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Electrical Level 3 > Line Y-3 > Flashcards

Line Y-3 Flashcards

1
Q

Define torque

A

Torque is a twisting or turning force that is capable of producing rotation about an axis

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2
Q

Define the motor effect principle of magnetism

A

Whenever a conductor carries current in the presence of a magnetic field, a force will act on the conductor at right angles to the field

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3
Q

Fleming’s right-hand motor rule is used to determine _____________________

A

The direction of force acting on a conductor

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4
Q

How does the operation of the armature differ between a DC generator and a DC motor?

A

In a generator, the armature is a source of emf. In a motor, the armature is a load

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5
Q

State two ways in which the direction of rotation of a DC motor can be reversed

A

Reverse the connection of the armature leads.
Reverse the connection of both the series and shunt field leads

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6
Q

What is torque directly proportional to?

A

The product of flux and armature current

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7
Q

The motion of armature conductors cutting a field results in an induced emf that ____________________ the applied emf

A

Opposes

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8
Q

The emf induced in armature conductors as they cut the magnetic field is called _____________

A

Counter emf

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9
Q

What is the counter emf of a motor proportional to?

A

Flux and speed

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10
Q

What is effective armature voltage the product of?

A

Armature current and armature resistance

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11
Q

The difference between applied voltage and counter emf is called ___________

A

Effective voltage

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12
Q

At standstill, the counter emf of a motor is _______________

A

Zero

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13
Q

At start-up, the current drawn by a DC motor is limited only by ____________

A

Armature resistance

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14
Q

How is the starting current limited to a safe value in large DC motors?

A

Resistance is placed in series with the armature

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15
Q

A locked rotor usually means that the motor will ________________

A

Fail to start when voltage is applied

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16
Q

Why is a locked rotor condition undesirable?

A

The persistence of high starting current may damage the motor

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17
Q

An increase in mechanical load on the shaft of a motor causes motor speed to ___________

A

Decrease

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18
Q

When the speed of a motor decreases due to an increase in mechanical load, the counter emf ___________

A

Decreases

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19
Q

A reduction in counter emf causes armature current to ______________

A

Increase

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20
Q

What would happen if an increase in mechanical load exceeded the ability of the motor to develop additional torque?

A

The motor would stall

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21
Q

In a motor, power is proportional to ________________ and ________________

A

torque and speed

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22
Q

The variation in motor speed with load is called ___________

A

Speed regulation

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23
Q

The speed at which a DC motor operates without speed control devices is called __________

A

Base speed

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24
Q

Why does increasing the resistance in the field circuit of a DC motor cause the speed to increase?

A

The increase in armature current (due to a decrease in counter emf ) is greater than the decrease in excitation

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25
Q

To operate a motor below base speed, it is necessary to decrease __________

A

Armature current

26
Q

A practical method of operating below base speed involves controlling ______________

A

The voltage applied to the armature

27
Q

List two adverse effects of armature reaction if it remains uncorrected

A

Weakening of the main field
Excessive brush sparking

28
Q

The most common solution for minimizing the effects of armature reaction involves the use of __________________

A

Interpoles

29
Q

Why are interpoles connected in series with the armature?

A

So that the strength of the interpole matches the strength of the armature field

30
Q

Shunt field windings consist of a:

A

Large number of turns of small-gauge wire

31
Q

T/F
Excitation of a shunt motor is provided by field coils connected in series with the armature

A

False

32
Q

When using NEMA subscripts, the convention of A1 positive indicates (CCW or CW) rotation?

A

CCW

33
Q

T/F
When a DC generator and a DC motor are operated with the same line voltage, polarity and direction of rotation, the respective armature currents will be in opposite directions

A

True

34
Q

State two ways to reverse the direction of the force acting on the armature conductors of a shunt motor.

A

Reverse the direction of current in the armature
Reverse the direction of current in the shunt field windings

35
Q

What is the most practical method of reversing the direction of rotation of a shunt motor?

A

Reverse the armature leads

36
Q

What happens to the direction of rotation if the polarity of the line terminals is reversed?

A

The rotation remains the same

37
Q

A shunt motor is commonly called a _____________________ motor

A

Constant speed

38
Q

Why must the field circuit of a shunt motor never be allowed to become open while the motor is in operation?

A

The motor can accelerate to unsafe speeds

39
Q

To prevent runaway in a shunt motor, an _______________ is used

A

Over-speed sensor (or loss of field relay)

40
Q

List three typical uses for a shunt motor

A

Drives for paper machines
Printing presses
Drill presses

41
Q

Series field windings consist of a:

A

Small number of turns of large-gauge wire

42
Q

What is the main difference between series and shunt excitation?

A

The magnetic strength of series field coils varies with load current

43
Q

State two ways to reverse the direction of rotation of a series motor

A

Reverse the connection of the armature leads
Reverse the connection of the series field leads

44
Q

Will changing the polarity of the line leads change the direction of rotation of a series motor? Why?

A

No. Reversing line polarity reverses both armature current and field polarity, maintaining torque in the same direction as before

45
Q

Why is the series motor unsuitable for constant-speed applications?

A

Since the torque varies with the square of the armature current, changes in mechanical load are accompanied by large changes in operating speed

46
Q

In practice, speed control of series motors is accomplished using _____________

A

Solid-state drives

47
Q

T/F
The main advantage of the series motor is its very high starting torque

A

True

48
Q

Why should a series motor always be mechanically coupled to the driven load?

A

The sudden removal of load may cause the motor to accelerate to an unsafe speed

49
Q

The main characteristics of the series motor are _______________ and _______________

A

High starting torque and poor speed regulation

50
Q

List three common uses for series motors

A

Cranes, hoists, locomotives

51
Q

A compound motor has both ___________________ and ___________________ field coils

A

Shunt and series

52
Q

In most compound motors, the motor characteristics are predominantly like the ___________________ motor

A

Shunt

53
Q

When the flux of the series field aids the flux of the shunt field, the connection is called a ___________________ compound

A

Cumulative

54
Q

When the flux of the series field opposes the flux of the shunt field, the connection is called a ___________________ compound

A

Differential

55
Q

Explain the difference between long shunt and short shunt connections

A

In the long shunt connection, the shunt field is connected across the line (armature and series field). In the short shunt connection, the shunt field is connected directly across the armature

56
Q

State two ways of reversing the direction of rotation of a compound motor

A

Reverse the connection of the armature leads
Reverse the connection of both the series and shunt field leads

57
Q

T/F
The speed of a compound motor can be increased by increasing resistance in the shunt field circuit

A

True

58
Q

Is it necessary to take precautions to prevent runaway of a compound motor?

A

Yes. The degree of compounding may not be sufficient to prevent dangerous over-speed in the event of an open in the shunt field circuit

59
Q

Is it acceptable to have belt-driven mechanical loads with a compound motor?

A

Yes. The no-load speed hazard of the series motor is not present

60
Q

When selecting a compound motor, the _______________ and _______________
characteristics must be matched to the specific application

A

Torque and speed

61
Q

Compound motors are more suitable than shunt motors for handling ______________

A

Sudden increases in mechanical load

62
Q

List three applications for compound motors

A

Presses, shears, compressors

Electrical Level 3 (9 decks)

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