electric systems Flashcards
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 (and higher current which requires heavier wires)
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 converted to 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?
Voltage
What does a parallel circuit boost, voltage or current?
Current
Are systems usually connected in series or parallel?
Parallel
What type and voltage are aircraft batteries typically?
12 or 24V, and lead acid or Nickel Cadmium (nicad)
What are the advantages and disadvantages of nicad batteries to lead acid
nicad is better for faster recharge, lighter, tougher, and don’t give off as much toxic gas, but are more expensive, and are more prone to thermal runaway.
What is a hot battery bus?
A bus that is always connected. Used for items such as the aircraft clock.
What is a bus tie?
Connects different electrical buses together
What is a split bus?
Only 1 generator can supply a bus at a time
What is a parallel bus?
multiple generators can supply one bus
What does a split system breaker (SSB) do?
Prevents paralleling (multiple generators connecting to same bus) with external devices. If it didn’t exist, damage would result because external devices wouldn’t be synchronised
What does a reverse current relay do?
In a parallel generator system, a weak generator may be ‘charged’ by the stronger generators, causing a reverse current flow. The reverse current relay prevents this from happening which prevents damage.
What is a diode?
An electric version of a check valve: Only lets current go in one direction
How are batteries typically charged?
Using the constant voltage reducing current method
What are the typical batteries found on aircraft?
12V or 24V lead acid or NiCad battery.
What is thermal runaway?
Overheating of the battery, causing the electrolytes to boil
In the event of a failure of the active sources of AC power generation in a large jet transport category aircraft, the component of the electrical system which supplies AC power is….
The static inverter
Batteries are often connected in parallel in an aircraft electrical system to….
Increase the total amp hour capacity
The function of a TRU in an aircraft electrical system is to….
Convert AC to DC at a lower voltage (115V to 28V)
The type of rectifiers normally used in large aircraft AC generators to provide DC for field excitation of the main generator are…
Full wave bridge rectifiers
When a generator field breaker trips, the associated generator….
excitation field is disconnected from the voltage regulator
An advantage of constant frequency alternators in an aircraft electrical system is….
load sharing (if frequencies are the same, you can distribute the load)
Which is more expensive, lead acid battery or NiCad?
NiCad
EMF is the same as…
Voltage
The function of a static inverter in an aircraft electrical system is to….
Convert DC to AD at a higher voltage (28V to 115V)
The component of an electrical system in which diodes would be found is a….
Rectifier (TRU)
If the constant speed drive is overheating, will it automatically disconnect?
No, the temperature needs to be monitored and disconnected if required
An advantage of a lead acid battery over a NICAD battery is…
Less chance of thermal runaway
