EASA Part 66 - Module 3 - Electrical Fundamentals - Part A

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JAR 66 CATEGORY B1 MODULE 3 (part A) ELECTRICAL FUNDAMENTALS

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Index

1 ATOMIC STRUCTURE ... 1-1 1.1 MATTER ... 1-1 1.1.1 States of Matter ... 1-1 1.1.2 Chemical classification of matter ... 1-1 1.1.3 Atomic classification of matter ... 1-1 1.2 MOLECULES ... 1-2

1.3 ATOMS ... 1-2

1.3.1 The Structure of an Atom ... 1-3 1.3.2 The Fundamental Particles ... 1-3 1.3.3 Particle function ... 1-4 1.3.4 ions ... 1-5 1.4 ELECTRICAL MATERIALS ... 1-5 1.4.1 Electron distribution ... 1-6 1.4.2 Ionisation ... 1-7 1.4.3 Energy levels ... 1-7 1.4.4 Conductors ... 1-7 1.4.5 Insulators ... 1-7 1.4.6 Semi-conductors ... 1-7 2 STATIC ELECTRICITY ... 2-1 2.1 ATTRACTION & REPULSION ... 2-3

2.2 UNIT OF CHARGE ... 2-3 2.3 STATIC ELECTRICITY & AIRCRAFT ... 2-3 3 ELECTRICAL TERMINOLOGY ... 3-1 3.1 VOLTAGE ... 3-1 3.1.1 Potential ... 3-1 3.1.2 Potential Difference ... 3-1 3.1.3 Electromotive Force – emf ... 3-2 3.2 CURRENT ... 3-2

3.2.1 Movement of charge ... 3-2 3.2.2 Conventional flow ... 3-3 3.2.3 Electron flow ... 3-3 3.3 RESISTANCE ... 3-3

3.3.1 Factors affecting resistance ... 3-4 3.3.2 Units of resistance ... 3-4 3.4 CONDUCTANCE AND CONDUCTIVITY ... 3-4 4 PRODUCTION OF ELECTRICITY ... 4-1 4.1 BY FRICTION ... 4-1

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4.3 BY MAGNETISM ... 4-2 4.4 BY HEAT ... 4-2 4.5 BY LIGHT ... 4-3 4.6 BY CHEMICAL ACTION ... 4-3 5 CELLS & BATTERIES ... 5-1 5.1 PRINCIPLES ... 5-1

5.1.1 Cell & Battery symbols ... 5-1 5.1.2 Construction & chemical action ... 5-1 5.1.3 Primary & secondary cells ... 5-2 5.1.4 Cell emf ... 5-2 5.1.5 Cell capacity ... 5-3 5.1.6 Interconnection of cells ... 5-3 5.2 LEAD ACID BATTERIES ... 5-4

5.2.1 Conventional construction ... 5-4 5.2.2 Solid block type construction ... 5-5 5.2.3 Chemical action ... 5-6 5.2.4 Voltage & Specific Gravity characteristics ... 5-7 5.2.5 Common lead acid battery faults ... 5-7 5.3 NICKEL CADMIUM BATTERIES ... 5-8

5.3.1 Construction ... 5-8 5.3.2 Chemical action ... 5-9 5.3.3 Advantages & disadvantages ... 5-10 5.3.4 Thermal runaway ... 5-11 5.4 SMALL ALKALINE CELLS ... 5-11 6 OHM’S LAW ... 6-1

6.1 TRANSPOSITION OF OHM’S LAW ... 6-1

6.2 THE OHM’S LAW TRIANGLE ... 6-2 7 ELECTRICAL MEASURING INSTRUMENTS ... 7-1 7.1 CONNECTING METERS TO A CIRCUIT ... 7-1

7.1.1 Voltmeters ... 7-1 7.1.2 Ammeters ... 7-2 7.1.3 Ohmmeters ... 7-2 7.2 ANALOGUE MULTIMETERS ... 7-3 7.2.1 DC voltage measurements ... 7-4 7.2.2 DC current measurements ... 7-5 7.2.3 DC high-current measurement ... 7-6 7.2.4 AC voltage measurements ... 7-7 7.2.5 Resistance measurements ... 7-7

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7.3 DIGITAL MULTIMETERS ... 7-12 7.3.1 DC voltage measurements ... 7-13 7.3.2 DC current measurements ... 7-13 7.3.3 High current measurements ... 7-14 7.3.4 AC voltage measurements ... 7-14 7.3.5 Resistance measurements ... 7-15 7.3.6 Capacitor measurements ... 7-15 7.3.7 Continuity testing ... 7-16 7.3.8 DO's & DON'Ts of using a digital multimeter ... 7-16 8 RESISTANCE & RESISTORS ... 8-1

8.1 RESISTIVITY ... 8-1

8.2 CHANGES OF RESISTANCE WITH TEMPERATURE ... 8-1

8.3 TEMPERATURE CO-EFFICIENT OF RESISTANCE ... 8-2

8.4 RESISTORS ... 8-3 8.4.1 Fixed resistors ... 8-3 8.4.2 Colour codes ... 8-4 8.4.3 Preferred values and tolerances ... 8-5 8.4.4 Letter & digit codes ... 8-6 8.4.5 Power rating ... 8-6 8.4.6 Potentiometers ... 8-7 8.4.7 Rheostats ... 8-7 8.4.8 Voltage Dependent Resistors ... 8-7 8.5 THERMISTORS ... 8-7 9 RESISTORS IN DC CIRCUITS ... 9-1 9.1 RESISTORS IN SERIES ... 9-1

9.1.1 Kirchoff’s Second Law... 9-2 9.1.2 Voltage division ... 9-3 9.1.3 The Potential Divider ... 9-3 9.1.4 Voltages relative to Earth ... 9-4 9.2 INTERNAL RESISTANCE ... 9-4

9.3 RESISTORS IN PARALLEL ... 9-6

9.3.1 Two resistors in parallel ... 9-7 9.3.2 Equal resistors connected in parallel ... 9-7 9.3.3 Effective value of resistors in parallel ... 9-8 9.3.4 Resistor size and current flow ... 9-8 9.3.5 Kirchoff’s First Law ... 9-8 9.4 RESISTORS IN SERIES / PARALLEL COMBINATIONS ... 9-9

9.4.1 Physical arrangement of resistors ... 9-9 9.4.2 Solution of resistor networks using Ohm’s Law ... 9-9 9.5 THE EFFECTS OF OPEN CIRCUITS... 9-11 9.6 THE EFFECTS OF SHORT CIRCUITS ... 9-12

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10 THE WHEATSTONE BRIDGE ... 10-1 10.1 CONSTRUCTION ... 10-1 10.2 CALCULATING UNKNOWN RESISTANCES ... 10-1

10.3 USES ON AIRCRAFT ... 10-2 11 ENERGY & POWER IN DC CIRCUITS ... 11-1 11.1 ELECTRICAL WORK ... 11-1

11.2 ELECTRICAL ENERGY ... 11-1

11.3 ELECTRICAL POWER ... 11-2

11.4 POWER RATINGS ... 11-2 11.4.1 Power ratings of resistors ... 11-3 11.4.2 Size and power rating ... 11-3 11.4.3 The Kilowatt Hour ... 11-3 11.5 MAXIMUM POWER TRANSFER ... 11-4 12 CAPACITANCE & CAPACITORS ... 12-1 12.1 CHARGING A BODY ... 12-1 12.2 THE BASIC CAPACITOR ... 12-2

12.3 CAPACITANCE ... 12-2

12.4 FACTORS AFFECTING CAPACITANCE ... 12-3

12.5 ENERGY STORED IN A CAPACITOR ... 12-4 12.6 CAPACITOR CONSTRUCTION ... 12-4 12.6.1 Fixed capacitors ... 12-4 12.6.2 Variable capacitors ... 12-4 12.6.3 Electrolytic capacitors ... 12-4 12.6.4 Safe working voltage ... 12-4 12.7 CAPACITOR SYMBOLS ... 12-6 13 CAPACITORS IN DC CIRCUITS ... 13-1 13.1 CAPACITORS IN SERIES ... 13-1 13.2 CAPACITORS IN PARALLEL ... 13-2

13.3 CAPACITORS IN SERIES / PARALLEL COMBINATIONS ... 13-3

13.4 CHARGE & DISCHARGE CHARACTERISTICS ... 13-3

13.4.1 Charging a capacitor ... 13-3 13.4.2 Time Constant ... 13-4 13.4.3 Discharging a capacitor... 13-5 13.4.4 A capacitor in a dc circuit ... 13-6 13.5 THE EFFECTS OF OPEN & SHORT CIRCUITS ... 13-6

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14.1 MAGNETIC THEORIES ... 14-1 14.1.1 Molecular Theory ... 14-1 14.1.2 Domain Theory ... 14-1 14.2 MAGNETIC PROPERTIES ... 14-2 14.2.1 Magnetic poles ... 14-2 14.2.2 Magnetic field ... 14-2 14.2.3 Lines of flux ... 14-3 14.3 THE EARTH’S FIELD ... 14-4

14.4 MAGNETIC MATERIALS ... 14-4 14.4.1 Ferromagnetic materials ... 14-4 14.4.2 Paramagnetic materials ... 14-5 14.4.3 Diamagnetic materials... 14-5 14.5 PRODUCTION OF A MAGNET ... 14-5 14.5.1 Stroke method ... 14-5 14.5.2 Induction ... 14-6 14.5.3 Use of electrical current ... 14-7 15 ELECTROMAGNETISM ... 15-1 15.1 PRODUCTION OF A BAR MAGNET ... 15-1

15.1.1 End Rule ... 15-2 15.1.2 Right Hand Gripping Rule ... 15-2 15.2 THE MAGNETIC CIRCUIT ... 15-2

15.2.1 Magnetomotive force (mmf) ... 15-2 15.2.2 Magnetising force ... 15-2 15.2.3 Flux & Flux density ... 15-3 15.2.4 Permeability ... 15-3 15.2.5 Reluctance ... 15-4 15.2.6 Composite paths and airgaps ... 15-4 15.3 BH CURVE ... 15-5

15.4 HYSTERESIS LOOP ... 15-5

15.5 COMPARISON OF ELECTRICAL & MAGNETIC CIRCUITS ... 15-7

15.6 MAGNETIC SCREENING ... 15-8 16 INDUCTION ... 16-1 16.1 ELECTRICITY FROM MAGNETISM ... 16-1

16.1.1 Factors affecting induced emf ... 16-1 16.1.2 Faradays Law ... 16-2 16.1.3 Lenz’s Law ... 16-2 16.1.4 Flemings Right Hand Rule ... 16-3 16.2 SELF INDUCTANCE ... 16-3

16.3 MUTUAL INDUCTANCE ... 16-4 16.4 COUPLING FACTOR ... 16-5

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16.5.1 Spark suppression ... 16-5 17 INDUCTORS ... 17-1 17.1 CONSTRUCTION ... 17-1 17.2 INDUCTOR SYMBOLS ... 17-2 18 INDUCTORS IN DC CIRCUITS ... 18-1 18.1 INDUCTORS IN SERIES ... 18-1 18.2 INDUCTORS IN PARALLEL ... 18-1 18.3 INDUCTORS IN A DC CIRCUIT ... 18-2

18.3.1 When dc current is applied ... 18-2 18.3.2 Time constant ... 18-3 18.3.3 The Effects of back emf on circuit current ... 18-4 18.3.4 When dc current is removed ... 18-5 18.3.5 Safety ... 18-6 19 CIRCUIT SYMBOLS ... 19-1

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1 ATOMIC STRUCTURE

1.1 MATTER

Matter is defined as anything that occupies space and may be classified in a number of ways.

1.1.1 STATES OF MATTER

There are three normal states of matter:

Solid. A solid has definite mass, volume and shape.

Liquid. A liquid has definite mass and volume but takes the shape of its container.

Gas. A gas has definite mass but takes the volume and shape of its container.

1.1.2 CHEMICAL CLASSIFICATION OF MATTER

From a chemical view we again have three divisions:

Elements. An element is a substance which cannot by any known chemical process be split into two or more chemically simpler substances.

Eg: Hydrogen; Oxygen; Copper; Iron; Aluminium; carbon.

Compounds. A compound is a substance which contains two or more elements chemically joined together.

Eg: Water (Hydrogen and Oxygen); Salt (Sodium and Chlorine); Sulphuric Acid (Hydrogen, Oxygen and Sulphur).

Mixtures. A mixture consists of elements or compounds which are brought together by a physical process.

Eg: Salt and Sand; Earth and Sawdust; Carbon and Iron Filings.

1.1.3 ATOMIC CLASSIFICATION OF MATTER

Material may also be classified according to the particles it contains, this is the atomic view of matter. This view gives us a better understanding of electrical and electronic phenomena and is the view we shall concentrate upon.

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1.2 MOLECULES

Let us take a piece of matter, for example, a drop of water and see what happens when it is sub-divided into smaller and smaller portions.

The drop is first cut in half, each half drop-let halved and so on indefinitely. The resulting smaller and smaller droplets will soon become invisible to the naked eye, but it is known what happens if the process could be carried far enough; a point would eventually be reached where the particles of water are of such a size that further sub-division would split them into the hydrogen and oxygen of which they are composed. These last minute particles of water are known as molecules and are the smallest particles of water which can exist alone and still behave chemically as water.

Every material is built-up from molecules and there are as many different molecules as there are different substances in existence.

Molecules. The molecule of an element or compound is the smallest particle of it which can normally exist separately. It consists of one or more atoms, of the same or different types joined together. The term ‘molecular structure’ is used when compounds are discussed.

1.3 ATOMS

If a water molecule could be magnified sufficiently it would be seen to consist of three smaller particles closely bound together. These three particles are ATOMS, two of hydrogen and one of oxygen.

The water is a compound, the oxygen and hydrogen are elements. Every element has atoms of its own type. There are 92 naturally occurring elements and therefore 92 types of naturally occurring atoms.

Every molecule consists of atoms. Molecules of elements contain atoms of the same types, for example the hydrogen molecule consists of two atoms of

hydrogen joined together, the oxygen molecule consists of two atoms of oxygen joined together, but the molecules of compound contain different atoms joined together.

Most molecules contain more than one atom but some elements can exist as single atoms. In such a case the atom is also the molecule. For example the Helium atom is also the Helium molecule.

An atom is the smallest indivisible particle of an element which can take part in a chemical change. The term ‘atomic structure’ is use when talking about

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1.3.1 THE STRUCTURE OF AN ATOM

The Nucleus and Electrons. Atoms themselves are also composed of even smaller particles. Let us take an atom of hydrogen as an example. A hydrogen atom is very small indeed (about 10 –10 in diameter), but if it could be magnified sufficiently it would be ‘seen’ to consist of a core or nucleus with a particle called an electron travelling around it in an elliptical orbit.

The nucleus has a positive charge of electricity and the electron an equal negative charge; thus the whole atom is electrically neutral and the electrical attraction keeps the electron circling the nucleus. Atoms of other elements have more than one electron travelling around the nucleus, the nucleus containing sufficient positive charges to balance the number of electrons.

Protons and Neutrons. The particles in the nucleus carrying a positive charge are called protons. In addition to the protons the nucleus usually contains electrically neutral particles called neutrons. Neutrons have the same mass as protons, whereas electrons are very much smaller – only ₁₈₃₆1 of the mass of a proton

1.3.2 THE FUNDAMENTAL PARTICLES

Although other atomic particles are known, the three fundamental ones are: Protons. The proton has unit mass and carries a unit positive charge. Neutron. The neutron has unit mass but no electrical charge.

Electron. The electron has only ₁₈₃₆1 unit of mass but it carries a unit negative charge.

Thus, although we have 92 types of naturally occurring atoms, they are all built-up from different numbers of these three fundamental particles.

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Thus our picture of the structure of matter is as shown below.

1.3.3 PARTICLE FUNCTION 1.3.3.1 Protons

The number of protons in an atom determines the kind of material: Eg. Hydrogen 1 proton

Helium 2 protons Lithium 3 protons Beryllium 4 protons etc Copper 29 protons etc Uranium 92 protons

The number of protons is referred to a the atomic number, thus the atomic number of copper is 29.

1.3.3.2 Neutrons

The neutron simply adds to the weight of the nucleus and hence the atom. There is no simple rule for determining the number of neutrons in any atom. In fact atoms of the same kind can contain different numbers of neutrons. For example chlorine may contain 18 – 20 neutrons in its nucleus.

The atoms are chemically indistinguishable and are called isotopes. The weight

Material

Molecules

Hundreds of different kinds

Atoms

92 Natural types

Neutrons

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1.3.3.3 Electrons

The electron orbits define the size or volume occupied by the atom. The

electrons travel in orbits which are many times the diameter of the nucleus and hence the space occupied by an atom is virtually empty! The electrical properties of the atom are determined by how tightly the electrons are bound by electrical attraction to the nucleus.

1.3.4 IONS

A neutral atom contains an equal number of positive charges (protons) and negative charges (electrons). It is possible for an atom to gain or loose an electron.

An atom (or possibly a group of atoms) which loses an electron has lost one of its negative charges and is therefore left with an excess of one positive charge; it is called a positive ion. An atom that gains an electron has an excess of negative charge and is called a negative ion.

1.4 ELECTRICAL MATERIALS

Materials which allow an electric current to flow easily are known as conductors and those which prevent the flow of an appreciable current are known as

insulators. Conductors and insulators are used in electrical circuits to provide paths for and to control the flow of, electric current. Practically all normal

materials are either good conductors or good insulators. There are, however, a few materials which fall between these two categories and these are called semiconductors. Semiconductors will be studied in detail when we begin the electronics phase of the course.

The best electrical conductor is silver, but for most purposes its high cost is prohibitive so copper is the standard conductor material. Aluminium is an alternative, but it is not such a good conductor. Brass, which is harder than copper, is commonly used for terminals, switches etc. Tungsten and nickel are used in the construction of lamps and thermionic valves.

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1.4.1 ELECTRON DISTRIBUTION

The atoms of a solid have electrons rotating in orbits around the positive nucleus. This is true of gases and liquids as well. These orbiting electrons exist in energy shells or levels.

The potential energy (energy of position) increases with distance out from the nucleus. The outermost occupied energy level is called the valence shell. This is a higher energy level than the energy levels of electrons in the other shells since the electrons are rotating further from the nucleus.

The electrons in the valence shell can most easily pass from one atom to another and thus constitute an electric current. Furthermore, the valence electrons are the ones that go into chemical reactions, or combinations, with other atoms. When an outside influence such as an electric field or heat is applied, a valence electron may acquire sufficient energy to jump through a forbidden (energy) gap and on into the conductor band where it is free of any influence of the positive nucleus and becomes a carrier of electricity, ready to take the place of another electron that has just left its own atom, in the same manner.

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1.4.2 IONISATION

If the amount of external energy is large enough the valence electron can gain sufficient kinetic energy (energy of movement) to be removed completely from its atomic orbit and may not be replaced by another accelerated electron. This process is known as ionisation, since an atom which now contains one more proton than can be neutralised by the remaining electrons is a positive ion. Gas-filled devices such as Neon tubes make use of this process. In a solid where atoms are close together, simple ionisation does not occur as with individual items.

1.4.3 ENERGY LEVELS

The energy levels, measured in electron volts (e.v.) in which orbiting electrons exist comply with a law of physics which states that energy can be given to electrons only in discrete amounts (quanta) which means that there are energy values that an electron cannot acquire. From this it can be deducted that there is a forbidden energy gap between each of the allowed energy bands K to O.

The width of the forbidden energy gap between the top of the valence band and the bottom of the conduction band determine the electrical conducting properties of materials.

1.4.4 CONDUCTORS

Elements with 1 or 2 electrons in their outer orbits readily transfer them from atom to atom, because there is an overlap between the valence and conduction bands. Silver and copper elements are good conductors.

1.4.5 INSULATORS

Elements with 6 to 8 valence electrons cannot have electrons-in the conduction bands because the forbidden gap is to large. Sulphur and rubber elements are insulators.

1.4.6 SEMI-CONDUCTORS

The elements Germanium and Silicon have four electrons in their valence shells. In conductivity they lie between good conductors and good insulators, ie; they are semi-conductors.

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2 STATIC ELECTRICITY

If electrons are removed from one material and placed on another, or if they are moved from one region of a piece of material to another, we have a separation of charge. The material, or area, that receives the electrons becomes negatively charged and the material or region that loses electrons becomes positively charged. If these accumulations of charge remain stationary after their transfer, they are referred to as static electricity.

Common examples of static electricity are the small shock you get when you touch a door handle having walked across a carpet, or the crackling you hear when you remove certain items of clothing. In both cases electrons have moved from one material to the other. This type of static charging between two or more dissimilar materials is known as triboelectric charging and is a very important factor in the design of aircraft and aircraft furnishings and equipment.

The nature and size of the charge produced depends on the materials, some loose or gain electrons more easily than others. The Triboelectric series on the next page list materials in the order in which they gain or loose electrons. The list is arranged such that, if any two materials are selected and rubbed together the one higher up the list will obtain a positive charge and the one lower down the list, a negative charge. So if a glass rod is rubbed with fur, the rod will become

negatively charged, but if it is rubbed with nylon it will become positively charged. When an insulating material is charged by rubbing it with another material, the electrons are not free to move through the material. The charge therefore remains at the point of friction. If a conductor is charged through rubbing, the electrons are free to move and the charge will dissipate unless the conducting material is insulated from its surroundings.

If two statically charged items are brought into contact with one another, electrons will transfer from the more negative to the more positive one. This movement of electrons constitutes a current flow, which will cease once the charges are equal. The region around the charged body may be detected and is called an electric field, the electric field is analogous to a magnetic field, which will be studied later in the course. The electric field is represented in magnitude and direction by electric lines of force. The density or magnitude of the force may be represented by the number of lines, and the direction is indicated by arrows that point from positive to negative.

Isolated positive and negative charges

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Triboelectric Series

Air Increasingly Positive Human Skin Asbestos Rabbit Fur Glass Mica Human Hair Nylon Wool Fur Lead Silk Aluminium Paper Cotton Increasingly Negative Steel Wood Amber Sealing Wax Hard Rubber Nickel, Copper Brass, Silver Gold, Platinum Sulphur Acetate Rayon Polyester Celluloid Orion Saran Polyurethane Polyethylene Polypropylene PVC (vinyl) Kelf (ctfe)

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2.1 ATTRACTION & REPULSION

It can be observed, that if two negatively charged bodies are brought together, there is a force of repulsion between them. Similarly if two positively charged bodies are brought together there is a force of repulsion. However, if a positively charged body is brought close to a negatively charged body, they attract each other. Hence:

Like Charges Repel, Unlike Charges Attract.

The force of attraction or repulsion is governed by an inverse square law 2.2 UNIT OF CHARGE

The charge on an electron is very small, therefore a more practical unit of charge called a Coulomb, has been chosen:

One Coulomb = 6.29 x 1018 electrons

2.3 STATIC ELECTRICITY & AIRCRAFT

As mentioned earlier, the effects of static electricity are of considerable

importance in the design of aircraft and aircraft equipment. An aircraft in flight picks up static charges as it flies through rain, cloud, snow, dust and other particles in the atmosphere. This build-up of statics is referred to as precipitation static.

The amount of charge that builds up in any particular part of the aircraft depends on the atmospheric conditions to which it is subjected, and the material of which it is made. If two adjacent pieces of material are able to build up charges at

different rates, a potential difference will exist between them. Eventually the potential difference will be sufficient to break down the insulation and current will jump as a spark between the 2 materials. This spark creates numerous

problems; it damages the materials, it causes corrosion, it radiates radio

frequencies that interfere with radio and navigation equipment and it could ignite fuel or oil vapour. In order to prevent this happening, it is essential that all of the aircraft structure and equipment is interconnected or bonded. Bonding allows small currents to continuously flow between materials and equipment, thereby preventing the build up of large static charges.

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An aircraft often accumulates very high electric charges, not only from

precipitation but also from the high velocity gases exiting the engine exhausts. When the charge is sufficiently large, it will start to dissipate into the surrounding atmosphere from any sharp or pointed parts of the aircraft, such as the trailing edges of aerofoil sections. The point at which this occurs is called the corona threshold. The corona discharge produces severe radio interference and needs to be controlled. This is achieved using special devices called wicks, that allow the charge to dissipate in a controlled manner from specific points on the aircraft so that it causes minimum interference.

The subject of static electricity can be considered amusing or annoying when one suffers from its effects. However, it must be taken very seriously by aircraft maintenance engineers. The following are a few points to consider.

• It essential to maintain the integrity of bonding when carrying out any maintenance work on aircraft.

• You can build up a charge on yourself as you move and work around the aircraft. Much of the equipment in modern aircraft is electronic, and can easily be destroyed by you discharging static through it.

• When an aircraft is refuelled, is the refuel vehicle at the same potential as the aircraft. If it isn’t, then it could be possible for a spark to ignite fuel vapour as the fuel nozzle comes into close proximity with the aircraft. It is essential that the two vehicles are interconnected electrically before any hoses or fillers are opened.

• An aircraft in flight can have a potential several thousand volts higher than the ground. This charge is dissipated through the tyres or special straps on the undercarriage when the aircraft lands.

• When an aircraft is inside a hangar for maintenance it should be correctly grounded.

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3 ELECTRICAL TERMINOLOGY

3.1 VOLTAGE

Voltage is the electrical equivalent of mechanical potential. If a person drops a rock from the first storey of a building, the velocity it will reach when dropped will be fairly small. However, if the rock is dropped from the twentieth floor, it will have reached a much greater velocity on reaching the ground. On the twentieth floor the rock had much more potential energy.

The potential energy of an electrical supply is given by its voltage. The greater the voltage of a supply source, the greater its potential to produce a current flow. Thus, a 115 volt supply has 115 times the potential to produce a current flow than a 1 volt supply.

3.1.1 POTENTIAL

If one coulomb of electrons is added to a body and one joule of work has been done, then the body will acquire of potential of – 1 volt. If the electrons had been removed, then the body would have acquired a potential of +1 volt. The unit of potential is the volt.

3.1.2 POTENTIAL DIFFERENCE

When charges move from one point to another, it is not the actual values of potential at those points which are Important, but the potential different (pd) through which the charge has travelled. Just as lifting weight in the gymnasium, the height above sea level is not important, but the distance between the gym floor and the height of one’s body. In cases where an actual level of potential is required, the zero of potential is taken as Earth and whenever the potential at a point is given, it means the difference in potential between the point and the earth’s surface.

If one coulomb of electricity requires one joule of work to move it between two points, then there is a potential difference of 1 volt between them. It is sometimes helpful to think of potential difference as a difference of ‘electrical pressure’

forcing a current through a load.

If a current flows round a circuit, then a potential difference must exist between any two points in that circuit and each point in the circuit must be at a different potential. However because there is very little opposition to current flow in conducting wires, very little potential difference is required to push the current along the wires and it is normally assumed to be zero. Whenever the opposition to current flow is not negligible, then a potential different exists across that component to push the electrons through the device.

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The converse is also true, if no current is flowing, then no potential difference exists. The larger the potential difference the larger the current.

3.1.3 ELECTROMOTIVE FORCE – EMF

To make use of electricity by provision of an electric current, the potential

different must be maintained. That is, the positive and negative charge must be continuously replenished. A cell (or battery) uses chemical energy to maintain the potential difference.

Another device used for this purpose is the generator, which uses

electro-mechanical energy to maintain the potential difference. The potential difference across the terminals of the source (cell, battery or generator) when it is not supplying current, is called Electromotive Force (emf), since this is a measure of the force available to push electrons around the circuit. In a circuit with a current flowing, the potential difference across the terminals of the source is always less than the emf and is referred to as the terminal voltage.

3.2 CURRENT

The SI unit of current is the ampere (A). Although it is known that electric current is a flow of electrons, this flow cannot be measured directly.

3.2.1 MOVEMENT OF CHARGE

Although electric current is referred to as the flow of electrons through a

conductor, it should be noted that more exactly, any movement of electric charge constitutes an electric current. Thus, passage of electricity may occur through a:

• conductor such as metal, due to the movement of the loosely held outer electrons of the atoms.

• vacuum or gas, due to the movement of electrons. • gas, due to the movement of the ionised gas molecules.

• liquid, due to the ionisation of certain molecules, particularly those of acids and salts in solution (e.g. Electrolytes).

The ampere may be defined in terms of the mechanical units of force and length, a more helpful picture is that of moving electrons. When a current of one ampere is flowing in a conductor, one coulomb of charge passes any point in the

conductor every second.

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The Coulomb and the Ampere

Since one coulomb = 6.29 x 1018 electrons, one ampere equals a flow rate of 6.29 x 1018 electrons per second,

Ampere =

3.2.2 CONVENTIONAL FLOW

An applied emf causes directional flow. Using conventional flow the charge carriers are considered to be positive, that is they leave the positive terminal of a supply and return to the negative terminal.

This form of flow was decided upon before anybody knew exactly what ‘current flow’ was, however it is still widely used in Britain and will be assumed throughout the course, unless stated otherwise.

3.2.3 ELECTRON FLOW

It is now known that current flow is a movement of negatively charged particles. I.e., electrons. Electrons flow from the negative terminal to the positive terminal. This form of flow is referred to as electron flow and is used extensively in the United States.

3.3 RESISTANCE

An electric current is a flow of free electrons through a conductor. The size of current flowing through a conductor for a given applied voltage depends on:

• The number of free electrons.

• The opposition to free movement of the electrons caused by the structure of the material.

These two factors taken together give an effective opposition to current flow which is called resistance. To simplify matters it is usual to ignore the second factor and equate good conductors to a large number of free electrons and poor conductors to fewer free electrons. Hence, a good conductor is a material which has low resistance, i.e. a large number of free electrons, and allows a large current to flow. Conversely a poor conductor has a high resistance, i.e. few free electrons and allows only a small current to flow for the same applied voltage. Because the value of the current flowing is determined by the resistance in the circuit, current flow can be controlled by varying the resistance.

Even the best conductors have resistance. T Q I or Seconds Coulomb ₌₌₌₌

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3.3.1 FACTORS AFFECTING RESISTANCE

The four factors that affect the resistance of a wire conductor are: • Material. Some materials conduct better than others.

• Length ( ). Resistance is directly proportional to length, thus if the length is doubled (other factors remaining constant), resistance is doubled.

• Cross Sectional Area (A). Resistance is inversely proportional to A. Thus if the cross sectional area is doubled, resistance is halved.

• Temperature. Temperature affects the number of free electrons and hence resistance.

3.3.2 UNITS OF RESISTANCE

Resistance is measured in ohms, symbol Ω (omega). The resistance of a piece of material is one ohm if a potential difference of one volt applied across it causes a current of one ampere to flow.

3.4 CONDUCTANCE AND CONDUCTIVITY

The conductance, G of a material is the reciprocal of its resistance and is;

The conductivity of a material is the reciprocal of its resistivity. It is given the Greek symbol σ (sigma) and has the units siemens per metre (s/m).

Thus at 0°C copper has a conductivity of;

Conductance and conductivity are rarely used in the course, but a mention is required. s/m 10 52 64 10 55 1 1 1 _-₈ ==== ⋅⋅⋅⋅ ×××× 6 ×××× ⋅⋅⋅⋅ ==== ρρρρ ==== σ σ σ σ λ λ a /a 1 R 1 G ==== σσσσ×××× ρρρρ ==== ==== λ

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4 PRODUCTION OF ELECTRICITY

Very large amounts of electrical energy lie dormant in the atoms of every speck of material in the universe. Whilst the atoms remain electrically balanced however, this electricity cannot be put to any practical use. What is needed is some form of external energy that will separate the electrons from their nuclei. In this way, the external energy that is applied will give rise to electrical energy.

There are six sources of external energy that are capable of separating the

electrons from their nuclei, these are friction, pressure, magnetism, heat, light and chemical action.

4.1 BY FRICTION

Static electricity, that is the separation and build-up of charge is an everyday phenomenon that is often caused by friction – the physical stripping of electrons from one body and depositing on another. Early examples in science were the rubbing of a glass rod (which loses electrons and gains a positive charge) with a silk stocking! (gains electrons, receives negative charge) and the rubbing of an ebonite rod (receives negative charge) with cats fur (becomes positively

charged). Everyday examples are:

• Combing the hair (dry). The comb attracts the individual hairs and the hairs repel each other and stand on end.

• Removing a shirt (especially nylon). The shirt crackles and sparks may be seen, the shirt is also attracted to the body.

• The receiving of ‘electric shock’ from cars (also aircraft) when touching them on the outside. Here the charge has been produced by the friction of air passing around the vehicle.

• The rapid collection of dust by records. The dust is attracted by the charge built up on the record produced by friction of handling and playing.

• Lightning flash is a result of the build up of static electricity in clouds. • Although not used to produce electricity for any aircraft systems, static

electricity is generated by friction as the aircraft moves through the air and will therefore be considered at various points throughout the course. 4.2 BY PRESSURE

Certain crystals and semiconductors produce an emf between two opposite faces when the mechanical pressure on them is either increased or decreased (the polarity of the emf is reversed when the pressure changes from an increase to a decrease). This emf is known as the piezoelectric emf.

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This effect is used in a number of devices including semi-conductor strain gauges and vibration sensors. As the mechanical pressure on the crystal is altered, a varying voltage which is related to the pressure is produced by the crystal. The voltage can be as small as a fraction of a volt or as large as several thousand volts depending on the crystal material and the pressure. Aircraft systems employing the piezoelectric effect generally only produce very small emf’s, the very high voltages produced by materials such as lead zirconate titanate are used in ignition systems for gas ovens and gas fires.

4.3 BY MAGNETISM

Magnetism itself is not used as the direct source of external energy. In a manner which will be studied in great detail later in the course, large amounts of electrical energy are produced by machines called generators. Energy is used to drive the generator, which when it turns, makes use of the properties of magnetism to produce the external energy necessary to break the electrons away from their nuclei and so make it possible for electric current to flow.

4.4 BY HEAT

The Seebeck effect – the thermocouple. When two different metals are brought into contact with one another, it is found that electrons can leave one of the metals more easily than they can leave the other metal. This is because of the difference in what is known as the work function of the two metals. Since electrons leave one metal and are gained by the other, a potential difference exists between the two metals; thus the emf is known as the contact potential or contact emf.

If two metals, say copper and iron, are joined at two points as shown in the diagram above, and both junctions are at the same temperature, the contact potentials cancel each other out and no current flows in the loop of wire.

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The magnitude of the voltage produced by this method is small – only a few millivolts per degree centigrade – but it is sufficient to be measured. The current flow is a measure of the difference in temperature between the ‘hot’ junction and the ‘cold’ junction.

Each junction is known as a thermocouple and if a number of thermocouples are connected in series so that alternate junctions are ‘hot’ and the other junctions are ‘cold’, the total emf is increased; this arrangement is known as a thermopile. On aircraft, thermocouples are used for temperature measurement and will be examined in more detail at a later date.

4.5 BY LIGHT

The Photovoltaic Cell or Solar Cell. A photovoltaic cell generates an emf when light falls onto it. Several forms of photovoltaic cell exist, one of the earliest types being the selenium photovoltaic cell in which a layer of selenium is deposited on iron and any light falling on the selenium produces an emf between the selenium and the iron.

Modern theory shows that the junction at the interface between the two forms, what is known as a semi-conductor n junction in which one of the materials is p-type and the other is n-p-type. The most efficient photovoltaic cells incorporate semi-conductor p-n junctions in which one of the regions is a very thin layer (about 1µm thick) through which light can pass without significant loss of energy. When the light reaches the junction of the two regions it causes electrons and holes to be released, to give the electrovoltaic potential between the two regions. A better understanding of this action will be obtained later in the course when semi-conductor materials and devices are studied.

4.6 BY CHEMICAL ACTION

The final method of producing electricity is by chemical action. It is the particular kind of chemical action that takes place in ‘electric cells’ and ‘batteries’ which is put to practical use in the production of electricity.

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5 CELLS & BATTERIES

To study electrical principles further we require a source of emf. Although an emf can be produced by any of the six methods discussed above, large amounts of useable power can only be produced chemically or by generation. Generation requires a more in depth study of magnetism and therefore cells and batteries will be studied first.

On an aircraft, the battery may be used for engine starting, but far more

importantly, the battery is the source of emergency power when the generator fails. Although aircraft battery systems and servicing will be studied at a later date, battery principles and battery construction will be studied now and will not be repeated.

5.1 PRINCIPLES

A Cell is a portable device which converts chemical energy into electrical energy. A group of interconnected cells is known as a battery. Cells operate on a

principle of the exchange of charges between dissimilar metals.

5.1.1 CELL & BATTERY SYMBOLS

The circuit symbols for cells and batteries are shown below. To identify the polarity of the terminals, a long thin line is used to represent the positive terminal and a short thick line the negative terminal. Sometimes the terminal voltage is indicated.

5.1.2 CONSTRUCTION & CHEMICAL ACTION

In cells, an electrolyte separates two charge collecting materials called electrodes, to which external connections are made. The electrolyte pushes electrons onto one of the plates and takes them off the other. This action results in an excess of electrons, or a negative charge, on one plate and a loss of electrons, or a positive charge, on the other plate.

Electrolytes are chemical solutions manufactured to allow the generation and free movement of both types of ions, and are normally acid or alkaline pastes or liquids.

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The action of the electrolyte in carrying electrons from one plate to the other is actually a chemical reaction between the electrolyte and the two plates. This action changes chemical energy into electrical charges on the cell plates and terminals.

With nothing connected to the cell terminals, the electrons would be pushed onto the negative plate until there was no more room. At the same time the electrolyte would take electrons from the positive plate to make up for those it had pushed onto the negative plate. Both plates would then be fully charged and the movement of electrons would cease.

If a wire were connected between the negative and positive terminals of the cell, electrons on the negative terminal would leave the terminal and travel through the wire to the positive terminal. The electrolyte would carry more electrons across from the positive plate to the negative plate. Whilst the electrolyte is carrying electrons you would see the negative plate being used up and you would see bubbles of gas at the positive plate.

5.1.3 PRIMARY & SECONDARY CELLS

In a primary cell, current will continue to flow until chemical action had dissolved the negative plate into the electrolyte, at which point the cell would be exhausted and of no further use.

In a secondary cell, the chemical action that takes place whilst the cell is producing a current flow is reversible, enabling the cell to be re-used. The process of reversing the chemical action is referred to as charging and entails passing a current through the cell in the opposite direction to the discharge current.

5.1.4 CELL EMF

The size of a cell has no bearing on the emf that it will produce, the generated emf being determined solely by the materials used in its construction. Another point to note is that the potential difference, or voltage measured across the terminals of a cell, is not the same as the emf generated by the cell. The terminal voltage of a cell depends on the:

• internal resistance of the cell. • size of the discharge current.

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• The size of the discharge current. As a general rule, whenever a cell is providing current, the terminal voltage will be less than the cell emf. The larger the discharge current, the greater the difference between the cell emf and its terminal voltage.

• The internal voltage of the cell. All sources of electricity have internal resistance which affects the terminal voltage, this will be examined in more detail later in the notes.

5.1.5 CELL CAPACITY

The amount of electrical energy that a cell can provide from new to the end of its useful voltage on load is called the cell capacity and is quoted in Ampere-hours (A-h).

Capacity varies with the amount of current drawn from the cell, the greater the current the lower the capacity, therefore capacity is normally quoted at a standard rate. The 1hr rate is the internationally accepted standard for Nickel Cadmium cells, with 10 hr or 20 hr rates being used for Lead Acid cells.

A cell quoted at 40A-h at the 10 hr rate will provide 4 Amps continuously for 10 hours.

A battery quoted at 40A-h at the 1 hr rate will provide 40 Amps continuously for 1 hour.

A 40 A-h cell will only be able to provide a discharge current of 80 amps for approximately 20 minutes, not 30 minutes as may be expected by calculation. Similarly, it will be able to supply a discharge current of 20 amps for longer than the expected 2 hrs.

The capacity of a cell is also affected by its age, the older a cell, the lower its capacity, therefore the only way of determining actual capacity is to measure it.

5.1.6 INTERCONNECTION OF CELLS

Cells may be connected in series, parallel or any combination of the two in order to form a battery. When cells are connected to form a battery they should be of similar construction, and have the same terminal voltage, internal resistance and capacity.

Series connection. When connected in series:

The battery voltage is the total of the individual cell voltages.

The battery resistance is equal to the total of the individual cell resistances. The battery capacity is the same as the capacity of a single cell.

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Parallel connection. When connected in parallel:

The battery voltage is the same as the voltage of a single cell.

The battery resistance is equal to the parallel total of the cell resistances. The battery capacity is equal to the total of the individual cell capacities. These rules can also be applied when connecting batteries together in series, parallel or any combination of the two.

5.2 LEAD ACID BATTERIES

Lead acid cells have a nominal voltage of 2 Volts, therefore a typical 24V aircraft battery would consist of 12 cells connected in series. The active material in the positive plates is Lead Peroxide (Pb02) the negative plates, Spongy Lead (Pb).

The electrolyte is dilute sulphuric acid (2H2SO4).

5.2.1 CONVENTIONAL CONSTRUCTION

There are two forms of Lead Acid battery construction, conventional and solid block, often referred to as a Varley type battery.

In the conventional battery the plates consist of lead grids into which the active materials are pressed. The positive and negative plates are then interleaved and connected to a lug that forms both a mechanical support and the terminal.

Cells are generally constructed with an additional negative plate, making both outside plates negative. This ensures that chemical action takes place on both sides of each positive plate. When chemical action only takes place on one side

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The plate arrangement is then inserted into a composite material container which is fitted with a lid. The inside of the container is ribbed to provide additional

support for the plates, which are raised clear of the bottom of the container to prevent shorting by any sediment that forms.

To provide further support for the plates and to ensure they cannot touch,

separators are fitted, these were originally cedar wood but modern batteries use micro-porous plastic materials.

Each cell is fitted with a special non spill valve that allows gasses to escape, but prevents the spillage of electrolyte, this valve can be removed for checking and adjusting the electrolyte level.

The electrolyte used is sulphuric acid diluted with pure distilled water, the specific gravity of the electrolyte used is determined by the manufacturer, however, it is generally lower than 1300.

5.2.2 SOLID BLOCK TYPE CONSTRUCTION

In the solid block type battery the electrolyte is completely absorbed into a compressed block consisting of porous plates and separators.

The plates are completely supported and therefore a more porous active material paste can be used, this gives better absorption and an enhanced electrochemical activity.

The support given to the plates means practically no distortion and no shedding, therefore no sludge gap is required, all the space inside the cells being used for the plates.

All of these advantage result in a battery that is stronger, less susceptible to vibration damage and has a higher capacity to weight ratio than its conventional counterpart.

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5.2.3 CHEMICAL ACTION

When the lead acid battery is delivering current, the sulphuric acid breaks up into Hydrogen ions (H2) carrying a positive charge and Sulphate ions (SO4) carrying a

negative charge. The SO4 ions combine with the lead plate (Pb) and form lead

sulphate (PbSO4). At the same time they give up their negative charge, thus

creating an excess of electrons on the negative plate.

The H2 ions go to the positive plate and combine with the oxygen of the lead

peroxide (PbO2) forming water (H2O), during the process they take electrons from

the positive plate. The lead of the lead peroxide combines with some of the SO4

ions to form lead sulphate on the positive plate.

The result of this action is a deficiency of electrons on the positive plate and an excess of electrons on the negative plate.

When a circuit is connected to the battery, electrons flow from the negative plate to the positive plate. This process will continue until both plates are coated with lead sulphate. The lead sulphate is highly resistive, and it is mainly the formation of the lead sulphate which gradually lowers the battery capacity until it is

discharged.

During charging, current is passed through the battery in a reverse direction. The SO4 ions are driven back into solution in the electrolyte, where they combine with

the H2 ions of the water, thus forming sulphuric acid. The plates are thus returned

to their original compositions.

The sulphuric acid is effectively used up as the battery is discharged, and returned to the electrolyte as it is charged, a test of the specific gravity of the

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5.2.3.1 A simple overview of the charge and discharge characteristics

During discharge the plates are converted into lead sulphate, the water content of the electrolyte increases, the internal resistance of the cell increases and the terminal voltage decreases.

By passing a current through the battery in the opposite direction these effects are reversed. The plates are converted back to their original form, the water content of the electrolyte decreases, the internal resistance decreases and the terminal voltage increases. The process of recharging takes approximately 8 to 10 hours.

During most of the charge and discharge cycle the battery terminal voltage

remains constant at 1.95V, it therefore gives no indication as to the battery’s state of charge.

The specific gravity of the electrolyte however changes at a regular rate as the battery is charged, or discharged and can therefore be used to determine the battery’s state of charge.

5.2.4 VOLTAGE & SPECIFIC GRAVITY CHARACTERISTICS

The voltage and specific gravity figures for a lead acid battery are: • Fully charged and still connected to the charging board charge:

2.5 to 2.7 Volts 1270 to 1280 SG • Fully charged and off charge:

2.2 to 2.5 Volts 1270 to 1280 SG • Fully Discharged:

1.8 Volts 1150 SG

The battery will be damaged if allowed to go below the above discharged values.

5.2.5 COMMON LEAD ACID BATTERY FAULTS

Careful treatment of lead acid batteries prevents damage and early failure, however, some common faults associated with lead acid batteries are:

Sulphation is the formation of hard, permanent lead sulphate on the plates and appears as random greyish white patches. Sulphation causes an increase in the internal resistance of the battery, leading to possible overheating and buckling of the plates.

Sulphation is caused by continually undercharging the battery or by discharging below 1.8 Volts or 1150 SG and is severe there is no cure, however if mild it can sometimes be cured by giving the battery a long low charge.

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Buckling is twisting and bending of the plates. Because the active material is squeezed out of the plates the capacity of the battery may be reduced, if severe it can lead to internal shorting of the battery.

Buckling is caused by excessive charge and discharge currents being imposed on the battery and by the effects of sulphation.

There is no cure for buckling only prevention.

Sedimentation is the collection of discarded active material from the plates at the bottom of the cell.

Sedimentation may result in shorting of the plates and complete loss of capacity, slight shedding is normal in a well maintained battery.

5.3 NICKEL CADMIUM BATTERIES

5.3.1 CONSTRUCTION

The plates of a nickel cadmium battery are made by sintering a nickel plated steel screen with nickel carbonyl powder. The resultant plaques are then impregnated with the active materials, Nickel salts on the positive, cadmium salts on the negative. The plaques are then placed in electrolyte and subjected to a small current to convert them to their final form.

After washing and drying the plaques are cut into plates, each one having a nickel tab welded to it. The plates are then stacked alternately to produce a cell. Whilst producing the stack a continuous separator is wound between the plates to prevent them shorting.

Terminals are then welded to the plates and the stack-up is inserted into its container, which is sealed and pressure tested.

The separator used is normally a triple layer type, one layer of cellophane, two of woven nylon cloth. Cellophane is used because it has a low resistance and is a good barrier material, it prevents metal particles from shorting the plates whilst allowing current to flow. The cellophane also acts as a gas barrier, preventing oxygen given off by the positive plate during overcharge, from passing to the negative plates. At the negative plates the oxygen combines with the cadmium, reducing the cell voltage and producing heat.

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The electrolyte, a solution of potassium hydroxide and distilled water, with a SG of between 1240 and 1300, is then injected into the cell under a vacuum. Fitted to the top of each cell is a special vent that allows the escape of gas but prevents electrolyte spillage.

In a typical Ni-Cad battery the cells are mounted in a metal case that incorporates 2 venting outlets, carrying handles, a quick release connector and a lid. Each cell is separated from its neighbour by its moulded plastic case and electrically

connected by nickel plated steel links between the terminals.

5.3.2 CHEMICAL ACTION

As the battery discharges, hydroxide ions (OH) from the electrolyte combine with the cadmium in the negative plates and release electrons to the plate. The

cadmium is converted to cadmium hydroxide during the process. At the same time, hydroxide ions from the nickel hydroxide positive plates go into the

electrolyte carrying extra electrons with them. Thus electrons are removed from the positive plate and delivered to the negative plate during discharge.

The composition of the electrolyte remains a solution of potassium hydroxide because hydroxide ions are added to the electrolyte as quickly as they are

removed. For this reason the specific gravity of the electrolyte remains essentially constant at any state of charge. It is therefore impossible to use the specific gravity as an indication of the charge state of the battery.

When the battery is charged, the hydroxide ions are caused to leave the negative plate and enter the electrolyte. Thus the cadmium hydroxide of the negative plate is converted back to metallic cadmium. Hydroxide ions from the electrolyte

recombine with the nickel hydroxide of the positive plates, and the active material is brought to a higher state of oxidation. This process continues until all the active material of the plates have been converted. If charging is continued, the battery will be in overcharge, and the water in the electrolyte will be decomposed by electrolysis. Hydrogen will be released at the negative plates and oxygen at the positive plates. This combination of gases is highly explosive.

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5.3.2.1 A simple overview of the charge and discharge characteristics

During charging and discharging the electrolyte acts only as an ionised

conductor, transporting electrons from one plate to the other, its specific gravity remaining constant.

On discharge the terminal voltage initially falls rapidly and then remains constant for most of the discharge cycle, dropping rapidly again when the battery is nearly fully discharged.

When charged, the terminal voltage initially rises rapidly and then settles to a gradual increase. A second rapid rise takes place as the battery reaches the fully charged condition, at this time gassing takes place, hydrogen being released at the negative plates, oxygen at the positive plates, this combination of gases is explosive. Prolonged gassing should be avoided as it reduces the water content of the electrolyte and causes overheating of the battery, a slight amount of gassing, however, is necessary to ensure charging is complete.

The terminal voltage remains constant for most of the batteries life and the

specific gravity of the electrolyte remains unchanged, the only way of determining the state of charge of the battery therefore, is to carry out a full charge followed by a capacity test.

During discharge the plates absorb electrolyte to such an extent that the level may disappear from view. As the battery is charged, the electrolyte is forced back out of the plates, a point to note when topping up the cells.

5.3.3 ADVANTAGES & DISADVANTAGES

A Nickel Cadmium battery has the following advantages over a Lead Acid battery: • They have a longer life

• The terminal voltage remains almost constant during the discharge cycle • They can be charged and discharged at much higher currents without

causing cell damage

• They can be discharged to a very low voltage without causing cell damage But have the following disadvantages:

• They are far more expensive to buy and maintain

• Each cell has a lower voltage, therefore more cell are required to produce a battery.

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5.3.4 THERMAL RUNAWAY

The battery looses heat by conduction and radiation. Provided the rate of heat loss is greater than the rate at which heat is generated there is no problem.

Should the battery not be able to loose heat so quickly it will start to get hot. As its temperature goes up the internal resistance decreases and the current increases. This increase in current leads to an increase in chemical activity within the

battery, this generates more heat and the cycle repeats.

Nickel Cadmium batteries are very susceptible to thermal runaway which can result in the battery boiling, or even being totally destroyed.

5.4 SMALL ALKALINE CELLS

Hermetically sealed Ni-Cad cells are produced in the same size and shape as their primary counterparts. They are small, portable and maintenance free, but have the added advantage of being rechargeable.

The plates are constructed in a similar manner to the larger Ni-Cad cells, the separator being a thin porous material. The electrolyte is fully absorbed by the plates and separator in a similar manner to the Varley type cell. With steel or plastic being used for the case.

Special vents are fitted to each cell, these allow the escape of gas but prevent the entry of oxygen and electrolyte leakage.

The nominal voltage of a fully charged cell is 1⋅25 volts and these can then be interconnected to form batteries.

A 10 hour rate capacity is generally used with an end of life voltage of 1.1 volts, it is possible to discharge the cells further but damage will occur if allowed to go below 1 volt.

Charging should be carried out using a constant current at the 10 hour rate, total charge taking approximately 14 hrs, the end of charge “on charge” voltage being 1⋅45 volts. Overcharging should be avoided, it produces heat and shortens the long term life of the cell.

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6 OHM’S LAW

So far you have been introduced to the concepts of electric current (as a

movement of free electrons through a conducting material), voltage (or potential) and potential difference and to the resistance to current flow by any conducting material. The relationship which exists between these quantities was discovered by a physicist called Ohm and is now referred to as Ohm’s Law. This is the most fundamental law in all electric’s and electronics.

Ohm’s law states: For a fixed metal conductor, with temperature and other conditions remaining constant, the current through it is proportional to the potential difference between its ends.

Mathematically this is expressed as: I ∝ V

Thus the ratio

and this ratio is called the resistance of the conductor. Hence we may write V = R where V is in volts

I I is in amperes R is in ohms 6.1 TRANSPOSITION OF OHM’S LAW

By transposition it is seen that Ohm’s law may be written in three forms:

thus resistance may be calculated if V and I are known.

thus current may be calculated if V and R are known.

thus voltage may be calculated if I and R are known. R = V I I = V R V = IR Constant I V ====

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6.2 THE OHM’S LAW TRIANGLE

One simple way of memorising Ohm’s law is the Ohm’s law triangle – see below.

By covering up the unknown quantity, the relationship between the remaining two is directly observed. You may check this against the equations in the above sub-chapter. This is not necessary if you are able to remember one form of the equation and derive the other two directly by transposition.

V I R

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7 ELECTRICAL MEASURING INSTRUMENTS

Quantities of electrical current, voltage and resistance are measured using instruments called meters. Until the advent of electronic displays and

semiconductor components, meters comprised a movement, working on the motor principle, driving a needle across a scale. These types of meters were called 'moving coil meters' or 'analogue meters'. Moving coil meters will be studied in some depth later in the course, because the principle behind their operation is the same as the principle employed in many aircraft instruments. Modern meters are referred to as a 'digital meters' or 'digital voltmeters', more commonly abbreviated to DVM's, although they measure far more than just voltage. Digital meters are cheaper, more reliable, more robust and generally considered more accurate than their analogue counterparts, although some would argue that, used correctly, an analogue instrument is just as accurate. It is essential that you are confident in the use of both types of meter. There are instances where a digital meter cannot be used, leaving no choice but to revert to an analogue meter.

7.1 CONNECTING METERS TO A CIRCUIT

Irrespective of whether the meter is digital or analogue, the way that it is connected to the circuit under test is the same.

7.1.1 VOLTMETERS

Voltmeters are used to measure emf's and more commonly potential differences. The two probes of the meter are therefore connected to the two points between which the potential difference is required.

If the potential at A with respect to B is required, the red lead is connected to point A, the black lead point B.

If the potential at B with respect to A is required, the red lead is connected to point B, the black lead point A.

If the potential between a point and Earth or ground is required. The red lead is connected to the point and the black lead is connected to ground or Earth.