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4th Year Electrical - Machines > Alternators > Flashcards

Flashcards in Alternators Deck (34)
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1

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

- Battery banks
- dc Generators
- Solid state rectifiers

2

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

Excitation

3

Describe two benefits of a rotating field alternator.

- higher kVa
- easier to cool

4

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

Salient pole
- short in length
- low speed

Cylindrical
- windings imbedded case
- high speed

5

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

Number of turns

6

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

120

7

What three factors determine the generated voltage of an alternator.

- Number of armature turns
- Speed of field
- Flux density of field

8

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

Change flux density of field

9

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

- number of turns in the field
- type of core material
- amount of current

10

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

Changing the amount of current by adjusting the field rheostat

11

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

- separately excited
- self excited

12

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

Brushless

13

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

Maintain constant terminal voltage to the load

14

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

Winding reactance
Armature reaction

15

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

The change between no load output and full load output voltage

16

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

Leading; Capacitive

17

What five (5) factors make up ac resistance?

Skin effect
Hysteresis
Resistance
Eddy Currents
Di Electric

SHRED

18

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

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

19

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

Eddy currents
- laminations

Hysteresis
- silicon steel

20

Copper losses are due to what two factors?

- current
- resistance

21

What are two practical methods of reducing copper losses.

Cooling armature conductors with hydrogen gas or water

22

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

More

23

What are three practical reasons for paralleling alternators?

- More kVa
- Maintenance
- Wider variety of loads

24

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

Half on each

25

Aspects of a rotating field

- 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

Aspects of rotating armature

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

27

Aspects of salient pole rotors

- large
- turns slow (1800 rpm or less)
- short in length

28

Aspects of cylindrical rotors

- turns fast PM (3600 rpm)
- windings embedded in case

29

What is AVR?

Auto Voltage Regulator

30

Three types of losses

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)