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ELECTRICAL MACHINE 2 (AC MOTOR & ALTERNATOR)

Total questions: 64

Worksheet time: 32mins

Name
Class
Date
1.

a synchronous motor can develop synchronous torque

a)

when under loaded

b)

below or above synchronous speed

c)

while over-excited

d)

only at synchronous speed

2.

an under-excited synchronous motor takes

a)

leading current

b)

lagging current

c)

both (A) and (B)

d)

none of the above

3.

a synchronous motor which works on a leading power factor and does not drive a mechanical load is called as

a)

static condenser

b)

synchronous condenser

c)

condenser

d)

none of the above

4.

a synchronous motor can be used as a synchronous capacitor when it is

a)

under-loaded

b)

under-excited

c)

over-loaded

d)

over-excited

5.

which of the following motors has a speed of 1174 rpm at certain instant, what is the supply frequency?

a)

D.C. series motor

b)

squirrel cage induction motor

c)

synchronous motor

d)

wound round induction motor

6.

under which of the following conditions hunting of synchronous motor is likely to occur?

a)

periodic variation of load

b)

over-loading for long period

c)

over-excitation

d)

small and constant load

7.

the maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as

a)

pull-up torque

b)

pull-in torque

c)

pull-out torque

d)

none of the above

8.

the voltage applied to the field of a rotating cylindrical rotor type alternators

a)

high dc voltage

b)

high ac voltage

c)

low dc voltage

d)

low ac voltage

9.

alternators are usually designed to generate

a)

definite currents

b)

variable frequencies

c)

definite power factor

d)

definite frequencies

10.

the power factor of an alternator is determined by its

a)

load

b)

speed

c)

excitation

d)

voltage rating

11.

what kind if rotor is most suitable for turbo-alternators which are designed to run at high speed?

a)

salient pole type

b)

non-salient pole type

c)

either A or B

d)

none of the above

12.

distribution factor Kd is defined as the ratio of emf's of

a)

distributed winding to full pitch winding

b)

distributed to concentrated winding

c)

concentrated winding to distributed winding

d)

full pitch winding to distributed winding

13.

increasing the speed of alternator, what will happen?

a)

decrease MW

b)

increase frequency

c)

increase MVAR

d)

decrease MVAR

14.

out of the following conditions, the one which does not have to be met by alternators working parallel is

a)

the machine must have equal rating

b)

terminal voltage of each machine must be the same

c)

the machines must have the same phase rotation

d)

the machine must operate at the same frequency

15.

it is never advisable to connect a stationary alternator to live bus-bars because

a)

it is likely to run as synchronous motor

b)

it will get short circuited

c)

it will decrease bus bar voltage through momentarily

d)

it will disturb generated emf of other alternator connected in parallel

16.

adjustment of field excitation on one of the two alternators operating in parallel will

a)

increase its load

b)

change its frequency

c)

decrease its load

d)

change its power factor

17.

the power factor of an alternator is 75% the operator is ordered to increase the power factor to 80% what shall he do?

a)

increase the voltage

b)

increase the exccitation

c)

operate the governor

d)

decrease the excitation

18.

in order to transfer the laod from one alternator to another alternator, when they are operating in parallel.

a)

field current is increased

b)

field current is decreased

c)

speed is decreased

d)

prime mover power input is increased

19.

in large capacity ac generator, the moving part is

a)

brushes

b)

armature

c)

poles

d)

frame

20.

salient pole type alternators are generally used on

a)

low voltage alternators

b)

high speed prime movers

c)

hydrogen cooled prime movers

d)

low and medium speed prime movers

21.

it plays an important role in over-speed protection of a generator?

a)

governor

b)

over current relay

c)

differential protection

d)

alarm

22.

one of the advantages of distributing the winding in alternator is to

a)

reduce harmonics

b)

improve voltage waveforms

c)

reduce the amount of copper

d)

decrease the value of the voltage

23.

synchronization of alternators means

a)

connecting alternators in series

b)

transferring load to one alternator to another

c)

connecting alternators in parallel

d)

adjusting the excitation of alternators

24.

damper or "Amortisseur" windings are used in alternators to

a)

achieve synchronism

b)

prevent hunting

c)

reduced windage losses

d)

start up

25.

increasing the power output of alternator working in parallel to a large system, the operator must adjust

a)

field excitation

b)

governor

c)

frequency

d)

power factor

26.

dampers in large generators

a)

increase stability

b)

reduce voltage fluctuations

c)

reduce frequency fluctuations

d)

none of the above

27.

overheating of generator's winding

a)

reduces life of the machine

b)

reduces generated voltage

c)

does not have any significant effect

d)

reduces power factor

28.

two alternators are to be put in parallel. which of the following factors should be identical for both?

a)

frequency

b)

voltage

c)

phase sequence

d)

all of the above

29.

a lower voltage alternator, for the same power rating, will be

a)

more costly

b)

more efficient

c)

larger in size

d)

operating at high r.p.m

30.

dirt accumulation in generator can cause all of the following except

a)

low power factor

b)

flashover

c)

poor voltage regulation

d)

overheating

31.

an alternator coupled to which prime mover will usually have the highest rotating speed?

a)

steam engine

b)

reciprocating diesel engine

c)

francis turbine

d)

steam turbine

32.

in which coil the harmonic component of the generated E.M.F. will be more?

a)

full pitch coil

b)

short pitch coil

c)

long pitch coil

d)

same in all coils

33.

the dark and bright lamp method is used for

a)

transfer of load

b)

synchronizing

c)

balancing of load

d)

phase sequence

34.

squirrel cage induction motor differs from the slip ring type in that it has no

a)

slip rings

b)

rotor winding

c)

windings on the stator

d)

rotating part

35.

the shaft of induction motor is made of

a)

high speed steel

b)

cast iron

c)

stainless steel

d)

carbon steel

36.

magnetic flux produced due to the stator winding with three phase current frequency f and which is of constant value, rotates at synchronous speed. if P is the number of poles of the motor, then the synchronous speed is given by

a)

Ns=120PfN_s=\frac{120P}{f}

b)

Ns=120fPN_s=\frac{120f}{P}

c)

Ns=120fPN_s=120fP

d)

Ns=60fPN_s=\frac{60f}{P}

37.

the difference between the synchronous speed Ns and the actual speed N of the rotor is known as slip. it can be expressed as a percentage and is given by

a)

%slip S=Ns−NNs ×100\%slip\ S=\frac{N_s-N}{N_s\ \times100}

b)

%slip S=N−NsN×100\%slip\ S=\frac{N-N_s}{N\times100}

c)

%slip S=N−NsNs ×100\%slip\ S=\frac{N-N_s}{N_s\ \times100}

d)

%slip S=Ns−S\%slip\ S=N_s-S

38.

the frequency of the rotor current in an induction motor is given by (if f is the supply frequency f' is the rotor frequency & S is the slip)

a)

f=Sff=Sf

b)

f=Sf′f=Sf'

c)

f′=Sff'=Sf

d)

S=ff′S=ff'

39.

the value of slip at maximum torque condition (under running condition) in terms R2 and X2 is

a)

S=X2R2S=\frac{X_2}{R_2}

b)

S=R2X2S=\frac{R_2}{X_2}

c)

S=2R2X2S=\frac{2R_2}{X_2}

d)

S=E22X2S=\frac{E_2}{2X_2}

40.

it is advisable to avoid line starting of induction motor and uses starter because

a)

starting torque is very high

b)

motor takes 5 to 7 times its full load current

c)

it will pick up very high speed and may go out of step

d)

it will run in reverse direction

41.

a squirrel cage motor is started at 50% its rated voltage. what is the starting torque relative to its rated voltage starting torque?

a)

50%

b)

100%

c)

25%

d)

75%

42.

a squirrel cage motor is started at 50% its rated voltage. what is the starting torque relative to its rated voltage starting torque?

a)

50%

b)

100%

c)

25%

d)

75%

43.

an autotransformer used for starting induction and synchronous motor to reduce the current drawn from the supply lines

a)

motor starter

b)

reducer transformer

c)

starting compensator

d)

four-point starter

44.

if the field of a synchronous motor is under-excited the power factor will be

a)

lagging

b)

leading

c)

unity

d)

more than unity

45.

when the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected

a)

it runs as a reluctance motor at a lower speed

b)

it runs as reluctance motor at the speed

c)

the motor stops

d)

none of the above

46.

what is the average slip in percent of a synchronous motor at running speed?

a)

2.5%

b)

3%

c)

2%

d)

0

47.

the advantage of a synchronous motor over a wound rotor motor is that

a)

the power factor may be varied

b)

its speed does not depend on the frequency of the supply

c)

it has no brushes to contend with

d)

its speed may be more readily controlled

48.

the induction motor differs from the synchronous motor is that it

a)

requires a dc source for its rotor

b)

does not require a rotating magnetic field

c)

current is induced in its rotor

d)

current is conducted in rotor

49.

the induction motor differs from the synchronous motor is that it

a)

requires a dc source for its rotor

b)

does not require a rotating magnetic field

c)

current is induced in its rotor

d)

current is conducted in rotor

50.

the angle between the rotor poles and stator poles, in a synchronous motor, is known as

a)

power factor angle

b)

synchronizing angle

c)

torque angle

d)

angle of retardation

51.

in a split phase motor, the running winding should have

a)

high resistance and low inductance

b)

low resistance and high inductance

c)

high resistance as well as high inductance

d)

low resistance as well as low inductance

52.

if the capacitor of single phase motor is short circuited

a)

the motor will not start

b)

the motor will burn

c)

the motor will run in reverse direction

d)

the motor will run in the same direction at reduced rpm

53.

the starting capacitor of a single phase motor is generally

a)

electrolytic capacitor

b)

paper capacitor

c)

ceramic capacitor

d)

none of these

54.

which of the following motor will give relatively high staring torque?

a)

capacitor start motor

b)

split phase motor

c)

capacitor run motor

d)

shaded pole motor

55.

in a capacitor start motor, the capacitor is replace by an inductor having the same reactance as in capacitor. then the motor will ____.

a)

be damaged

b)

start and run at its rated speed

c)

start but run at a slow speed

d)

not start

56.

which of the following is NOT a reason why single-phase induction motor run?

a)

two windings displaced from each other

b)

hysteresis

c)

one of the current is made out of phase between the lines

d)

shaded poles

57.

which of the following motor will give relatively high-power factor?

a)

capacitor start motor

b)

split phase motor

c)

capacitor run motor

d)

shaded pole motor

58.

if a particular application needs high speed and high starting torque, then which of the following motor will be preferred

a)

universal motor

b)

capcitor start motor

c)

shaded pole type motor

d)

capacitor start and run motor

59.

the motor used for the compressors is

a)

DC series motor

b)

capacitor start capacitor run motor

c)

shaded pole motor

d)

reluctance motor

60.

if starting winding of a single-phase induction motor is left in the circuit. it will

a)

run faster

b)

draw excessive current & overheat

c)

run slower

d)

spark at light load

61.

if a single-phase motor runs hot the probable cause cannot be

a)

blown fuses

b)

low voltage

c)

high voltage

d)

shorted stator coils

62.

direction of rotation of split phase motor can be reversed by reversing the connection of

a)

running winding only

b)

starting winding only

c)

either A or B

d)

both A & B

63.

which of the following type of motor is used in electric toys

a)

split phase motor

b)

shaded pole motor

c)

capacitor start motor

d)

permanent split

64.

what is the instrument often used to check the degree of motor shaft alignment?

a)

micrometer

b)

voltmeter

c)

clamp-on ammeter

d)

dial indicator

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