AC Generation Flashcards
AC Generation in Modern AC
115V 3 phase at 400Hz for distribution system
26V at 400Hz for some flight instruments
115/200V
Where Is AC commonly from in AC?
Engine
APU
Hydraulically
RAT (Emergency Only)
DC TO AC in AC Generation
Inverter
Advantages to AC
No loss of power and transfer over longer distance - AC Changed from one voltage to another via transformer
Simple and light weight construction than DC
Changed into DC using TRU
Constant Frequency AC Generation
Provide constant freq
Engine drive hydraulic pump which drives a hydraulic motor
Output of pump and motor depends on swash plate angle
What controls swash plate in constant freq CSDU
Controlled by a device called a speed governor
Speed governor controlled by load controller
AC Generator Frequency Wild
Direct driven by engine gearbox rpm variation
Used only for purely resistive components
Cannot connect 2 frequency gens in parallel
CSDU or IDG Disconnect protocol
Disconnect can only be operated when engines are running
Only reset by engineer on ground
Guarded switch
CSDU failure
Quill drive - weak link which break if damaged
Low oil P
Low oil level
High oil Temp
IDG
Integrated drive generator
CSDU and generator combined
- Generator stator can be cooled by oil rather than air
Smaller unit for same amount of power output
3 Phase AC Generator (Alternator)
More efficient way of generating electricity using 3 phase AC generator
Magnetic field on rotor
Conductors (armature) on stator
3 phase rectifier:
- Consists of 6 diodes
- Only 4 diodes use at one time
Capacitor used for output smoothing
Advantage of multi phase AC
Entire stator used
Motors run more smoothly
High power spreads the load across all 3, but current at each phase is lower than single
Generator Control Breaker or Generator Load Contractor role
Connects generator to generator busbar
CSDU/IDG Oil Leaks
Small engine oil leak could be a large CSDU or IDG oil leak
Permanent Magnet Generators
3 Phase ac Gen - Brushless
Provide emergency power
Made of hard iron rather than electromagnet
EG RAT system
Brushless AC Generator - no physical connection between rotor and static side
+ No need for slip rings and brushes
+ Reliable
+ High power to weight ratio
- PMG - Only used to start the main field when generator starts up
- Main exciter Normal Ops put voltage through coils
- Main generator
Also provides excitation voltage for generators and control output voltage -
What is a CSD
Constant Speed drive
Provide constant speed to rotor irrespective to engine speed
Within 5% of 400hz
Engine must be running to disconnect CSDU
CSDU Disconnecting principles
Not to be used on ground or when engines not running
CSD cannot be reset in flight needs to be reset by engineers
Physically separates CSD from engine preventing damage
Y star connection
Allows for unbalanced loads and can lose a phase without impacting other two
Output 115v at each line
A differential fault is…
Difference between generator output current and the bus bar input current
Suggesting earth fault between the two
Generators in parallel must…
Be the same voltage sync frequency and phases aligned
Then can connect to tie bus bar
Generator failure and bus bar
Generator control breaker will disconnect the generator from it bus
Frequency of current provided by alternator depends on
Rotation speed
Freq = (RPM x number of pairs) / 60
Frequency is defined as
The number of oscillations per second in hertz
Equation for gen output frequency
Rotor RPM x number of pole pairs
————————————————
60
How do you calculate pairs of poles
Frequency / RPM x 60
Generator Control Unit does what
Fault sensed by GSU and open circuit breaker - used in parallel bus systems for protection - connected to each to monitor all in phase
Frequency of an AC generator is dependent on
RPM of the rotor
Number of pair poles or phases
Hz = rpm x poles/60
Generator control unit protects against what
AC gen over/under voltage
Under frequency
Over speed
Excess current
Differential fault
Frequency Formula
RPM X Number of pole Paris
60
Reactance
Measured in ohms - form of resistance reaction to current flow - back EMF
Apparent Power
Measured in volt-amps
AC generators are rated in
KVA
Reactive power vs real power
Reactive power is wasted
Real power is what is being used
Frequency of an AC generator is dependent upon
RPM of the rotor
Number of phase windings in the stator
To increase the real load taken by paralleled ac generator
Generator drive torque is increased
PMG in rotating field AC generator
Referred to as the pilot exciter provides the voltage regulator with power to feed main exciter.
Feeds voltage to field rotor and generator can start producing power
PMG is no longer required after start up
Where would you find a frequency wild generator on what type of engine?
Turbo prop
IDG vs CSD
Integrated drive generator is a electrical generator which provides electricity for the aircraft. IDG provides electrical power for all the aircraft systems and consists of a CSD
CSD is part of an IDG
