Electrical SystemsChapter 9

Aim

Describe principals of operation of the aircraft electrical system

Objectives1.State the principals of operation of electrical

components2.Describe typical electrical system layout

1. ComponentsElectrical System Overview

The electrical system is made up of a number of individual components, these may include:• Batteries• Alternator/Generator• External power source• Voltage Regulator• Inverters• Bus Bar• Ammeter• Volt meter• Master switch• Starter motor• Circuit Breakers

1. ComponentsBatteries

The battery provides the initial electrical energy required to start the engine and provides a back up should the alternator/generator fail or electrical load is too largeMost batteries are of the lead acid type where sulphuric acid interacts with lead plates in the batteries cellsSulphuric acid is very corrosive and dangerous to the aircraft structureFlammable hydrogen gas is given off by the battery therefore it must be secured in a sealed, vented container

1. ComponentsBatteries

Cells are connected in series to provide 12 or 24 voltsBatteries are rated on their ability to supply an amount of current for a given time. A battery rated at 60 ampere hours should be able to provide a current of 60 amps for 1 hour, or 10 amps for 6 hours, or any combination which when multiplied together equals the given ratingLead acid batteries can be tested using a hydrometer, the specific gravity of the electrolyte is a measure of the chargeCharge reduces with a decrease in temperature

1. ComponentsAlternator

Alternators turn mechanical energy from the engine into electrical energy required to power electrical components and recharge the batteryThe alternating current produced by the alternator is rectified into direct current via use of diodesThe rotor of the alternator is mechanically driven however the system relies on electro magnets to produce current. Should the battery fail during flight and the alternator subsequently switched off there may not be enough charge in the system to re-energise the system resulting in a complete electrical failure

1. ComponentsGenerator

Some older aircraft utilize generators instead of alternator. Here we have rotating magnets with a stationary commutatorThe advantage of the generator is it produces DC current so there is no need to rectify the currentThe disadvantages are:• Generators are larger and heavier than alternators• Generators less efficient, producing less current per revolution

1. ComponentsExternal Power Source

Many aircraft allow external power to be connected to the system. This will typically be used:• If the battery is completely flat or charge too low to start the engine• To conserve battery power• For starting some larger engines• To run electrical components on the ground without the engine running for

extended periodsThe system normally consists of a bank of batteriesSpecial procedures apply when starting with external power, consult the flight manual

1. ComponentsVoltage Regulator

A voltage regulator senses the voltage produced by the alternator and acts to maintain output voltage at a present valueThe amount of energy produced is dependant on the number of wires in the coil of the electro magnet, the strength of the magnetic field and engine RPM. The number of wires can be considered a constant and the rotor is directly drive by the crankshaft so the easiest way of regulating the voltage is to adjust the strength of the magnetic fieldThis is done by adjusting he amount of current sent to the coils of the electro magnetA volt meter and over voltage relays are normally incorporated into the system to detect failure of the voltage regulator. Should failure occur the alternator should be switched off to prevent damage to electrical components

1. ComponentsInverters

Inverters are used to turn DC into AC required for some aircraft systemsOlder models used a DC motor to drive a second alternator however these were highly inefficientNewer systems utilize semiconductors and are known as static inverters

1. ComponentsBus Bar

The bus bar is the main conductor and distributor for the electrical systemThe aircraft may have multiple bus bars to provide redundancyIt consists of a number of interconnected terminals

1. ComponentsCircuit Breakers

Fuses, circuit breakers or overload switches are used to prevent damage to electrical components from electrical current overloadIf there is an overload or short-circuit:• Fuses will melt• Circuit breakers will pop• Overload switches will switch offShould any of the above occur it is possible to reset the system however multiple resets are not advisable, see the aircraft flight manualIf any indication of smoke, fire or burning is detected do not reset the system

1. ComponentsLoad Type Ammeter

The ammeter measures the amount of current in amperes (amps) flowing through the electrical systemThe load type or left zero ammeter measures only output of the alternatorThe scale increases from left to right or may be calibrated to display percentage of alternators rated loadThis type of ammeter can be seen as a load meter:• With the battery on, engine running and alternator off the ammeter will

indicate zero amps• With the engine running and alternator on the ammeter will indicate

alternator output current• After start the ammeter will read high as the battery is recharging, when

fully charged the ammeter indication should reduceMost systems will incorporate an annunciator to indicate alternator failure

1. ComponentsCentre Zero Ammeter

The canter zero ammeter indicates flow of current to and from the battery• If the battery is charging the ammeter will indicate a positive charge• If current is flowing out of the battery a negative charge will be indicated• With no current flow into or out of the battery a 0 indication will be shown

1. ComponentsVoltmeter

An increasing number of aircraft are fitted with voltmeters as well as ammetersIn a 24 volt system:• With the alternator off the voltmeter will

display battery charge, around 24 volts• With the alternator on and supplying charge

the voltmeter will typically indicate around 28 volts

1. ComponentsMaster Switch

The master switch activates the aircrafts electrical system. It normally comprises of two switches• The battery switch connects the battery to the

bus bars• The alternator switch connects the bus bar to

the alternator field

1. ComponentsStarter Motor

The starter motor is a small electric motor which rotates the crank shaft during engine startIt can be activated by use if the ignition key or secondary push-button switchesVery large current is required therefore a heavy duty relay or solenoid is utilisedShould you have difficulty starting the engine it is advisable to allow time between attempted starts to allow the starter motor to cool down, refer to the aircraft flight manual

2. Electrical System LayoutC172 SP

Here we can see the layout of the electrical system in the C172 SP, this is indicative of modern light aircraft, older aircraft systems will typically be a lot simpler

2. Electrical System LayoutC172 SP

Here we can see the layout of the electrical system in the C172 SP, this is indicative of modern light aircraft, older aircraft systems will typically be a lot simplerThis diagram shows the layout of the various buses and components attached to each

2. Electrical System LayoutC172 SP

Here we can see the layout of the electrical system in the C172 SP, this is indicative of modern light aircraft, older aircraft systems will typically be a lot simplerThis diagram shows the layout of the various buses and components attached to eachThis final diagram shows the standby battery circuit and associated buses

Questions?