aircraft power gene ratio 2

8/8/2019 Aircraft Power Gene Ratio 2

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AIRCRAFT POWERGENERATION, 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


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iron has permeability (comparable with conductivity) which is some 1000

times higher than air, and this property is used in relays and motors.

Temporary

Some materials are easily magnetized, they also loose this propertywith the same ease. Soft iron is known for this. Which makes them verysuitable for temporary magnets to be used in for example relays,solenoid switches and electro magnets.

Permanent

Other materials are not so easily magnetized (hard iron, steel) but whenthey are magnetized they will not loose that property that easy. Usableas a permanent magnet. You will need to hammer or heat them tomake them loose their magnetic field.

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.


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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


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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


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aircraft power gene ratio 2

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