AIRCRAFT POWER GENERATION, I

Most aircraft require some form of electrical power to operate

navigation-, taxi-, landing-, strobe lights, one or more com and

nav radio's, transponder, intercom and other electronic systems.

The electrical system consist of a battery and an alternator or

generator on older aircraft. All of this is connected through

several meters (kilometers in large aircraft) of wire.

All matter on earth is made up from molecules and they consist

of atoms. Atoms are made of electrons, protons and neutrons.

Electricity is about the flow of electrons attracted to protons and

repelled by other electrons.

Electricity

The most basic part of electricity is the electron, to be more

precise the free electron. Every atom consist of a nucleus of

protons and electrons in orbit around the nucleus, the electrons

are held in orbit by attraction of an equal and opposite charge of

the protons.

Conductors

A substance with a good number of free electrons is said to be a

good conductor and allowing electricity to flow through. Very

good conductors are: silver, copper and aluminum. Materials with

a few or no free electrons at all are insulators like: rubber,

ceramics and plastics. And these are used to separate

conductors from each other preventing a short circuit.

Current and electron flow

The electrical charge of an electron is negative and the proton is

positive. The battery has two connecting terminals: a plus pole

(protons) and a negative pole where the surplus of electrons are.

When a circuit is connected across the poles of a battery the

electrons will flow from the negative pole through the circuit to

the plus pole (attracted by the protons).

This continues until the chemical action in the battery, and the

production of free electrons, stops. The electrical charge on both

poles is then equal and the battery is said to be empty. Until a

charging current reverses the chemical process and 'reloads' the

battery.

The flow of electrons is from the negative terminal to the positive

terminal, but in general speaking terms the current flow is said to

be from the positive terminal to the negative terminal. This is a

result of a convention long ago when the workings of electrons

and protons was not understood properly.

Volts and Amps

The characteristics of electrical measurement are expressed in

volts (force) and amps (current). To have a flow (current) of

electrons, we need a force (volts) to push them around the

circuit. The higher this force, the higher the number of electrons

at the negative terminal and the higher the voltage. The amount

of electrons flowing through the circuit is called amperage

(amps) and is directly proportional with the voltage and inversely

proportional with the resistance in that circuit.

Ohms law

All conductors have an internal resistance, this depends on the

material they consist of (copper, alu) and the thickness, size and

length of that conductor. The relation between volt, current and

resistance is known as Ohms Law. Which states that one voltage

applied over a resistance of one ohm results in a current of one

amps, in formula: U = I x R . Where U is the voltage (V), I is the

current (A) and R is the resistance in Ohm (Ω).

This formula can be transformed to obtain the current if voltage

and resistance are known: I = U / R .

Power

Of course, when a current of 1 amp flows through a resistor of 1

Ohm the voltage is 1 V. But what about power? The formula to

calculate power is P (W) = U (V) x I (A) . Thus in our 1 Ohm

resistor with 1 V applied and a current of 1 A the power

generated is: 1 Watt (W). So with a 12 V battery over a 1 Ohm

resistor the current is 12 amps and the power dissipated is: 12 V

times 12 A = 144 W.

AC/DC

Batteries create a direct current/voltage, the current can only

flow in one direction. It will not change direction without us

changing a wire. The alternator in an aircraft generates an

alternating voltage and that is rectified by solid state devices

(diodes) thus creating direct current flow or voltage.

This DC voltage is then used to recharge the battery and to

supply power to the rest of the aircraft electrical system. Aircraft

with a FADEC driven engine must have a failsafe electrical

system by either using a standby battery or a second alternator

on the engine.