Aircraft Electrical Diagram

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.