Flashcards in electric systems Deck (27):
In flight and after engine start, what form of electrical power is delivered?
3 phase alternating current (115V 400Hz)
What does the APU do in regards to electrical supply?
The turbine powers an AC generator, and bleed air powers pneumatics (usually used for air-start of main engines)
Where does the APU get power from to start?
A DC battery, which can also be used by other systems in the event of a serious emergency
Why is DC seen as inferior to AC in aircraft?
DC typically requires heavier equipment, and per unit size is not as efficient as AC, it is also lower voltage .
What is a series wound generator?
generator field in series, as load increases, voltage increases
What is a shunt wound generator?
Generator in parallel, as load increases, voltage decreases
What is a compound wound generator?
Both series and parallel (both series and shunt), and it maintains a constant voltage for varying loads.
What is the most common type of AC generator?
A compound wound generator (because it provides a constant voltage for a range of loads)
What is meant by the term "aircraft generators are self-excited"?
They produce their own magnetic field once the circuit is complete (once they're switched on).
Once the Constant Speed Drive is disconnected, it can only be reconnected....
on the ground
total power (Kva) =
kW (real power e.g. lights and heaters) + kVAR (reactive power e.g. transformers and motors)
How many generators does a twin engine transport aircraft generally have, and how many generators need to be lost before heavy loads need to be shed?
Typically have 4, and loads need to be shed if 2 are lost, unless APU can provide sufficient backup.
If a generator fails, how will power from other generators be supplied to that failed generator's bus?
through a bus tie
AC generators can be split bus or parallel bus, explain the terms.
split bus is where only one generator supplies a bus at any one time, and parallel is where multiple generators may be connected to the same bus.
What is the purpose of the sycnhronising bus?
Ensures that generator voltage and frequency outputsare in phase.
In monitoring the electrical system, what values are usually monitored?
Most of the time it is kw, but to check on the status of the voltage regulator, Kvar can be checked as well.
What is the purpose of a Transformer Rectifier Unit (TRU)?
transforms 115V AC to 28V AC, and then convertedto 28V DC through Diodes.
Where do DC buses get their power from?
The AC generators but THROUGH TRUs and Diodes.
What does the generator field switch do?
Automatically trips (opens) the circuit when a fault exists
What are trip free circuit breakers?
Circuit breakers which are always free to trip, and cannot be overridden
What is a rectifier?
Diodes which convert AC to DC
What is a static inverter?
Converts 28V DC to 115V AC trhough use of transistors.
What does a series circuit boost, voltage or current?
What does a parallel circuit boost, voltage or current?
Are systems usually connected in series or parallel?
What type and voltage are aircraft batteries typically?
12 or 24V, and lead acid or Nickel Cadmium (nicad)