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Level 3 Electrical, BCIT > DC Generators/Motors > Flashcards

DC Generators/Motors Flashcards

1
Q

Define a generator

A

A device that converts mechanical energy into electrical energy

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2
Q

Voltage produced in the armature is DC or AC?

A

AC

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3
Q

Which part of the generator produces AC?

A

Armature

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4
Q

Which part of the generator produces DC?

A

Commutator

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5
Q

What is the purpose of the commutator?

A

To convert AC to DC by reversing the current flow in the winding

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6
Q

In a DC generator, increasing the number of loops has what purpose?

A

To create a smoother output voltage. More waves evens it out.

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7
Q

What is the smooth effect of multiple loops called?

A

Ripple

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8
Q

What makes up the armature?

A

Iron core, commutator, windings

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9
Q

What are the three types of armatures?

A

Lap-Wound, Wave-Wound, Frogleg-Wound

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10
Q

What is the most common type of armature?

A

Frog leg-Wound

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11
Q

What are the voltage and current properties of lap-wound armatures?

A

Low voltage, high current. Larger wire gauge.

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12
Q

What are the voltage and current properties of wave-wound armatures?

A

High voltage, low current. Smaller wire. Connected in series.

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13
Q

What are brushes?

A

Armature leads, carbon pieces that sit against the commutator to connect Eg to external circuit

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14
Q

What are the properties of the pole pieces?

A

Provide magnetic field for operation, made of soft iron or silicon steel.

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15
Q

Which windings have larger wire and fewer turns?

A

Series

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16
Q

Which windings have smaller wire and more turns

A

Shunt

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17
Q

What are the three types of DC generators?

A

Series, shunt, and compound

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18
Q

Which type of generator must be self-excited?

A

Series

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19
Q

What does a self-excited generator mean?

A

Residual magnetism produces an initial Power Out Voltage

The pole pieces retain magnetism

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20
Q

What are the three factors that determine Vout in a series generator?

A
  1. # of turns
  2. Strength of magnetic field
  3. Speed of rotation
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21
Q

What is Saturation of Iron in a series generator?

A

The max voltage that can be produced, max amount of flux produced. Past saturation the voltage output will decrease.

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22
Q

What is rated load in series generator?

A

Rated load is the point of saturation, where max voltage is produced.

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23
Q

What is the difference between a self excited and separately excited generator?

A

Self excited holds residual magnetism, separately excited needs an external source of energy to start

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24
Q

T or F: shunt generators can be both self excited and separately excited

A

True

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25
Q

T or F: series generators can be both self excited and separately excited

A

F. Only self excited.

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26
Q

What is the difference between series and shunt self-excited generators in regards to current?

A

A load is required to start current flow in a series SE generator, in a shunt SE the windings act as a loop so current is already generated before a load is connected. Full voltage can be obtained by a shunt SE generator before a load is connected.

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27
Q

What are the two advantages of separately excited shunt generators?

A
  1. Better voltage regulation
  2. Less voltage drop when a load is added
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28
Q

What is a difference between series SE and shunt SE generators in regards to voltage?

A

Series SE will only produce a small amount of voltage with no load, Shunt SE can produce full voltage with no load because the windings of the shunt act as a load creating a loop

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29
Q

Why are separately excited machines superior to self excited machines?

A

On a SE generator, once saturation is reached, when a load is added it decreases the voltage and current output because there is a maximum amount that can be produced through the magnetic field. On a Sep Ex generator, an external source keeps the flux magnetic field constant, so it doesn’t have the same amount of drop.

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30
Q

What is the difference between field poles and inter poles?

A

Field poles: permanent magnets that create magnetic field
Inter poles: windings connected in series with the armature. Made of smaller windings than the field poles

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31
Q

What is the purpose of interpoles?

A

Helps reduce arching between brushes and commutator

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32
Q

What are the connection codes for series and shunt?

A

S2 and S1 are series, F2 and F1 are shunt.

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33
Q

What is the formula to find Regulation?

A

Enl-Efl / Efl x100

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34
Q

When armature leads are reversed, what happens to the direction of rotation?

A

It also reverses.

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35
Q

What type of switch can be used to reverse the rotation of a shunt motor?

A

DPDT

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36
Q

When the shunt leads are reversed, what happens?

A

The field changes from cumulative to differential or vv

37
Q

What is the most common compound configuration for a generator?

A

Short shunt cumulative compound. More magnetic flux and torque.

38
Q

Your thumb indicates which rule?

A

Direction on force

39
Q

Your forefinger indicates which rule?

A

Magnetic field

40
Q

Your middle finger indicates which rule?

A

Direction of current

41
Q

Your left hand rule is for _____

A

Generators

42
Q

Your right hand rule is for _____

A

Motors

43
Q

The standard rotation for a DC generator is CW or CCW?

A

CW

44
Q

The standard rotation for a DC motor is CW or CCW?

A

CCW

45
Q

What type of generator can be run in either direction?

A

A separately excited generator

46
Q

How do you find E gen of a short shunt generator?

A

E across armature resistance + E across shunt.

47
Q

How do you find efficiency of a generator?

A

Add both of the wattages produced in the generator (armature and resistor). Divide terminal power by the armature and resistor power.

Pa + Par / Pload

48
Q

What are the four generator losses?

A

Windage/mechanical, copper/line, iron/core, brush

49
Q

What are copper losses of a motor?

A

Resistance in the windings/loads. I^2 x R

50
Q

In a DC generator, what is cumulative?

A

When shunt and series connections are made to aid in the development of the magnetic field.

Stronger magnetic field, more torque.

51
Q

In a DC generator, what is differential?

A

When series and shunt fields are wound to oppose each others magnetic fields, reducing torque.

52
Q

What is the formula to find frequency?

A

F = P x N / 120

Frequency = number of poles x RPM / 120

53
Q

Generator EMF is proportional to what factors?

A

Field strength, number of turns in armature windings, rpm of armature

54
Q

What is the formula for Output Voltage of a generator?

A

E gen - IR drop in armature and brushes

E gen - E of armature and brushes.

55
Q

T or F: (for a generator) voltage regulation is determined by the resistance of the armature?

A

True

56
Q

What are the four types of compounding?

A

Over, flat, under, differential

57
Q

What is over compounding?

A

When voltage at full load is greater than no load.

Series windings are increased - higher voltage on long cable runs offsets I^2 x R losses at the load.

58
Q

What is flat compounding?

A

When full load and no load output are equal

59
Q

What is under compounding?

A

When output voltage is less at full load than no load.

60
Q

How do you adjust the no load voltage of the generator?

A

With a field rheostat

61
Q

How do you control the compounding and adjust the full load voltage of the generator?

A

With a diverter rheostat

62
Q

How do you calculate power output of a motor?

A

Power input - all losses

63
Q

What is the difference of A1 and A2 between a generator and motor?

A

Generator: A1 is usually positive, A2 is usually negative

Motor: determined by the source leads.

64
Q

What is the purpose of a long shunt differential compound generator?

A

Lower the voltage to maintain a higher current.

65
Q

Define counter torque

A

The turning resistance of the armature because of the attracting magnetic fields

66
Q

What is dynamic or regenerative braking?

A

When the motor very quickly disconnects the armature from the source and reconnects it to a load resistor. Current now flows through the resistor instead of the source, slowing down the armature. This only works if the current is still flowing.

67
Q

What is armature reaction?

A

Twisting or bending of magnetic lines of flux of the pole pieces, causing neutral plane to move, causing brushes to no longer connect to commutator.

68
Q

Amount of armature reaction is proportional to what?

A

Armature current.

69
Q

What are the three ways to correct armature reaction?

A

Move the brushes, interpoles, and compensating windings.

70
Q

How does adjusting the brushes fix the armature reaction ?

A

Moving them to align with the neutral plane

71
Q

How do interpoles correct the armature reaction?

A

The interpoles restore the magnetic field to its original condition

72
Q

Interpoles in a motor must have the same polarity as ____

A

As the main field pole directly behind them (N or S)

73
Q

Interpoles in a generator must have the same polarity as ____

A

As the main field pole directly ahead of them.

74
Q

Interpoles are connected in 1. series or 2. parallel, and with what?

A

In series with the armature.

75
Q

Full load speed of a shunt motor generally remains within ____% of no-load speed

A

10

76
Q

What is speed regulation?

A

The change in speed from no load to full load

77
Q

Speed regulation is proportional to _____

A

Supply voltage

78
Q

What are the properties of series windings?

A

Less turns, larger wire, low resistance.

79
Q

What are the properties of shunt windings?

A

More turns, smaller wire, higher resistance.

80
Q

How do you determine total losses due to resistance?

A

Find all wattages across all loads and resistors, add them all together.

81
Q

What is the formula for force of a motor?

A

F = mass x acceleration

82
Q

What is the difference between load torque and acceleration torque?

A

Load torque is the amount of torque constantly required for application.

Acceleration torque is the torque required just for the maximum acceleration and deceleration rate for the load.

83
Q

What is the formula for Torque?

A

T = Kt x I
Or
T = F x r

84
Q

What is the formula for Kt?

A

2 x pi x RPM / 60

85
Q

The strength of EMF can be measured at any moment with what formula

A

E = Blv

86
Q

Maximum cutting of flux is reached at what degrees?

A

90 and 270

87
Q

What is the formula to find instantaneous EMF?

A

E = BLVsintheta

88
Q

If sin theta is not provided, what is it usually?

A

90

Level 3 Electrical, BCIT (7 decks)

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