ATPL
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CHAPTER 1.
Basic DC Terminology
Introduction... 1-1 The Electrical Circuit... 1-1 Current (I) ... 1-2 Electromotive Force (EMF)... 1-2 Potential Difference (PD) ... 1-2 Voltage (V)... 1-2 Resistance (R)... 1-3 Connecting Resistances in Series or Parallel in a DC Circuit ... 1-3 Ohms Law ... 1-5 Loads... 1-5 Kirchhoff’s Laws ... 1-5 Electrical Power (P) ... 1-6 Electrical Work... 1-6 Electrical Unit Prefixes... 1-6 Typical Circuit Symbols ... 1-7
CHAPTER 2.
Electrical Components
Introduction... 2-1 Electrical Systems ... 2-1 Electrical Circuit Faults ... 2-2 Busbars ... 2-3 Protection Devices... 2-3 Reverse Current Circuit Breaker (RCCB) ... 2-5 Switches ... 2-5 Electrical Generator ... 2-9 Electrical Alternator ... 2-9 Electrical Motor... 2-9 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 ... 3-6 Emergency Use ... 3-7 Connection of Batteries ... 3-7 Spare Batteries... 3-8 Battery Compartment Inspection ... 3-8
CHAPTER 4.
Magnetism
Introduction... 4-1 Fundamental Laws of Magnetism... 4-2 Classification of Magnetic Materials ... 4-5 Magnetic Flux ... 4-6 Flux Density... 4-6 Reluctance... 4-6 Permeability... 4-6 Hysteresis... 4-6 Saturation ... 4-7 Magnetism Produced by Current Flow... 4-7 The Electromagnet ... 4-10 The Relay ... 4-12 Electromagnetic Induction ... 4-13 CHAPTER 5.
DC Generator Systems Introduction... 5-1 Basic Generator Theory... 5-1 Simple AC Generator... 5-1 Conversion of AC to DC ... 5-2 DC Generator System Architecture ... 5-4 DC Generator Construction ... 5-4 Principle of Operation of a DC Generator ... 5-5 Types of DC Generator... 5-5 Voltage Regulator... 5-7 Cut-out... 5-8 Reverse Current Circuit Breaker... 5-9 Busbars ... 5-9 Power Failure Warning ... 5-10 Ground Power ... 5-10 DC Generator System Fault Protection ... 5-11 Twin Engine DC Electrical System ... 5-11 Operation of DC Generators in Parallel ... 5-12 DC Load Sharing ... 5-13 Operation of an Equalising Circuit ... 5-13 Single Engine Aeroplane DC Electrical System... 5-14 Operation of the Alternator ... 5-15
CHAPTER 6.
DC Motors
Introduction... 6-1 The Motor Principle... 6-1 DC Motors ... 6-2 Back EMF ... 6-3 Direction of Rotation ... 6-4 Motor Speed Control... 6-4 Types of DC Motor... 6-5 Actuators ... 6-7 Split-Field Series Motor ... 6-8 Electromagnetic Brakes... 6-9 Clutches... 6-10 Instrument Motors... 6-10 Architecture of a Starter/Generator System... 6-10
Inverters... 6-13 Multiple Inverter Installations ... 6-14
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... 7-5 The Charging of a Capacitor... 7-6 Discharging of a Capacitor ... 7-8 The Time Constant of a Capacitor... 7-8 Capacitors in Series and Parallel in a DC Circuit... 7-8
CHAPTER 8.
Basic AC Theory Introduction... 8-1 Advantages of AC over DC... 8-1 Generating AC... 8-1 Simple AC Generator... 8-2 AC Terminology... 8-3 Relationship Between Radians and Degrees ... 8-5 Phase and Phase Angle ... 8-5 Phasor Representation ... 8-6
CHAPTER 9.
Single Phase AC Circuits
Introduction... 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 ... 9-2 Inductive Reactance (Xl)... 9-3 The Effect of Ac on a Purely Capacitive Circuit ... 9-3 Power in an AC Capacitive Circuit... 9-4 Capacitive Reactance (Capacitors Ac Resistance) ... 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 ... 9-8 AC Series Circuit Example ... 9-10 AC Parallel Circuit Example... 9-11
CHAPTER 10.
Resonant AC Circuits
Introduction... 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... 10-6
CHAPTER 11.
Transformers
Introduction... 11-1 Construction and Operation... 11-1 Types of Transformers... 11-2 Transformer Rectifier Units... 11-5
CHAPTER 12.
AC Power Generation
Introduction... 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 ... 12-3 Voltage and Frequency of AC Generators... 12-3 Phase Rotation ... 12-4 Faults on Three-Phase AC Generators ... 12-4 Generator Real and Reactive Load Sharing ... 12-5 Types of AC Generator... 12-5 Brushless Three Phase AC Generator ... 12-7 Constant Speed Drive Unit ... 12-8 Operation of the Hydro-Mechanical CSDU ... 12-10 Protection of the Hydro-Mechanical CSDU... 12-11 Integrated Drive Generator ... 12-12 Variable Speed Constant Frequency Power Systems ... 12-13 Auxiliary Power Unit... 12-13 Emergency Ram Air Turbine ... 12-14
CHAPTER 13.
AC Power Generation Systems
Introduction... 13-1 Piston-Engine Frequency Wild AC System Architecture... 13-1 Operation of a Piston-Engine Frequency Wild AC System ... 13-1 Fault Protection in a Piston-Engine Frequency Wild AC System... 13-2 Twin-Engine Turbo-Propeller Frequency Wild AC System Architecture ... 13-2 Operation of a Twin-Engine Turbo-Propeller Frequency Wild AC System... 13-3 Fault Protection in a Twin-Engine Turbo-Propeller Frequency Wild AC System ... 13-4 The Constant Frequency Split Busbar AC System ... 13-5 Operation of a Constant Frequency Split Busbar AC System... 13-5 Regulation and Protection of Constant Frequency Units ... 13-6 Faults on a Constant Frequency Split Busbar AC Generator System ... 13-6 Emergency Supplies... 13-7
Battery Power ... 13-8 Ground Handling Bus ... 13-8 Constant Frequency Parallel AC System... 13-8 Operation of a Constant Frequency Parallel AC System ... 13-9 Reactive Load Sharing ... 13-11 Real Load Sharing... 13-11 Paralleling... 13-11 Fault Protections in a Constant Frequency AC Parallel System ... 13-12 DC Power Supplies... 13-13
CHAPTER 14.
AC Motors
Introduction... 14-1 Stator-Produced Rotating Magnetic Field ... 14-1 Induction (Squirrel Cage) Motor... 14-2 Two-Phase Induction Motor... 14-5 Split-Phase Motor ... 14-5 The Synchronous Motor ...14-5
CHAPTER 15.
Semiconductor Devices
Introduction... 15-1 Advantages and Disadvantages of Semiconductor Devices... 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... 15-3 Use of Diodes ... 15-5 Bi-Polar Transistors ... 15-7 Operation of a PNP Bi-Polar Transistor ... 15-8 Operation of a NPN Bi-Polar Transistor... 15-8 Disadvantages of Diodes and Transistors ... 15-9 Transistor Applications ... 15-9 Integrated Circuits ... 15-10 The Advantages and Disadvantages of Integrated Circuits ... 15-11 Types of Integrated Circuits... 15-11
CHAPTER 16.
Logic Circuits
Introduction... 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 Analogue Computers ... 17-1 Digital Computers ... 17-1 Computer Architecture... 17-2
Input Devices... 17-3 Central Processing Unit ... 17-3 Output Devices ... 17-4 Storage Devices ... 17-5 Operating Systems ... 17-5 Programming ... 17-5
CHAPTER 18.
HF and Satellite Airborne Communications
Long Range Communications (Up to 4000 Km) ... 18-1 Short Range Communications (Up to 450 Km)... 18-3 Selective Calling (SELCAL) System ... 18-4 Satellite Communications (SATCOM)... 18-4 Satellite Aircom (SITA) ... 18-6
Chapter 1.
Basic DC Terminology
IntroductionThis chapter covers the basic Direct Current (DC) terminology, the connection of resistances in electrical circuits, and any associated laws.
The Electrical 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 provides the force necessary to influence the flow of electrons around the circuit when the switch is closed, whilst the load is an electrical device that performs a useful function, eg. a lamp, a motor or a heating element. The switch makes and breaks the flow of current to the load, which only performs a useful function when current flows through it, whilst the conductor provides a low resistance path for the current to flow. In aeroplanes these conductors are usually formed from aluminium or copper or aluminium, or even the metal structure of the aeroplane.
When the switch is closed, the force from the supply causes electrons to flow outside the source from the negative to the positive terminals, and is known as ‘Electron Flow’. Current flowing from the positive to the negative terminals outside the source is alternatively known as ‘Conventional Current Flow’.
Current (I)
Current is an indication of the flow of electricity and is measured in amperes (amps). This is the rate at which electrons flow in a conductor, such that when one Coulomb (C) or 6.25 x 1018 electrons pass a point in a conductor in one second, a current of one ampere is said to flow.
Amperes =
Seconds
Coulombs
Current in a circuit is measured by connecting an ‘Ammeter’ in line, or in series with the load, as shown below.
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. Potential Difference (PD)
Even though a circuit is open, and no current is flowing, a power source still has the potential for current flow. Thus whether a battery is connected in a circuit or not, a potential difference will still exist between its terminals.
Voltage (V)
Voltage is the basic unit of electrical pressure and is measured in ‘Volts’, where one volt is the amount of pressure (EMF) that will cause one Coulomb of charge to move from one point to the other. A ‘Voltmeter’ is used for measuring voltage, and is connected across the load,
Resistance (R)
Resistance is measured in Ohms, where one Ohm is the amount of resistance that will allow one ampere of current to flow in a circuit to which one volt EMF is applied.
Resistance opposes current flow and causes a reduction in the voltage. In doing so it produces heat, and power is consumed. Rubber and glass are examples of ‘Insulators’, and offer a great deal of opposition to the flow of electricity, ie. high resistance. These materials prevent conductors coming into contact with other objects, which could be harmed or damaged. Other materials such as silver and copper have very little opposition to current flow, ie. low resistance, and are known as ‘Conductors’. Alternatively materials, which offer some resistance to the flow of electricity, are known as ‘Semi-Conductors’.
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
A
x L
Resistors can have either fixed or variable values. An example of a variable resistor is a rotary switch, which is used to control* the intensity of a lighting circuit.
Temperature also affects the resistance of a material. The resistance of most materials increases with increasing temperature, and exhibit a ‘Positive Temperature Coefficient
(PTC)’. The resistance of a few materials, however, decreases with increasing temperature
and exhibit a ‘Negative Temperature Coefficient (NTC)’. Generally most conductors have a ‘PTC’, whilst insulators and semi-conductors have a ‘NTC’. Another form of resistor is a ‘Thermister’, which is a NTC device, and is used for measuring temperatures in aeroplanes, ie. the higher the temperature the lower the resistance.
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 the same current flows through each of them, and the total opposition to current flow is thus equal to the sum of the individual resistances.
Total Resistance
(R
T) = R
1+ R
2+ R
3+ ……
= 15 + 22 + 31 = 68Ω
If the resistances are alternatively connected in parallel with each other, the current will flow along two or more paths, as shown on the next page-.
The greater the number of resistors connected in parallel the lower the overall resistance, and the greater the current flow from the supply. In a parallel circuit the supply voltage is common to all resistors, and the total resistance is calculated using the following method:-
T R
1
= 1 R1
+
2 R1+
R1
3
T R
1
=41
+
61+
121=
162 RT =12
6
= 2 ohmsBy firstly calculating the equivalent resistance in the parallel part of the circuit, and then adding this value to the series resistance enables the total resistance to be found. 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
=
3.6Ω(i) Total Circuit Resistance
RT = RTP + R3 = 3.6 + 2.4 = 6 ohms
Ohms Law
Ohms 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. Thus the higher the voltage the higher the current, and the higher the resistance the lower the current. Ohms Law may thus be stated by the following formulae:-
R = V ohms, V = IR volts or I = V amps I R
Loads
Loads are items of electrical equipment that have varying amounts of resistance, and are normally connected in parallel with the supply. Thus the amount of current being drawn from the supply will increase as more items of equipment are switched on.
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 drops always equals the supply voltage.
In the circuit shown above a 10 volt battery is connected across a lamp, and as current flows through the circuit, a voltage drop will be developed across the lamp. The lamp will thus consume the same amount of energy as the battery provides, and the voltage drop across the lamp will equal the supply voltage.
If two identical lamps are connected in series, then the voltage drop across each will be the same, and the sum of the voltage drops will similarly equal the supply voltage.
Electrical Power (P)
Electrical power is the amount of work done in a specific time, and is the ability of an electrical device to produce work. Power is measured in ‘Watts’ (746 watts equals 1 horsepower). One Watt is also equal to one Joule per Second (J/s), which is the work done in one second by one volt of EMF, in moving one Coulomb of charge, ie. when one volt causes one ampere to flow, a power of 1 watt will be consumed. Power is represented by the following formulae:-
P=VI or I2R or
R 2 V .
Electrical Work
Electrical work is defined by the product of force x the distance an object moves under the influence of electrical power.
Electrical 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
Typical Circuit Symbols
Chapter 2.
Electrical Components
IntroductionElectrical circuits form an integral part of an aeroplane, and must be adequately protected. The flight crew must also be able to select and operate any equipment’s safely.
Electrical Systems
The possible electrical system layouts are:-
Single Pole or Earth Return System. This system is used on aeroplanes 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 also gives a reduction in aeroplane weight.
Dipole or Two-wire System. This system is used on aeroplanes, which are 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 thus increases the aeroplanes overall mass.
Ground (Earth). This is simply a zero or reference point within an electrical circuit and is the
metal frame or chassis on which the various electrical circuits are constructed. On an aeroplane the metal airframe is called ground or earth.
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. For example 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, then the other terminal of the battery will be +12 volts.
Electrical Circuit Faults
The following faults can occur in an electrical circuit:-
Short Circuit. This fault will occur in:-
an earth return system, if the live conductor touches the metal airframe, or in a di-pole circuit, if both conductors touch each other.
If a short circuit occurs due to a fault a low resistance path will be created across the supply. This will cause an extremely high current to flow, causing possible damage to the circuit and any associated wiring. It may even burn the cables, and cause a fire.
Open Circuit. This type of fault will occur in:-
an earth return circuit if the conductor breaks, or becomes disconnected, or in a dipole circuit if either of the conductors becomes broken or disconnected.
Busbars
A busbar is a current distribution point from which individual circuits take their power, and is simply a strip of metal, which is supplied with a voltage from the main power generating system or one particular element thereof. The busbars are also sub divided into vital, essential and non-essential, which indicates their power source or their importance in the overall system. The vital busbar or battery bus is supplied direct from the aeroplane battery, and supplies power to the vital systems that may be required in a crash situation, eg. fire extinguishers and fuel shut off valves. The essential busbars supply the systems required for the safe flight of the aeroplane, ig. Navigation lights and instrumentation, whilst the non-essential busbars supply the systems, which can be safely switched off in an emergency, eg. galley ovens.
Protection Devices
The following protection devices exist in an aeroplane electrical circuit:-
Electrical fuse. This protection device will open or break the electrical circuit when
excessive current flows. This is because the magnitude of the 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 burn out. 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, which is the maximum current they can carry without overheating and rupturing. They are located as near the supply (busbar) as possible, so that if an excessive current flows, due to a short circuit, the fuse will rupture protecting all of the cable to the load. In practice aircraft are required by law to carry spare fuses; minimum stocks of each type of fuse being 3 or 10%, whichever is the greater.
Actions to be taken 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
If the fuse blows again, switch off the circuit and do not attempt a further replacement. National and company regulations must be followed in this respect, but in either case the fault must be reported on landing.
Current Limiters. These devices are used mainly to protect heavy-duty power distribution
circuits and consist of a high melting point filament of tinned copper encased in a ceramic housing.
The central portion of the filament in some types is wasted to form the fusible area. The time/current characteristics of the device allow a considerable overload current to flow in the circuit before rupturing occurs.
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, which can be opened (tripped) via a bi-metallic strip, as shown on the next page. If an overload current exists the bi-metallic strip will heat up and distort, causing the latch mechanism to be released. This will cause the main contacts of the circuit breaker to open, and a push-pull button to pop out. A white band will also be revealed, and indicates that the circuit breaker has tripped. To reset the circuit breaker the button that protrudes when it trips needs to be pushed in again. On modern aeroplanes circuit breakers are fitted in preference to fuses, and are referred to as ‘trip-free’, ie. they can not be reset whilst the fault still exists, regardless of whether the button is held in or not.
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
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.
Circuit Breakers have the following advantages over fuses:-
¾ No spares have to be carried.
¾ They can be used as a switch, eg. when carrying out aeroplane maintenance.
A circuit breaker that can be physically held in against the fault is known as a Non-Trip Free Circuit Breaker. If this is allowed to happen it may cause severe damage to the aircraft wiring, and in extreme circumstances may even lead to a fire.
Reverse Current Circuit Breaker (RCCB)
Reverse current circuit breakers are used in DC power supplies to protect against short circuits and prevent extremely high currents flowing towards the power source. They operate at high speed, and are manually reset. 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 aeroplane electrical installations, switches and relays principally perform the function of installing and controlling the operating sequences of circuits. Circuit breakers, though they control the flow of current to and within systems, are regarded as only circuit protection devices.
In its simplest form, a switch consists of two contacting surfaces, which can be isolated from each other or brought together by a moveable-connecting link, called a ‘Pole’, and the number of circuits it controls is known as its ‘Throw’. Some examples of these are shown below.
If a switch has only one operating toggle it is known as a ‘Single Pole Switch’, but a switch where two or three toggles have been grouped together is known as a ‘Double’ or ‘Triple
Pole Switch’. Switches which use 2 or 3 position switches, may be fitted with guards or
latches to hold them in their normal operating positions with cover plates, spring loaded sliding guards or physical restraints, all of which have to be moved to operate the switch.
The following types of switches are used on aeroplanes:-
Toggle switches (tumbler switches). These are general-purpose switches and are
extensively used.
They have simple ON/OFF functions and may be ganged, guarded or be 2 or 3 position devices, as shown on the next page.
Push switches. These switches are used for short duration operations. ie. when a circuit is
to be completed or interrupted for a short duration. Other types are designed to close one or more circuits (through separate contacts) whilst opening another circuit. They may be designed for either push to make or push to break operation. Some contain small lamps which, illuminate legends. These are typically used in turbo-prop engine start and stop circuits, and operate either manually or electro-magnetically.
In a typical start circuit the button is normally held latched in place until the start sequence is complete, whilst in a stop circuit the button usually operates the circuits which stop the fuel supply, remove electrical power from other engine systems and shut the engine down.
Rocker button switches. These switches combine the action of both toggle and push
button switches, eg. in a generator system a single switch may allow ‘ON, ‘OFF’ (selectable) and reset (spring loaded) selections to be made using the same switch.
Rotary switches. These switches are manually operated and are often used in place of
toggle switches. A typical use is the selection of a single voltmeter between several busbars, generators or batteries.
Micro-switches. These are a special type of switch, and are the type most extensively used
in aeroplanes. It is a switch in which the travel between make and break is in the order of a few thousandths of an inch.
Activation of micro switches varies with the designs of the system, but are usually by a lever, roller or cam. They are used in various applications such as:-
¾ Landing gear systems to indicate the position to the indicator lights, ¾ Door warning systems,
¾ Power lever sequencing of system operation (arming of power augmentation systems)
¾ Nose wheel or main wheel weight on switches to ensure that systems do not operate on the ground.
Rheostats. These switches are used to alter the amount of current in a circuit by varying the
overall resistance, eg. to vary the intensity of panel or flight deck lighting. They normally also have an ‘OFF’ position, to completely remove the current.
Time switches. These switches are required to operate pre-determined controlled time
sequences. They are usually linked to, and are controlled by an electric motor. For example the switching of power between the heater mats on propellers, or between pairs of propeller blades to achieve de-icing. In some sequences the time switch operations can be varied, which is done by a rocker or toggle switch, via a continuous operating time switch that selects power for different time sequences.
Mercury switches. These switches are glass tubes in which stationary contacts and a pool
of loose mercury are hermetically sealed, as shown on the next page. Tilting the tube causes the mercury to flow and close or open a gap, thus make or breaking a circuit. A typical application is in the torque motor circuits of Artificial Horizons where the gyro must be forced to, and be maintained in the vertical position.
Pressure switches. These switches are used to indicate high and low pressure in systems
where pressure measurement is involved, eg. Hydraulic systems. They are usually linked to warning captions to indicate high or low pressure outside normal limits. Pressure switches
are also installed in cabin pressurisation systems to indicate high differential pressure and cabin altitude above set limits.
Thermal switches. These switches are used in systems where warning of excessive heat is
required, eg. in engine fire and over-heat warning systems.
Proximity switches. These switches are used in some aeroplanes to give warning of
whether or not passenger doors, freight doors, etc. are fully closed and locked. They have certain advantages over micro switches in that they have no moving parts, which might break or malfunction.
Bi-metallic switches. These switches are used in temperature sensitive areas where
smaller devices than thermal switches may be required. Two different metals with different co-efficient rates of expansion are fastened together. The different metals cause the combined plates to bend and make or break contacts. They may be used in instruments, especially electronic instruments, to operate cooling fans that maintain internal temperatures within limits.
Electrical Generator
An electrical generator is a mechanical device that changes mechanical energy into electrical energy by using permanent magnets or electromagnets with moving conductors. Generally engine driven generators produce a voltage or EMF, which causes current to flow when the electrical circuit is completed.
Electrical Alternator
An electrical alternator is sometimes incorrectly referred to as a DC generator, because the alternating current output it produces is changed directly into DC within the alternator itself.
Electrical Motor
An electrical motor is an electrical or mechanical device that changes electrical energy back into mechanical energy. These are extensively used in many aeroplane electrical systems.
Chapter 3.
Aircraft Batteries
IntroductionAll aircraft electrical systems include a battery, which is 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, which are immersed in a liquid known as electrolyte.
All cells and batteries store energy in a chemical form, which can then be released as electrical energy. The following basic types of cells exist:-
¾ Primary Cell. This type of cell is not rechargeable and only has a limited use in aircraft’s, where it is mainly used for emergency lighting.
¾ Secondary Cell. Batteries made up of secondary cells are rechargeable and are the type mainly used in aircraft’s. They are either of the lead-acid or Nickel-Cadmium (Ni-Cd) / alkaline variety.
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 containing an electrolyte of distilled water and concentrated Sulphuric acid, to a level just above the plates. Each cell is 2.2 volts fully charged and 1.8 volts fully discharged. In aircraft’s batteries of this type consist of either six cells (12 volts), or twelve cells (24 volts).
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 and the net result of the chemical reaction is that lead sulphate forms on both plates. At the same time the electrolyte is diluted by the formation of water, 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 volts’ respectively. Any change in the temperature of the
is non-standard. The Specific Gravity of the electrolyte also determines its freezing point, and a discharged battery will freeze at a lower temperature than a fully charged battery. Batteries constructed from this type of cell must not be left in a discharged condition for extended periods of time since the Lead Sulphate will harden on the plates and cut down their active area. This process is known as ‘Sulphation’, which can drastically shorten the life expectancy of a battery.
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 placed in a stainless steel or plastic container.
Each cell is ‘1.2 volts (fully charged)’ and ‘1.1 volts (fully discharged)’. Batteries of this type for use on an aircraft consist of either twenty cells (24 volts), or twenty-two cells ( 26 volts). 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 can be 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.
Battery Venting
When charging batteries 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 distilled 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:-
¾ Non-Spill Vent. This type of vent is most commonly used on small aircraft’s and allows the hydrogen gas to escape, whilst retaining the electrolyte.
¾ Cross-Flow Cell System. This system is used on larger aircraft’s, 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 towards the end of charging. Similar to lead-acid batteries different types of alkaline battery also exist:-
¾ Semi-Open Batteries. In this type of battery the cells are allowed to gas freely in order to avoid overheating, which can result from overcharging, and 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 intervals with distilled water.
¾ Sealed Batteries. In these batteries, the cells are completely sealed and require no maintenance.
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 - A solution of Bicarbonate of Soda. ¾ Alkaline Battery - A solution of Boric Acid.
It is important that once the area is neutralised that copious quantities of fresh water are used to cleanse the area, and prevent corrosion setting in.
Battery Capacity
The capacity of a battery is a measured in ‘Ampere-Hours (Ahr)’, and is a measure of the total amount of energy that it contains. It is based on the maximum rated current in amperes, which will be delivered by a battery for a known time 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. This is the manufacturers stated capacity that is usually stamped on the side of the battery, eg. 40 Ahr, which 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. This is the capacity of the battery, which is determined by a Capacity Test.
Batteries used in aircraft are normally removed, and capacity checked every 3 months 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 amperage to a permissible minimum voltage level, and time how long it takes.
¾ Compare this value against the batteries ‘Rated Capacity’. Eg. ActualRatedCapacityCapacity x 100 = 4038 x 100 = 95%
Note: For continued use in aircraft this value must be 80%, or more.
Battery Charging
The following methods are used to charge the batteries whilst installed in the aircraft:-
¾ Constant Voltage. This method 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 12 Volt batteries is 2 volts, ie. the generator voltage is normally regulated at 14 volts.
¾ Pulse Charging. This method is used mainly on alkaline batteries, and aircraft using this method are fitted with a battery charger, which is supplied by Alternating Current (AC). This source is then rectified to provide a constant Direct Current (DC) of approximately 50 amps that continues flowing until the battery is nearly fully charged. The charger then 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. In the event of these values being exceeded ‘Thermal Runaway’ can occur, which causes violent gassing and boiling of the electrolyte. If this condition is allowed to continue the temperature of the battery will rise to such a level that it may melt, or even explode, and may cause damage to the aircraft structure. The reason for this effect is that when a battery exceeds a certain temperature its internal resistance drops, thus allowing a higher charging current to flow, and the battery temperature to rise. This effect is self-perpetuating, and in some aircraft’s, particularly those employing alkaline batteries, temperature-sensing devices are located within the batteries to provide a battery overheat warning on the flight deck, which indicates that the battery should be electrically isolated by the flight deck crew.
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.
The state of charge of alkaline batteries is however, not easily ascertained by these methods, and must therefore be assumed to be serviceable.
Battery Condition Check
An aircraft battery is a vital piece of equipment, and must therefore be checked for serviceability prior to flight. The check involves the following:-
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’s:-
¾ Main batteries. These batteries must last at least 30 minutes after total failure of the electrical generating system. (Refer JAR 25.1303).
¾ Emergency Lighting Batteries. These batteries must last for at least 10
minutes. Connection of Batteries
Batteries, which are connected together, must be of the same type, ie. Acid and alkaline batteries must never be mixed. Batteries may be joined 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.
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
Some aircraft carry spare batteries, but no attempt should be made to change the batteries in flight.
Battery Compartment Inspection
Prior to flight the battery compartment should be checked as follows:- ¾ Check the batteries for security.
¾ Check the electrical connections. ¾ Check for any electrolyte spillage.
Chapter 4.
Magnetism
IntroductionMagnetism is so closely allied with electricity that without it the means of creating electrical power would be greatly reduced. A magnet attracts small pieces of iron or steel, and is surrounded by a magnetic field, which is made up of invisible lines of magnetic force, or magnetic flux. This is best demonstrated by sprinkling iron filings on a piece of paper placed over a magnet.
This illustrates that magnetism is concentrated at a magnet's extremities, called poles, and if it is freely suspended it will always align itself in a north-south orientation.
The north-seeking or red pole will always point north, and the south-seeking or blue pole will always point south.
The earliest known form of magnetism was the Lodestone, which was 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 likewise align itself in a north-south direction.
This characteristic led to its use as a compass and the name Lodestone, meaning leading stone. This occurs because the earth itself is a huge magnet with it's 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; and all other magnets are produced artificially. For example an iron bar will become magnetised if it is repeatedly rubbed against a piece of Lodestone. Another type of magnet is the Electromagnet, which is produced when an electric current is passed through a conductor. 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 ‘Degaussing’ it with an alternating magnetic field.
Fundamental Laws of Magnetism
¾ The line joining the poles is called the magnetic axis. ¾ Red or blue poles cannot exist separately.
¾ Like poles repel each other, and unlike poles attract, as shown on the next page.
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 the reason why like poles repel, since lines of magnetic flux having the same polarity can neither connect nor cross.
Thus when one field intrudes into another, as shown on the next page, the lines will repel, and the magnets will tend to move apart.
¾ They tend to form the smallest possible loops, which is the reason why unlike poles attract. Lines of magnetic flux having the same polarity will link up, as shown below, and the resulting loops will 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 thus attracted towards the magnet, and strengthens its overall magnetic field.
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. This is the property of a material that enables it to become a permanent magnet, ie. Ferromagnetic materials when placed in a magnetic field will develop a very strong internal field and will 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:-
¾ Hard Iron. This is a material which is difficult to magnetise, but once magnetised will retain it magnetism unless it is subjected to a strong demagnetising force. This is known as a ‘Permanent’ magnet.
¾ Soft Iron. This is a material, which is easily magnetised, but also easily loses it magnetism when it is not subjected to a strong magnetising force. This is known as a ‘Temporary’ magnet.
¾ Paramagnetic. This is the property of a material, which when placed in a magnetic field, will have an internal field stronger than that outside, and will thus slightly attract lines of magnetic force. However once the magnetic field is removed the magnetism will be destroyed by random thermal motion. Typical materials are platinum, manganese, and aluminium.
¾ Diamagnetic. This is the property of a material, which when placed in a magnetic field will have an internal field proportional to, but less than that outside, and will thus slightly repel lines of magnetic force. Typical materials are copper and bismuth.
Magnetic Flux
Magnetic Flux (φ) is produced by a force known as the ‘Magneto-Motive Force (MMF)’, whose magnitude is determined by the product of the current and the number of turns of wire that link together the magnetic circuit. Thus the greater the current, and the greater the number of turns, the greater the resulting flux. The unit of magnetic flux is the ‘Weber (Wb)’.
Flux Density
Flux density is the number of ‘Webers per square metre (Wb/m2)’, and is alternatively
known as the ‘Tesla (B)’.
Reluctance
Reluctance is the opposition to magnetic flux, and is similar, in nature 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) being produced, ie.
Reluctance = MMF
φ
Permeability
Permeability (µ) is the ease by which a material will accept lines of magnetic flux and may be compared to conductance in an electrical circuit, which is the ease with which a material or circuit will allow 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.
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
It is possible to take a iron ring, completely de-magnetized, and measure the value of flux density (B) with respect to increasing values of magnetizing force (H). This relationship is expressed by the curve OC.
If the magnetizing force is reduced from this maximum value the flux density will follow the curve CD, and the flux remaining in the iron is called the ‘Remnant Flux’. To totally remove the remnant flux the magnetizing force needs to be reversed, which in this case is to a value of OE, whose value is called the ‘Coercive Force’. Further negative increases in H will cause B to grow in the reverse direction until saturation occurs, following the line EF.
Decreasing the value of H, and subsequently increasing H in a positive direction completes a symmetrical figure, CDEFGC, which is termed the ‘Hysteresis loop’. The word
‘Hysteresis’ means to lag behind, and this is what happens to the flux density as it lags
behind the changing values of the magnetising force.
Saturation
Saturation plays an important role in ferro-magnetic circuits, where the magnitude of magnetism being induced in a piece of iron is proportional to the current creating it. If the current is however increased beyond a certain point, no further appreciable increase in magnetism will occur, 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.
The direction of the field depends on the direction of current flow, and 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 point in the direction of the magnetic field. In explaining some aspects of electromagnetism, it is also useful to show current flow in a third dimension, which can be done using two further symbols. If a wire is viewed from the end, the tail of the arrow will indicate current flowing into the wire, and a dot on the point of the arrow will indicate current flowing out of the wire, as shown on the next page.
If two pieces of wire are placed side by side the resulting magnetic fields will act together, and will either attract or repel each other, which depends on the direction of the currents, as shown below.
Currents flowing in opposite directions will produce opposing fields and will repel each other, whilst currents flowing in the same direction will produce fields, which add together, and attract each other.
The magnetic field produced in a straight piece of wire is of little practical use, and has direction, but no north or South Pole. Unless the current is extremely high, the magnetic field
REPULSION
will have little useful strength, but by shaping the wire into a loop its magnetic characteristics can be greatly improved.
The coil of wire, as shown above, will now possess the following characteristics:- ¾ The lines of flux will be closer together.
¾ The majority of lines of flux will be concentrated in the centre of the loop.
¾ North and south poles will be created at the ends of it, and it will assume the same magnetic characteristics as that of a permanent magnet, ie. lines of magnetic flux will emerge from the north pole, and return via the south pole, as seen below.
The Electromagnet
The principle of an electromagnet is that when current passes through a loop of wire, a magnetic field is established, as shown above, and by increasing the number of turns in the wire, ie. by forming a coil, the individual fluxes will add together to produce a stronger magnetic field.
This is known as a ‘Solenoid’, and the more current that flows through the coil, the greater the number of lines of flux. The strength of the magnetic field around a coil (electromagnet) thus increases with either an increase in current, or an increase in the number of turns. Another method of increasing the strength of the magnetic flux around a coil is to insert a bar of ferromagnetic material into it, ie. soft iron, as shown below.
This has the effect of concentrating the magnetic lines of flux because an iron core is much more permeable than air, and the polarity of a coil can be determined if the direction of current through the coil is known, using the ‘Right Hand Grasp Rule’.
If the fingers of the right hand are wrapped around the coil in the direction of current flow, the thumb will point in the direction of the north pole.
The Relay
A relay uses the principle of the electromagnet (solenoid), and is typically used to remotely control a high current/voltage circuit using a low current/voltage circuit, as shown below.
Electromagnetic Induction
If relative motion exists between a conductor and a magnetic field an electromotive force (EMF) will be induced in the conductor, whose magnitude is determined by the following factors:-
¾ The strength of the magnetic field.
¾ The speed of the conductor with respect to the field. ¾ The angle at which the conductor cuts the field. ¾ The length of the conductor in the field.
These factors are all a natural consequence of ‘Faraday's law’, which states that the voltage (EMF) induced in a conductor is directly proportional to the rate at which the conductor cuts the magnetic lines of force. This principle is used in generators, and ‘Fleming’s Right Hand
Rule’ can be used to establish the polarity of the induced EMF. This rule involves the thumb
and the first two fingers of the right hand being placed at 90° to each other, as shown on the next page. The thumb points in the direction of motion of the conductor, the index finger points in the direction of the lines of magnetic flux, and the middle finger points to the positive end of the conductor. The middle finger also shows the direction of current flow, when an external circuit is connected across the two ends of the conductor.
Chapter 5.
DC Generator Systems
IntroductionModern aircraft electrical power systems are extremely complex and varied. DC generator systems have now been mostly superseded by AC generator systems, although it is still necessary to understand the operation of a DC system.
Basic Generator Theory
All generators work on the principle of magnetic induction, and it is purely the method by which the resulting voltage (EMF) is converted which determines its type. An AC generator is a device, which converts mechanical energy into AC electrical energy using this principle, where a voltage (EMF) is induced in a conductor as it moves through a magnetic field.
The magnitude of the voltage produced is dependent on the following factors:- ¾ The strength of the magnetic field.
¾ The speed at which the conductor cuts the magnetic field. ¾ The length of the conductor within the magnetic field. ¾ The angle at which the conductor cuts the magnetic field.
The polarity of the induced voltage can be found using ‘Fleming’s Right Hand Rule’ for generators, which involves the thumb and the first two fingers of the right hand being placed at 90° to each other. The thumb points in the direction in which the conductor is moving, the first finger points in the direction of the magnetic field (N to S), and the second finger indicates the polarity of the induced voltage (+ Ve). The second finger also points in the direction in which conventional current is flowing in the conductor when it is connected across a load.
Simple AC Generator
In its simplest form an AC generator consists of a single loop of wire, which is mounted, so that it can rotate within a magnetic field. When the loop (Armature) is rotated an AC voltage is induced in it, which can be transferred easily to an external circuit by means of carbon
brushes, which bear on 2 copper or brass slip rings that are connected directly to the loop. When the armature moves through 360°, or one revolution at a constant speed the output voltage and current rises to a maximum in one direction and back to zero, before reversing in polarity. It then rises to a maximum in the opposite direction, before again returning to zero. The paths plotted out by the voltage and current are in the shape of a sine wave.
The magnitude and polarity of the induced EMF is related to the actual position of the armature, as shown below.
Conversion of AC to DC
The AC is converted to DC by replacing the slip rings with a commutator, which consists of 2 segments insulated from each other and connected to the ends of the loop, as shown on the next page.
The commutator is a device, which is connected across the output in such a way that the connections to the load are reversed every time the polarity of the voltage in the loop changes thus maintaining the current to the load in the same direction. The load is connected to the loop by brushes, which bear on opposite sides of the commutator.
When the loop is at 90° to the magnetic field no EMF is induced, and thus no current flows. With the loop in this position the brushes will be in contact with both segments of the commutator the loop will thus be short-circuited.
As the loop rotates the short circuit is removed; the left brush becomes connected to the down- going segment, whilst the right hand brush becomes connected to the up-going segment. The right hand brush is thus in contact with the segment, which is positive, since the current flows away from this side of the loop, and the left-hand brush is alternatively connected to the negative segment. As the brushes are always connected to the conductors moving in the same direction in relation to the magnetic field the output is always DC. The change over from one segment to the other takes place at the instants when the voltage induced in the loop is zero, ie. at positions A, C and E. The commutator is therefore a switching device, which reverses the direction of the current during alternate half-cycles in the output. To produce a smoother and more constant output voltage, as shown on the next page, by fitting additional wire loops and commutator segments.
DC Generator System Architecture
All aeroplane generator systems must be capable of supplying a constant voltage for varying engine speed and load conditions, which is achieved by varying the field strength (excitation) of the generator. The components of a basic single engined generator system are shown below:-
DC Generator Construction
The construction of a typical DC generator is shown on the next page, and consists of the following components:-
¾ The Yoke. This is a cylinder of cast iron, which supports the pole pieces of the electromagnetic field.
¾ The Armature. This is driven by the aircraft engine and holds the windings in which the output voltage of the machine is induced.
¾ The Commutator. The voltage induced in the armature is AC. The commutator changes the AC voltage into DC voltage.
¾ The Quill Drive. This is a weak point, which is designed to shear and protect the engine, if the generator seizes.
¾ The Suppressor. This reduces radio interference, which may be caused by sparking between the brushes and commutator.
Principle of Operation of a DC Generator
When the armature is rotated in the magnetic field a DC voltage is collected at the brushes, and if this voltage is applied to a load, a current will flow in the armature. This will produce a motoring torque in the generator, and this will act in opposition to the driving torque. This effect is noticeable on a car engine tachometer, when the headlights are switched on and off. When the lights are switched on more current is drawn from the generator, thus increasing the motoring effect, and slowing the engine down. Alternatively switching the lights off will reduce the load on the generator, thus reducing the motoring effect, and reducing the overall load on the engine.
Types of DC Generator
Three basic types of DC generator exist, with each differing in how the armature and field windings are electrically connected:-
Shunt Wound. In this arrangement the field windings are connected in parallel with the
It is used in all aeroplane DC generators, and at a constant speed has a slightly falling voltage output with increasing load.
Series Wound. In this arrangement the field windings are connected in series with the
armature windings, as shown below.
This type of generator is however not used on aeroplanes, because at a constant speed it has a rising voltage output characteristic with increasing load. It is thus difficult to regulate the voltage output from this type of generator.
Compound Wound. In this arrangement some of the field windings are connected in shunt,
This arrangement is used on larger more expensive types of aeroplanes, where it's output characteristics depend on the actual generator design, ie. the ratio of shunt to series windings.
Voltage Regulator
The voltage regulator is designed to maintain a constant generator output voltage for varying loads, and engine speeds. Many different types of voltage regulators are fitted in aeroplane generator systems, although the following are the most common types.
Carbon Pile Voltage Regulator. A diagram of a typical carbon pile voltage regulator is
shown on the next page. In this device two forces act on a pile of carbon discs fitted in series with the generator field coil. The first of these forces is due to a moveable iron armature, which is attached to a leaf spring, and holds the pile in compression, whilst the second force is due to an electromagnet, which tends to pull the pile apart. Any variation in the magnitude of pile compression will vary the resistance, and thus the excitation current being supplied to the generator field.
For example the regulator is set to control a generator to give an output of 28 volts DC, if the output voltage falls below 28 volts DC, the current through the electromagnet will reduce, thus causing the leaf spring to compress the pile. This in turn will cause a reduction in the
piles resistance, and the amount of current to the field coil will increase, thus bringing the generator output back to 28 volts.
The opposite will occur if the generator voltage output exceeds 28 volts.
Transistorised Voltage Regulator. This type of regulator is a solid state device, and has
the following advantages over the carbon pile voltage regulator:- ¾ Less maintenance
¾ Less Weight ¾ More reliable
¾ Little or no radio interference
Cut-out
The DC generator in an aeroplane electrical supply system has to be protected from the battery voltage whenever the engine is shut down, or when its output alternatively fails. This is normally achieved by a ‘Cut-out’, which is fitted between the generator and the busbar. Many different types of Cut-out exist, of which the most common is the ‘Differential Current
Cut-Out’, as shown on the next page. The main components in this device are a ‘Series (Current) Coil (DCO)’ that is wound physically on top of a ‘Differential (Voltage) Coil’, and
which in turn controls the ‘Generator Line Contactor (GLC)’.
The contacts in the cut-out are initially closed via the ‘Differential Coil’ when the generator output voltage is approximately 0.5 volts greater than that being already supplied by the battery. This in turn causes the GLC to close, and allows the generator to feed the busbar via the ‘Series Coil’. The resulting magnetic field produced by this coil then adds to that already being produced by the Differential coil, and helps to hold the GLC in its closed position.
Conversely if the generator voltage output drops, for whatever reason, a reverse current will flow in the current coil, and a corresponding magnetic field will be produced. This field will act in opposition to the magnetic field being produced by the Differential coil, and will weaken the overall combined magnetic field. This will cause the GLC to open, and will stop the generator feeding the busbar. If the generator voltage subsequently returns to its normal output value, the generator will automatically feed the busbar again, as the DCO contacts close, via the Differential coil.
Alternatively if the engine is shut down or fails the Differential Cut-out will cause the GLC to open when the reverse current reaches a value of 20-30 amps.
Reverse Current Circuit Breaker
The Differential Cut-out does not provide complete protection to the generator system, since any short circuit on the outgoing side of the Differential Cut-out will not ‘open’ or ‘Trip’ the GLC. A ‘Reverse Current Circuit Breaker (RCCB)’ is thus fitted between the main contactor and the aeroplane busbars to provide complete protection. The RCCB is designed to operate at a very high speed if the reverse currents reach a value of approximately 500 amps, and will mechanically ‘lock’ itself out until reset. Some RCCB's are additionally fitted with auxiliary contacts, which are used to open the generator field and provide further protection against overload or fault conditions.
Busbars
Busbars are current distribution points and are usually standard rectangular sections of high conductivity copper or aluminium, which are categorised as follows.-
¾ Vital Busbar. This busbar is powered directly from the aeroplane battery, and is used for emergency undercarriage selection, and also to provide power for fire extinguishers and emergency lighting.
