AC Flashcards
AC SYSTEM ON AN AIRCRAFT
115V - 400Hz - 3 separate phases
AC has extra resistance but it’s known as
Reactance measured in ohms
If the Hz drops then
The resistance drops
On the aircraft to ensure we have constant frequency we have
Generator RPM - controlled by the CSU
Voltage is determined by
Voltage regulator
Split Bus system
No two AC sources are on the same bus bar
PARALLEL BUS SYSTEM
Normally all generators are on all busbars
When a fault is sensed
The GCU automatically opens the GCB
GCU protects
Ac generator over and under voltage
Excess current
Under frequency
Over frequency
Internal short
When operating the fire handle
We never touch the bus tie breaker
To ensure generators are sharing the same load
They must have the same voltage and frequency and current - test by using a current transformer
Correct Real load shedding - parallel systems
Make sure the torque is the same
Correct reactive load shedding - parallel systems
We must make sure the voltage is the same
AC in aircraft
400Hz
Frequency formula
Number of poles /2 * RPM/60seconds
If there’s a difference between generator output current and bus bar input current
There is a differential fault
When the engine fire shut off handle is pulled then
The exicter control relay and generator breaker open
hot buses and direct bus bars
Directly connected to the battery
Capatance
Is the ability of the circuit to hold charge
Starter generator is used to
Start the core engine until reaching self sustaining speed
Rotating armature ac generators
Rotating coils are heavy
Coils are difficult to cool
Resistance across the brushes to the slip ring is high
Stationary armature generators (AC)
Stator windings are connected in series
Max output depends the amount of heat that can be absorbed by armature
Rated in Va or kVa
AC generator have ___ phases usually
3 (3 stator armatures)
3 phase spacings are x degrees
120
3 phase system advantages
Simple and efficient
Easier to connect in parallel
Greater power to weight ratio
2 phase ac generator is known as
Line voltage
Brushed ac generator
Initial excitation current is supplied by the battery
Brushless generator has
A main generator and an exciter generator on the same shaft
CSD keeps
Generator speed constant regardless of engine speed
If CSU displays
High temp and low oil then it should be disconnected
If a bus bar short circuits
It’s isolated and consumers are lost
Isolation of a bus bar
Decreases its consumption
