# Electrical Systems Chapter 9.

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Electrical Systems Chapter 9

Aim Describe principals of operation of the aircraft electrical system

Objectives State the principals of operation of electrical components
Describe typical electrical system layout

1. Components Electrical 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. Components Batteries 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 large Most batteries are of the lead acid type where sulphuric acid interacts with lead plates in the batteries cells Sulphuric acid is very corrosive and dangerous to the aircraft structure Flammable hydrogen gas is given off by the battery therefore it must be secured in a sealed, vented container

1. Components Batteries Cells are connected in series to provide 12 or 24 volts Batteries 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 rating Lead acid batteries can be tested using a hydrometer, the specific gravity of the electrolyte is a measure of the charge Charge reduces with a decrease in temperature

1. Components Alternator
Alternators turn mechanical energy from the engine into electrical energy required to power electrical components and recharge the battery The alternating current produced by the alternator is rectified into direct current via use of diodes The 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. Components Generator Some older aircraft utilize generators instead of alternator. Here we have rotating magnets with a stationary commutator The advantage of the generator is it produces DC current so there is no need to rectify the current The disadvantages are: Generators are larger and heavier than alternators Generators less efficient, producing less current per revolution

1. Components External 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 periods The system normally consists of a bank of batteries Special procedures apply when starting with external power, consult the flight manual

1. Components Voltage Regulator
A voltage regulator senses the voltage produced by the alternator and acts to maintain output voltage at a present value The 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 field This is done by adjusting he amount of current sent to the coils of the electro magnet A 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. Components Inverters Inverters are used to turn DC into AC required for some aircraft systems Older models used a DC motor to drive a second alternator however these were highly inefficient Newer systems utilize semiconductors and are known as static inverters

1. Components Bus Bar The bus bar is the main conductor and distributor for the electrical system The aircraft may have multiple bus bars to provide redundancy It consists of a number of interconnected terminals

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

The ammeter measures the amount of current in amperes (amps) flowing through the electrical system The load type or left zero ammeter measures only output of the alternator The scale increases from left to right or may be calibrated to display percentage of alternators rated load This 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 reduce Most systems will incorporate an annunciator to indicate alternator failure

1. Components Centre 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. Components Voltmeter An increasing number of aircraft are fitted with voltmeters as well as ammeters In 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. Components Master 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. Components Starter Motor
The starter motor is a small electric motor which rotates the crank shaft during engine start It can be activated by use if the ignition key or secondary push-button switches Very large current is required therefore a heavy duty relay or solenoid is utilised Should 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 Layout
C172 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 Layout
C172 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 This diagram shows the layout of the various buses and components attached to each

2. Electrical System Layout
C172 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 This diagram shows the layout of the various buses and components attached to each This final diagram shows the standby battery circuit and associated buses

Questions?