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Electrician - Fourth Period > Alternators > Flashcards

Alternators Flashcards

1
Q

What is an Alternator?

A

A machine that converts mechanical energy into electrical energy

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2
Q

How is the electrical energy created in an Alternator?

A
  • It is created with the relative motion between the conductor and a magnetic field
    • Relative motion indicates that either the conductor is moving through the magnetic field, or the magnetic field is moving past the conductor
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3
Q

What is the rotor of an alternator?

A
  • The rotating componet of the alternator
  • The rotor is driven by the primer mover
  • An Alternator can be contructed with either the field windings or the armature windings on the rotor
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4
Q

What is the stator of an alternator?

A
  • The stationary armature assembly of an alternator
  • This consists of armature conductor windings fitted into slots cut into an iron core
  • The armature conductor windings consists of coils insulated from each other and from the armature core
  • Mechanical to electrical energy conversion happens in the armature winding
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5
Q

What creates or excites the magnetic field of an alternator?

A
  • The exciting current
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6
Q

Which type of alternator is able to produce a higher kVA output?

Rotating Armature or Rotating Field Alternator?

A
  • Rotating Field Alternator
  • Slip rings and brushes aren’t required to connect the armature output to the external load
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7
Q

What are the advantages of a Rotating Field Alternator over a Rotating Armature Alternator?

A
  1. Slip rings and brushes are not required to connect the armature output to the external load
  2. The field requires relatively low currents and voltages, so smaller brushes and slip rings are sufficient
  3. The rotating field structure can be physically smaller, which results in the mechanical advantages such as reduced bearing sizes and reduced vibrations
  4. Most of the heat generated in an alternator comes from the armature windings which are easier to cool, because of their position
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8
Q

What are the two distinct types of synchronous-generator/alternator rotating field structures ?

A
  1. Cylindrical Rotors - smooth exterior, relatively long, must be well balanced, usually have two or four magnetic poles, windings must be firmly embedded in slots in the rotor core
  2. Salient Pole Rotors - driven by slower speed prime movers such as internal combustion engines, water turbines, or electric motors. These are typically shorter in length and larger in diameter than cylindrical rotors
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9
Q

What is the formula for finding the frequency of the generated voltage of an Alternator?

A

Frequency = (Poles x Speed) / 120

  • Frequency = (cycles / seconds) or Hertz (Hz)
  • Poles = The number of poles on the rotor
  • Speed = The rotational frequency of the rotor in r/min
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10
Q

What is the voltage generated in each winding in a stator proportional to?

A

The number of turns in the winding

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11
Q

Which formula is used to find the rated line current, and the current in each winding of an alternator?

A

VA = VLINE x ILINE

ILINE = VA / VLINE

  • If an alternator is rated at 3kVA and 120V, calculate the rated line current and the current in each winding
  • Rated ILINE = 3000VA / 120V
  • Rated ILINE = 25A
  • Current in each winding = 25A ÷ 2 = 12.5A
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12
Q

If the load on an alternator is increased by a factor of two, how would this affect the alternator’s copper losses?

A

Increase by a factor of four

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13
Q

What are some of the basic componets of the rotor of a rotating magnetic field alternator?

A

Slip Rings and Field Windings

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14
Q

How many electrical degrees apart are the armature windings of a three-phase, two-pole alternator placed from each other?

A

120 degrees apart

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15
Q

What happens when there is relative motion between the armature conductors and the field of an alternator?

A

An EMF is generated in the armature conductors

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16
Q

What two values of losses remain fixed as load is added or removed from an alternator?

A
  1. Core Losses
  2. Mechanical Losses
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17
Q

A six-winding, 12 lead alternator is series connected for 277/480 volt operation

How many volts would each winding generate?

A

139 volts

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18
Q

If the load of an alternator is reduced by a factor of two, how would this affect the alternator’s core losses?

A

It’s core losses would remain fixed

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19
Q

A 75 kVA six-winding, 12 lead alternator is series-delta connected for 240V operation

What would the phase current rating be for this alternator?

A
  1. 75,000VA ÷ (240V x 1.732) = ILine = 180.42A
  2. 180.42A ÷ 1.732 = 104.17A

Note: 1.732 is equal to the square root of 3

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20
Q

How many cycles of voltage are generated when a south and north pole of a rotor move past an armature winding in an alternator?

A

1 cycle of voltage in generated

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21
Q

What will laminating the magnetic structure of an alternator reduce?

A

Eddy Current Losses

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22
Q

If the load on an alternator is reduced by a factor of two, how would this affect the alternator’s rotational losses?

A

It will remain fixed

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23
Q

What type of excitation system uses a three-phase rotating rectifier to provide the excitation current to the alternator field?

A

Brushless Excitation

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24
Q

Why does an alternator with a rotating field require slip rings and brushes?

A

To connect the DC source to the field windings

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25
Q

Of a running alternator, what are the two factors that can be altered that would affect the value of the voltage generated in the armature windings?

A
  1. The amount of flux produced by the field winding
  2. The speed of the rotating field
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26
Q

How do voltage regulators of alternators control the output voltage?

A

They control the current in the field windings

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27
Q

In a large self-excited AC alternator, what current does the voltage regulator directly control?

A

The exciter field current

28
Q

When there is no load on an AC alternator, the generated voltage is the same as the terminal voltage

True or False?

A

True

29
Q

What is the formula for finding the voltage regulation of an alternator?

A

VREG = [(VNL - VFL) ÷ VFL ] x 100

  • VNL = No Load Voltage
  • VFL = Full Load Voltage
30
Q

In a synchronous alternator, synchronous reactance results in a terminal volt drop

What are the other two items that are combined into the quantity called synchronous reactance?

A
  1. Armature inductive reactance
  2. Armature reaction
31
Q

The voltage induced in an AC generator depends upon the field flux and the speed of the rotor

True or False?

A

True

32
Q

What does a synchroscope indicate when used to parallel alternators?

A

The relative frequency and phase position of the two alternators

33
Q

When paralleling alternators, what must be done when the frequency of the voltage of an incoming alternator needs to be increased?

A

The rotor rotational speed of the incoming alternator must be increased

34
Q

What must be done to reduce the reactive circulating current of paralleled alternators?

A

The field excitation currents must be adjusted to keep the power factor of the alternators the same

35
Q

What formula is used to find the rated lined current of a three-phase alternator?

A

S = 1.732 x VLINE x ILINE

  • S = VA
  • 1.732 = the square root of 3
36
Q

Every rotating alternator has mechanical losses, what is included in mechanical losses?

A
  1. Bearing Friction
  2. Friction between the brush and the slip rings
  3. Windage friction of the rotor
  4. The power required to drive the built-in cooling fan

These losses are supplied by the Prime Mover that attached to the alternator when any mechanical to electrical energy conversion takes place

37
Q

In an alternator, the brushes are stationary

True or False?

A

True

38
Q

High speed rotors are called?

A

Cylindrical rotors

39
Q

A rotor with projecting poles is called?

A

A salient pole rotor

40
Q

A 60Hz, four-pole alternator operates at what r/min?

A

1800 r/min

41
Q

A 300 r/min, 60Hz alternator has how many poles?

A

24 poles

42
Q

A six-pole alternator rotating at 1200r/min generates a frequency of?

A

60Hz

43
Q

Once an alternator is constructed, the amount of voltage that is generated in the armature of a rotating field alternator can be practically varied by changing the…?

A

The amount of flux produced by the field

44
Q

Using a stationary solid-state rectifier to supply current to an alternators field is an example of which type of excitation system?

A

Separately excited system

45
Q

A relatively small shunt or compound DC generator mounted on the same alternator shaft that supports the main alternator rotor and field windings is an example of what type of system?

A

A self-excited system

46
Q

In a brushless excitation system, the exciter output is connected to the alternator field via…?

A

Diodes

47
Q

What do Voltage Regulators maintain at a constant value?

A

Alternator output voltage

48
Q

Synchronous reactance is equal to armature reactance plus armature reaction

True or False?

A

True

49
Q

VR(Arm) is always in-phase with IArm

True or False?

A

True

50
Q

What is the percent voltage regulation at unity power factor, of a 240V, single-phase alternator if VNL = 269.3V and VFL = 240V?

A

[(VNL - VFL) / VFL ] x 100

= 12.2%

51
Q

If the load on an alternator is increased by a factor of two, how would this affect the alternator’s copper losses?

A

Copper losses will increases by a factor of four

Copper Losses = I2R

52
Q

If the load on an alternator is reduced by a factor of two, how would this affect its core losses?

A

They will remain fixed

  • Iron losses/core losses occur within the iron of the alternator and they are not affected by the changing of load
  • The two main groups are:
    • Eddy Currents - these flow within the iron of any changing magnetic circuit, these produce a heat loss in the core. To reduce Eddy Currents laminations are used in the alloy of the alternator to increase the overral resistance of the current path
    • Hysteresis - occur because the iron used in the alternator is not fully permeable, as the iron is forced to become magnetized to opposite polarities from an AC source every 1/120 second, some power is used to overcome the residual magnetism. To reduce Hysteresis losses, allows containing iron, such as silicon are use in the core materials of the alternators
53
Q

When comparing the location of the armature windings of a DC generator to the most common type of AC alternator, they are on the rotor of the DC machine and on the stator of the AC machine

True or False?

A

True

54
Q

In an alternator, how do salient pole rotors compare to cylindrical rotors?

A

Salient Pole rotors are shorter in length and larger in diameter

55
Q

What are the two quantities combined into the quantity called Synchronous Reactance?

A
  1. Armature Inductive Reactance
  2. Armature Reaction
56
Q

In a large self-excited AC alternator, what current does the voltage regulator directly control?

A

The exciter field current

57
Q

When there is no load on an alternator, the generated voltage is the same as the terminal voltage

True or False?

A

True

58
Q

Of a running alternator, what are the two factors that can be altered that would affect the value of the voltage generated in the armature windings?

A
  1. The amount of flux produced by the field winding
    1. Adjusting the voltage regulator or rheostat
    2. This controls the output voltage of the alternator
  2. The speed of the rotating field
    1. Speeding up or slowing down the prime mover
    2. This controls the frequency of the output voltage of the alternator
59
Q

Why does an alternator with a rotating field require slip rings and brushes?

A

To connect the DC source to the field windings

60
Q

What does a Synchroscope indicate when used to parallel AC alternators?

A

The relative frequency and phase position of the two alternators

61
Q

What conditions must be met before alternators can be safely paralleled?

A
  1. Same Voltage
  2. Same Phase Sequence
  3. Same Phase Position
  4. The frequency of the incoming alternator should be slightly greater than that of the running alternator
62
Q

Two three-phase alternators are to be paralleled

Using the three-dark lamp method, it is found that one lamp is on, one lamp is off, and one is dim

What is the cause of this situation?

A

Different Phase Sequence

63
Q

What would happen to an alternator once it is paralleled to a power system if the field excitation is increased?

A

The power factor of that alternator will change

64
Q

If one of the prime movers of two alternators running in parallel stops running, what will happen to the alternator driven by that prime mover?

A

The alternator will appear to be a motor load to the other alternator

65
Q
A

Electrician - Fourth Period (24 decks)

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