Aircraft Electrical Diagram

Aircraft Power Generation, II

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.

Magnetism

Magnetism enables certain materials (iron based) to attract (opposite poles) or repel (equal poles) each other. It is widely used in aircraft in applications as the compass, alternators/generators, starter motors, navigation and instruments.

Magnetic material consist of minute particles each having a north and a south pole and they are aligned in one direction. Non magnetized material have the same particles: they are only not aligned but in random order.
Around a magnet is a magnetic field, this can be shown with a compass (also a magnet wich aligns itself with the magnetic field of the earth). This magnetic field is called flux and can be 'guided' by soft iron. Soft iron has permeability (comparable with conductivity) which is some 1000 times higher than air, and this property is used in relays and motors.

Electricity generation

When a current flows through a wire, a small weak detectable magnetic field exists around that wire. If that wire is formed into a coil the resultant magnetic field is concentrated, the lines of magnetic force of the separate wires will all align.

The same principle works in reverse too: when a wire passes through a magnetic field a voltage is generated. Form that wire into a coil and rotate a magnet through it and an even higher alternating voltage is generated.

Other forms of generating electricity are: friction (static electricity), heat (thermocouple with two dissimilar metals), pressure (piezo crystals), light (photo/light sensitive cells) and chemical (battery).

Generators

In a generator the magnetic field is generated by a stationary permanent magnet and a coil is rotated within the field. Two sliprings are used to pickup the AC voltage. If a DC voltage is required the sliprings are replaced by a commutator. A commutator makes sure that the same polarity voltage is pickup by the brushes. It rectifies the alternating voltage.

In practice the permanent magnet is assisted by a field coil and this strenghtens the field of the permanent magnet, the generator is said to be self exciting. A drawback is that the aircraft engine RPM must be above 1200 for the generator to start charging the battery. During taxi and other low RPM activity the battery is the main power source.

Alternators

In contrary to the generator, an alternator uses a rotating magnetic field in a stationary coil to generate electricity. This rotating magnetic field can be supplied by a magnet but normally a coil is used and is therefore called an electromagnet. The ALT part of the main switch energizes the field coil of the alternator with power from the battery until the alternator comes online. The voltage generated is alternating and rectified internally by diodes to a DC voltage.
This illustrates that if the battery failes during the flight and the pilot switches the ALT switch off and on, the magnetic field will no longer exists and the alternator will not produce any power.

One of the advantages is that the alternator generates more electricity, even when the engine is idling and it even weighs less than the generator. There is no heavy magnet inside the alternator. Both will need a voltage regulator to keep their output constant at 13.8 V (28), current regulation is by design in the alternator but the generator needs an external one combined with reverse current flow protection.

Batteries

There are two types of batteries: primary and secondary cell. The primary can not be recharged where as the secondary can be. Primary cells are: zinc-carbon and alkaline type batteries. Secondary cells are: lead acid, nickel cadmium, nickel metal-hydride, lithium-ion, lithium-polymer. These are all rechargable.

Chemical processes

The principle of a lead acid battery is as follows: two dissimilar electrodes are placed in a electrolyte, they are all conductors. The chemicals react with the electrodes and electrons attract to the negative electrode and a shortage of electrons exists at the positive terminal and a voltage of 2 V is build up at each cell. Batteries are made up from 6 cells for a 12 V model.

Each cell in a NiCad battery has a voltage of 1.2 V, you will need 10 cells for a 12 V model. NiCads are based on a strong alkaline for their electrolyte.