generator having duplex lap winding is

a)

2

b)

6

c)

4

d)

8

15.

the direction of rotation of a dc motor is reversed by

a)

reversing armature connections

b)

adding resistance to the field circuit

c)

interchanging the armature and field connection

d)

reversing supply connection

16.

which of the following loss in a DC generator varies significantly with the load current?

a)

field copper loss

b)

windage loss

c)

armature copper loss

d)

none of the above

17.

if Bmax is the maximum flux density, then eddy current loss will vary as

a)

βmax⁡\beta\max

b)

(βmax⁡)1.2\left(\beta\max\right)^{1.2}

c)

(βmax⁡)2\left(\beta\max\right)^2

d)

(βmax⁡)2.4\left(\beta\max\right)^{2.4}

18.

the hysteresis loss in a DC generator varies with the frequency of magnetic reversals as

a)

1f\frac{1}{f}

b)

f1.6f^{1.6}

c)

ff

d)

f2f^2

19.

the number of armature parallel paths in two-pole DC generator having duplex winding is

a)

2

b)

6

c)

4

d)

8

20.

if the armature of an eight-pole dc machine were wound with a simplex wave winding. how many parallel paths would there be?

a)

16 paths

b)

2 paths

c)

8 paths

d)

4 paths

21.

in case of DC machines, mechanical losses are primary function of

a)

current

b)

speed

c)

voltage

d)

none of the above

22.

in which of the following applications DC series motor is invariably used?

a)

start for a car

b)

drive for a water pump

c)

fan motor

d)

motor operation in AC or DC

23.

a DC series motor. as compared to shunt and compound motors, has the highest torque at the start because of its comparatively

a)

stronger series field

b)

lower armature resistance

c)

large armature current

d)

fewer series turns

24.

a series motor is best suited for driving

a)

machine tools

b)

shear and punches

c)

cranes and hoists

d)

none of the above

25.

which DC motor will be suitable along with flywheel for intermittent light and heavy loads?

a)

series motor

b)

shunt motor

c)

cumulatively compounded motor

d)

differentially compounded motor

26.

differentially compound DC motors can find applications requiring

a)

high starting torque

b)

low starting torque

c)

variable speed

d)

frequent on-off cycles

27.

in an overload motor main danger arises dure to

a)

winding getting overheated

b)

bus bars getting heated

c)

starter getting damaged

d)

bearings getting overheated

28.

N-load speed of which of the following motor will be highest?

a)

shunt motor

b)

series motor

c)

cumulative compound motor

d)

differential compound motor

29.

a shearing machine has cyclic load consisting of intermittent light and heavy loads. which of the following DC motor will be suitable for this purpose?

a)

series motor

b)

shunt motor

c)

cumulative compound motor

d)

differential compound motor

30.

The ratio of starting torque to full-load torque is least in case of

a)

series motor

b)

shunt motor

c)

compound motor

d)

none of the above

31.

The type of DC motor used for shears and punches is

a)

shunt motor

b)

series motor

c)

differential compound DC motor

d)

cumulative compound DC motor

32.

Which DC motor will have least percentage increase of input current, for the same percentage increase in the torque?

a)

shunt motor

b)

series motor

c)

cumulative compound motor

d)

separately excited motor

33.

These day DC motors are widely used in

a)

pumping sets

b)

air compressor

c)

electric traction

d)

machine shops

34.

The direction of rotation of a DC series motor can be changed by

a)

interchanging supply terminals

b)

interchanging field terminals

c)

either of (a) and (b) above

d)

none of the above

35.

if a DC motor is to be selected for conveyors, which motor would be preferred?

a)

series motor

b)

shunt motor

c)

differentially compound motor

d)

cumulative compound motor

36.

Which DC motor will be preferred for constant speed line shafting?

a)

cumulative compound motor

b)

differentially compound motor

c)

shunt motor

d)

series motor

37.

A DC generator works on the principle of

a)

Lenz's law

b)

Ohm's law

c)

Faraday's law of electromagnetic induction

d)

series motor

38.

For generating large current on DC generators which winding is generally preferred?

a)

progressive wave winding

b)

lap winding

c)

retrogressive wave winding

d)

current depends on design

39.

In case of DC 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

40.

The purpose 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

41.

In a DC generator the critical resistance refers to the resistance of

a)

brushes

b)

armature

c)

field

d)

load

42.

In a DC generator in order to reduce sparking at brushes, the self-induced emf in the coil is neutralized by all of the following except.

a)

interpoles

b)

dummy coils

c)

compensating winding

d)

shifting of axis of brushes

43.

Interpoles are connected ___ with the armature and compensating windings are connected in ___ with the armature.

a)

series, series

b)

series, shunt

c)

shunt, series

d)

shunt, shunt

44.

In a DC machine stray loss is the sum of

a)

total copper loss and mechanical loss

b)

shunt field copper loss and mechanical loss

c)

armature copper loss and iron loss

d)

iron loss and mechanical loss

45.

Which of the following losses are significantly reduced by laminating the core of A DC generator?

a)

hysteresis losses

b)

Eddy current losses

c)

copper losses

d)

windage losses

46.

if residual magnetism of a shunt generator is destroyed accidentally, it may be restored by connecting its shunt field

a)

in reverse

b)

to earth

c)

to a battery

d)

to an alternator

47.

one dc motor drives another dc motor. The second dc motor when excited and driven

a)

runs

b)

runs as a generator

c)

does not run when excited and driven

d)

also runs ad a motor

48.

in over compounded generator, full load terminal voltage is

a)

almost zero

b)

less than no-load terminal voltage

c)

more than no-load terminal voltage

d)

equal to no-load terminal voltage

49.

The series field of a shirt-shunt DC generator is excited by

a)

external current

b)

shunt current

c)

armature current

d)

load current

50.

when the shunt field of a compound generator is connected across both series field and armature such a connection is known as

a)

short shunt

b)

long shunt

c)

cumulative compounding

d)

differential compounding

51.

drop in speed of a DC generator due to increase in load can be compensated by

a)

cooling the armature

b)

increasing the armature resistance

c)

reducing the load voltage

d)

increasing the input to prime mover

52.

Lamination of core are generally made of

a)

cast iron

b)

carbon

c)

silicon steel

d)

stainless steel

53.

The field coils of DC generators are usually made of

a)

mica

b)

copper

c)

cast iron

d)

carbon

54.

The bearings used to support the rotor shafts are generally

a)

ball bearings

b)

bush bearings

c)

magnetic bearings

d)

needle bearings

55.

Brushes of DC machines are made of

a)

carbon

b)

soft copper

c)

hard copper

d)

all of the above

56.

The insulating material used between the commutator segments is normally

a)

graphite

b)

paper

c)

mica

d)

insulating varnish

57.

The purpose of providing a dummy coil in a generator is 

a)

to enhance flux density

b)

to amplify voltage

c)

to provide mechanical balanced for the rotor

d)

to reduced eddy current

58.

Welding generator will have 

a)

lap winding

b)

wave winding

c)

delta winding

d)

duplex wave winding

59.

In case of DC machine winding, number of commutator segments is equal to

a)

number of armature coils

b)

number of armature coils sides

c)

number of armature conductors

d)

number of armature turns

60.

which one of the following generators is used for charging the batteries?

a)

shunt

b)

series

c)

compound

d)

any of the above

61.

shunt generators are most suited for stable parallel operation because of their

a)

rising voltage characteristics

b)

identical voltage characteristics

c)

drooping voltage characteristics

d)

linear voltage characteristics