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ELECTRICAL SCIENCE > DC Generators > Flashcards

DC Generators Flashcards

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1
Q

A _____ is a machine that converts mechanical energy into electrical energy by using the principle of Faraday’s law of electromagnetic induction and Lenz’s law.

A

generator

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2
Q

T or F. When a conductor is moved within a magnetic field or vice versa, it cuts across magnetic lines of flux and thus, current is generated in the conductor.

A

FALSE.
VOLTAGE

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3
Q

The amount of voltage generated (Eg) depends on:

A
  1. magnetic flux density (B)
  2. angle at which the conductor cuts the magnetic field (θ)
  3. length of the conductor within the magnetic field (L)
  4. speed at which the conductor is moved (v)

e = BLvsinθ

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4
Q

Related to the effective area cutting the flux

A

angle, length

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5
Q

Related to the amount of flux, Φ

A

flux density, angle, length

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6
Q

T or F. The stronger the field or the more flux lines are cut for a given period of time, the larger the induced EMF.

A

TRUE

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7
Q

The polarity of the generated voltage depends on the direction of the magnetic flux and the direction of movement of the conductor (Lenz’s law). To determine the direction of the current in a given situation, _______ for generators is used.

A

Fleming’s Right Hand Rule

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8
Q

Fleming’s Right Hand Rule
thumb - ______
forefinger - _______
middle finger - ______

A

motion
field
EMF & current

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9
Q

An ______ consists of a loop of wire placed so that it can be rotated in a stationary magnetic field. This will produce an induced emf in the loop.

A

elementary generator

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10
Q

provide the magnetic field

A

pole pieces

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11
Q

usually made of carbon, with wires attached to them, ride against the rings

A

brushes

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12
Q

loop of wire that rotates thru the field

A

armature

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13
Q

the conductor ends of the armature loop are connected to rings called ____.

(AC Generator)

A

slip rings

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14
Q

T or F. Number of poles of any generator is not always an even number.

A

False
always

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15
Q

S or R

pole pieces

A

stator

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16
Q

S or R

armature

A

rotor

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17
Q

S or R

brushes

A

stator

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18
Q

S or R

slip rings

A

rotor

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19
Q

S or R

pole pieces (High AC generators)

A

rotor

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20
Q

S or R

armature (High AC generators)

A

stator

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21
Q

(AC Generator) At 0° position, armature loop is ______ to the magnetic field

A

perpendicular

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22
Q

When motion is parallel to the field

A
  1. no flux lines cut
  2. no emf produced (0V)
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23
Q

another term for Neutral Plane

A

Magnetic Neutral Axis (MNA)

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24
Q

(AC Generator) At 90° position, ______ has been cut

A

maximum flux

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25
Q

(AC Generator) At 90° position, induced emf at gold conductor is in _____ w/ the induced emf with the black conductor

A

series

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26
Q

T or F. (AC Generator) At 180° position, coil is moving parallel to the direction of the flux, thus, no flux cutting.

A

True

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27
Q

(AC Generator) From 0° to 180° the conductors of the armature loop have been moving in the same direction thru the magnetic field. Therefore, the polarity of the induced emf is _____.

A

the same (+)

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28
Q

(AC Generator) At 180° to 360° (back to initial point) position, the terminal voltage will be the same as it was from 0° to 180° but the polarity is _____.

A

reversed (-)

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29
Q

(DC generator) Each terminal is connected to a segment of a two-segment metal ring called ________.

A

commutator or split rings

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30
Q

The ______ mechanically reverses the armature loop connections to the external circuit. This occurs at the same instant that the polarity of the voltage in the armature loop reverses.

A

commutator

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31
Q

T or F. In commutation, commutator changes the generated DC voltage to an AC voltage.

A

False. AC to DC

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32
Q

The resulting induced voltage at the armature windings is always ______ regardless if the generator is AC or DC.

A

AC

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33
Q

(DC Generator) At 90° position, voltage reaches its ______ value.

A

maximum

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34
Q

(DC Generator) At 180° position, each brush makes contact with both segments of the commutator. Thus, the generated voltage and the current _____.

A

fall to zero

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35
Q

(DC Generator) As the armature loop rotates to 270° position, a voltage is again induced in the loop but of ______.

A

opposite polarity

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36
Q

the voltage across the brushes

A

pulsating DC or unidirectional

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37
Q

The pulsating DC voltage varies twice during each revolution between zero and maximum. This variation is called ______.

A

Ripple

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38
Q

Ways to lower the ripple voltage of DC generators

A
  1. add more armature coils
  2. add more magnetic poles
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39
Q

Nearly all practical generators use ________ instead of the permanent magnets used in the elementary generator.

A

electromagnetic poles

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40
Q

Advantages of using electromagnets as field poles

A
  1. increases field strength
  2. means of controlling the field strengths
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41
Q

T or F. In general, electromagnets are cheaper than permanent magnets.

A

False. Permanent magnets are cheaper

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42
Q

The brief period during which the coil remains short-circuited

A

commutation period

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43
Q

current reversal (+I -> 0 -> -1) is completed by the end of commutation period

A

ideal commutation

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44
Q

when current reversal is not completed by the end of commutation period, _____ in the brushes occurs (non-ideal commutation)

A

sparking

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45
Q

other term for non-ideal commutation

A

improper commutation

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46
Q

Possible causes of improper commutation

A
  1. Defective brush holder
  2. Worn out carbon brushes
  3. Defective armature winding
  4. Uncompensated armature reaction
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47
Q

For proper commutation, the coil short-circuited by the brushes must be in the ________.

A

neutral plane

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48
Q

the shift in the neutral plane caused by the interaction between the magnetic field produced by the current in the armature and the main field.

A

armature reaction

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49
Q

T or F. To prevent arcing, the brushes must be shifted to a new neutral plane.

A

True

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50
Q

used to address the issue of armature reaction

A

Compensating Winding
or
Interpoles

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51
Q

consists of a series of coils embedded in slots in the pole faces and these coils are connected in series with the armature.

A

Compensating Windings

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52
Q

The compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to ______ the effects of armature reaction.

A

cancel

53
Q

small auxiliary poles between the main field poles. They have a few turns of large wire and are connected in series with the armature.

A

interpoles

54
Q

other term for interpoles

A

commutating poles

55
Q

T or F. Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole behind of it, in the direction of rotation.

A

False.
ahead

56
Q

T or F. The amount of shift caused by the interpoles will equal the shift caused by armature reaction.

A

True

57
Q

A current-carrying conductor experiences a force when placed in a magnetic field.

A

Lorentz Law

58
Q

formula to compute the magnitude of the Lorentz force

A

F = BILsinθ

59
Q

When a generator delivers current to a load, the armature current creates a magnetic force that opposes the rotation of the armature.

A

Motor Reaction

60
Q

The system/device that provides the turning force applied to the armature.

A

Prime mover

61
Q

T or F. The prime mover must be able to overcome the armature reaction force in the generator.

A

False.

motor reaction force

62
Q

Examples of Prime mover

A
  1. Electric motor
  2. Gasoline engine
  3. Steam turbine
63
Q

Losses in DC Generator

A
  1. Copper Losses
  2. Eddy-Current Losses
  3. Hysteresis Losses
  4. Mechanical Losses
  5. Stray Power Losses (SPL)
64
Q

The mechanical input of generator is the same as the

A

power output of prime mover

65
Q

power lost as heat in the windings

A

Copper Losses

66
Q

T or F. Copper loss is minimized in armature windings by using small diameter wire.

A

False
large

67
Q

The currents that are induced in the generator armature core when it is rotated in a magnetic field.

A

Eddy Currents

68
Q

Eddy current losses are kept low when the core material is made up of many ______.

A

thin sheets of metal

69
Q

Laminations in a small generator armature may be as thin as _______.

A

1/64 inch (equiv. to 4 thin sheets of paper)

70
Q

To compensate for hysteresis losses, heat-treated ______ laminations are used in most dc generator armatures

A

Silicon steel

71
Q

Rotational or mechanical losses are caused by:

A
  1. Friction between bearings and armature shaft
  2. Friction between brushes and commutator segments
  3. Windage/air friction
72
Q

These are additional losses that cannot be easily accounted for

A

Stray Power Losses (SPL)

73
Q

SPL typically occur when the machine is _____

A

loaded

74
Q

On small machines, SPL are usually ____

A

ignored

75
Q

On large machines, SPL are computed as ______

A

1% Pout

76
Q

T or F. Gramme-ring Armature DC Generator differs in construction with elementary DC generator

A

True

77
Q

The windings of a Gramme-Ring Armature DC Generator are placed on an _____

A

Iron ring

78
Q

T or F. Gramme-ring is more efficient than drum-type

A

False
drum type > gramme-ring

79
Q

T or F. Drum-type armature dc generator has a more complex construction than Gramme-Ring

A

True

80
Q

used in dc generators designed for high current applications

A

LAP winding

81
Q

LAP windings are connected to provide several _____ paths for current in the armature

A

parallel

82
Q

used in dc generators designed for high voltage applications

A

WAVE Winding

83
Q

T or F. WAVE winding only requires one pair of brushes.

A

True

84
Q

When a dc voltage is applied to the field windings of a dc generator, current flows thru the windings and sets up a steady magnetic field.

A

Field Excitation

85
Q

Field winding is energized from a source of DC external to the machine called exciter

A

Separately-excited DC Generator

86
Q

T or F. Exciter may be a battery or another dc generator of bigger capacity

A

False
smaller

87
Q

Field winding is energized by its own armature

A

Self-excited DC generator

88
Q

T or F. Self-excited DC generator is possible only if the the field pole pieces have retained a slight amount of permanent magnetism called remanent/residual magnetism

A

True

89
Q

Armature of dc generator is laminated to reduce _______ losses

A

Eddy-current

90
Q

Types of Self-Excited DC Generator

A
  1. Series-wound DC Generator
  2. Shunt-wound DC Generator
  3. Compound-wound DC Generator (Long-shunt & Short-shunt)
91
Q

uses very low resistance field coils which consists of a few turns of large diameter wire

A

Series-wound DC generator

92
Q

T or F. In a series generator, changes in load current do not affect the generator output voltage.

A

False
drastically affect

93
Q

A series generator has a ____ voltage regulation, thus, it is not used for _______ loads

A

poor, fluctuating

94
Q

T or F. Series-wound generator has the poorest voltage regulation among DC generators.

A

True

95
Q

used for constant voltage application like in electric welding

A

Shunt-wound DC Generator

96
Q

The field coils used in shunt generator have _____ resistance, consisting of many turns and ____ wire

A

high, small

97
Q

T or F. Current in the field windings of a shunt-wound generator is dependent of the load current

A

False
independent

98
Q

In actual use, the output voltage in a DC shunt-wound generator varies ______ as load current varies.

A

inversely

99
Q

Series-wound and shunt-wound generators have a disadvantage in that changes in load current cause changes in output voltage. One means to supply a stable output voltage is by using a ______ generator.

A

Compound

100
Q

Compound-wound DC Generators have both the series and shunt field windings connected in either _______ or ________ configurations.

A

long-shunt, short-shunt

101
Q

T or F. The two windings of the compound generator are made such that their magnetic fields will either aid or oppose one another.

A

True

102
Q

The series field flux opposes the shunt field flux

A

Differentially Compounded Compound-wound DC Generator

103
Q

The series field flux aids the shunt field flux

A

Cumulatively Compounded Compound-wound DC Generator

104
Q

Types of Cumulatively Compounded Compound-wound DC Generator

A
  1. Under-compounded
  2. Flat-compounded
  3. Over-compounded
105
Q

Full-load VT < No-load VT

A

Under-compounded

106
Q

is used when the load is located near the Cumulatively Compounded Compound-wound DC Generator

A

Under-compounded

107
Q

Full-load VT = No-load VT

A

Flat-compounded

108
Q

is used when the load is at a medium distance from the Cumulatively Compounded Compound-wound DC Generator

A

Flat-compounded

109
Q

Full-load VT > No-load VT

A

Over-compounded

110
Q

is used when the load is far from the Cumulatively Compounded Compound-wound DC Generator

A

Over-compounded

111
Q

refers to the voltage change that takes place when the load changes

A

Voltage Regulation

112
Q

Voltage Regulation is usually expressed as the change in voltage from a ______ to a ______, and is expressed as a percentage of _______.

A

no-load condition -> full-load condition, full-load

113
Q

T or F. The higher the percent of voltage regulation, the better the generator.

A

False
lower

114
Q

T or F. It is better when the no-load VT is almost the same (or fairly constant) as the full-load VT.

A

True

115
Q

The 3 factors that may be able to vary generated voltage (Eg)

A
  1. Number of conducting loops in series (in the armature)
  2. Speed of the rotor/armature
  3. Magnetic field strength
116
Q

_______ is the internal action that is occurring within the generator whenever the load changes

A

Voltage Regulation

117
Q

______ is an imposed action, usually thru an external adjustment for the purpose of increasing or decreasing the voltage

A

Voltage Control

118
Q

Voltage control can be provided ______ and ______.

A

manually, automatically

119
Q

A type of voltage control where hand-operated field rheostat is connected in series with the field.

A

Manual Voltage Control

120
Q

Most field rheostats for generators use resistors of _____ wire.

A

alloy
(copper, nickel, manganese, chromium)

121
Q

T or F. Some very large generators use cast-iron grids (in place of rheostats), and motor-operated mechanisms to provide voltage control.

A

True

122
Q

It senses changes in output voltage and causes a change in field resistance to keep output voltage constant.

A

Automatic voltage control device

123
Q

The shunt field winding current is ______ when the resistance of rheostat is low

A

high
(I=V/R)

124
Q

When two or more generators are supplying a common load, they are said to be operating in ______.

A

parallel

125
Q

Parallel operation of generators is done on ________

A

Compound wound & Shunt Generators

126
Q

Purposes of Parallel operations of generators

A
  1. Greater supply capacity
  2. Redundancy
127
Q

A disabled or faulty generator may be taken off-line and replaced without interrupting normal operations

A

Redundancy

128
Q

Conditions for parallel operation of DC Generators

A
  1. The terminal voltage is the same
  2. The polarities of the generator must be identical
  3. The prime movers driving the armature of the generators must be similar and stable rotational characteristics (same speed regulations)
  4. The change of voltage with the change of load must have the same character (same drooping voltage characteristics, same percent voltage regulation)
129
Q

If generators are used in the purpose of redundancy, maximum load power must be equal to the power rating of the _______ generator.

A

smallest

ELECTRICAL SCIENCE (2 decks)

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