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iii
PREFACE_______________________
As the world moves toward a single standard for international pilot licensing, many nations have adopted the syllabi and regulations of the “Joint Aviation Requirements-Flight Crew Licensing" (JAR-FCL), the licensing agency of the Joint Aviation Authorities (JAA).
Though training and licensing requirements of individual national aviation authorities are similar in content and scope to the JAA curriculum, individuals who wish to train for JAA licences need access to study materials which have been specifically designed to meet the requirements of the JAA licensing system. The volumes in this series aim to cover the subject matter tested in the JAA ATPL ground examinations as set forth in the ATPL training syllabus, contained in the JAA publication, “JAR-FCL 1 (Aeroplanes)”.
The JAA regulations specify that all those who wish to obtain a JAA ATPL must study with a flying training organisation (FTO) which has been granted approval by a JAA-authorised national aviation authority to deliver JAA ATPL training. While the formal responsibility to prepare you for both the skill tests and the ground examinations lies with the FTO, these Jeppesen manuals will provide a comprehensive and necessary background for your formal training.
Jeppesen is acknowledged as the world's leading supplier of flight information services, and provides a full range of print and electronic flight information services, including navigation data, computerised flight planning, aviation software products, aviation weather services, maintenance information, and pilot training systems and supplies. Jeppesen counts among its customer base all US airlines and the majority of international airlines worldwide. It also serves the large general and business aviation markets. These manuals enable you to draw on Jeppesen’s vast experience as an acknowledged expert in the development and publication of pilot training materials.
We at Jeppesen wish you success in your flying and training, and we are confident that your study of these manuals will be of great value in preparing for the JAA ATPL ground examinations. The next three pages contain a list and content description of all the volumes in the ATPL series.
ATPL Series
Meteorology (JAR Ref 050)
• The Atmosphere • Air Masses and Fronts
• Wind • Pressure System
• Thermodynamics • Climatology • Clouds and Fog • Flight Hazards
• Precipitation • Meteorological Information
General Navigation (JAR Ref 061)
• Basics of Navigation • Dead Reckoning Navigation • Magnetism • In-Flight Navigation
• Compasses • Inertial Navigation Systems • Charts
Radio Navigation (JAR Ref 062)
• Radio Aids • Basic Radar Principles • Self-contained and • Area Navigation Systems
External-Referenced • Basic Radio Propagation Theory Navigation Systems
Airframes and Systems (JAR Ref 021 01)
• Fuselage • Hydraulics
• Windows • Pneumatic Systems
• Wings • Air Conditioning System
• Stabilising Surfaces • Pressurisation
• Landing Gear • De-Ice / Anti-Ice Systems • Flight Controls • Fuel Systems
Powerplant (JAR Ref 021 03)
• Piston Engine • Engine Systems
• Turbine Engine • Auxiliary Power Unit (APU) • Engine Construction
Electrics (JAR Ref 021 02)
• Direct Current • Generator / Alternator • Alternating Current • Semiconductors
v
Instrumentation (JAR Ref 022)
• Flight Instruments
• Automatic Flight Control Systems • Warning and Recording Equipment
• Powerplant and System Monitoring Instruments
Principles of Flight (JAR Ref 080)
• Laws and Definitions • Boundary Layer • Aerofoil Airflow • High Speed Flight • Aeroplane Airflow • Stability
• Lift Coefficient • Flying Controls
• Total Drag • Adverse Weather Conditions • Ground Effect • Propellers
• Stall • Operating Limitations
• CLMAX Augmentation • Flight Mechanics • Lift Coefficient and Speed
Performance (JAR Ref 032)
• Single-Engine Aeroplanes – Not certified under JAR/FAR 25
(Performance Class B)
• Multi-Engine Aeroplanes – Not certified under JAR/FAR 25
(Performance Class B)
• Aeroplanes certified under JAR/FAR 25 (Performance Class A)
Mass and Balance (JAR Ref 031)
• Definition and Terminology • Limits
• Loading
• Centre of Gravity
Flight Planning (JAR Ref 033)
• Flight Plan for Cross-Country • Meteorological Messages
Flights • Point of Equal Time • ICAO ATC Flight Planning • Point of Safe Return
• IFR (Airways) Flight Planning • Medium Range Jet Transport • Jeppesen Airway Manual Planning
Air Law (JAR Ref 010)
• International Agreements • Air Traffic Services
and Organisations • Aerodromes • Annex 8 – Airworthiness of • Facilitation
Aircraft • Search and Rescue • Annex 7 – Aircraft Nationality • Security
and Registration Marks • Aircraft Accident Investigation • Annex 1 – Licensing • JAR-FCL
• Rules of the Air • National Law • Procedures for Air Navigation
Human Performance and
Limitations (JAR Ref 040)
• Human Factors
• Aviation Physiology and Health Maintenance • Aviation Psychology
Operational Procedures (JAR Ref 070)
• Operator • Low Visibility Operations • Air Operations Certificate • Special Operational Procedures • Flight Operations and Hazards
• Aerodrome Operating Minima • Transoceanic and Polar Flight
Communications (JAR Ref 090)
• Definitions • Distress and Urgency • General Operation Procedures Procedures
• Relevant Weather Information • Aerodrome Control • Communication Failure • Approach Control • VHF Propagation • Area Control
Table of Contents
Electrics vii
CHAPTER 1 Basic DC Terminology
Introduction ...1-1 The Electric Circuit ...1-1 Current (I)...1-2 Electromotive Force (EMF) ...1-3 Potential Difference (PD) ...1-3 Voltage (V) ...1-3 Resistance (R) ...1-4 Connecting Resistances in Series or Parallel in a DC Circuit ...1-5 Ohm’s Law ...1-7 Loads ...1-7 Kirchhoff’s Laws ...1-7 Electric Power (P) ...1-8 Electrical Work ...1-9 Electric Unit Prefixes ...1-9 Typical Circuit Symbols ...1-9
CHAPTER 2 Electrical Components
Introduction ...2-1 Electric Systems ...2-1 Electric Circuit Faults ...2-2 Busbars ...2-3 Protection Devices ...2-4 Reverse Current Circuit Breaker (RCCB)...2-6 Switches ...2-6 Electric Generator ...2-10 Alternator ...2-10 Electric Motor ...2-10 CHAPTER 3 Aircraft Batteries Introduction ...3-1 Lead Acid Battery ...3-2 Alkaline Battery (Nickel-Cadmium) ...3-3 Battery Venting ...3-4 Electrolyte Spillage ...3-5 Battery Capacity ...3-5 Battery Charging ...3-6 Thermal Runaway ...3-6 Battery State of Charge ...3-6 Battery Condition Check ...3-7 Emergency Use ...3-7 Connection of Batteries ...3-7 Spare Batteries ...3-8 Battery Compartment Inspection ...3-8
Table of Contents
CHAPTER 4 Magnetism
Introduction ... 4-1 Magnetism ... 4-2 Fundamental Laws of Magnetism ... 4-2 Characteristics of Lines of Magnetic Flux... 4-3 Classification of Magnetic Materials ... 4-5 Magnetic Flux ... 4-5 Flux Density ... 4-5 Reluctance ... 4-5 Permeability ... 4-5 Hysteresis ... 4-6 Saturation ... 4-7 Magnetism Produced by Current Flow ... 4-7 The Electromagnet ... 4-10 The Relay ... 4-12 Electromagnetic Induction ... 4-12 CHAPTER 5 DC Generator Systems Introduction ... 5-1 Generator Systems ... 5-1 Basic Generator Theory ... 5-1 A Simple Generator ... 5-2 Conversion of AC to DC ... 5-3 DC Generator System Architecture ... 5-5 DC Generator Construction ... 5-5 Principle of Operation of a DC Generator ... 5-6 Types of DC Generator ... 5-6 Voltage Regulator ... 5-8 Cut-Out ... 5-9 Reverse Current Circuit Breaker ... 5-10 Busbars ... 5-10 Power Failure Warning ... 5-11 Ground Power ... 5-11 DC Generator System Fault Protection ... 5-12 Twin-Engine DC Electrical System ... 5-13 Operation of DC Generators in Parallel ... 5-14 DC Load Sharing ... 5-14 Operation of an Equalising Circuit ... 5-15 Single-Engine Aircraft DC Electrical System ... 5-15 Operation of the Alternator ... 5-16
Table of Contents Electrics ix CHAPTER 6 DC Motors Introduction ...6-1 Motors...6-1 The Motor Principle ...6-1 DC Motors ...6-2 Back EMF ...6-3 Direction of Rotation ...6-3 Types of DC Motor ...6-4 Motor Speed Control ...6-6 Actuators ...6-8 Split-Field Series Motor ...6-9 Electromagnetic Brakes ...6-10 Clutches ...6-10 Instrument Motors ...6-10 Architecture of a Starter/Generator System ...6-11 Operation of a Starter/Generator System ...6-11 Inverters ...6-14 Multiple Inverter Installations ...6-15
CHAPTER 7 Inductance and Capacitance
Introduction ...7-1 Inductance ...7-1 Self Induction ...7-2 Inductors ...7-2 Time Constant of an Inductor ...7-3 Inductors in Series and Parallel ...7-4 Capacitance ...7-5 Factors Affecting Capacitance ...7-5 Types of Capacitor ...7-6 The Charging of a Capacitor ...7-7 Discharging of a Capacitor ...7-8 The Time Constant of a Capacitor ...7-8 Capacitors in Series and Parallel in a DC Circuit ...7-9
CHAPTER 8 Basic AC Theory Introduction ...8-1 Alternating Current...8-1 Advantages of AC Over DC ...8-1 Generating AC ...8-1 Simple AC Generator ...8-2 AC Terminology ...8-3 Phasor Representation ...8-6
Table of Contents
CHAPTER 9 Single Phase AC Circuits
Introduction ... 9-1 Single Phase AC Circuits ... 9-1 The Effect of AC on a Purely Resistive Circuit ... 9-1 Power in an AC Resistive Circuit ... 9-1 The Effect of AC on a Purely Inductive Circuit ... 9-2 Power in an AC Inductive Circuit ... 9-3 Inductive Reactance (XL)... 9-3 The Effect of AC on a Purely Capacitive Circuit ... 9-4 Power in an AC Capacitive Circuit ... 9-4 Capacitive Reactance ... 9-5 Relationship Between Voltage and Current in Capacitive and Inductive AC Circuits ... 9-5 Resistive and Inductive (RL) Series AC Circuit ... 9-5 Resistive and Capacitive (RC) Series AC Circuit ... 9-6 Phase Shift ... 9-6 Resistive, Inductive, and Capacitive (RLC) Series AC Circuits ... 9-6 Impedance (Z) in a Resistive, Inductive, and Capacitive (RLC) Series AC Circuit ... 9-7 Resistive, Inductive, and Capacitive (RLC) Parallel AC Circuit ... 9-7 Impedance (Z) in a Resistive, Inductive, and Capacitive (RLC) Parallel AC Circuit ... 9-7 Power in a Resistive, Inductive, and Capacitive (RLC) AC Circuit ... 9-8 Power Factor ... 9-9 AC Series Circuit Example ... 9-9 AC Parallel Circuit Example ... 9-11
CHAPTER 10 Resonant AC Circuits
Introduction ... 10-1 Resonant Circuit... 10-1 Series Resonant Circuit ... 10-1 Q Factor in a Series Resonant Circuit ... 10-3 Parallel Resonant Circuit (Tank Circuit) ... 10-3 Q Factor in a Parallel Resonant Circuit ... 10-5 Self Resonance of Coils ... 10-5 Use of Resonant Circuits ... 10-5 Tuning Circuits ... 10-7
CHAPTER 11 Transformers
Introduction ... 11-1 Transformers... 11-1 Construction and Operation ... 11-1 Types of Transformers ... 11-3 Transformer Rectifier Units ... 11-5
Table of Contents Electrics xi CHAPTER 12 AC Power Generation Introduction ...12-1 Power Generation ...12-1 Simple Three Phase Generator ...12-1 Star Connection ...12-2 Delta Connection ...12-3 Advantages of Three Phase Over Single Phase AC Generators ...12-4 Voltage and Frequency of AC Generators ...12-4 Phase Rotation ...12-4 Faults on Three-Phase AC Generators ...12-4 Generator Real and Reactive Load Sharing ...12-5 Types of AC Generator ...12-6 Brushless Three Phase AC Generator ...12-7 Constant Speed Drive Unit ...12-9 Operation of the Hydro-Mechanical CSDU ...12-11 Protection of the Hydro-Mechanical CSDU ...12-11 Integrated Drive Generator ...12-13 Variable Speed Constant Frequency Power Systems ...12-13 Auxiliary Power Unit ...12-14 Emergency Ram Air Turbine ...12-15
CHAPTER 13
AC Power Generation Systems
Introduction ...13-1 A Typical Frequency-Wild AC System Architecture ...13-1 Operation of a Typical Frequency-Wild AC System ...13-2 Fault Protection in a Typical Frequency-Wild AC System ...13-2 The Constant Frequency Split Busbar AC System ...13-4 Operation of a Constant Frequency Split Busbar AC System ...13-4 Regulation and Protection of Constant Frequency Units ...13-5 Faults on a Constant Frequency Split Busbar AC Generator System ...13-5 Emergency Supplies ...13-7 Battery Charger ...13-7 Battery Power ...13-7 Ground Handling Bus ...13-7 Constant Frequency Parallel AC System ...13-8 Operation of a Constant Frequency Parallel AC System ...13-8 Reactive Load Sharing ...13-11 Real Load Sharing ...13-11 Paralleling ...13-12 Fault Protections in a Constant Frequency AC Parallel System ...13-12 DC Power Supplies ...13-14
CHAPTER 14 AC Motors
Introduction ...14-1 Alternating Current Motors ...14-1 Stator-Produced Rotating Magnetic Field ...14-1 Induction (Squirrel Cage) Motor ...14-2 Two-Phase Induction Motor ...14-4 Split-Phase Motor ...14-4 The Synchronous Motor ...14-5
Table of Contents
CHAPTER 15 Semiconductor Devices
Introduction ... 15-1 Semiconductor Devices ... 15-1 Advantages and Disadvantages ... 15-1 Construction of a Semiconductor ... 15-1 Doping ... 15-2 P-Type Material ... 15-2 N-Type Material ... 15-3 P-N Junction Diode ... 15-4 Use of Diodes ... 15-6 Zener Diode ... 15-6 Variable Capacitance (VARICAP) Diode... 15-7 Bi-Polar Transistors ... 15-7 Operation of a PNP Bi-Polar Transistor ... 15-8 Operation of a NPN Bi-Polar Transistor ... 15-9 Disadvantages of Diodes and Transistors ... 15-9 Transistor Applications ... 15-10 Integrated Circuits ... 15-11 The Advantages and Disadvantages of Integrated Circuits ... 15-11 Types of Integrated Circuits ... 15-11
CHAPTER 16 Logic Circuits Introduction ... 16-1 Logic Circuits... 16-1 Number Systems ... 16-1 Binary Representation ... 16-2 Basic Logic Gates ... 16-2 Adder and Subtracter Circuits ... 16-4 Digital Latch and Flip-Flop Circuits ... 16-6
CHAPTER 17 Computer Technology Introduction ... 17-1 Computers ... 17-1 Analogue Computers ... 17-1 Digital Computers ... 17-1 Computer Architecture ... 17-3 Input Devices ... 17-3 Central Processing Unit ... 17-4 Output Devices ... 17-5 Storage Devices ... 17-5 Operating Systems ... 17-5 Programming ... 17-5
CHAPTER 18
Electrics 1-1
INTRODUCTION
It is essential to know the basic terminology applied to electricity and electrical components before studying further into specific functions and systems. This chapter explains the most common terminology to provide a basis for further study.
THE ELECTRIC CIRCUIT
An electrical circuit usually consists of a power source, a load, a switch, and a conductor, which connects the components together.
The power source, which can be a battery or a generator, provides the pressure that causes electrons to flow in a circuit. When electrons flow, it is referred to as an electric current. In the picture above with the switch open, electrical pressure can be measured on the positive side of the switch but no current flows because there is not a complete circuit and so the filament will not illuminate. As the switch is closed, the circuit is completed and current can flow through the closed contacts of the switch and through the filament, causing it to illuminate. Notice that current has to flow from the supply, through the load, and back to the supply to form a circuit.
The filament is considered a load as it uses power and creates heat in the process. Notice that it does not matter whether the switch is between the positive and the load, or between the load and the negative. Also notice that with the switch closed the voltage can be measured on the positive side of the load (indicated in red), but not on the negative side. This is because all the voltage is dissipated across the load.
Chapter 1 Basic DC Terminology
Wires made from copper or steel normally provide the path for the current to flow, and in most cases, the airframe structure is used to complete the circuit back to the supply. A distinction must be made here between electron flow and conventional current flow. In the earliest days of electrical experimentation, electricity was considered to share the same properties as fluid in motion. Fluid flows from high pressure to low, and as voltmeters measure the positive side of the supply as high, the assumption was made that electricity flows from positive to negative, and came to be accepted as conventional flow. With further scientific study came the realisation that electrons, which carry a negative charge, are attracted to the positive end of a supply, and therefore flow from negative to positive. By this time, however, conventional flow theory had become the rule, as it is today. In all diagrams, unless specified otherwise, conventional flow is assumed.
CURRENT (I)
Electric current is the flow of electrons in a conductor, but there must be a means to measure this flow. The Coulomb is a charge of 6.25 x 1018 electrons, so it is convenient to use this charge as a yardstick. Therefore, 1 Coulomb passing a given point in 1 second equals 1 ampere, often abbreviated to as amp.
Amperes = CoulombsSeconds
Current in a circuit is measured by connecting an ammeter in line, or in series, with the load, as shown below.
Basic DC Terminology Chapter 1
Electrics 1-3
ELECTROMOTIVE FORCE (EMF)
EMF is the force or pressure that sets electrons in motion, and is a natural result of Coulomb's Law, which states that like charges repel and unlike charges attract. EMF is measured in terms of voltage.
POTENTIAL DIFFERENCE (PD)
Even though a circuit is open, and no current is flowing, a power source still has the potential for current flow. Therefore, whether a battery is connected in a circuit or not, a potential difference still exists between its terminals. The same is true within a circuit or between circuits. For instance, if one part of a circuit is at higher voltage than another part a potential difference exists, and current would flow if a connection was made between them. As with EMF, potential difference is expressed as a voltage.
VOLTAGE (V)
The volt is the basic unit of electrical pressure. In order to understand how to measure one volt, it is important to know about resistance. Using fluid flow as an analogy, if water flowing in a pipe meets any resistance, the water flow decreases. Electricity behaves in the same way. Therefore, if current flows through an electrical resistance, the flow rate decreases. One volt of electrical pressure forces 1 ampere through 1 unit of resistance. Voltage is measured using a voltmeter, which must be connected in parallel with (or across) the load or supply.
Chapter 1 Basic DC Terminology
RESISTANCE (R)
The unit of resistance is the Ohm. One Ohm exists when it restricts the current flow to 1 amp when a pressure of 1 volt is applied.
Resistance opposes current flow and in doing so dissipates the voltage across it, which is why it is said that voltage is dropped across a load. A low resistance would allow a relatively large current to pass through it which in turn creates heat, and heat is energy. Energy over time gives power so it can be said that if heat is being produced then power is being developed. On the other hand, if the value of resistance is very high then little or no current will flow.
Metals such as silver and copper have virtually no resistance and are used to conduct electricity. As they have virtually no resistance they must be in series with a resistive load and should be thick enough to withstand the expected current flow, otherwise heating of the conductor will occur. Rubber has a very high resistance and is a non-conductor used for insulation between conductive materials. Cables and wires are comprised of both materials: the metal conductor permits the flow of current along a given path, and the insulation covering it stops the voltage from forcing current out into other paths causing short circuits.
A material that is half way between being a conductor and an insulator is known as a semi-conductor. On their own, semi-conductors are not particularly useful, but when doped with other elements and fused together, they form the basis of the electronic age.
The resistance of a material at a constant temperature is affected by its:
¾ Specific Resistance (ρ), the resistance offered by a cube of material at 0°C ¾ Length (L)
¾ Cross Sectional area (A)
R =
ρ
x
A
L
Resistors can have either fixed or variable values. An example of a variable resistor is a rheostat, which is used to control the intensity of a lighting circuit.
A material’s temperature can affect its resistance. The resistance of most materials increases with increasing temperature, and these materials have a Positive Temperature Coefficient (PTC). A few materials however, exhibit a decreasing resistance with increasing temperature, and these have a Negative Temperature Coefficient (NTC). In general, most resistive components have a PTC characteristic, and semi-conductors and insulators have a NTC characteristic. NTC is used to advantage with semi-conductors called thermistors, which have a greater change in resistance with temperature than normal resistors, and are used to sense temperature changes, such as in fuel low-level warning systems.
Basic DC Terminology Chapter 1
Electrics 1-5
CONNECTING RESISTANCES IN SERIES OR PARALLEL IN A DC CIRCUIT
Resistances can either be connected in series, in parallel, or in series-parallel combinations. When resistors are connected in series as shown below, the same current flows through each of them, and the total opposition to current flow is equal to the sum of the individual resistances. The supply voltage dissipates across all of the resistors in the series network, and therefore each individual resistor will have a different amount of voltage dropped across it. The sum of all the voltages dropped across each resistor will equal the supply voltage.Total Resistance (RT) = R1 + R2 + R3
= 15 + 22 + 31 = 68 Ω
If the resistances connect in parallel with each other, the current flows along two or more paths, as shown below.
As the number of resistances in parallel is increased, the total resistance will decrease, which will draw more current from the supply. The total resistance of a parallel network will always be lower than the smallest resistor in the network. The supply voltage is the same across each resistor in the network. To calculate total resistance of a parallel network, use the following formula:
T
R1 = R11 + R12 + R13 T
R
1
= 41 + 61 + 121 = 162Chapter 1 Basic DC Terminology
In many circuits, a parallel circuit is connected in series with one or more resistors.
To find the total resistance, first calculate the equivalent resistance in the parallel part of the circuit, and then add this value to the series resistance. In the circuit shown above, the total resistance is calculated as follows:
PARALLEL PART OF CIRCUIT
TP R
1
=
1 R1 +
2 R1
=
91+
61=
185Total Parallel Resistance (RTP) = 185
=
3.6 ΩTOTAL CIRCUIT RESISTANCE RT= RTP + R3= 3.6 + 2.4 = 6 ohms An alternative and easier method to calculate two resistors in parallel is:
RT = R1 X R2 ÷ R1 + R2.
For instance, in the parallel circuit above, 9 x 6 ÷ 9 + 6 = 54 ÷ 15 = 3.6. Remember, this only works for two resistors in parallel.
Basic DC Terminology Chapter 1
Electrics 1-7
OHM’S LAW
Ohm’s law states that the current flowing in a circuit is directly proportional to the applied voltage, and inversely proportional to the resistance through which the current flows. Simply stated this means that as voltage is increased current will increase, and as resistance is increased current will decrease. Ohm’s law may be stated by the following formulae:
V = IR, R = V/I, or I = V/R
It is handy to remember the Ohm’s law triangle. To find an unknown value from the triangle, cover it with your finger and the required formula remains:
Here, V = IR
LOADS
The term load refers to any electrical component which consumes power. Loads are connected across the supply voltage, and as more loads are switched on across the supply, the total current increases. Remember that with resistors in parallel, the total resistance is always less than the lowest resistor in the network.
KIRCHHOFF’S LAWS
The first law states that the sum of the currents entering a junction must equal the sum of the currents leaving the junction.
The second law states that in a closed circuit, the sum of the voltage drop always equals the supply voltage.
Chapter 1 Basic DC Terminology
In the circuit shown above, a 10-volt battery is connected across a lamp, and as current flows through the circuit, a voltage drop develops across the lamp. The lamp therefore consumes all the energy provided by the battery, and the voltage drop across the lamp equals the supply voltage.
If two identical lamps are connected in series, each consumes half the power in the circuit and there is an equal voltage drop across each. The sum of the voltage dropped across each lamp equals the supply voltage.
ELECTRIC POWER (P)
The terms work and power are often confused with each other. To clarify the difference, imagine that two people dig holes of equal dimensions. Both have done an equal amount of work. However, if person A dug the hole in 1 hour, and person B dug his in 2 hours, person A used double the power in order to complete the job sooner. Therefore, power can be seen as the work done divided by the time. As 1 ampere represents work done in 1 second with 1 volt applied, increasing the voltage increases the amount of work done, which equals power. Power is measured in Watts and is calculated using the following formula:
Basic DC Terminology Chapter 1
Electrics 1-9
ELECTRICAL WORK
If a potential difference of IV is applied to the ends of a conductor and one coulomb of electricity passes along it, one Joule of work has been done. Electrical work done creates heat and can also result in electromagnetic radiation, as well as motion.
ELECTRIC UNIT PREFIXES
For ease of usage and display, electrical units are normally divided into multiples and sub-multiples. Some of the most commonly used prefixes are as follows:
Multiples Sub-multiples
Kilo - 1 x 103 Milli - 1 x 10-3
Mega - 1 x 106 Micro - 1 x 10-6
Giga - 1 x 109 Nano - 1 x 10-9
Tera - 1 x 1012 Pico - 1 x 10-12
TYPICAL CIRCUIT SYMBOLS
Electrics 2-1
INTRODUCTION
Electrical circuits form an integral part of an aircraft and must be adequately protected. The flight crew must also be able to select and operate any electrical system safely.
ELECTRIC SYSTEMS
Current can return to the source by two methods: the single pole system, better known as earth return, and the dipole system.
Single Pole or Earth Return System is used on aircraft constructed from metal, where the airframe acts as a return path between the load and the power source.
This gives an overall reduction in the amount of wiring required and reduces aircraft weight. Dipole or Two-wire System is used on aircraft constructed from non-conductive or non-metallic materials.
In this system, one wire connects the electrical supply to the load, whilst a second wire provides the return path from the load to the power source. This increases the aircraft’s overall mass.
Chapter 2 Electrical Components
Ground (Earth) is simply a zero or reference point within an electrical circuit and is the metal frame or chassis to which all the various electrical circuits are connected. On an aircraft, the metal airframe is called ground or earth and is at zero volts.
All voltages are measured with respect to the metal structure. In electrics, ground is important because it allows us to have both negative and positive voltages with respect to the metal structure. If a 12-volt battery has a PD between its terminals of 12 volts, then it is not referred to as +12 or -12 volts but simply as 12 volts. The ground reference allows us to express voltages as positive and negative with respect to ground. Remember, ground is a reference point that is considered to be zero or neutral. For example, if the positive terminal of a 12 volt battery is ground, the negative terminal is 12 volts more negative. It follows that the voltage at this terminal with respect to ground is -12 volts. Conversely, if the negative terminal of the battery is connected to ground, the other terminal of the battery will be +12 volts.
ELECTRIC CIRCUIT FAULTS
The following faults can occur in an electrical circuit: Short Circuit
If the insulation around a wire breaks down or is damaged, it exposes an area of bare conductor. If the wire is at a voltage higher than earth, and the damaged area contacts the airframe in an earth single pole system or the return line in a dipole system, a path of very low resistance will exist. In such circumstances, a very high current flows from the supply through the short circuit bypassing the load and back to the supply.
It is not always guaranteed that a fuse will rupture, or circuit breaker trip straight away. It may happen that the short to earth will initially have a value of resistance which causes a higher than normal current to flow, but just within the rating of the protection device. This could lead to burned wiring and possible fire.
Electrical Components Chapter 2
Electrics 2-3
Open Circuit
In any situation where a wire becomes disconnected or breaks, an open circuit fault exists. Its effect is exactly like turning off a switch. Current does not have a complete circuit to flow around, so the system does not work. As there is no current flow, the circuit protection devices do not operate. When a positive wire disconnects, an open circuit exists, but if that wire subsequently touches the airframe, a short circuit occurs from the initial open circuit fault.
Static interference
During flight, significant levels of static voltage build up on the airframe. If two adjacent areas are electrically isolated from each other, the potential difference between the two could build up to the point that the voltages equalise by discharging across the gap between them, creating a spark in the process. The spark is heard as interference on the radio equipment. The cure for this is bonding. All parts of the airframe and equipment are kept at the same electrical potential using metal braided straps. It is important not to confuse the need for bonding with the static discharge wicks found at the extremities of the airframe. These are fitted to help reduce the build up of static voltage on the airframe by continually discharging it to atmosphere as far away as possible from sensitive equipment.
Induced interference
All forms of electricity generate magnetic fields. In AC circuits especially, pulsating magnetic fields are created and this can give rise to voltages being induced into adjacent wires, a condition known as cross talk. Obviously, if the circuits affected are sensitive signal circuits such as radio navigation systems, it is important to protect them against such interference. The most common methods of protection are to use twisted pairs or bundles of wires, and enclose them in a metal braided sleeve or screen, which is connected to earth at one end. This accepts the induced voltages and feeds them away to earth.
BUSBARS
Busbars form the distribution points from which various systems derive their power and are formed from a solid copper bar. On a simple light aircraft DC system, there may be only one DC busbar, fed from the battery, or the generator if online. On aircraft that are more complex there are many busbars distributed around the aircraft. Busbars are categorised as either AC or DC. Within these categories, there is further sub-division depending on the relative importance of the systems being supplied. For instance, the battery is likely to be the last remaining power source in an emergency, so there will be a vital or emergency busbar, which is hard-wired to the battery. This supplies the most vital services in an emergency, such as the fire extinguishers, shut off valves, etc. Other busbars will be categorised as essential or non-essential. The difference is that the non-essential services, such as galley ovens, can be switched off as a group by disconnecting the non-essential busbar when it is required to reduce the overall load on the generator(s).
Chapter 2 Electrical Components
PROTECTION DEVICES
Aircraft electrical circuits use the following protection devices: Fuse
This protection device opens or breaks the electrical circuit when excessive current flows. Too much current may ultimately damage either the circuit itself or the system to which it is connected. A fuse is designed to form a weak link in an electrical circuit to protect the majority of the cable between the supply and the load against overheating and burnout. In its simplest form, it consists of a strip or filament of low melting point metal, which is encased in a glass or ceramic envelope.
Fuses are rated in amperes, according to the maximum current they can carry without overheating and rupturing. They are located as near to the supply (busbar) as possible, so that if an excessive current flows due to a short circuit, the fuse can protect all of the cable to the load. Aircraft are required by law to carry spare fuses; minimum stocks of each type being 3 fuses or 10%, whichever is the greater.
If a fuse ruptures in flight: ¾ Switch off the circuit.
¾ Replace the fuse with one of the same value. ¾ Switch on the circuit.
A ruptured fuse may be replaced only once. If the fuse ruptures a second time, the flight must continue without the affected system.
Current Limiters
These devices protect high power circuits where transient high-current conditions may exist, such as certain electric motors that draw a heavy current on initial switch on. They consist of a filament of tinned copper that has a relatively slow temperature rise, allowing an initial over current condition to exist but will rupture if the high current condition persists. The filament is contained in a ceramic housing.
Electrical Components Chapter 2
Electrics 2-5
Circuit breaker
This device has the same function as a fuse but can be used to restore a circuit when it is reset. Like fuses, circuit breakers are also rated in amperes and are fitted as close to the supply as possible. A circuit breaker is basically a switch that can be opened (tripped) via a bi-metallic strip, as shown below. If an overload current exists, the bi-metallic strip will heat up and distort, causing the latch mechanism to release and open the main contacts of the circuit breaker. A push-pull button pops out, revealing a white band that indicates the circuit breaker has tripped. To reset the circuit breaker, push in the button. Early circuit breaker designs could be manually held in against a fault condition. Although tempting for the pilot on the last leg home, this practice carried inherent risks of system damage or fire. Modern designs are known as trip free and cannot be held in against a fault condition. It is important to learn the difference between trip free and non-trip free.
If a circuit breaker trips:
¾ Switch the circuit off.
¾ Allow a period of approximately 20-30 seconds to allow the bi-metallic element to cool.
¾ Reset the circuit breaker. ¾ Switch the circuit on.
Chapter 2 Electrical Components
If the circuit breaker trips again, switch off the circuit and do not attempt a further reset. In either case, report the fault on landing.
Unlike fuses, circuit breakers can be reset after tripping, so there is no requirement to carry spares. The circuit breaker button functions just like a switch; however, this facility should only be used by ground crew carrying out maintenance in order to isolate a system from the supply.
REVERSE CURRENT CIRCUIT BREAKER (RCCB)
Reverse current circuit breakers are used in DC power supplies to protect against short circuits within the generator, and between it and the busbar, which would cause dangerously high currents to flow from the busbar. They operate at high speed and once operated, they mechanically lock and can only be reset manually on the ground with the engine off. Some of the more sophisticated types of RCCB have a separate thermal overload as an additional precaution against a forward current in excess of the power sources safe working capacity.
SWITCHES
In most electrical systems, switches are the means of control. Selecting a system on may be made using a simple on/off switch, however some systems or sub-systems should not be selected on together, in which case more complex switches may ensure that one system is isolated before another system can be enabled.
A simple switch consists of two contacting surfaces, which can be isolated from each other or brought together by a moveable-connecting link, called a pole. A switch may have an effect on more than one circuit and the number of contacts that can be switched by moving the pole is called the throw. Some examples of these are shown below.
If a switch has only one set of contacts, it is a single pole switch. A switch that operates two or three sets of contacts in one switching action is a double or triple pole switch. Switches operating emergency systems or for non-normal operations are often guarded. A guard must be moved to gain access to operate the switch thereby minimising the risk of inadvertent operation.
Electrical Components Chapter 2
Electrics 2-7
The following types of switches may be found on aircraft:
Toggle switches (tumbler switches) are general-purpose switches and are used extensively. They range from simple on/off selectors to ganged double or triple throw switches often incorporating a spring-loaded position for intermittent selections, for instance selecting test.
Push switches are used for momentary actions when a circuit is to be completed or interrupted for a finite time. An example of this type of circuit is the start circuit of many turbine aircraft. The start push is held in electromagnetically when operated and released when the engine has reached self-sustaining speed. Many push switches incorporate illuminated lens caps to indicate that the specific circuit has been selected. As with toggle switches, the contacts can be arranged to make or break when operated.
Chapter 2 Electrical Components
Rocker switches are single throw or double throw and may have one or both throws spring loaded to the centre-off position. The spring return allows one shot operations such as reset circuits.
Rotary switches are manually operated and are often used as selector switches, such as when selecting a single voltmeter to measure voltage across different busbars or generators.
Micro switches are extensively used throughout aircraft systems in both remote control circuits and remote position sensing and indication. The switch has a snap action and is operated by a spring leaf or roller and cam impinging on the switch-actuating plunger. The actual movement of the spring is very small, typically in the range of a few millimetres.
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Some typical circuits using micro switches include: ¾ Landing gear systems
¾ Door warning systems
¾ Power lever sequencing of system operation (arming of power augmentation systems)
¾ Weight on wheels sensing, which isolates circuits that should not operate on the ground
Rheostats are used to alter the amount of current in a circuit by varying the total resistance (e.g. to vary the intensity of panel or flight deck lighting). They normally also have an OFF position to completely remove the current.
Time switches are used to perform timing functions. They can be operated by a clockwork mechanism, electric motors or electronically. Obviously, clockwork mechanisms are old technology and are only found in older generations of aircraft. Examples of aircraft-specific timed operations are power switching between heater mats and propeller de-icing, where an electric motor is often the drive for the timing switch and turbine-engine start systems, where electronic timing and switching is employed in modern aircraft. The timing cycles of both electric motor- driven time switches and electronic timers can be varied to suit different operating conditions. Mercury switches rely on the fluid properties and electrical conductivity of mercury. Contained in a slightly curved tube of insulating material such as glass or ceramic, mercury can electrically connect between two or three electrodes fixed into the container, forming a switch which is dependent on the tilt of the switch. They are found in instruments such as the Artificial Horizon, where gravity is used as the controlling force, and in any other circuit where gravity is a controlling force.
Pressure switches are used in control and protection circuits and indication circuits where pressure is an important parameter. There are many different types of pressure switches dependent on their application and on the systems in which they are fitted. For instance, a pressure switch installed in a hydraulic circuit is subjected to very high pressure, so the switch itself has to be very robust. They often take the form of solid metal cylinders containing the switch mechanism. An altogether different pressure switch is employed in cabin pressurisation circuits where the weight of a solid metal container can be saved by using a much lighter construction. Thermal switches are sensitive to temperature. Such switches are employed where temperature must be measured or sensed. Most switches in common use are either electronic or are based upon the bending properties of a metallic strip which in turn operates a micro switch (see bi-metallic switches below).
Chapter 2 Electrical Components
Proximity switches are similar to micro switches in application. They are either magnetically or electronically operated when a steel or ferrous metal is brought into close proximity to the sensing element. Their reliability is greater than micro switches because they contain no moving parts. Bi-metallic switches are also thermal switches, but specifically use the principle of a bi-metallic strip. Two different metals with different rates of expansion with temperature are fastened together so that the strip will bend when subjected to varying temperatures. By careful design, the strip can be made to operate a snap spring to open or close a micro switch at a specific temperature. They are most often found in cooling circuits for either control or indication.
ELECTRIC GENERATOR
An electrical generator is a mechanical device that changes mechanical energy into electrical energy by using permanent magnets or electromagnets with rotating conductors. Engine driven generators produce a voltage that causes current to flow when electrical circuits are switched on. Depending on design, generators may produce DC or AC.
ALTERNATOR
As with a generator, an alternator produces electricity. Unlike the generator, alternators are DC machines only. However, the method of producing DC differs from the DC generator as discussed later.
ELECTRIC MOTOR
These are electro mechanical devices that convert electrical energy into mechanical energy and are employed extensively throughout aircraft systems.
Electrics 3-1
INTRODUCTION
All aircraft electrical systems include a battery used to:
¾ Supply power to essential services in the event of generator failure ¾ Stabilise the power supplies during switching of transitory loads ¾ Supply power for engine starting
Batteries are made up of a number of units called cells. Each cell consists of a series of negative and positive plates, immersed in a liquid known as electrolyte.
All cells and batteries store energy in a chemical form, which can be released as electrical energy. The following basic types of cells exist:
¾ A Primary Cell is not rechargeable and has a limited use in aircraft, where it is mainly used for emergency lighting.
¾ Secondary Cell batteries are rechargeable, and are the type mainly used in aircraft. They are either of the lead-acid or Nickel-Cadmium (Ni-Cd)/alkaline variety.
Chapter 3 Aircraft Batteries
LEAD ACID BATTERY
Each cell of a lead acid battery consists of positive plates of lead peroxide and negative plates of spongy lead, as shown below.
The plates are interleaved, and insulated from each other by plastic separators. An odd number of negative plates is used with one positioned either side of the positive plates to prevent buckling by evening out the thermal distribution. The complete structure is supported in an acid-resistant casing that contains an electrolyte of distilled water and concentrated sulphuric acid to a level just above the plates. Each cell is 2.2 V fully charged and 1.8 V fully discharged. Aircraft batteries of this type consist of either six cells (12 V) or twelve cells (24 V).
When a battery is connected to an external circuit, electrons in each cell are transferred through the electrolyte from the spongy lead to the lead peroxide. The net result of the chemical reaction is that a voltage is created across the cells of the battery. Consequently, lead sulphate forms on both plates of each cell. At the same time, the formulation of water dilutes the electrolyte, which takes place during the chemical reaction. For practical purposes, each cell is fully discharged when the specific gravity (SG) or relative density of the electrolyte falls from 1.27 SG (fully charged) to 1.1 SG (fully discharged), which equates to 2.2 and 1.8 V respectively. Any change in the temperature of the electrolyte also varies its specific gravity, so a correction must be made if the temperature is non-standard. The specific gravity of the electrolyte also determines its freezing point, therefore, a discharged battery is more prone to freezing.
Aircraft Batteries Chapter 3
Electrics 3-3
Lead acid batteries may be recharged by connecting the positive and negative terminals respectively, to the positive and negative terminals of a DC source of a slightly higher voltage than the battery. All of the fore-going reactions are reversed; the lead sulphate is removed from both plates, the positive plate is restored to lead peroxide, the negative plate is restored to spongy lead, and the electrolyte is restored to its original Specific Gravity (SG).
ALKALINE BATTERY (NICKEL-CADMIUM)
Each cell of a nickel-cadmium battery in a fully charged condition consists of positive plates of Nickel Oxide and negative plates of pure Cadmium, as shown below.
The plates are interleaved and fully immersed in an electrolyte of dilute potassium hydroxide. The plates and electrolyte are housed in a stainless steel or plastic container. Each cell is 1.2 V (fully charged) and 1.1 V (fully discharged). Batteries of this type for use on an aircraft consist of either twenty cells (24 V) or twenty-two cells (26 V).
During discharge, the negative plates turn into cadmium hydroxide, and the positive plates turn into nickel hydroxide. The electrolyte in an alkaline cell has a specific gravity of 1.26, which remains constant whether it is in a charged or discharged condition.
Like lead-acid batteries, alkaline batteries are recharged by connecting the positive and negative terminals respectively to the positive and negative terminals of a DC source of slightly higher voltage than the battery. The chemical reaction is reversed, and the plates return to their former states; the negative plates to cadmium, and the positive plates to nickel oxide.
Chapter 3 Aircraft Batteries
BATTERY VENTING
As batteries are charged, their temperature increases and volatile hydrogen gas is given off, which is safely vented to atmosphere by way of various systems. In each case, however, a certain amount of water is lost by evaporation, and it is, therefore, necessary to top the battery up to a specific level from time to time with distilled water.
¾ Lead-Acid Battery Venting
Lead-acid batteries are vented using one of the following methods:
• The Non-Spill Vent is most commonly used on small aircraft and allows the hydrogen gas to escape, whilst retaining the electrolyte.
•
• The Cross-Flow Cell System is used on larger aircraft where cabin pressurisation air flows over the tops of the cells and vents the battery to atmosphere.
¾ Alkaline Battery Venting
Alkaline batteries give off a mixture of hydrogen and oxygen gases toward the end of charging. As with lead-acid batteries, there are different types of alkaline batteries:
• Semi-Open Battery cells are allowed to gas freely in order to avoid overheating, which can result from overcharging. The gases given off during the chemical reaction are vented safely to atmosphere using a cross-flow venting system. These batteries must also be topped up at regular servicing
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ELECTROLYTE SPILLAGE
Any electrolyte spilled from a battery, normally due to heavy landings and severe turbulence, must be neutralised before it damages the aircraft structure. The neutralising agents for this purpose are as follows:-
¾ Lead-Acid Battery - Use a solution of Bicarbonate of Soda. ¾ Alkaline Battery - Use a solution of Boric Acid.
It is important that once the area is neutralised, copious quantities of fresh water are used to cleanse the area and prevent corrosion from setting in.
BATTERY CAPACITY
The capacity of a battery is measured in Ampere-Hours (AH), and is a measure of the total amount of energy that it contains. It is based on the maximum rated current in amperes delivered by a battery for a known period until it has discharged to a permissible minimum voltage level, which varies according to the size and number of plates in each cell. The following definitions apply:
¾ Rated Capacity is the manufacturer’s stated capacity that is usually stamped on the side of the battery (e.g. 40 AH). This signifies that the battery is designed to last 10 hours when discharged at a 4-Ampere rate, or 1 hour when discharged at a 40-Ampere rate.
¾ Actual Capacity is the capacity of the battery as determined by a Capacity Test. Batteries used in aircraft are normally removed and their capacity checked at specified intervals in a specific battery-charging bay, where the following process takes place:
¾ Fully discharge the battery. ¾ Fully charge the battery.
¾ Discharge the battery at known current level to a minimum permissible voltage level, and note the time taken.
¾ Multiply the current by the time taken to obtain the Actual Battery Capacity. ¾ Compare this value against the battery’s Rated Capacity.
Capacity Rated
Capacity Actual
x 100 = 4038 x 100 = 95%
Chapter 3 Aircraft Batteries
BATTERY CHARGING
The following methods are used to charge the batteries whilst installed in the aircraft:
¾ Constant Voltage is used mainly on aircraft fitted with lead-acid batteries. The battery-charging rate is proportional to the difference between the battery and the generator voltage, which in aircraft using 24 V batteries is 4 V (i.e. the generator voltage is normally regulated at 28 V).
¾ Pulse Charging is used mainly on alkaline batteries. Aircraft using this method are fitted with a battery charger supplied by Alternating Current (AC). This source is rectified to provide a constant Direct Current (DC) of approximately 50 amps that continues flowing until the battery is nearly fully charged. The charger goes into a pulse DC current mode to keep the battery topped up. A temperature sensor within the battery is normally designed to reduce or even stop the charging if the battery starts to overheat.
THERMAL RUNAWAY
Batteries are capable of performing to their rated capacity when the temperature conditions and charging rates are maintained within the values specified by the manufacturer. If these values are exceeded, Thermal Runaway can occur, which causes violent gassing and boiling of the electrolyte. If this condition continues, the temperature of the battery rises to such a level that it may melt or even explode and cause damage to the aircraft structure. When a battery exceeds a certain temperature, its internal resistance reduces, allowing a higher charging current to flow and the battery temperature to rise. This effect is self-perpetuating, and in some aircraft, particularly those employing alkaline batteries, temperature-sensing devices are located within the batteries to provide a battery-overheat warning on the flight deck. This indicates that the battery should be electrically isolated.
BATTERY STATE OF CHARGE
The state of charge of lead-acid batteries can be found by: ¾ Measuring the terminal voltage
¾ Measuring the specific gravity of the electrolyte
Alkaline batteries have a fairly constant voltage output until they are discharged, and the specific gravity of the electrolyte does not alter significantly from fully charged to fully discharged. This makes it difficult to ascertain the state of a battery using the above means, so an alkaline battery must be assumed serviceable unless the voltage is obviously low.
Aircraft Batteries Chapter 3
Electrics 3-7
BATTERY CONDITION CHECK
An aircraft battery is a vital piece of equipment. Check the following for serviceability prior to flight:
¾ Examine the battery OFF load and note its voltage reading. ¾ Select a specified load and note the new voltage reading.
¾ Compare both ON and OFF load readings, and ensure that the difference between the readings is within a set tolerance.
EMERGENCY USE
In an emergency, the aircraft batteries must be capable of maintaining a supply for a minimum period of time, according to JAR:
¾ Main batteries must last at least 30 minutes after total failure of the electrical generating system. (Refer JAR 25.1303).
¾ Emergency Lighting Batteries must last for at least 10 minutes.
CONNECTION OF BATTERIES
Batteries that are connected together must be of the same type (i.e. acid and alkaline batteries must never be mixed). Batteries may be connected together as follows:
SERIES CONNECTION
If three identical batteries are connected in series, their voltages are added together, but their capacity remains the same as that of an individual battery, as shown below.
Chapter 3 Aircraft Batteries
PARALLEL CONNECTION
If identical batteries are connected in parallel, their capacities are added together, but the voltage remains the same as that of an individual battery.
SPARE BATTERIES
Spare batteries are sometimes carried for operations away from ground servicing facilities, and no attempt should be made to change the batteries in flight.
BATTERY COMPARTMENT INSPECTION
Prior to flight, check the battery compartment as follows:¾ Check the batteries for security. ¾ Check the electrical connections. ¾ Check for any electrolyte spillage.
Electrics 4-1
INTRODUCTION
Any study of electricity cannot be conducted without considering the close relationship between it and magnetism. When a current flows in a wire, a magnetic field develops around it. If a magnetic field has movement near a wire, an electric voltage develops in the wire. A magnet has a magnetic field around it that attracts metal objects toward it and can be visualised by sprinkling iron filings on a piece of paper over a magnet. This also shows that the invisible lines of magnetic force flow in circuits or loops, and that they do not cross each other. Magnetic flux is the flow of magnetism around a circuit.
The above illustration also shows that magnetism is concentrated at the extremities of a magnet, called the poles. If it is freely suspended, a magnet always aligns itself in a North-South orientation.
The North-seeking or red pole always points North, and the South-seeking or blue pole always points South.
The earliest known form of magnetism is Lodestone, which is a natural mineral found in Asia. It was found that if a piece of this ore was suspended horizontally by a thread, or floated on a piece of wood in water, it would align itself in a North-South direction.
Chapter 4 Magnetism
This characteristic led to its use as a compass. Lodestone means leading stone.
The north-south alignment occurs because the Earth itself is a huge magnet with its own magnetic field. The fields interact with each other and the Lodestone aligns itself according to the fundamental laws of magnetism. Other than the Earth itself, Lodestone is the only natural magnet. All other magnets are produced artificially. For example, an iron bar becomes magnetised if it is repeatedly rubbed against a piece of Lodestone, and a magnetic field is created if an electric current is passed through a coil of wire. Magnets are additionally classified by their shape and can exist as horseshoe, bar or even ring magnets.
Conversely a magnet can be demagnetised by:
¾ Heating it to a temperature known as its Curie Point ¾ Hitting it with a hammer
¾ Placing it in an alternating field created by feeding an alternating current through a coil, known as Degaussing
MAGNETISM
FUNDAMENTAL LAWS OF MAGNETISM
The fundamental laws of magnetism are as follows:¾ The line through the poles is called the magnetic axis. ¾ Red or blue poles cannot exist separately.
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CHARACTERISTICS OF LINES OF MAGNETIC FLUX
Lines of magnetic flux have the following characteristics:¾ They have direction or polarity, and the lines of magnetic flux travel externally from the North Pole to the South Pole, as indicated in the following diagram.
¾ They always form complete loops, where each line of magnetic flux travels back through the body of the magnet to form a complete loop.
¾ They never cross each other; which is why like poles repel, since lines of magnetic flux having the same polarity can neither connect nor cross. When one field intrudes into another, as shown below, the lines repel, and the magnets tend to move apart.
Chapter 4 Magnetism
They tend to form the smallest possible loops, which is why unlike poles attract. Lines of magnetic flux having the same polarity link up, as shown below, and the resulting loops attempt to shorten by pulling the two magnets together.
¾ They can be distorted by interacting with other flux lines, as shown below. This is because the lines of magnetic flux pass through soft iron more readily than air, and at the same time, the lines tend to contract to make the smallest possible loops. The iron bar is attracted toward the magnet, and strengthens its overall magnetic field.
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CLASSIFICATION OF MAGNETIC MATERIALS
Theoretically, all materials are affected to some extent by a magnetic field, and can be placed in one of the following categories:
¾ Ferromagnetism is the property of a material that enables it to become a permanent magnet (i.e. when placed in a magnetic field, ferromagnetic materials will develop a very strong internal field and retain some of it when the external field is removed). The most common ferromagnetic substances are iron, cobalt, nickel, and alloys of these metals. Above the Curie temperature, thermal agitation destroys the domain structure and the substance becomes paramagnetic. In practice, it is convenient to sub-divide ferromagnetic materials into two classes:
• Hard Iron is a material that is difficult to magnetise. However, once magnetised, it will retain its magnetism unless subjected to a strong demagnetising force. This is a Permanent magnet.
• Soft Iron is a material, which is easily magnetised, but also easily loses it magnetism when not subjected to a strong magnetising force. This is a Temporary magnet.
¾ Paramagnetic is the property of a material that has an internal field stronger than that outside and slightly attracts lines of magnetic force when placed in a magnetic field. However, once the magnetic field is removed, random thermal motion destroys the magnetism. Typical materials are platinum, manganese, and aluminium.
¾ Diamagnetic is the property of a material that has an internal field proportional to, but less than that outside and slightly repels lines of magnetic force when placed in a magnetic field. Typical materials are copper and bismuth.
MAGNETIC FLUX
Magnetic flux can be considered the equivalent of electric current and is the flow of magnetism. It moves under the influence of Magneto-motive force which can be considered the magnetic equivalent of voltage. The ease with which it flows through a medium is dependent on the material’s reluctance, the equivalent of electrical resistance. Magnetic flux is measured in Webers (Wb).
FLUX DENSITY
Flux density is the number of Webers per square metre (Wb/m2) and is known as the Tesla (β).
RELUCTANCE
Reluctance is the opposition to magnetic flux, and is similar to resistance in an electrical circuit. It is the ratio of the Magneto-Motive Force (MMF) acting on a magnetic circuit to the magnetic flux (Φ) produced.
Reluctance = MMF
φ
PERMEABILITY
Permeability (µ) is the ease by which a material accepts lines of magnetic flux and may be compared to conductance in an electrical circuit, which is the ease with which a material or circuit allows current to flow. It is the ratio of B/H, where B is the induced magnetic flux, and H is the magnetising force. The table on the next page shows how the permeability of a material determines its characteristic.
Chapter 4 Magnetism
Material Permeability Characteristic Action
Bismuth Water Copper Air Aluminium Cobalt Nickel Iron 0.999833 0.999991 0.999995 1.000000 1.000021 170 1000 7000 Diamagnetic Diamagnetic Diamagnetic Paramagnetic Paramagnetic Ferromagnetic Ferromagnetic Ferromagnetic Slightly Repelled Slightly Repelled Slightly Repelled Non-Magnetic Slightly Attracted Strongly Attracted Strongly Attracted Strongly Attracted
HYSTERESIS
Any ferrous material becomes magnetised to some degree when subjected to a magnetising force. In the diagram below, it is possible to see the effect of a magnetising force on a non-magnetised iron bar.
The H-H axis is the magnetising force and is assumed to be an electromagnet whose magnetic strength can be increased by increasing the electric current through the coil, and vice-versa. As the magnetising force increases from O to +H, the magnetism induced in the bar increases along the curved line O-C. Notice at C that although current could be increased further, the curve has flattened out, indicating that the bar cannot be further magnetised. This is known as saturation.
If the magnetising force is now reduced to zero, there is a residual magnetism left in the bar at D, which is known as remnant flux. If it is intended to completely remove the remnant flux, the magnetising force would have to be applied in the opposite polarity, known as the coercive force. At E, the magnetism in the bar has been removed, but any further increase in the magnetising force toward -H will magnetise the bar in the opposite polarity to that originally achieved, and as before, saturation will eventually be reached. It is worth pointing out that the shape of this hysterisis loop is dependent on the magnetic properties of the bar. Consider a bar which is easy to magnetise, but loses its magnetism on removal of the magnetising force; a paramagnet. The
Magnetism Chapter 4
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SATURATION
Saturation plays an important role in ferromagnetic circuits, where the magnitude of magnetism induced in a piece of iron is proportional to the current creating it. However, if the current is increased beyond a certain point, no further appreciable increase in magnetism occurs, as the iron becomes fully saturated. This is a very important property, and is the principle on which a magnetic amplifier operates.
MAGNETISM PRODUCED BY CURRENT FLOW
When current flows through a conductor, a magnetic field is produced around the conductor, and its magnitude is proportional to the current flow.
Chapter 4 Magnetism
The direction of the field depends on the direction of current flow. The Right Hand Grasp Rule is used to determine the direction of the field when a conventional current is flowing.
The thumb points in the direction of the current flow, whilst the fingers wrapping around point in the direction of the magnetic field. In explaining some aspects of electromagnetism, it is also useful to picture current flow looking at the end of a wire, by visualising a feathered arrow. If a cross sectional view of a wire is shown, a cross would indicate flow into the wire, like looking at the back of a feathered arrow. Current flowing out of the wire would be shown as a dot, like looking at the pointed end of an arrow. This principle is illustrated below.
If two wires are placed side by side, the resulting magnetic fields either attract or repel each other dependent on the direction of the current flow, as shown below.
