Smallest particle of any matter is called Atoms.
Atom has electrons, which are charged negative & protons, which are charged positive
Some electron, which are negative and protons, which are positive combine to form Neutrons.
In an Atom amount of Electrons, & Protons, are equal therefore an Atom has no charge.
Electrons, which are in the circular orbit can be taken out or added.
If no. of electrons is more, Atom becomes negatively charged.
If no. of Protons, are more Atom becomes positively charged.
Charge can be created by rubbing.
Rubbing glass rod with Silk, glass becomes positive and silk becomes negative.
On rubbing Ebonite with wool, Ebonite becomes positive and wool becomes negative.
Charge can be created by
Rubbing
Touching
Induction
If positive glass is brought close to or touched to another material negative electrons are attracted to it & it becomes positively charged (Material).
Potential Energy is due to the position of a body.
Moving charge can be made to do some work, because it has kinetic energy.
Difference of potential – is the situation which determines the tendency of free electrons negative to move from one place to another.
Electric Current – is the flow of the free electrons, which are negative flowing in a continuous motion or
It is a continuous motion of electrons, which are negative in one direction in a controlled manner.
CONDUCTORS – substance which permits a practical amount of electric current to flow through them.
INSULATORS – A substance which do not permit the flow of current through them.
SEMICONDUCTORS – neither good conductors nor insulators.
E.M.F – Electro Motive force, electric unit is voltage.
Potential differences – differences in potential between 2 points in an electrical system, unit is volts.
Electric Current – Rate of flow of electricity.
It is measured in Amperes.
1 Ampere is when one coulomb of charge moves past a point in one second.
Effect of electric current
1) Heating
2) Magnetic
3) Chemical
4) Light
Resistance – opposite to the motion of Electrons,
Unit is ohms.
One ohm is the resistance which limits the current to one ampere, when applied voltage is one voltage.
Factors affecting resistance –
1) Types of Materials – Every substance has resistance called Specific Resistance.
Specific Resistance – is resistance of 1 cubic cm of substance at 20oC (68oC F). If specific resistance is low resistance is less.
2) Length – If length increases resistance decreases
3) Diameter – Resistance is invariably proportional to the diameter.
4) Temperature – If temperature increases resistance decreases.
Ohm’s low – In a circuit, current varies directly with the applied voltage, At a constant temperature or when Resistance is constant.
In a circuit Power – Voltage & Current
Kirchoff’s 1st law – Sum of all current flowing to a junction is equal to the sum of all current flowing away from a junction.
Kirchoff’s 2nd law – deals with distribution of voltage in a circuit, ohm of voltage drop across each resistance is equal to the applied voltage.
Starter Generators – device which is both a Starter & a Generator.
When engine is off, It acts as a Starter.
When engine is running, it acts as a Generator.
Before engine starts, current flows from Battery to Starter Generator.
After engine starts, current flows from Generator to Battery.
Therefore, if generator fails, current flows from Battery to Starter Generator & therefore starts behaving like motor.
In A/c electrical system, Battery is charged by Generator.
Therefore Generator voltage is higher than Battery.
In flight if Generator fails voltage can flow from Battery to generator, therefore to avoid this to happen Reverse Current Relays are provided to prevent current flowing from battery to generator.
When External power supply is connected, Generator & Battery are disconnected from electrical system of the A/c.
As long as external power is ON A/c Generator can’t be put ON.
A.P.U./APP Auxiliary Power Unit/Auxiliary Power Plant- runs with some fuel as A/c.
A.C. is supplied by INVERTERS
Inverter Converts DC into A.C.
It gives 115V 400hz, 3 phase A.C.
Inverter is used to supply fixed frequency A.C. to Instruments & Avionics.
Each Inverter has its own dedicated load.
Unlike Generator, Inverters cannot be connected in parallel to share the load, because:-
1) Problem of matching phases of 2 different inverters.
2) Problem of matching frequencies of 2 inverters.
But in case one Inverter fails, its services can be transferred to another inverter.
Number of inverters depends upon power required.
Inverter draws current from Generator & not from Engine.
Alternators – are engine driven, each engine has its own Alternator & each Alternator shows voltage & its frequency & will have an Ammeter showing load on alternator.
It gives 200V variable cycles 3 phase A.C. (Cycles of A.C. depends upon engine rpm.
Alternator is used to drive heavy loads like heating, system, De icing/Anti icing system. It is installed for the electrical equipments that do not require fixed frequency).
Difference between Generator & Alternator is that a Generator has a permanent magnet.
Alternator has a Electro Magnet – soft “FE” core, Initially current is fed from battery, but once this existing current comes ON, alternator runs on it’s own.
Weight of Alternator, is less as compared to a Generator, therefore an Alternator is used for low Performance planes.
Voltage regulation of Alternator is easier.
D.C. ALTERNATOR
DC Alternator is a DC source, It is basically producing A.C. but has an Inbuilt Rectifier that gives D.C. output.
(It has a soft “FE” core instead of permanent magnet like Geneator).
It is light in weight.
Since battery is charged with D.C. It can provide battery charging current even at a very low rpm.
But it cannot charge a fully discharged battery. Therefore, on starting engine, if battery is fully discharged D.C. Alternator will not come ON because it requires initial current from battery to start.
A/c electric system is electric pole system.
Return path of current is provided by A/c body.
Bonding wire – wire is used to connect the equipment to A/c body & it provides return path for current.
Negative grounding – Negative of battery is connected to A/c body.
Positive grounding – Positive of battery is connected to A/c body.
All a/c have negative grounding.
Bonding also equalizes electrostatic charge on body of A/c without causing Arcing.
STATITC DISCHARGERS – is used to dissipate electrostatic charge on body of A/c into the atmosphere.
They equalize electrostatic charge on the body of A/c with surrounding atmosphere.
The metal surfaces of on a/c are an essential part of a/c Electric grounding system (grounding of electric system) & for this reader it is important that all metal parts of A/c be connected together. If they are not electrical fields may be generated around isolated metal components that not only may cause radio Interference but may also be conducive to the formation of a spark that could cause a fire.
Static Discharger Wicks are usually attached to the trailing edge of the wing to discharge static electricity into the atmosphere.
Any residual charge is dissipated by a small wire/chain hanging from Tail as Main wheels, which dissipates charge on landing.
In a modern sophisticated Aircraft, this dissipation of charge is done by a conducting nose wheel.
If voltage regulator malfunctions, battery will be discharged, because voltage regulator maintains 28Voltage DC under load conditions.
It steps up voltage, but when it fails, It will not step-up Generator voltage, sufficient voltage will not be provided to all the systems & now they will start taking current from the battery & start getting discharged.
In case of Generator failure, Generator warning light comes ON
Voltmeter of failed Generator shows O
Ammeter/Load meter of failed Generator shows O.
Ammeter is Ampere rated to give Rated voltage.
Lenz Law – Property of circuit by which it opposes any change in circuit or current.
If current in circuit is 1, it will induce a current which will prevent this reduction & vice versa.
Inductance/ (Inductive Reactance – Property of a circuit, which indicates its ability to oppose the charge in the current.
(Z) Impedance – the sum of inductive reactance & resistance.
Household voltage is 220V RMS & not peak voltage.
RMS – Root mean square – It is generally 70% of peak value.
Doll’s eye – are magnetic Indicators located in A/c, to indicate current is flowing through circuit.
When- Power is ON it is Black.
Power is OFF it is white.
Battery is an Electrical Accumulator.
Earth Returning circuit – is In which a/c body is used as returning path for Static wick dischargers.
It discharges static from a/c body to the atmosphere & prevents Radio Interference.
Total fuses to be carried are 10% of total or 3 whichever is more.
Example- If 10 nos. of 5 Ampere fuses are used in the A/c then extra 5 ampere fuses to be carried is 10% of 10 fuses.
Shielding – is used to prevent the ignition current interfering with the radio, the whole ignition system, magnetos, plugs & wiring are surrounded with a metal covering which is grounded.
Bonding – is a process of connecting all metal parts together of an a/c since they are essential parts of a/c electric ground system. If they are not connected together, electric fields may be generated around isolated metal components that may not only cause radio interference but may be conducive to formation of a spark that could cause a fire.
Therefore, static discharges wicks are usually attached to the trailing edge of the wings to discharge static electricity into the atmosphere.
In series voltage increases, but current remains the same.
In parallel voltage remains the same, but the current increases.
Primary cells can not be recharged.
In Primary cells, Electrolyte used is Sulfuric Acid.
Disadvantage of having a DC Alternator is that it cannot charge a fully discharged battery.
Ebonite rod when rubbed with wool – Ebonite rod becomes negative, wool becomes positive.
A glass when rubbed with silk – glass becomes positive, silk becomes negative.
When the Ammeter shows negative, the battery is being discharged.
When the Ammeter shows Positive, the battery is being charged.
Therefore, the Ammeter should never go to the negative side.
Trip free Circuit breaker can be reset & held in the position to check the fault in the electric circuit by force.
Non trip free Circuit breaker cannot be reset in ON position until the short circuit has been removed.