basic complementary course 33
TRANSCRIPT
-
8/18/2019 Basic Complementary Course 33
1/218
Electrical Systems
Basic Complementary Course)
E1-0-COM
EgyptAir
Technical Training Center
By Engineer: Amr Eissa
-
8/18/2019 Basic Complementary Course 33
2/218
2 of 219
Course Outlines
Part I: Electrical Fundamentals.Duration : 18 Hours
Part II: Electrical Power (ATA 24).
Duration : 9 Hours
Part III: Light System (ATA 33).Duration : 3 Hours
-
8/18/2019 Basic Complementary Course 33
3/218
-
8/18/2019 Basic Complementary Course 33
4/218
-
8/18/2019 Basic Complementary Course 33
5/218
5 of 219
ATOMIC CLASSIFICATION OF MATTER
ATOMIC CLASSIFICATION OF MATTER view gives a better understanding
of electrical and electronic phenomena.
The electrical properties of the atom are determined by how tightly the
electrons are bound by electrical attraction to the nucleus.
-
8/18/2019 Basic Complementary Course 33
6/218
6 of 219
The Periodic Table of Elements
-
8/18/2019 Basic Complementary Course 33
7/2187 of 219
ATOMIC STRUCTURE - Electron shells (EL)
-
8/18/2019 Basic Complementary Course 33
8/2188 of 219
ATOMIC STRUCTURE - Electron shells (EL)
-
8/18/2019 Basic Complementary Course 33
9/2189 of 219
IONS A neutral atom contains an equal number of positivecharges (protons) and negative charges (electrons).
It is possible for an atom to gain or loose an electron.
-
8/18/2019 Basic Complementary Course 33
10/21810 of 219
POSITIVE IONS
An atom (or possibly a group of atoms) which loses anelectron has lost one of its negative charges and istherefore left with an excess of one positive charge; it iscalled a positive ion.
-
8/18/2019 Basic Complementary Course 33
11/21811 of 219
NEGATIVE IONS
An atom that gains an electron has an excess of negativecharge and is called a negative ion.
-
8/18/2019 Basic Complementary Course 33
12/21812 of 219
GENERATION OF ELECTRICITY
There are six sources of external energy that are capableof separating the electrons from their nuclei, these are:
Friction, Static Electricity
Pressure, Piezoelectric emf Magnetism, Generators
Heat, The Seebeck effect – the thermocouple
Light, The Photovoltaic Cell or Solar Cell
Chemical Action, Battery
-
8/18/2019 Basic Complementary Course 33
13/21813 of 219
STATIC ELECTRICITY
If electrons are removed from one material and placedon another, or if they are moved from one region of apiece of material to another, we have a separation of
charge. If these accumulations of charge remain stationary
after their transfer, they are referred to as staticelectricity .
-
8/18/2019 Basic Complementary Course 33
14/21814 of 219
This type of static charging between two or moredissimilar materials is known as Triboelectric chargingand is a very important factor in the design of aircraft
and aircraft furnishings and equipment.
The nature and size of the charge produced depends onthe materials, some loose or gain electrons more easily
than others.
STATIC ELECTRICITY
-
8/18/2019 Basic Complementary Course 33
15/218
15 of 219
Triboelectric Series Cats That Found Out About StaticElectricity The Hard Way
-
8/18/2019 Basic Complementary Course 33
16/218
16 of 219
STATIC ELECTRICITY (Cont.)
If two statically charged items are brought intocontact with one another, electrons will transfer fromthe more negative to the more positive one.
This movement of electrons constitutes a current flow, which will vanish once the charges are equal.
-
8/18/2019 Basic Complementary Course 33
17/218
17 of 219
Like Charges Repel, Unlike Charges Attract
The force of attraction or repulsion is governed by aninverse square law
UNIT OF CHARGE
The charge on an electron is very small, therefore amore practical unit of charge called a Coulomb, hasbeen chosen:
One Coulomb = 6.29 x 10^18 electrons
ATTRACTION & REPULSION
-
8/18/2019 Basic Complementary Course 33
18/218
18 of 219
STATIC ELECTRICITY & AIRCRAFT
During flight, a build-up of electrical energy occursin the Structure of an aircraft, developing in two ways:
by Precipitation Static Charges. by Charges due to Electrostatic Induction.
One of the hazards is the possibility of discharges
occurring within the aircraft as a result of differencesbetween the potentials of the separate parts ofaircraft.
-
8/18/2019 Basic Complementary Course 33
19/218
19 of 219
A System is called bonding system which will
form a continuous low-resistance link between allparts and in so doing will:
1. Limit the potential difference between allparts.
2. Eliminate spark discharges and fire risks.
3. Reduce interference with radio andnavigational aid signals.
4. Prevent the possibility of electrical shockhazards to persons contacting equipment andparts of the aircraft.
-
8/18/2019 Basic Complementary Course 33
20/218
20 of 219
The continuous link is formed by:
Metal Strip conductors joining fixed metal parts. Short-Length Flexible Braid Conductors for joining moving parts.
-
8/18/2019 Basic Complementary Course 33
21/218
21 of 219
Bonding Classifications
Depend on the magnitude of current which isexpected from the electrostatic charges.
Primary Bonding:
used between major components, engines,external surfaces, e.g. flight control surfaces, andthe main structure or earth.
Secondary Bonding:
used between components and earth for whichprimary conductors are not specifically required,e.g. pipelines, metal conduits, door plates, etc.
-
8/18/2019 Basic Complementary Course 33
22/218
22 of 219
Grounding & Earthing Discharge Of StaticCharges On Touch Down :
This is achieved by:
1. the nose or main wheel tires which contain a highproportion of carbon in the rubber.
2. Or provides a leakage path via short flexible steel wires secured to the nose wheel or main wheel axlemembers and making physical contact with thegrown.
During the refueling operation:
physical contact between the hose nozzle and tankfiller is always maintained.
-
8/18/2019 Basic Complementary Course 33
23/218
23 of 219
Static Wicks
They are fitted to reduce static build up on theairframe .
They allow static electricity to disperse from theminto the atmosphere “ Corona Discharge
breakdown”.Static Wick is a small wire brush or a straightmetal stick.
They are located on the trailing edge of theaircraft control surface and on the tips of wingsand stabilizers.
-
8/18/2019 Basic Complementary Course 33
24/218
24 of 219
It essential to maintain the integrity of bonding whencarrying out any maintenance work on aircraft.
You can build up a charge on yourself as you move and work around the aircraft.
Much of the equipment in modern aircraft is electronic,and can easily be destroyed by you discharging static
through it.
Safety
-
8/18/2019 Basic Complementary Course 33
25/218
25 of 219
ELECTRICAL TERMINOLOGY
VOLTAGE “ volt”
CURRENT “ampere”
RESISTANCE “Ohm”
-
8/18/2019 Basic Complementary Course 33
26/218
26 of 219
ELECTRICAL TERMINOLOGY
VOLTAGE “ volt”
If one coulomb of electricity requires one joule of work to move it between two points, then there is a
potential difference of 1 volt between them.
It is sometimes helpful tothink of potential difference
as a difference of ‘electricalpressure’ forcing a currentthrough a load.
-
8/18/2019 Basic Complementary Course 33
27/218
27 of 219
Electromotive Force (emf ) “ volt”
To make use of electricity by provision of an electriccurrent, the potential different must be maintained.
ELECTRICAL TERMINOLOGY
-
8/18/2019 Basic Complementary Course 33
28/218
-
8/18/2019 Basic Complementary Course 33
29/218
29 of 219
CURRENT “ampere”
An electric current is a flow of free electrons through aconductor.
When a current of one ampere is flowing in aconductor, one coulomb of charge passes any point inthe conductor every second.
Since one coulomb = 6.29 x 10^18 electrons, one
ampere equals a flow rate of 6.29 x 10^18 electrons persecond.
ELECTRICAL TERMINOLOGY
-
8/18/2019 Basic Complementary Course 33
30/218
30 of 219
RESISTANCE “Ohm”
An electric current is a flow of free electrons through aconductor.
The size of current f lowing through a conductor for agiven applied voltage depends on:
The number of free electrons.
The opposition to free movement of the electrons caused by
the structure of the material. The opposition to current flow is called resistance of a
Conductor.
ELECTRICAL TERMINOLOGY
-
8/18/2019 Basic Complementary Course 33
31/218
31 of 219
FACTORS AFFECTING RESISTANCE
The four factors that affect the resistance of a wireconductor are:
Material. Some materials conduct better than others.
Length (l ). Resistance is directly proportional tolength, thus if the length is doubled (other factorsremaining constant), resistance is doubled.
Cross Sectional Area (A). Resistance is inverselyproportional to A. Thus if the cross sectional area is
doubled, resistance is halved.
Temperature. Temperature affects the number of freeelectrons and hence resistance.
-
8/18/2019 Basic Complementary Course 33
32/218
32 of 219
CHANGES OF RESISTANCE WITH TEMPERATURE
The resistance of all materials changes with changes intemperature.
The resistance of all pure metal increases with
temperature. The resistance of electrolytes, insulators, carbon and
semi-conductors decreases with increasingtemperatures.
-
8/18/2019 Basic Complementary Course 33
33/218
33 of 219
TEMPERATURE CO-EFFICIENT OF RESISTANCE
The temperature co-efficient of resistance is defined as;
The Fractional change in resistance from 0ºC, per degreetemperature change.
The temp. co-efficient of resistance equals:α = R t-R o/R 0(t-to)
THERMISTORS
thermally sensitive resistor whose resistance alters withtemperature; a negative temperature coefficient (n.t.c.)thermistor is one whose resistance reduces with increasein temperature.
-
8/18/2019 Basic Complementary Course 33
34/218
34 of 219
Resistor’s COLOUR CODES Table
-
8/18/2019 Basic Complementary Course 33
35/218
35 of 219
OHM’S LAW
The relationship which exists between electric current(as a movement of free electrons through a conductingmaterial), voltage (or potential) and potential differenceand the resistance to current flow by any conducting
material quantities
-
8/18/2019 Basic Complementary Course 33
36/218
36 of 219
For a fixed metal conductor, with temperature andother conditions remaining constant, the currentthrough it is proportional to the potential differencebetween its ends.
Mathematically this is expressed as:
I ∝ V
OHM’S LAW
Georg Ohm
-
8/18/2019 Basic Complementary Course 33
37/218
37 of 219
RESISTORS IN SERIES:
RESISTORS IN PARALLEL:
RESISTORS IN DC CIRCUITS
-
8/18/2019 Basic Complementary Course 33
38/218
38 of 219
ENERGY & POWER IN DC CIRCUITS
ELECTRICAL WORK
Whenever a force of any kind causes motion, work isaccomplished.
Electrical work is done if a quantity of charge (coulombs)is moved between two points which are at differentelectrical potentials.
Electrical Work (joule) = Charge (coulomb) × PotentialDifference (volt)
-
8/18/2019 Basic Complementary Course 33
39/218
39 of 219
ELECTRICAL POWER
Electrical power (symbol P) is the rate at which work isdone or the rate of conversion of energy by an electricalsystem.
The SI unit of power is the watt which is a rate of workof 1 joule per second.
P = V × I
That is watts = volts × amps
By substituting V = IR in the above formula, two otherexpressions for electrical power are obtained:
P = VI = I^2R = V^2/R watts
-
8/18/2019 Basic Complementary Course 33
40/218
40 of 219
POWER RATINGS
Current flow through a resistive material causes heat.
An electrical component can be damaged if thetemperature is too high.
Electrical equipment can only stand a certain amountof heat production without damage and the safepower which a piece of equipment can consume without damage is its ‘Power Rating’ or ‘ Wattage
Rating’. Each component is given a wattage rating and if this is
exceeded the component will overheat.
-
8/18/2019 Basic Complementary Course 33
41/218
41 of 219
Power systems are designed to have the minimuminternal resistance to minimize loses in the powersupply.
Power Sources’ Internal Resistance
-
8/18/2019 Basic Complementary Course 33
42/218
42 of 219
CELLS & BATTERIES
To study electrical principles further we require asource of emf.
Although an emf can be produced by any of the six
methods mentioned earlier, large amounts of useablepower can only be produced chemically or bygeneration. Generation requires a more in depth studyof magnetism and therefore cells and batteries will be
studied first.
-
8/18/2019 Basic Complementary Course 33
43/218
43 of 219
Batteries Principle of OperationPrinciple based on:
Change Of Chemical Energy To Electrical Energy.
The exchange of electrons between the Electrodes through an Electrolyte due to chemical reactions.
-
8/18/2019 Basic Complementary Course 33
44/218
44 of 219
Batteries usage at
1. Transient conditions.
(At starting large D.C motor…).
2. Supplying power for short term to heavy loads.( when generator or ground power is not available, at
Engine, APU starting).
3. Emergency conditions.(to operate flight instruments…For 30 min as the
capacity of the batteries allow).
-
8/18/2019 Basic Complementary Course 33
45/218
45 of 219
An aircraft battery consists of a number of individualcells interconnected together.
It contain separators between plates.
Each cell consists of an odd number of -Ve plates andeven number of +Ve plates.
the plate assemblies are supported
in acid-proof container.
Battery Construction
-
8/18/2019 Basic Complementary Course 33
46/218
46 of 219
Battery Construction Cont.
Terminal posts are connected by cell straps andbrought out to a main receptacle for connection intothe aircraft’s main wiring.
- +
- + - + - +
- + - +
Cell StrapCell
Main Receptacle
-
8/18/2019 Basic Complementary Course 33
47/218
47 of 219
Types of Cells
Primary Cell Secondary Cell
OutputPower
Higher Output Power Lower Output Power
Charging Can’t Be charged Can Be Charged Many Times
Active
Material
Is destroyed duringdischarging
Is not destroyed duringdischarging but converts to
another form
Life Time Short Life Time Long Life Time
-
8/18/2019 Basic Complementary Course 33
48/218
48 of 219
Types of Batteries
Lead Acid Battery.
Nickel Cadmium Battery.
Battery type is derived from the plate material(electrodes) and liquid (electrolyte) that is usedduring construction.
-
8/18/2019 Basic Complementary Course 33
49/218
49 of 219
Capacity Rating
The maximum current, in amperes, which the battery will deliver for a known time period, in hours, untilthe output voltage has fallen to minimum value,measured in
AMPERE-HOURS (AH).
What’s the factors The Battery capacity Rating depend
on ???
-
8/18/2019 Basic Complementary Course 33
50/218
50 of 219
Capacity Rating Cont.
-
8/18/2019 Basic Complementary Course 33
51/218
51 of 219
Lead Acid Vs. Ni-Cad Batteries
A Nickel Cadmium battery has the following advantagesover a Lead Acid battery:
They have a longer life
The terminal voltage remains almost constant during
the discharge cycle They can be charged and discharged at much higher
currents without causing cell damage
They can be discharged to a very low voltage without
causing cell damage
-
8/18/2019 Basic Complementary Course 33
52/218
52 of 219
Lead Acid Vs. Ni-Cad Batteries
But have the following disadvantages: They are far more expensive to buy and maintain
Each cell has a lower voltage, therefore more cell arerequired to produce a battery.
They are more susceptible to thermal runaway.
-
8/18/2019 Basic Complementary Course 33
53/218
53 of 219
Battery Charging & Discharging
Battery will be CHARGING when ??
Battery will be DISCHARGING when ??
Discharge Rate
It is the time taken to discharge until a permissibleminimum voltage.
The two methods of battery charging are:-
Constant Voltage.
Constant Current (used).
-
8/18/2019 Basic Complementary Course 33
54/218
54 of 219
Thermal Runaway
It is the condition which causes violent gassing,boiling of electrolyte and finally melting of plates.
Increase
Charging
Current
Increase
Battery
Temperature
-
8/18/2019 Basic Complementary Course 33
55/218
55 of 219
Constant Current Charging Method
-
8/18/2019 Basic Complementary Course 33
56/218
56 of 219
THE BASIC CAPACITOR
If we have two metal plates close together, but separatedby an insulator or dielectric (which could be air) and weapply a voltage across them, electrons are removed fromone plate and applied to the other and each becomescharged.
Thus, a capacitor is a device which
opposes voltage change in a circuit
through its capacity to store electricalenergy (or charge) in the form of an
electric field.
-
8/18/2019 Basic Complementary Course 33
57/218
57 of 219
CAPACITANCE “Farad”
If we increase the voltage between the plates, the chargeincreases, but the ratio of charge to voltage remains thesame. This ratio gives the capacitance (C) of thecapacitor.
Charge/Voltage = A constant called capacitance
The Farad is a huge unit and smaller units are used inpractice.
1 microfarad (μF) = 10^-6 farad 1 picofarad (pF) = 10^-12 farad
-
8/18/2019 Basic Complementary Course 33
58/218
58 of 219
FACTORS AFFECTING CAPACITANCE
The factors which affect the capacitance of a parallel-plate capacitor are:
Overlapping area of the plates (A).
Distance between the plates (d).
Material between the plates. This introduces a constantcalled the absolute permittivity (ε).
-
8/18/2019 Basic Complementary Course 33
59/218
59 of 219
The constant ε is actually the product of two constants, thepermittivity of space (εo) which has a value of 8·85 x 10^-12Fm-1 and the relative permittivity (εr), which is basically amultiplication factor (no units) that indicates how manymore times the material is able to concentrate the electric
flux compared with space. We may summarize this in equation form as:
The units of ‘C’ are Farads if the units of the otherquantities are:
Area (a) – square metres (m^2). Distance between plates (d) – metres (m).
Absolute permittivity (ε) – farads per metre (Fm-1).
FACTORS AFFECTING CAPACITANCE
-
8/18/2019 Basic Complementary Course 33
60/218
60 of 219
CAPACITORS IN DC CIRCUITS CAPACITORS IN SERIES:
CAPACITORS IN PARALLEL:
-
8/18/2019 Basic Complementary Course 33
61/218
61 of 219
CHARGE & DISCHARGE CHARACTERISTICS
If we had a perfect d.c. circuit and a perfect capacitor,then only an instantaneous current would flow, chargingthe capacitor instantaneously to equal the applied voltage (but in the reverse sense) and so preventing
further current f low.However, in any real circuit, resistance is present in theform of:
the connecting wires.
Internal resistance within the d.c. source.This causes the capacitor to take a finite time to chargeup.
-
8/18/2019 Basic Complementary Course 33
62/218
62 of 219
CHARGING A CAPACITOR
It is found that the time taken to charge up the capacitordepends on the product of capacitance and resistance.This product is called the ‘time constant’ of the circuitand its value is in seconds, providing R is in ohms and Cin farads.
TIME CONSTANT = CR
-
8/18/2019 Basic Complementary Course 33
63/218
63 of 219
The time constant is defined as either: The time which would be taken for the capacitor
voltage to reach its maximum value if it continued toincrease at the initial value, or
The time for the capacitor voltage to reach 0.632 of itsmaximum
TIME TO FULLY CHARGE = 5CR
CHARGING A CAPACITOR
-
8/18/2019 Basic Complementary Course 33
64/218
64 of 219
DISCHARGING A CAPACITOR
-
8/18/2019 Basic Complementary Course 33
65/218
65 of 219
A CAPACITOR IN A DC CIRCUIT
It can be seen that although current does flow for aperiod of time in a d.c. circuit containing a capacitor(until the capacitor is fully charged), the current iseventually reduced to zero.
Thus, a capacitor inserted in a d.c. circuit preventscurrent flow and is sometimes called a dc blockingcapacitor.
-
8/18/2019 Basic Complementary Course 33
66/218
66 of 219
Faraday discovered thatelectricity could be made bymoving a magnet inside a wire coil, which allowed himto build the first electricmotor. From this knowledgehe later built the firstgenerator and transformer.
Michael Faraday
-
8/18/2019 Basic Complementary Course 33
67/218
67 of 219
Faraday's experiments and discovery of electromagneticinduction paved the way for changing mechanical energyinto electrical energy.
He also introduced words we still use in the electric trade
today: Ion
Electrode
Electrolytes
Cathode
Anode
The farad, a unit of electricity, was also named in his honor.
Michael Faraday
-
8/18/2019 Basic Complementary Course 33
68/218
68 of 219
Tesla was responsible for a great many inventions anddevices as well as principles we still use today.
Nikola Tesla
-
8/18/2019 Basic Complementary Course 33
69/218
69 of 219
His work with gas-filled lamps led to the creation offluorescent lighting.
His work with electromagnetic waves led to theinvention of the radio, radar and the MRI, a type of x-rayenabling us to look inside the human body.
Tesla's greatest achievement, the invention of thealternating current motor, led to the creation of the
electric utility.
Nikola Tesla
-
8/18/2019 Basic Complementary Course 33
70/218
70 of 219
Tesla Noted inventions:
Alternating current induction motor
Polyphase transmission system
Multiphase power system (we use this today) Wireless transmission of energy
Hydroelectric generator
Nikola Tesla
-
8/18/2019 Basic Complementary Course 33
71/218
71 of 219
Radio
Radar
Fluorescent light
Vacuum tubes Loud speaker
MRI x-rays
Nikola Tesla
-
8/18/2019 Basic Complementary Course 33
72/218
72 of 219
MAGNETISM
DOMAIN THEORY it is assumed that magnetic materials are composed of
tiny individual magnets called domains, a singledomain is very small - about 10^12 atoms.
Considering each atom - orbital electrons not onlyorbit the nucleus but spin axially on their own axis.
In non magnetic materials the same number ofelectrons spin clockwise as anti-clockwise.
In magnetic materials more electrons spin one waythan the other way
-
8/18/2019 Basic Complementary Course 33
73/218
73 of 219
The unbalanced spin creates twists called magneticmoments.
In unmagnetised state the moments of the electronsare in the same direction in a single domain, but the
domains produce random pockets of magnetism.
As the magnetic material becomes magnetised thedomains become partially aligned.
In fully magnetised material all domains become fullyaligned.
MAGNETISM
-
8/18/2019 Basic Complementary Course 33
74/218
74 of 219
-
8/18/2019 Basic Complementary Course 33
75/218
75 of 219
MAGNETIC PROPERTIES
The region around a magnet in which it exerts a force iscalled the ‘magnetic field’.
The magnetic field is three-dimensional and it may beshown visually by drawing imaginary lines called ‘linesof magnetic flux’.
-
8/18/2019 Basic Complementary Course 33
76/218
76 of 219
Electromagnetism
An electromagnetic field is a magnetic field generatedby current flow in a conductor.
Whenever current flows a magnetic field exists around
the conductor.
-
8/18/2019 Basic Complementary Course 33
77/218
77 of 219
THE MAGNETIC CIRCUIT
COMPARISON OF ELECTRICAL & MAGNETICCIRCUITS
-
8/18/2019 Basic Complementary Course 33
78/218
78 of 219
MAGNETOMOTIVE FORCE (MMF) a flux is established due to the existence of amagnetomotive force.
The mmf is produced by the current flowing in the coiland its value is the product of the current and thenumber of turns on the coil.
Magnetomotive Force = Current x Number of Turnson the Coil
-
8/18/2019 Basic Complementary Course 33
79/218
79 of 219
MAGNETISING FORCE
is a measure of the intensity of the magnetic effects atany given point in the magnetic field.
Magnetising Force (H) = Magnetomotive Force
/Length of magnet
-
8/18/2019 Basic Complementary Course 33
80/218
80 of 219
FLUX & FLUX DENSITY
A magnetising force produces a certain amount ofmagnetic flux (Φ), measured in Webers.
The magnetic field is represented by imaginary lines ofmagnetic flux.
The number of lines of flux passing though a given areais called the ‘flux density’.
Flux density is denoted by the symbol B and given the
unit Tesla.
-
8/18/2019 Basic Complementary Course 33
81/218
81 of 219
PERMEABILITY
When an mmf produces a magnetizing force H, a certainflux density B is established.
Ratio B/H is termed ‘the permeability of thematerial'.
Permeability is an indication of the ability of the flux topermeate the material.
If a flux is established in any material other than air orfree space, then the flux density will increase.
The number of times by which the flux density increasesis called the ‘relative permeability of the material’denoted by the symbol μr.
-
8/18/2019 Basic Complementary Course 33
82/218
82 of 219
The product of μo ‘the permeability of free space, 4 ×10^-7 H/M’ and μr is called the ‘absolute permeability’and is denoted by the symbol μ.
PERMEABILITY
-
8/18/2019 Basic Complementary Course 33
83/218
83 of 219
RELUCTANCE
The opposition experienced by a magnetizing force tothe creation of a flux is called ‘reluctance’ and denotedby the symbol S.
-
8/18/2019 Basic Complementary Course 33
84/218
84 of 219
BH CURVE and HYSTERESIS LOOP
When a material is subjected to a changing magnetizingforce, the flux density is affected by its previousmagnetic history.
There is tendency for the magnetic conditions to lag
behind the magnetizing force that is producing them.This is known as ‘hysteresis’.
If a piece of material is taken through a complete cycleof magnetizing and demagnetizing the graph of Bagainst H is called a hysteresis loop.
-
8/18/2019 Basic Complementary Course 33
85/218
85 of 219
HYSTERESIS LOOP
-
8/18/2019 Basic Complementary Course 33
86/218
86 of 219
The area of the loop represents the energy loss duringeach magnetic cycle, or the power dissipated.
It’s size is dependent upon the type of material andfrequency at which the magnetizing force is switched.
Materials with large loops are used for permanentmagnets .
Materials with small loops are used for temporarymagnets .
HYSTERESIS LOOP
-
8/18/2019 Basic Complementary Course 33
87/218
87 of 219
INDUCTION
Michael Faraday discovered that an electric current wasproduced by the relative movement of a magnet and acoil, a phenomenon which is known as electromagneticinduction.
-
8/18/2019 Basic Complementary Course 33
88/218
88 of 219
ELECTRICITY FROM MAGNETISM
If a magnet is moved into or out of a coil of wire and ifthe coil is connected to a meter, the meter records a flowof current as long as the magnet is moving.
FACTORS AFFECTING INDUCED EMF: The faster the magnet (or coil) is moved, the greater is
the deflection obtained on the meter.
-
8/18/2019 Basic Complementary Course 33
89/218
-
8/18/2019 Basic Complementary Course 33
90/218
90 of 219
FACTORS AFFECTING INDUCED EMF
Using the south pole of the magnet instead of thenorth results in meter deflections in the oppositesense.
If more turns are used on the coil, meter deflection isgreater and is proportional to the number of turns(N).
-
8/18/2019 Basic Complementary Course 33
91/218
91 of 219
LENZ’S LAW
A change of flux in a closed circuit induces an emf andsets up a current.
The direction of this current is such that its magneticfield tends to oppose the change of flux.
-
8/18/2019 Basic Complementary Course 33
92/218
92 of 219
When current through a coil changes, the changing fluxinduces an emf that opposes the current flow.
This emf is the result of self inductance and is called
‘back emf ’.The term ‘self inductance’ is often replaced merely byinductance.
SELF INDUCTANCE
-
8/18/2019 Basic Complementary Course 33
93/218
93 of 219
The value of back emf is given by: Where L is the inductance in henries, and
dI/dt the rate of change of current.
The minus indicates back emf.
N = Number of Turns
μo μr= Absolute Permeability
A = Area in square metres
I = Length of coil in metres (not wire)
SELF INDUCTANCE
-
8/18/2019 Basic Complementary Course 33
94/218
94 of 219
MUTUAL INDUCTANCE
If the changing flux in a coil links with the turns of asecond coil, the two coils are said to be mutually coupledand mutual inductance exists .
If the primary current, changing at a rate of 1 amp persecond, induces a secondary voltage of 1v, then themutual inductance is 1 henry.
Thus:
Es = M × dIprimary /dt between them.
-
8/18/2019 Basic Complementary Course 33
95/218
95 of 219
INDUCTORS IN DC CIRCUITS
INDUCTORS IN SERIES With no mutual coupling:
LT = L1 + L2 etc
If the coils are positioned so that the mutual inducedemf’s in each coil aid the self induced emf’s then thecoils are said to be series aiding, and
LT = L1 + L2 + 2M
If the coils are positioned so that mutually inducedemf’s in each coil oppose the self induced emf’s, thecoils are said to be in series opposing, and
LT = L1 + L2 - 2M
-
8/18/2019 Basic Complementary Course 33
96/218
96 of 219
INDUCTORS IN PARALLELIf inductors are connected in parallel, the totalinductance decreases. With no mutual coupling:
INDUCTORS IN DC CIRCUITS
-
8/18/2019 Basic Complementary Course 33
97/218
97 of 219
INDUCTORS IN A DC CIRCUIT
Time Constant = LR Seconds Maximum Current flows after 5L/R
-
8/18/2019 Basic Complementary Course 33
98/218
98 of 219
WHEN DC CURRENT IS REMOVED
-
8/18/2019 Basic Complementary Course 33
99/218
99 of 219
SAFETY
As the current increases through an inductor, flux buildsup and energy is stored in the magnetic field. On short circuiting an inductor, the magnetic field
collapses and the energy is returned to the circuit in theform of an emf that tries to maintain the current flow.
If the circuit is open-circuited rather than short-circuitedby a resistor, then the collapsing flux will produce a largeback-emf that may cause sparking across the switchcontacts as they are opened. The sparks damage thecontacts, produce heat, could ignite fuel vapor andtransmit electromagnetic radiation which interferes with
communication and navigation equipment. The large emf’s can also cause electric shocks on what are
considered safe, low voltage d.c. circuits
-
8/18/2019 Basic Complementary Course 33
100/218
100 of 219
-
8/18/2019 Basic Complementary Course 33
101/218
101 of 219
-
8/18/2019 Basic Complementary Course 33
102/218
102 of 219
DC MOTORS
If a current carrying conductor is placed at right anglesto a magnetic field, a force will be exerted on it, causingit to move.
The direction of the force and the resultant movement
depends on two factors,the :
direction of current flow in the conductor
direction of the magnetic field
-
8/18/2019 Basic Complementary Course 33
103/218
103 of 219
The direction of the force and the resultant movementcan be found by using Fleming’s left hand rule
DC MOTORS
-
8/18/2019 Basic Complementary Course 33
104/218
-
8/18/2019 Basic Complementary Course 33
105/218
105 of 219
DC MOTORS CONSTRUCTION
-
8/18/2019 Basic Complementary Course 33
106/218
106 of 219
BACK EMF
When a conductor moves in a field, an emf is induced inthe conductor.
The armature coils of the motor are moving in amagnetic field and therefore must have an emf
induced in them, this emf acts against the applied voltage and is called back emf .
The resultant of the two voltages is called theeffective voltage.
The armature current is due to the effective
voltage, not the applied voltage.
-
8/18/2019 Basic Complementary Course 33
107/218
107 of 219
When running, the back emf is almost equal to theapplied voltage, therefore the effective voltage and thecurrent taken from the supply are both small.
BACK EMF
-
8/18/2019 Basic Complementary Course 33
108/218
108 of 219
STARTING D.C. MOTORS
On starting, the rotor is stationary and thereforeproducing no back emf, this results in a high effective voltage and a large current being taken from the supply.
To limit the current, a starting resistor is often used,
the resistor being removed from the circuit once themotor is running.
-
8/18/2019 Basic Complementary Course 33
109/218
109 of 219
TORQUE
The Torque produced by a d.c. motor is directlyproportional to the armature current and themagnetic field strength.
.
-
8/18/2019 Basic Complementary Course 33
110/218
110 of 219
SPEED CONTROL
The speed of a d.c. motor can be varied by controllingthe field current or by controlling the armature current.
Field control
With field control, a decrease in field current causesan increase in motor speed;
main field decreases
back emf across armature decreases
effective voltage increases
-
8/18/2019 Basic Complementary Course 33
111/218
111 of 219
SPEED CONTROL
armature current increases motor torque increases over load torque
motor speed increases
This occurs because a small change in the main fieldstrength causes a large change in the armature current.
Field control is generally used for speed control ofnormal running speed and upwards.
-
8/18/2019 Basic Complementary Course 33
112/218
112 of 219
Armature control With armature control, an increase in armature
current causes an increase in motor torque over loadtorque and an increase in motor speed.
A decrease in armature current causes a decrease inmotor speed.
Armature control is generally used for control ofnormal running speed and downwards.
SPEED CONTROL
-
8/18/2019 Basic Complementary Course 33
113/218
113 of 219
CHANGING THE DIRECTION OF ROTATION
To change the direction of rotation it is only necessary tochange the direction of the main field or the armaturecurrent.
-
8/18/2019 Basic Complementary Course 33
114/218
114 of 219
MOTOR CLASSIFICATIONS
-
8/18/2019 Basic Complementary Course 33
115/218
115 of 219
SERIES MOTOR
A series motor has a low resistance, heavy gauge field winding in series with the armature winding.
In series motors the field strength depends on thearmature current, so the torque produced is
approximately proportional to the square of thearmature current.
-
8/18/2019 Basic Complementary Course 33
116/218
-
8/18/2019 Basic Complementary Course 33
117/218
117 of 219
There is a short period of high current drain on thesupply.
Applications include starter motors, winches andaircraft actuators.
Some series motors are fitted with two separate windings. This enables motor rotation to be quicklyreversed.
Applications include fuel valves and landing lights.
SERIES MOTOR
-
8/18/2019 Basic Complementary Course 33
118/218
118 of 219
SHUNT MOTOR
Shunt wound motors have a high resistance field winding connected in parallel with the armature.
-
8/18/2019 Basic Complementary Course 33
119/218
119 of 219
Applications - Shunt motors are used where a constantspeed is required and will be found in inverter drives and windscreen wipers.
SHUNT MOTOR
S d t l
-
8/18/2019 Basic Complementary Course 33
120/218
120 of 219
Speed control
The speed of a shunt motor is normally controlled by a variable resistor placed in series with the field winding.
When the resistance is increased, the field current isreduced, the back-emf decreases and the effective
voltage increases. The increase in effective voltage produces an increase
in armature current and an increase in speed.
When required to reduce the speed of the motor, the
field resistance is decreased.
-
8/18/2019 Basic Complementary Course 33
121/218
-
8/18/2019 Basic Complementary Course 33
122/218
122 of 219
STARTER GENERATORS
operates as a starter motor to drive the engine duringstarting, and after the engine has reached aselfsustaining speed, operates as a generator to supplythe electrical system power.
-
8/18/2019 Basic Complementary Course 33
123/218
123 of 219
STARTER GENERATORS
The starter-generator unit is basicallya shunt generator with an additional
heavy series winding.
This series winding is electricallyconnected to produce a strong field
and a resulting high torque for starting.
DC GENERATORS
-
8/18/2019 Basic Complementary Course 33
124/218
124 of 219
DC GENERATORS
If a conductor is moved at right angles to a magneticfield, an emf is induced in the conductor.
If an external circuit is then connected to theconductor a current will flow.
The direction of the current f low depends on twofactors, the:
direction of the magnetic field
direction of relative movement between the conductorand the field
DC GENERATORS
-
8/18/2019 Basic Complementary Course 33
125/218
125 of 219
The size of the generated emf depends on three factors,the:
strength of the magnetic field - B
effective length of the conductor in the field - l
linear velocity of the conductor - v
DC GENERATORS
-
8/18/2019 Basic Complementary Course 33
126/218
COMMUTATION
-
8/18/2019 Basic Complementary Course 33
127/218
127 of 219
COMMUTATION
DC GENERATOR CONSTRUCTION
-
8/18/2019 Basic Complementary Course 33
128/218
128 of 219
DC GENERATOR CONSTRUCTION
GENERATOR INTERNAL RESISTANCE
-
8/18/2019 Basic Complementary Course 33
129/218
129 of 219
GENERATOR INTERNAL RESISTANCE
A d.c. machine has resistance due to the: armature windings
brushes
brush to commutator surface contact
Internal resistance causes the generators terminal voltage to vary with changes in the load current.
As the load current increases, the voltage dropped acrossthe internal resistance increases and the terminal voltage decreases.
The generated emf E = Ir + V
GENERATOR CLASSIFICATIONS
-
8/18/2019 Basic Complementary Course 33
130/218
130 of 219
GENERATOR CLASSIFICATIONS Generators are usually classified by the method ofexcitation used.
There are three classifications; permanent magnet,separately excited and self excited.
A permanent magnet generator has a limitedoutput power and an output voltage that is directlyproportional to speed.
GENERATOR CLASSIFICATIONS
-
8/18/2019 Basic Complementary Course 33
131/218
131 of 219
GENERATOR CLASSIFICATIONS A separately excited generator has its field supplied
from an external source. The output voltage beingcontrolled by varying the field current.
Self excited generators supply their own fieldcurrent from the generator output, again the output voltage is controlled by varying the field current.
This group may be subdivided into three sub-groups;
series, shunt and compound.
GENERATOR CLASSIFICATIONS
-
8/18/2019 Basic Complementary Course 33
132/218
132 of 219
GENERATOR CLASSIFICATIONS
SERIES GENERATOR
-
8/18/2019 Basic Complementary Course 33
133/218
133 of 219
SERIES GENERATOR
The series generator has a field winding consisting of afew turns of heavy gauge wire connected in series withthe armature.
A series generator therefore has a rising characteristic
and is generally only used as a line booster.
SHUNT GENERATOR
-
8/18/2019 Basic Complementary Course 33
134/218
134 of 219
SHUNT GENERATOR
The shunt generator has a field consisting of many turnsof fine wire connected in parallel with the armature.
The shunt generator has a falling characteristic and isused for d.c. generation on aircraft.
SELF EXCITATION GENERATORS
-
8/18/2019 Basic Complementary Course 33
135/218
135 of 219
SELF EXCITATION GENERATORS
For a d.c. generator to self excite, certain conditionsmust be met:
The generator must have residual magnetism.
The excitation field, when formed, must assist the
residual magnetism.
AC THEORY
-
8/18/2019 Basic Complementary Course 33
136/218
136 of 219
AC THEORY
PRODUCTION OF A SINEWAVEThe only practical way of generating an electromotiveforce (emf) by mechanical means is to rotate aconductor in a magnetic field.
THE SINEWAVE
-
8/18/2019 Basic Complementary Course 33
137/218
137 of 219
THE SINEWAVE
AC VOLTAGE & CURRENT
-
8/18/2019 Basic Complementary Course 33
138/218
138 of 219
AC VOLTAGE & CURRENT
The type of load (resistive, capacitive or inductive)placed on an a.c. power supply affects the phase anglerelationship between the voltage and current.
‘ac resistance’ is
called
‘reactance’
SERIES L/C/R CIRCUITS
-
8/18/2019 Basic Complementary Course 33
139/218
139 of 219
SERIES L/C/R CIRCUITS
INDUCTANCE AND RESISTANCE IN SERIES:
CAPACITANCE AND RESISTANCE IN SERIES:
SERIES L/C/R CIRCUITS
-
8/18/2019 Basic Complementary Course 33
140/218
140 of 219
INDUCTANCE, CAPACITANCE AND RESISTANCEIN SERIES:
SERIES L/C/R CIRCUITS
IMPEDANCE
-
8/18/2019 Basic Complementary Course 33
141/218
141 of 219
IMPEDANCE
When inductance, capacitance and resistance appeartogether in an a.c. circuit, in any combination, thetotal opposition to current flow is referred to asimpedance and given the symbol Z.
APPARENT POWER & ACTUAL CURRENT
-
8/18/2019 Basic Complementary Course 33
142/218
142 of 219
APPARENT POWER & ACTUAL CURRENT
PRACTICAL GENERATOR CONSTRUCTION
-
8/18/2019 Basic Complementary Course 33
143/218
143 of 219
PRACTICAL GENERATOR CONSTRUCTION There are two types of alternating current generator, arotating field type and a rotating armature type.
PRACTICAL GENERATOR CONSTRUCTION
-
8/18/2019 Basic Complementary Course 33
144/218
144 of 219
rotating field generator has several advantages over the rotating
armature type: Because the output windings are now stationary they are no
longer subject to high centrifugal forces and can therefore belarger.
By having the output windings on the outside of the machine
there is more room for good insulation and higher voltages canbe used.
With the output windings on the outside of the machine theyare more easily cooled and can therefore carry larger currents.
Using a rotating field only requires the use of two slip rings andtwo brushes, also the current required is relatively small.
These advantages mean a larger output can be obtained from asmaller machine.
PRACTICAL GENERATOR CONSTRUCTION
TWO PHASE GENERATOR
-
8/18/2019 Basic Complementary Course 33
145/218
145 of 219
TWO PHASE GENERATOR
A two phase generator has two output windings woundon separate pairs of poles positioned 90 degrees apart asshown.
The output from the generator will be two voltages of
equal amplitude and frequency, but phase displacedfrom each other by 90°.
THREE PHASE GENERATOR
-
8/18/2019 Basic Complementary Course 33
146/218
146 of 219
THREE PHASE GENERATOR
A three phase a.c. generator has three sets of output windings, each physically displaced from the othertwo by 120°.
The windings are normally connected together in one
of two ways, called star or delta.
STAR & DELTA SYSTEMS
-
8/18/2019 Basic Complementary Course 33
147/218
147 of 219
STAR & DELTA SYSTEMS
STAR Connection DELTA Connection
V Line = 1.73 * V Phase V Line = V Phase
ILine = IPhase ILine = 1.73 * IPhase
The voltage from the neutral line, or star point, to theother end of each phase winding is called the phase voltage, the voltage from one phase to another iscalled the line voltage.
STAR & DELTA SYSTEMS
-
8/18/2019 Basic Complementary Course 33
148/218
148 of 219
STAR & DELTA SYSTEMS
In aircraft a.c. systems, the phase voltage is 115V andthe line voltage is 200V.
On some aircraft systems the frequency is variable(wild), however, on the majority of modern aircraft,
the frequency is kept constant at 400 Hz.
AC MOTORS
-
8/18/2019 Basic Complementary Course 33
149/218
149 of 219
AC MOTORS
the operation of an a.c. motor relies on the productionof a rotating magnetic field.
To create a rotating field, the current in one pair offield windings must be 90 degrees out of phase with
the current in the other pair of field windings.
TYPES OF AC MOTOR:
induction motor.
synchronous motor.
Hysteresis motor.
INDUCTION MOTOR
-
8/18/2019 Basic Complementary Course 33
150/218
150 of 219
INDUCTION MOTOR
The rotor of an induction motor consists of a numberof copper or aluminum bars connected by two endrings to form a cage.
The cage is enclosed in a laminated iron core to
reduce its reluctance.
INDUCTION MOTOR
-
8/18/2019 Basic Complementary Course 33
151/218
151 of 219
INDUCTION MOTOR When the rotor is placed in a rotating magnetic field,the bars are cut by the rotating f lux, causing emf's to beinduced in them, because the bars are shorted by theend rings, currents then flow in the bars.
Current flow in the bars produces a magnetic fieldaround them, which reacts with the main field of themachine, causing the rotor to turn.
INDUCTION MOTOR
-
8/18/2019 Basic Complementary Course 33
152/218
152 of 219
It is not possible for the rotor to rotate at synchronousspeed (the speed of the field), because there would beno emf’s induced in the rotor bars, no current flow andno magnetic field produced.
The difference between synchronous speed and rotorspeed is called ‘Slip Speed’ and is usually expressed as apercentage of the synchronous speed.
INDUCTION MOTOR
SYNCHRONOUS MOTOR
-
8/18/2019 Basic Complementary Course 33
153/218
153 of 219
SYNCHRONOUS MOTOR
The synchronous motor gets its name from the factthat the rotor runs at synchronous speed (the speed ofthe field), for it to do this, the rotor must be apermanent magnet or an electro-magnet.
SYNCHRONOUS MOTOR
-
8/18/2019 Basic Complementary Course 33
154/218
154 of 219
SYNCHRONOUS MOTOR
In order for the magnet to lock-on to the field, it mustbe brought up to about 75% of synchronous speed, toachieve this the majority of synchronous motors havethe cage of an induction motor built into them.
The motor starts as an induction motor and whensufficient speed has been attained, the electromagnetis energized, allowing the rotor to lock onto the field.Once running, no emf's are induced in the rotor bars,
however, they are useful in holding the rotor and rotor windings in place and also assist in smooth runningduring load changes.
HYSTERESIS MOTOR
-
8/18/2019 Basic Complementary Course 33
155/218
155 of 219
HYSTERESIS MOTOR
The motor is so named because the material used forthe rotor has a large hysteresis loop.
This type of motor requires a two phase a.c. supplyand is often used as a servo motor, one phase being
supplied from a reference source, the other from acontrol circuit.
The current in the control phase is made to either leador lag the reference phase by 90 degrees, depending
on the direction of rotation required.
TRANSFORMERS
-
8/18/2019 Basic Complementary Course 33
156/218
156 of 219
TRANSFORMERS
Transformers are electromagnetic devices that transferelectrical energy from one circuit to another bymutual induction.
Because the flux must be changing state, static
transformers can only be used on alternating current. In order for a transformer to be used on direct current,
part of the transformer must be rotated.
POWER TRANSFORMERS
-
8/18/2019 Basic Complementary Course 33
157/218
157 of 219
A simple transformer consists of two coils, a primaryand a secondary, wound on a high permeability, softiron core.
The changing current
in the first coil createsa changing magnetic
field that induces an
alternating voltage inthe secondary coil.
POWER TRANSFORMERS
-
8/18/2019 Basic Complementary Course 33
158/218
158 of 219
All of the energy transferred from the primary windingto the secondary must be stored in the magnetic fieldcreated in the core, therefore, sufficient iron must beprovided to store the energy of each half cycle of the a.c. waveform
CIRCUIT SYMBOLS & DOT CODES
-
8/18/2019 Basic Complementary Course 33
159/218
159 of 219
The basic symbol used for a transformer with oneprimary winding and one secondary winding.
The two dots are used to indicate the phaserelationship between the two windings, the terminals
marked with a dot are always in phase with each other.
a ferrite core - used on
medium to high
frequencies.
air cored - used on
very high frequencies
(VHF) and above
iron core -used at low
frequencies
TURNS RATIO
-
8/18/2019 Basic Complementary Course 33
160/218
160 of 219
If the number of turns on the secondary is less thanthe number of turns on the primary, the output voltage will be less than the input voltage, and thetransformer is called a step-down transformer.
If the number of turns on the secondary is greaterthan the number of turns on the primary, the
transformer is a step-up type and the output voltage will be greater than the input voltage.
TURNS RATIO
-
8/18/2019 Basic Complementary Course 33
161/218
161 of 219
when writing the transformation ratio, thesecondary voltage is put before the primary .
therefore a 4:1 transformer is a step-up transformer, thesecondary voltage being 4 times the primary voltage.
AUTOTRANSFORMERS
-
8/18/2019 Basic Complementary Course 33
162/218
162 of 219
Auto transformers have only one winding, this servingas both the primary and secondary.
They may be used as "step up" or "step down“transformers.
AUTOTRANSFORMERS
-
8/18/2019 Basic Complementary Course 33
163/218
163 of 219
Auto transformers are used for: line boosters to compensate for the voltage drops in
long cable runs.
motor starting.
Several tappings being used in sequence to apply an increasing voltage to the motor.
to step the 115V a.c. aircraft supply down to 26V forlighting circuits.
AUTOTRANSFORMERS
-
8/18/2019 Basic Complementary Course 33
164/218
164 of 219
The major disadvantage of auto transformers, especiallystep down types, is that should the common portion ofthe winding go open circuit, the primary voltage isapplied directly to the load on the secondary.
CURRENT TRANSFORMERS
-
8/18/2019 Basic Complementary Course 33
165/218
165 of 219
CURRENT TRANSFORMERS
Current transformers (CT's) are designed to enablecircuit currents to be measured without breaking thecircuit.
The outputs are applied directly to instruments, or
used in control circuits.
THREE PHASE TRANSFORMERS
-
8/18/2019 Basic Complementary Course 33
166/218
166 of 219
The preferred methods of connection are the last two.
-
8/18/2019 Basic Complementary Course 33
167/218
DIFFERENTIAL TRANSFORMERS
-
8/18/2019 Basic Complementary Course 33
168/218
168 of 219
The magnitude of the signals produced is dependent on
the amount of movement, and the phase of the signal onthe direction of movement.
All three devices are used in control systems,
-
8/18/2019 Basic Complementary Course 33
169/218
SERIES RESONANCE
-
8/18/2019 Basic Complementary Course 33
170/218
170 of 219
At the resonant frequency, the applied voltage and the
circuit current are in phase, and the impedance of thecircuit equals the resistance.
In a Series Circuit at Resonant Frequency (f O):
X L
= X C
X L = V C
V L and V C are in antiphase and therefore cancel eachother out.
V R = Applied Voltage V. Z = R.
SERIES RESONANCE
-
8/18/2019 Basic Complementary Course 33
171/218
171 of 219
The only opposition to the flow of current comes from
the resistive element of the circuit, therefore currentrises to a maximum value.
Because I is a maximum, this series resonant circuit isknown as an ‘acceptor circuit’.
BANDWIDTH
-
8/18/2019 Basic Complementary Course 33
172/218
172 of 219
The bandwidth (B) of a circuit is the difference between
two frequencies either side of the resonant frequency at which the power has fallen to half its value at resonance,i.e. the half power points.
PARALLEL L/C/R CIRCUITS
-
8/18/2019 Basic Complementary Course 33
173/218
173 of 219
In a Parallel Circuit at Resonant Frequency (f O):
X L = X C
X L = V C
V L and V C are in antiphase and therefore cancel each
other out. V R = Applied Voltage V.
Z =L/CR and current is a minimum.
Because the impedance is a maximum, the parallel
resonant circuit is known as a ‘rejecter circuit’.
Parallel RESONANCE
-
8/18/2019 Basic Complementary Course 33
174/218
174 of 219
if R is very small, the term involving resistance may beignored and for most practical purposes the resonant
frequency is given by:
FILTERS
-
8/18/2019 Basic Complementary Course 33
175/218
175 of 219
Filter circuits are four terminal networks designed topass a band of frequencies from the input to theoutput terminals, and to filter-off or attenuate, theremaining unwanted frequencies present at the inputterminal.
Such circuits are made from capacitors and inductors whose reactance changes with change in frequency.
Filter circuits take four main forms:High pass, Low pass, Band pass, and Band stop
HIGH PASS FILTERS
-
8/18/2019 Basic Complementary Course 33
176/218
176 of 219
High pass filters allow all frequencies above a certain
cut-off frequency to be passed from the inputterminals to the output terminals.
All frequencies below the cut-off frequency arefiltered off or attenuated.
LOW PASS FILTERS
-
8/18/2019 Basic Complementary Course 33
177/218
177 of 219
Low pass filters allow all frequencies below a certain
cut-off frequency to be passed from the inputterminals to the output terminals.
All frequencies above the cut-off frequency are filteredoff or attenuated.
BAND PASS FILTERS
-
8/18/2019 Basic Complementary Course 33
178/218
178 of 219
These circuits allow a certain narrow band of
frequencies to be passed onto the output terminalsand filter off, or attenuate the frequencies above andbelow this band.
BAND STOP FILTERS h h l ll
-
8/18/2019 Basic Complementary Course 33
179/218
179 of 219
These circuits pass onto the output terminals all
frequencies except a certain narrow band which isattenuated or filtered off.
FILTERS FREQUENCY RESPONSE
-
8/18/2019 Basic Complementary Course 33
180/218
180 of 219
-
8/18/2019 Basic Complementary Course 33
181/218
Electrical Power Outlines
-
8/18/2019 Basic Complementary Course 33
182/218
182 of 219
Power Distribution
Cables
Emergency Supplies AC and DC
Voltage regulation
Inverters
TR Units
Ground Power Supplies Typical Aircraft Power Distribution Network.
Power Distribution
-
8/18/2019 Basic Complementary Course 33
183/218
183 of 219
In order for the power available at the appropriategenerating source, to be made available at theterminals of the power-consuming equipment thenclearly, some organized form of distributionthroughout an aircraft is essential.
Busbars
In most types of aircraft, the output from thegenerating sources is coupled to one or more low
impedance conductors referred to as busbars Toprovide a convenient means for connecting positivesupplies to the various consumer circuits.
Split Busbar Systems
-
8/18/2019 Basic Complementary Course 33
184/218
184 of 219
a distribution system must meet requirements whichconcern a power source, or a power consumer systemoperating either separately or collectively, underabnormal conditions.
The requirements and abnormal conditions are:1. Power-consuming equipment faults must not
endanger the supply of power to other equipment.
Split Busbar Systems
-
8/18/2019 Basic Complementary Course 33
185/218
185 of 219
2. Power-consuming equipment must not be deprivedof power in the event of power source failures unlessthe total power demand exceeds the available supply.
3. Faults on the distribution system (e.g. fault currents,
grounding or earthing at a busbar) should have theminimum effect on system functioning, and shouldconstitute minimum possible fire risk.
it is usual to categorize all consumer services into theirorder of importance and, in general, they fall intothree groups: vital, essential and non-essential
consumer services category:
-
8/18/2019 Basic Complementary Course 33
186/218
186 of 219
Vital services are those which would be required afteran emergency wheels-up landing, e.g. emergencylighting and crash switch operation of fire extinguishers.
These services are connected directly to the battery.
Essential services are those inquired to ensure safeflight in an in-fight emergency situation.
They are connected to d.c. and a.c. busbars, as
appropriate, and in such a way that they can always besupplied from a generator or from batteries.
consumer services category:
-
8/18/2019 Basic Complementary Course 33
187/218
187 of 219
Non-Essential services are those which can beisolated in an in-flightemergency for load
shedding purposes, and areconnected to d.c.
and a.c. busbars, asappropriate,
supplied from a generator.
Emergency Supplies, A.C. and D.C.
-
8/18/2019 Basic Complementary Course 33
188/218
188 of 219
In the event of total loss of generated power, it isnecessary to resort to emergency services.
These can be provided by:
A battery which supplies essential d.c. loads and a
static inverter which supplies the a.c. essential busbar. An electrical generator driven by a ram air turbine.
An electrical generator driven by a hydraulic motor.
GENERATORS PARALLELING
-
8/18/2019 Basic Complementary Course 33
189/218
189 of 219
Emergency Supplies, A.C. and D.C.Prior to paralleling there are quite a few conditions that must
-
8/18/2019 Basic Complementary Course 33
190/218
190 of 219
Prior to paralleling, there are quite a few conditions that must
be followed: The frequencies on either side of the bus-tie breaker
must be within specified limits.The frequency difference must be less than 6 Hz.
The voltage on either side of the bus-tie breaker must be within specified limits.
The voltage difference must be less than 10 V.
The phase angle difference must be less than 90 degrees.
The phase rotation of polyphase generators must be
identical. The generators must share the load on tie bus within
specified limits.
Voltage Regulator
-
8/18/2019 Basic Complementary Course 33
191/218
191 of 219
The voltage is regulated to maintain a constantoutput regardless of engine speed or Electricalloading.
Regulation is achieved by adjusting the strength of
the magnetic field by altering the Field Current.
Types of Voltage Regulators
-
8/18/2019 Basic Complementary Course 33
192/218
192 of 219
Automatic adjustment of the field current isachieved by using one of two types of voltageregulator:-
• Carbon pile voltage regulator.
• Semi-conductor (transistors).
Carbon Pile Regulator
-
8/18/2019 Basic Complementary Course 33
193/218
193 of 219
Carbon Pile Regulator Construction
-
8/18/2019 Basic Complementary Course 33
194/218
194 of 219
A device in which a number of carbon coatedmetal discs are placed together to form a cylinder.
This is placed in the field winding circuit, and will vary the field current by varying its resistanceas a function of applied pressure.
Multi-Generator Operation When two or more generators are connected in
-
8/18/2019 Basic Complementary Course 33
195/218
196 of 219
g
parallel to a power system, the generators shouldshare the electrical load.
If the voltage of one generator is higher than that ofthe other, then that generator will take a greater part
of the electrical load, which may lead to failure of thegenerator.
Equalizing Circuit
-
8/18/2019 Basic Complementary Course 33
196/218
197 of 219
INVERTERS
S f i f i i l
-
8/18/2019 Basic Complementary Course 33
197/218
198 of 219
Some of aircraft equipments require a 115 volts AC, these include:-
• Fluorescent lighting.
• Radio/radar/navigation & autopilot equipment.
• Engine instrument, motors and actuators. INVERTER is used for converting d.c power from
batteries to a.c power.
INVERTERS
-
8/18/2019 Basic Complementary Course 33
198/218
199 of 219
There are two types of inverter, which will convert 28 volt d.c. to 115 volts a.c., they are the:-
rotary inverter,
static inverter.
THE ROTARY INVERTER The inverter consists of a d c motor and an a c
-
8/18/2019 Basic Complementary Course 33
199/218
200 of 219
The inverter consists of a d.c. motor and an a.c.generator mounted on a common shaft.
A fan attached to the shaft draws cooling air throughthe unit.
As the motor armature rapidly rotates under theinfluence of motor action, the a.c. output windingsrotate through an electro magnetic field producing 115 volts3 phase a.c. at a standard frequency of 400 Hz.
Static Inverter It’s a non-rotating inverter which utilizes
-
8/18/2019 Basic Complementary Course 33
200/218
201 of 219
It s a non rotating inverter which utilizes
transistorized electronic circuit to convert 28 Vd.c to 115 V a.c.
The advantages of a static inverter are:-
• High efficiency & Low weight.
• Low maintenance and long life. • Does not require warm up time.
• Quiet in operation.
• Has a fast response to load changes.
Static Inverter (Cont’d)
-
8/18/2019 Basic Complementary Course 33
201/218
202 of 219
Square WaveGenerator
Pulse Shaper Power Amp. And Filter
Voltage &Frequency
Sensor
TRANSFORMER-RECTIFIER UNITSTransformer rectifier units (T RU;'s) are combinations of
-
8/18/2019 Basic Complementary Course 33
202/218
203 of 219
Transformer-rectifier units-(T.RU; s) are combinations of
static transformers and rectifiers, and are utilized insome a.c. systems as secondary supply units, and also asthe main conversion units in aircraft having rectified a.c.power systems.
TRANSFORMER-RECTIFIER UNITS
-
8/18/2019 Basic Complementary Course 33
203/218
204 of 219
GROUND POWER SUPPLYt i ft h th f ilit t b t d t
-
8/18/2019 Basic Complementary Course 33
204/218
205 of 219
most aircraft have the facility to be connected to anexternal power source during servicing or maintenance.This allows systems to be operated without having tostart the engines or use the battery.
GROUND POWER SUPPLY
-
8/18/2019 Basic Complementary Course 33
205/218
206 of 219
In its simplest form, aground power supplysystem consists of aconnector located in the
aircraft at a convenientlyaccessible point'( at the sideof a fuselage for example)and a switch for completing
the circuit between theground power unit and thebus bar system.
Electric Cables Wires and cables constitute the framework of power
-
8/18/2019 Basic Complementary Course 33
206/218
207 of 219
p
distribution systems conducting power in its variousforms and controlled quantities, between sectionscontained within consumer equipment, and alsobetween equipment located in the relevant areas of
an aircraft.
-
8/18/2019 Basic Complementary Course 33
207/218
Wire Vs. Cable
A Wire is described as:-
-
8/18/2019 Basic Complementary Course 33
208/218
209 of 219
A single solid conductor or as a stranded
conductor covered with an insulating material.
A Cable is described as:-
Two or more separate wires in the same jacket
-
8/18/2019 Basic Complementary Course 33
209/218
210 of 219
or twisted together or covered with a metallicshield.
Types of Wires and Cables The wire and cable are derived from the names
-
8/18/2019 Basic Complementary Course 33
210/218
211 of 219
of the various Insulating materials used."NYVIN" is derived from NY lon and frompoly VIN yl-chloride (P.V.C.).
"TERSIL" is derived from polyesTER andSILicone.
"EFGLAS" is derived from GLASs braid andpolytetraflouroethylene (ptFE).
The insulation materials used for wires and cablesmust be:
-
8/18/2019 Basic Complementary Course 33
211/218
212 of 219
Flexibility over a fairly wide temperature range. Resistance to fuels, lubricants and hydraulic
fluids.
Ease of stripping for terminating.
No flammability.
Minimum weight.
Special Purpose Cables
-
8/18/2019 Basic Complementary Course 33
212/218
213 of 219
Ignition Cables.
Thermocouple Cables.
Co-axial Cables.
Electro Magnetic Interference
-
8/18/2019 Basic Complementary Course 33
213/218
214 of 219
Methods to reduce interference to the minimum:Use of metallic shielded cables, connected to
the airframe earth.
Twisting wires together.
Grouping specific wires together in bundles.
Routing of Wires and Cables
-
8/18/2019 Basic Complementary Course 33
214/218
215 of 219
Routing of Wires and Cables
-
8/18/2019 Basic Complementary Course 33
215/218
216 of 219
Types of Routing:-1. Open Loom.
2. Ducted Loom.
3. Conduits.
Earthing Or Grounding It refers to the return of current to the
-
8/18/2019 Basic Complementary Course 33
216/218
217 of 219
conducting mass of the earth or ground. Since in most aircraft the structure is of metal
and of sufficient mass to remain electricallyneutral, then it can function as an earth or"negative busbar" and so provide the return pathof current.
power supply and consumer circuits can be
completed by coupling all negative connectionsto the structure at various "earth stations“.
The selection of types of connection for earth returncables is based on:
-
8/18/2019 Basic Complementary Course 33
217/218
218 of 219
Mechanical strength, Current to be carried,Corrosive effects.
In aircraft in which the primary structure is ofnon-metallic construction, a separate
continuous main earth and bonding system isprovided.
It consists of four or more soft copper strip-type
conductors extending the whole length of thefuselage and disposed so that they are not morethan six feet apart.
Typical Aircraft Power Distribution Network
-
8/18/2019 Basic Complementary Course 33
218/218