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Fourth Year Electrician > Theory > Flashcards

Theory Flashcards

Math Applications

1
Q

Inductance in a circuit opposes a change of current by introducing a _____ force into the circuit.

A

Counter-electromotive

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2
Q

In a series phasor diagram, what value is used as the horizontal reference?

A

Current

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3
Q

In a series phasor diagram, what is in phase with the current?

A

Volt drop across resistance

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4
Q

In a series phasor diagram, current (leads or lags?) Inductive voltage drops by what angle?

A

Lags, 90 degrees

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5
Q

In a series phasor diagram, current (leads, lags?) Capacitive voltage drop by what angle?

A

Leads, 90 degrees

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6
Q

In a series phasor diagram, current (leads, lags?) The voltage drop across the coil with both R and XL by what angle?

A

Lags, phase angle of coil

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7
Q

In RLC circuits, total current is the ____ of the currents that flow through _____ of the parallel circuit.

A

Phasor sum, each branch

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8
Q

A dynamo can function as a DC _____ or _____

A

Generator, motor

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9
Q

A generator is rated by _____.

A

Kilowatt (or megawatt) of power that can be delivered at rated voltage.

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10
Q

An alternator is also called a _____.

A

Synchronous generator.

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11
Q

Alternators are rated by _____.

A

VA (or kVA, or mVA)

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12
Q

3 examples of a prime mover are _____.

A

1: turbines powered by falling water, steam, wind
2: diesel engines
3: natural gas engines

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13
Q

Motors are rated by the power in _____.

A

KW or HP at rated V or r/min

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14
Q

What’s the difference between the field winding on an AC versus a DC machine? What do they create?

A

On a DC, usually on stator. On AC, usually on the rotor.

They create magnetic flux.

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15
Q

What develops in the armature of a DC generator? And what in a DC motor?

A

Generator: voltage

Motor: torque

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16
Q

Where is the armature windings on an AC machine?

A

On the stator

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17
Q

Where are slip rings mounted?

A

Shaft of an AC machine. Will rotate with coil and shaft.

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18
Q

What is another name for commutator?

A

Split ring

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19
Q

What is the purpose of the commutator in a DC motor?

A

Convert applied DC voltage to AC

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20
Q

What is the purpose of a commutator in a generator?

A

Convert induced AC voltage to DC.

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21
Q

Commutation is the process of __________ of current in the armature coil

A

Reversing the direction

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22
Q

Neutral plane is _____.

A

Where the armature cuts the fewest lines of flux

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23
Q

Brushes on a DC machine _____

A

Form the sliding electrical connection between armature windings and an external circuit

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24
Q

Brushes in an AC machine _____

A

Are an electrical connection between field windings and an external circuit.

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25
Q

Arcing is reduced during commutation by _____.

A

Having armature conductors cut fewest lines of force.

26
Q

What is the equation for electron flow?

A

6.242 x 10(exp 18) electrons per second

27
Q

What are brushes made of?

A

Carbon

28
Q

The field winding on a DC machine is located on the _____.

A

Stator

29
Q

Energy input to a machine is supplied by the _____ _____.

A

Prime mover

30
Q

A DC motor converts ______ into _____.

A

Electrical energy to mechanical energy

31
Q

Are the field current and generated voltage after saturation point directly proportional?

A

No

32
Q

The field current and generated voltage are almost _____ until the _____ is reached.

A

Linear

Knee

33
Q

What is the “Knee” in relation to generated voltage and field current?

A

It is the abrupt change in the angle of the curve, where the core begins to reach saturation point.

34
Q

At which angles will the current to the load be at it’s maximum in an AC generator?

A

90 and 270

35
Q

What does it mean when induced voltages are series-adding?

A

Current in both halves of the armature loop is flowing in the same direction

36
Q

What action does a DC commutator provide?

A

Switching/rectifying which allows current to flow in only one direction

37
Q

What is the output voltage at any instant of a multiple coil winding that is connected in series?

A

Sum of the voltages in each winding

38
Q

How do you produce a smoother output waveform in a DC generator?

A

Increasing number of armature coils and commutator bars.

39
Q

Adding more coils to the armature will produce a waveform with less _____.

A

Ripple

40
Q

Most generators are designed to operate at a relatively ______ speed.

A

Constant

41
Q

What does a rheostat do?

A

Controls generated voltage by limiting the amount of current in the winding

42
Q

If there is no field current, will there also be no voltage? Why?

A

No, there will be a small amount.

Residual magnetism

43
Q

Generators are rated by their _____ Kilo or Mega Watt capacity.

A

Output

44
Q

An IEC rated generator would be rated in _____.

A

Kilowatts

45
Q

A Nema rated generator would be rated in _____.

A

Horsepower

46
Q

Temperature rise rating is min __ and max __ degrees C

A

40, 50

47
Q

If the ambient temperature exceeds a specified value, th allowable temperature rise must be _____.

A

Reduced

48
Q

Rotational frequency is measured in _____ and sometimes also called _____.

A

R/min

RPM

49
Q

Examples of DC generator speed ratings are: (4 items)

A

1800, 1200, 900, 720 r/min

50
Q

Examples of DC motor speed ratings are: (5 items)

A

2500, 1750, 1150, 850, 500 r/min

51
Q

Three phase alternators typically range from _____ V to ______ V.

A

208 - 13800

52
Q

Typical DC voltage ratings for generators are: (3 items)

A

125, 250, 500

53
Q

Typical DC voltage ratings for motors are: (6 items)

A

90, 120, 180, 240, 500, 550

54
Q

Duty (ie continuous, standby) is determined based on the _____ rating.

A

Temperature rise

55
Q

What is X’d SAT?

A

X prime d saturation.

=Reactance value indicating ability of generator to absorb sudden loads without having unacceptable levels of voltage drop.

56
Q

What is X’‘d SAT?

A

X double prime d saturation.

=Reactance value indicating ability of generator to provie short circuit current.

57
Q

FLC rating means the max current while operating at rated voltage and speed, with full load. T or F

A

True

58
Q

The polarity of induced emf in a DC generator depends on what?

A

Direction of the relative motion between the conductor and the magnetic field.

59
Q

What type of waveform is produced by a rotating loop of wire connected to slip rings?

A

Sine wave

60
Q

Magnitude of induced voltage depends on which three factors?

A

1: Length of the conductor in the field
2: Velocity
3: Flux density

61
Q

What type of waveform is produced by a single armature loops connected to a split ring?

A

Pulsating DC

Fourth Year Electrician (7 decks)

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