Alternators

Question 1

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

Answer 1

• Battery banks
• dc Generators
• Solid state rectifiers

Question 2

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

Answer 2

Excitation

Question 3

Describe two benefits of a rotating field alternator.

Answer 3

• higher kVa

- easier to cool

Question 4

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

Answer 4

Salient pole

• short in length
• low speed

Cylindrical

• windings imbedded case
• high speed

Question 5

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

Answer 5

Number of turns

Question 6

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

Answer 6

120

Question 7

What three factors determine the generated voltage of an alternator.

Answer 7

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

Question 8

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

Answer 8

Change flux density of field

Question 9

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

Answer 9

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

Question 10

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

Answer 10

Changing the amount of current by adjusting the field rheostat

Question 11

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

Answer 11

• separately excited

- self excited

Question 12

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

Answer 12

Brushless

Question 13

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

Answer 13

Maintain constant terminal voltage to the load

Question 14

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

Answer 14

Winding reactance

Armature reaction

Question 15

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

Answer 15

The change between no load output and full load output voltage

Question 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?

Answer 16

Leading; Capacitive

Question 17

What five (5) factors make up ac resistance?

Answer 17

Skin effect 
Hysteresis
Resistance 
Eddy Currents 
Di Electric 

SHRED

Question 18

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

Answer 18

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

Question 19

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

Answer 19

Eddy currents
- laminations

Hysteresis
- silicon steel

Question 20

Copper losses are due to what two factors?

Answer 20

• current

- resistance

Question 21

What are two practical methods of reducing copper losses.

Answer 21

Cooling armature conductors with hydrogen gas or water

Question 22

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

Answer 22

More

Question 23

What are three practical reasons for paralleling alternators?

Answer 23

• More kVa
• Maintenance
• Wider variety of loads

Question 24

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

Answer 24

Half on each

Question 25

Aspects of a rotating field

Answer 25

- 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

Question 26

Aspects of rotating armature

Answer 26

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

Question 27

Aspects of salient pole rotors

Answer 27

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

Question 28

Aspects of cylindrical rotors

Answer 28

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

Question 29

What is AVR?

Answer 29

Auto Voltage Regulator

Question 30

Three types of losses

Answer 30

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)

Question 31

In an alternator, the brushes are stationary True or False?

Answer 31

True

Question 32

In an alternator, the slip rings are stationary True or False?

Answer 32

False

Question 33

The excitation current that creates the magnetic field in an alternator is from an ac source True or False?

Answer 33

False

Question 34

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

Answer 34

True