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Ac Machines > Ac machines > Flashcards

Ac machines Flashcards

1
Q

If rotor slip increases what happens to PF of the rotor

A

PF WILL INCREASE

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2
Q

If the mechanical load on a motor increases what happens to the motors PF

A

PF improves

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3
Q

What happens to the cemf in the stator if the rotor flux increase

A

Opposed by 180° so cemf will decrease as rotor flux increases

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4
Q

If rotor slip rises what happens to stator current

A

Current will increase

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5
Q

If the motors load is lightened what happens to the rotor speed

A

Increases

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6
Q

If rotor speed is Increased how is induced voltage affected

A

Output voltage will decrease

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7
Q

How is torque affected by an increase in stator current

A

Current increases
Torque increases

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8
Q

What will happen to the rotor speed if the frequency iof the stator voltage decreases

A

Rpm decreases

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9
Q

If the motor stalls what happens to sync speed

A

Sync speed is resultant of frequency and number of poles so there is not affect on sync

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10
Q

If a motors rated FLT is 50ftlbs but the motors supply voltage is reduced to 75% of rated voltage how much torque is produced

A

Torque is equal to applied voltage²
.50²x50 =12.5ftlbs

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11
Q

What will happen to the slip% when the load increases on the motor

A

Increases

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12
Q

When slip% increases what happens to rotor frequency

A

It will increase

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13
Q

How would a larger %speedreg affect rotor current

A

Current would increase

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14
Q

If the rotor PF goes down what happens to output torque

A

Less PF equals less torque

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15
Q

How is line current affected if the induced rotor voltage decreases

A

Less induced=higher cemf =lowers current

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16
Q

What happens to a motors efficiency when load is reduced

A

Shouldn’t have much change

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17
Q

If the rotor speed drops from 1795 to 1725 how will this affect the torque

A

Torque increases

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18
Q

A motor stalls what happens to rotor flux

A

Rotor flux is only changed by frequency and pole numbers

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19
Q

Does a 3phase scim operate at higher power factor when it is operating no load or full load

A

Full load

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20
Q

A fully loaded synchronous motor has excitation adjusted so that its power factor is unity. The excitation is decreased slightly without making any other change what would happen?

A

The pf would become lagging

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21
Q

What are two advantages of a high reactance scim

A

Low inrush
Low speed reg

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22
Q

What two items found on a nameplate can you use to correctly determine the size of overload device

A

Service factor
FLA

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23
Q

The rotor of a 3phase double squirell cage induction motor has __________ resistance bars beat the rotor surface

A

High

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24
Q

What are two advantages of choosing a 3phase scim having high resistance rotor

A

Low inrush
Strong starting torque

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25
Q

State two disadvantages of a 3phase scim high resistance rotor

A

High% speed reg
Low efficiency

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26
Q

If you reversed two of the rotor circuit leads for a 3phase wound rotor motor what would happen

A

Operate as previous

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27
Q

How could you determine if there is a faulty bearing in a scim

A

Vibration displacement indicator

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28
Q

What would the likely result should the centrifugal switch of a split phase motor fail to open during start

A

Less full load torque
Slower speed
Poorer speed regulation
Burn out of start winding

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29
Q

What would be the likely result of an open circuit in the rheostat circuit of a 3phase wrim

A

Motor won’t start

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30
Q

What test can you do to find out the problem with a WRIM if you suspect there is an open in the rheostat circuit

A

Short out m1 M2 m3

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31
Q

What are two advantages of a scim with a high reactance design over a standard scim

A

Low inrush
low %speed ref

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32
Q

What is a disadvantage of a high reactance scim

A

Low starting torque

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33
Q

What is are two advantages of a scim with a low Z

A

Strongest starting torque
Low%speed reg

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34
Q

What is one disadvantage of a low Z scim

A

Large inrush current

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35
Q

Should a 3phase wrim be started max or min resistance

A

Max

36
Q

What is the two purposes of the amortisseur winding

A

Start winding
Dampening winding

37
Q

What is the advantage of a double squirrel cage motor when compared to other types of induction motors

A

Strong start torque
Low%speed reg

38
Q

The torque angle between the centre’s of the stator field and rotor field of a synchronous motor are determined by what

A

The load

39
Q

A double squirrel cage rotor is what nema design

A

Nema c

40
Q

What term is meant by single phasing of a 3phase scim

A

One of the lines of the motor has developed an open

41
Q

To reverse rotation of a split phase motor you would

A

Reverse the star winding leads

42
Q

When the field rheostat of a synchronous motor is adjusted to produce the lowest stator line current the pf would be

A

At unity

43
Q

Is it possible to vary the torque of a wound rotor motor

A

Yes by adjusting the external resistance

44
Q

Is it possible to vary the speed of a wound rotor motor

A

Yes adjusting external resistance

45
Q

What would happen to the speed of a wound rotor motor if the frequency of the applied voltage was increased

A

The speed would increase due to speed being directly proportional to frequency in an induction motor

46
Q

What is the difference between the stator of a scim and the stator of a WRIM

A

No different from eachother

47
Q

If we compared the starting winding of a standard split phase motor to the running winding we would see

A

That the starting winding is placed over the top of the running winding in the stator slots

48
Q

What would be the main reason one would expense a WRIM over a scim

A

High start torque
Smoother acceleration of heavy loads

49
Q

Will a 3phase wrim start if the rotor circuit is opened

A

No
No current flow in rotor circuit

50
Q

If a 3phase wrim fails to initially start but rotates when the 3 slip rings are shorted what would the problem be

A

Open in the secondary resistor circuit

51
Q

What happens if the centrifugal switch contacts fail to close when the motor stops

A

The motor will not start again

52
Q

How would you reverse the rotation of a split phase induction motor

A

Interchange the start winding terminals

53
Q

What is the primary difference of a split phase and a split phase capacitor start

A

Capacitor start has better starting torque

54
Q

What is one limitation of a capacitor start induction motor

A

Needs heal time cants be started frequently

55
Q

A 3phase Ac motor is said to be single phasing when

A

No current flows in one of the line

56
Q

Will a universal motor operate faster or slower on a DC or single phase Ac supply

A

Would be the same

57
Q

What precaution must be watched when operating a single phase universal motor

A

Loss of load causes runawayb

58
Q

If the excitation is adjusted so that the power factor of the fully loaded synch motor is at unity what would happen if excitation is decreased

A

PF would decrease and start lagging

59
Q

How could we increase the speed of a synch motor

A

Increase frequency

60
Q

If a fully loaded synchronous motor is at unity and only mechanical load is reduced what would happen

A

PF would lead

61
Q

When the line current of a synchronous motor is at Minimum what would the pf be

A

Unity

62
Q

When should the DC excitation be applied to the rotor of a synch motor

A

About 95% rated speed

63
Q

What is meant by the term pull out torque

A

Torque value that causes synch motor to drop out of synch and drop to a standstill

64
Q

When operating an alternator at no load a decrease in the speed of the prime mover will cause a

A

Decrease in frequency and voltage

65
Q

When a synchronous motor is operated with little or no load and its field excitation is adjusted for a leading pf it can be called a

A

Synchronous condenser

66
Q

A fully loaded synchronous motor is operated at unity of if only the mechanical load is reduced you would expect the pf to

A

Go leading

67
Q

What rating does a vfd need to be if driving a motor

A

Inverter rated

68
Q

Is a synchronous motor an induction motor

A

No

69
Q

a synchronous motor is a ______________ motor

A

Electromagnet

70
Q

Are the stator winding of a synchronous motor the same as a squirrel cage and a WRIM

A

Yes

71
Q

What sizes in hp do synchronous motors come in

A

200-20000hp

72
Q

What percentage do you put a synch motor into synch speed

A

Around 96-98% synch rpm

73
Q

The ____________ of ______________ produces motor torque

A

Stretching

Flux linkages

74
Q

Synchronous motors have an efficiency of around

A

97-98%

75
Q

What prevents voltage from travelling down the DC source path of a synchronous motor

A

A field discharge resistor

76
Q

How do you reverse a split phase motor

A

Swap terminals on start winding

77
Q

What is the slotted ring which holds fixed windings in an AC machine

A

Stator core

78
Q

What is the revolving part of an AC machine called

A

The rotor

79
Q

Where is the revolving magnetic field produced In an AC machine

A

Stator iron

80
Q

What causes the revolving magnetic field in the rotor of an AC machine

A

Mutual induction

81
Q

What other Ac equipment can a scim be compared too besides another motor

A

Transformer

82
Q

What is the relationship between slip speed and induced rotor voltage

A

Directly proportional

83
Q

What is the relationship between reactance and frequency

A

Directly proportional

84
Q

What approximate %slip is motor torque the greatest

A

25%

85
Q

What is the relationship between resistance and reactance when torque is at its strongest point

A

Both values should be equal

Ac Machines (1 decks)

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