Alternators Flashcards

1
Q

What are three examples of dc supplies in a separately excited ac alternator?

A
  • Battery banks
  • dc Generators
  • Solid state rectifiers
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2
Q

The current used to create the magnetic field is called __________ current.

A

Excitation

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3
Q

Describe two benefits of a rotating field alternator.

A
  • higher kVa

- easier to cool

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4
Q

List the two types of rotors used in alternators, and describe one benefit of each.

A

Salient pole

  • short in length
  • low speed

Cylindrical

  • windings imbedded case
  • high speed
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5
Q

The amount of voltage generated by each winding in a stator is proportional to the ________ of ______ in the winding.

A

Number of turns

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6
Q

In a three phase alternator, each phase winding must be physically placed _____ degrees apart.

A

120

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7
Q

What three factors determine the generated voltage of an alternator.

A
  • Number of armature turns
  • Speed of field
  • Flux density of field
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8
Q

What is the most common method of controlling the output voltage of an alternator.

A

Change flux density of field

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9
Q

The main field flux of an alternator can be changed by changing what three values.

A
  • number of turns in the field
  • type of core material
  • amount of current
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10
Q

What is the most common method of controlling the field flux of an alternator.

A

Changing the amount of current by adjusting the field rheostat

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11
Q

What two basic sources of field excitation are typically found in A/C alternators.

A
  • separately excited

- self excited

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12
Q

What type of excitation system would be the most reliable and easiest to maintain

A

Brushless

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13
Q

The purpose of a voltage regulator in an ac alternator is to

A

Maintain constant terminal voltage to the load

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14
Q

Synchronous reactance in an ac alternator is made up of what two factors

A

Winding reactance

Armature reaction

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15
Q

Voltage regulation is a percentage between what two values in an alternator?

A

The change between no load output and full load output voltage

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16
Q

An alternator has a voltage regulation of 0% when supplying full rates values, what type of load is most likely connected to this machine?

A

Leading; Capacitive

17
Q

What five (5) factors make up ac resistance?

A
Skin effect 
Hysteresis
Resistance 
Eddy Currents 
Di Electric 

SHRED

18
Q

What are the three examples of mechanical losses in an ac alternator?

A

Friction (bearing/brush and slip rings)
Windage
Power req’d to drive cooling fan

19
Q

List two types of iron losses and a method of reducing both

A

Eddy currents
- laminations

Hysteresis
- silicon steel

20
Q

Copper losses are due to what two factors?

A
  • current

- resistance

21
Q

What are two practical methods of reducing copper losses.

A

Cooling armature conductors with hydrogen gas or water

22
Q

Generally speaking, an alternator running at full load is (more/less) efficient than the same machine at half load.

A

More

23
Q

What are three practical reasons for paralleling alternators?

A
  • More kVa
  • Maintenance
  • Wider variety of loads
24
Q

If 2 alternators have the exact same ratings, what is the preferred method of load sharing?

A

Half on each

25
Q

Aspects of a rotating field

A
  • Field winding on the rotor
  • dc via brushes and slip rings
  • a rotating winding with stationary magnetic field
  • high output kVa
  • most common
  • easy to cool
26
Q

Aspects of rotating armature

A
  • field winding in the stator
  • armature winding in the rotor
  • large slip rings and brushes
  • hard to cool (I2R losses)
  • low output kVa
27
Q

Aspects of salient pole rotors

A
  • large
  • turns slow (1800 rpm or less)
  • short in length
28
Q

Aspects of cylindrical rotors

A
  • turns fast PM (3600 rpm)

- windings embedded in case

29
Q

What is AVR?

A

Auto Voltage Regulator

30
Q

Three types of losses

A

Mechanical
- bearings, brushes and slip rings, friction, windage, cooling fan

Iron losses
- eddy currents, hysteresis

Copper losses
- armature and field (I2R -> changes as load changes)

31
Q

In an alternator, the brushes are stationary

True or False?

A

True

32
Q

In an alternator, the slip rings are stationary

True or False?

A

False

33
Q

The excitation current that creates the magnetic field in an alternator is from an ac source

True or False?

A

False

34
Q

To establish relative motion between the armature and the field, an alternator requires a rotor and a stator.

True or False?

A

True