Large Aircraft Electrical Systems

What type of electrical system do large aircraft primarily use?

AC

- AC alternators common for lighter aircraft

- AC generators used when weight is less vital

- Some aircraft require DC (AC gets converted)

Why do large aircraft systems primarily use AC?

- Their high demand for electrical power

- Lots of wiring needed

- AC wiring (high voltage/low current) is smaller in diameter

Why is AC wiring better than DC wiring (aside from diameter)?

AC's higher voltage/lower current loses less voltage so better for longer wires (more potential to get to the end)

What are the components of a DC generator?

- Rotating conductor (armature)

- Stationary bar/electromagnet

- Split rings (pulsating DC) and brushes (commutator)

What are the components of an AC generator?

- Rotating conductor (armature)

- Stationary bar/electromagnet

- Slip rings (AC)

What are the components of an alternator?

- Stationary conductor (stator)

- Rotating bar/electromagnet (rotor)

- Slip rings

What defines AC and DC?

Brushes and rings

What is the downside of split and slip rings?

Arcing and energy losses

What is the main difference between an alternator and a generator?

Alternator load current comes from the stator (outside windings) while the generators load current comes from the armature (rotating part) and has to flow through the commutator

What is the process of a generator?

Generator -> AC wild -> CSD, GCU, IDG or VSCF -> AC/DC conditioned

How do you start an alternator?

- DC current energises rotor coil for magnetic field

- Provided by battery

What is a 3-phase alternator?

- 3 coils

- Individually produces single-phase AC

- Produces 120V AC across each phase (208V AC across two phases)

- Generally 400Hz

What is the voltage for large aircraft?

110V, 120V or 210V

How is a generator controlled?

Components for

- Current limitations

- Over voltage protection

- Prevention of reverse flow of current from battery through alternator/generator

How is a generator operated over a range of speeds?

Hydraulic or mechanical constant speed generator drive unit (CSGD) or variable speed constant frequency output (VSCF)

What is a Constant Speed Drive (CSD)?

- Has a design input speed

- At lower/higher input speeds, hydraulic link adds/subtracts speed

- Normally between the engine gearbox and generator

What is an Integrated Drive Generator (IDG)?

- Generator and CSD together

- Lighter

- Aka Integrated Constant Speed Drive (ICSD)

Can the CSD/IDG be disconnected in flight?

- Yes, by a switch

- If fault in generator/gearbox

- Can't be reset

What is a Variable Speed Constant Frequency System (VSCF)?

- No CSD

- Generator produces 'wild' frequency power

- Conditioned to give constant frequency AC

- Less moving parts

- Cyclo-converter system or DC link

What is a Generator Control Unit (GCU)?

- For generator to be starter

- Controls starter motor-generator

- Like VSCF by more functions

What is AC wild power?

- Power that has frequency variation

- Some electrical equipment can use it

What are the controls of a generator?

- Sensing if frequency drops (so current doesn't increase)

- Sensing voltage (so it stays within limits)

- Real and reactive load equalisation (for parallel generators otherwise they might overheat etc)

What is a rectifier?

- Converts AC to DC

- One or more semi-conductor diodes

- Bridge rectifier has diodes stopping current going one way

What is a transformer-rectifier?

- Rectifies AC to DC

- Reduces AC voltage to required level (28V)

What is an inverter?

Converts DC to AC

What is an inductor?

- Like resistor in AC circuit (changes voltage and current)

- In DC circuits, filters voltage fluctuations

- Stores energy in magnetic field and current flows through wires

- If a spike occurs, magnetic field cuts through itself and opposes change in current

What is the purpose and the three basic types of large aircraft AC power distribution systems?

Allows load sharing and redundancy

- Split-bus system (twins)

- Parallel system (three/four engine)

- Split/parallel system

What is a split-bus system?

Generator 1 and 2 each supply bus 1 and 2 respectively, is one of then fails, it is isolated by the generator breaker and a bus tie will close so that the other generator can feed both busses

What is a parallel system?

All generators are connected to a common bus and share the load equally. If a generator fails, it gets isolated and a bus tie breaker selected so the bus stays working

What is a compound system?

- Two RH alternators in parallel

- Two LH alternators in parallel

- LH and RH busses kept isolated by a split-system breaker

How is emergency power supplied?

- DC from battery inverted to AC

- Ram Air Turbine (RAT), wild frequency)

- APU (constant speed gas turbine)

- Pneumatic air system

- Hydraulic system

What is the ground power supply?

- Electrical power for starting

- Also for maintenance

- Battery cart

How does a 28V DC plug and socket work?

- Has two longer pins and one shorter pin

- Two pins must be fully engaged before the power turns on

- Shorter pin activates a solenoid to generate power so there is no sparking