Alternators Flashcards
What are three examples of dc supplies in a separately excited ac alternator?
- Battery banks
- dc Generators
- Solid state rectifiers
The current used to create the magnetic field is called __________ current.
Excitation
Describe two benefits of a rotating field alternator.
- higher kVa
- easier to cool
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
The amount of voltage generated by each winding in a stator is proportional to the ________ of ______ in the winding.
Number of turns
In a three phase alternator, each phase winding must be physically placed _____ degrees apart.
120
What three factors determine the generated voltage of an alternator.
- Number of armature turns
- Speed of field
- Flux density of field
What is the most common method of controlling the output voltage of an alternator.
Change flux density of field
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
What is the most common method of controlling the field flux of an alternator.
Changing the amount of current by adjusting the field rheostat
What two basic sources of field excitation are typically found in A/C alternators.
- separately excited
- self excited
What type of excitation system would be the most reliable and easiest to maintain
Brushless
The purpose of a voltage regulator in an ac alternator is to
Maintain constant terminal voltage to the load
Synchronous reactance in an ac alternator is made up of what two factors
Winding reactance
Armature reaction
Voltage regulation is a percentage between what two values in an alternator?
The change between no load output and full load output voltage
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
What five (5) factors make up ac resistance?
Skin effect Hysteresis Resistance Eddy Currents Di Electric
SHRED
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
List two types of iron losses and a method of reducing both
Eddy currents
- laminations
Hysteresis
- silicon steel
Copper losses are due to what two factors?
- current
- resistance
What are two practical methods of reducing copper losses.
Cooling armature conductors with hydrogen gas or water
Generally speaking, an alternator running at full load is (more/less) efficient than the same machine at half load.
More
What are three practical reasons for paralleling alternators?
- More kVa
- Maintenance
- Wider variety of loads
If 2 alternators have the exact same ratings, what is the preferred method of load sharing?
Half on each
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
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
Aspects of salient pole rotors
- large
- turns slow (1800 rpm or less)
- short in length
Aspects of cylindrical rotors
- turns fast PM (3600 rpm)
- windings embedded in case
What is AVR?
Auto Voltage Regulator
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)
In an alternator, the brushes are stationary
True or False?
True
In an alternator, the slip rings are stationary
True or False?
False
The excitation current that creates the magnetic field in an alternator is from an ac source
True or False?
False
To establish relative motion between the armature and the field, an alternator requires a rotor and a stator.
True or False?
True
