021 02 Electrics & Electronics

The shaft receives mechanical work from either the accessory gear box of the engine/APU or via a v-ribbed belt.

The coil turns within a stationary magnetic field. By rotating the coil, voltage is induced in the coil (Lorentz Force).

By rotating the coil, alternating voltage is induced. Alternating voltage causes alternating current.

The brushes act as an electrical contact on the rotating slip rings (part of the coil) in order to transfer the current from the rotating coil to stationary wiring for further use.

A Commutator is used instead of slip rings: the commutator changes its connection to brushes when polarity changes. This process is called rectifiying (of the alternating current).

1. A rotating (permanent) magnet (within a stationary coil/loop)
2. A diode (semi-conductor aka "the check-valve of electrical applications")

The Full Wave Bridge rectifier uses 4 diodes, connected together in a “bridge” configuration, to rectify both half-cycles of a sine wave.

Without the excitation system the alternator would have no way of building its voltage as it starts to rotate, nor would not be able to regulate its voltage to the pre-set nominal level while running at its rated speed

1. RPM of generator/alternator
2. Number of loads which require electrical energy
3. (Variable) Magnetic Field Strength of the generator/alternator

Controlled elec. system  shedding

higher (due to the law of induction)

1. drops
2. RPM (of generator/alternator)

Ut = Rt + It, while:

• Ut remains constant,
• Rt decreases (in parallel circuit, sum of resistors smaller than single resistors),
• It increases

As It increases and internal resistance in generator remains the same, the voltage drop over the internal resistor increases, thus the voltage drop over the generator (in series with the inernal resistance) decreases.

By adjusting the resistor (which is in parallel with the generator and in series with the field winding), more or less current will flow through the field winding, which in return controls the excitment of the generator.

Switching type (relay or transistor), which starts/stops current through field winding.

A plane builds up static electrical charge during the flight by friction between the air and the surfaces of the plane.
--> Static dischargers reduce the build up of static electricity by dispensing the charge into the atmosphere.

ACFT is statically charged due to...

1. friction between the air and the ACFT surfaces - in-flight
2. friction between the fuel and the fuel pumps, pipes and filters it runs through causes positive charged fuel ions to enter the fuel tank

--> ACFT parts are connected by flexible wire strips to distribute the static charge equally across the whole ACFT to keep it at similar potential.
--> On ground ACFT is grounded to dispose of the static charge

= difference of potential
--> On one side there is a surplus of electrons (=Minuspol), on the other side there is a shortage of electrons (=Pluspol)

= electrons per time

Physical current = Movement of electrons from Minuspol to Pluspol
Technical current = Movement of electrons from Pluspol to Minuspol

DC = Direct current --> For low voltage consumers, e.g. PFD
AC = Alternating current --> For high voltage consumers, e.g. generators/motors

Every conductor has a maximum allowable current. --> Depends on the material and cross section

= obstruction of the flow of electrons

Depends on...

• Specific resistance of the material \(\rho\) (Depends on the number of free electrons)
• Lengt L of the conductor (Longer = Higher resistance)
• Cross section A of the conductor (Larger = Lower resistance)
• Temperature
  • Metalls: High temperature = higher resistance (and vice-versa)
  • Semi-conductors: High temperature = lower resistance (and vice-versa)

W = U * I * t [kWh]

P = W / t = U * I [W]

Fuses protect the circuit from overcurrent conditions.

--> A strip of metal melts at a specified current [A] and opens the circuit

Case material: Glass / Ceramic
Element material: Tin, Lead, Silver, Copper, Tin bismuth alloy, Copper silver alloy

Similar to fuse but withstands a considerable overload for a short period of time.

Element material = Copper

Break the circuit to stop the current when predetermined value is exceeded.
--> CBs can be reset, fuse and current limiter must be replaced

Non Trip Free CB: Can be held in when tripped --> Closed circuit despite overload current
Trip Free CB: Cannot be held in when tripped

Push Type CB: After bimetallic strip has cooled, it can be reset
Push Pull & Toggle Switch Type CB: Can be pulled manually

Electrical storage device serving as a back-up to provide electrical energy by a chemical process.

Are charged with DC voltage --> Charge unit must have a higher voltage than the battery.
 

Consists of two conductor elements separated by an electrolyte through which electrons flow from one element (Cathode) to the other (Anode).

Primary cell: Non-rechargeable elements (Batteries)

Secondary cell: Rechargeable (Akku)

Elements: Lead Peroxid --> Lead

Electrolyte: Water (70%) & Sulphuric Acid (30%)

Cell voltage: 2.0 - 2.1 V
--> min. 12 cells in series required to supply the standard 24V

Elements: Nickel Oxyde --> Cadmium

Electrolyte: Potatssium Hydroxide (KOH)

Cell voltage: 1.2 V
--> min. 20 cells required for the standard 24V