DC Alternators/Generators Flashcards
DC Alternators Components
Diodes
Rotary electromagnet
Stator is the field coils
Rectifier unit
How do we increase power generation capability in alternator
3 pairs of coils for 3-phase AC
Benefits of DC Alternator
No commutator and brush system so less weight less maintenance
Reverse current protection in DC alternator
Provided by the diodes in rectifier unit
Voltage Control in DC Alternator
Depends on speed of rotor
Number of turns of wire
Strength of magnetic field
RPM increases root increase - voltage regulator adjusts voltage supply to electromagnet to reduce/increase strength as required
Field Current/Excitation Current
Refers to the current passed through the field to control the output via the voltage regulator
Difference Between DC Generator and Alternator
DC Gen has -
Rotating Armature
Stationery field
Split Ring commutator
Low Power/weight ratio
High Maintenance
Can self excite
DC Alternator -
Stationery Armature
Rotating field
AC/DC bridge rectifier unit
High power to weight ratio
Low Maintenance
Cannot self excite
Rectifier Bridges
Made of diodes
Pass in one direction
Block half the since curve of AC
AC to DC
Sensitive to Static E limit current flow
Disadvantage of both DC Alternator and generator
Low engine RPM low current to exciter winding = weak magnetic field reducing V output
Issues with loads/consumers consistent power
Advantages of DC Alternator over DC generator
Save weight
Smaller Parts
Less brush wear and arcing due to lower current
Higher power/weight ratio
Output voltage on DC generator and alternator vary with…
RPM of engine driving generator
Loads attached
Field strength
Number of windings on rotor and stator
Starter Generators Function?
Unit starts as motor supplied with current via field changeover relay
Engine running self sustaining speed motor circuit isolated from power
Functions as conventional DC generator and connected to bus bar
Typically 300a AT 28V
Voltage Control in DC Systems
Varying current through exciter field to vary strength of magnetic field
- Increase exciter current if output too low
- Decrease exciter current if output is too high
Reverse Current Cut Out Relay
If generator V falls below bus bar voltage reverse current could occur
Disconnects generator from busbar or battery when reverse current flows
Redundancy usually back up reverse current circuit breaker
DC Gen Series Wound
Has poor voltage regulation not used for fluctuating loads
DC Generators Shunt Wound
Field Windings parallel with armature/loads
Output V decrease with increasing load
Used in Light DC generators that need lots of initial power
DC Generator Compound Wound
Connected in series and parallel with armature
Expensive to make
Constant output with changes in load
What is a Bus Bar?
Low resistance conductor service as a distribution point for loads
Consumers connected in parallel to busbar
Essential Bus Bar Controls?
Compass/Radio/Flight controls/Flight instruments
Non Essential Bus Bar
Galley Services
Hot Bus Bar
Connected directly to the battery - hard wired cannot be switched off
Emergency lights etc
Battery Bus Bar
Connected to battery
Checks/Start up
Can be disconnected
What is a contractor
Mechanically latched solenoid
High voltage/high load
Connects generators to bus bars
Used as bus tie breakers
Armature runaway
Rotor runaway current and torque decrease to try and slow
Everything decreases
Rotor seizure
Everything increases - current/torque
DC Generator summary
Has a rotating armature
Stationary field
Split ring commutator for AC to DC
Generated current has to pass through brushes and split ring commutator (arching and sparking)
Low power to weight ratio
High maintenance
DC Alternator Summary
Rotating field magnetic
Stationary armature
AC to DC via a bridge rectifier unit
Only field current has to pass through brushes and slip rings
High power to weight ration
Voltage Regulator
RPM load changes so voltage fluctuation therefore voltage sensitive variable resistor varies to provide constant field coil current to maintain voltage
Resistors in series with shunt wound field coil
Starter Generator
Operate as starter motor to start engine using battery then operates as DC generator after start
+ Supplies 300A at 28V
+ Saves weight