AIRCRAFT ELECTRICAL SYSTEM INTRODUCTION ⚫What is Electricity? ⚫•A form of energy associated with stationary or moving electrons, ions, or other charged particles. ⚫Electricity provides electrical power throughout the aircraft WHAT IS AC & DC SYSTEM ? ALTERNATING CURRENT is an electric current which periodically reverses direction, in contrast to direct current which flows only in one direction. DIRECT CURRENT is the unidirectional flow of an electric charge. A battery is a prime example of DC power. ELECTRICAL SYSTEM ⚫Most aircraft are equipped with either a 14- or a 28-volt direct current electrical system. A basic aircraft electrical system consists of the following components: • Alternator/generator • Battery • Master/battery switch • Alternator/generator switch • Bus bar, fuses, and circuit breakers • Voltage regulator • Ammeter/loadmeter • Associated electrical wiring Aircraft Electrical System Power Sources Battery •Stores electrical energy •Provides electrical power to start the engine •Stores a limited supply of backup power if alternator or generator fails •Recharges when engine is running (turning Alt/Gen) ⚫ NiCad,LeadAcid,GelType,Sea led LeadAsid Alternator ⚫ Produces alternating current (AC) which is converted to direct current (DC) ⚫ Belt driven (engine driven)Recharges battery while engine is running ⚫ Creates sufficient current to operate entire electrical system, even at low Engine R.P.M. ⚫ Electrical current is more constant throughout different engine speeds than a generator Generator ⚫ Produces Alternating current electrical charge (AC) ⚫ Belt driven by the engine ⚫ Doesn’t produce enough current at low R.P.M. to operate entire electrical system ⚫ Aircraft with generators use battery power at low engine R.P.M. • TYPES OF GENERATORS 1. AC Generator 2. DC Generator All generators produce alternating current naturally. The method which is used to take it from the coil will determine if the generator provides ac or dc to the circuit. If a commutator is used for this purpose it will be a dc generator. If a slip ring is used it will be an ac generator. DC GENERATORS Commutator in the generator converts the ac current to dc current. DC generator is designed to supply nearly constant voltage. Usually the voltage is 28vdc, but there are 270vdc systems developed recently. They supply current up to 400 A. Most aircraft do not have a separate dc generator, but ac produced by ac generator is converted to dc to power dc systems. It is important to remember that the voltage being produced by this basic generator is AC, and AC voltage is supplied to the slip rings. Since the goal is to supply DC loads, some means must be provided to change the AC voltage to a DC voltage. Generators use a modified slip ring arrangement, known as a commutator, to change the AC produced in the generator loop into a DC voltage. The action of the commutator allows the generator to produce a DC output. By replacing the slip rings of the basic AC generator with two half cylinders (the commutator), a basic DC generator is obtained. DC GENERATORS • There are three methods of excitation, and thus three main Types of DC Generators 1. Permanent Magnet DC Generators – Field coils excited by permanent magnets 2. Separately Excited DC Generators – Field coils excited by some external source 3. Self Excited DC Generators– Field coils excited by the generator itself AC GENERATOR (ALTERNATOR) • On most large aircraft high-load electrical devices are usually AC powered. • AC is produced by ac generator which is usually called alternator. • AC generators use slip rings instead of commutators to provide the alternating current to the circuits. • Alternator generates three-phase current, and standard aircraft voltage is 115 vac with 400 Hz. AC POWER GENERATION • AC generators are used as the primary source of electric power in almost all transport category aircraft. • The AC system supplies most of the electrical power required for the aircraft. Magneto ⚫ Self contained ignition unit that provides pulses of electricity to the two spark plugs ⚫ Used in internal combustion ignition engines ⚫ Doesn’t require a battery to function ⚫ Each spark plug has a separate magneto system ⚫ Converts mechanical energy to electrical energy Spark Plugs ⚫ Receives ‘spark’ electricity from magneto ⚫ Ignites compressed fuel/air mixture inside engine cylinder ⚫ Aircraft engines have two spark plugs (two magneto systems) per cylinder ⚫ Electricity travels from magneto to magneto spark plug in heavily insulated wires –HIGH VOLTAGE Circuit Breaker/Fuses ⚫ Protects the electrical system from electrical overload ⚫ Overload can cause damage to components in the electrical system ⚫ Fuses have an internal metal wire or strip that melts when too much electrical current flows through it ⚫ Circuit breakers have a switch that trips if too much electrical current flows through it ⚫ The switch can be manually reset instead of replacing it ⚫Breakers and Fuses do Same Job ⚫Difference: ⚫Replace a Fuse ⚫Reset a Circuit breakers ⚫Technology Improvement ⚫Fuses-Older Technology ⚫Circuit Breakers –Newer Technology Voltage Regulator ⚫ Controls the rate of charge to the battery ⚫ Stabilizes the generator or alternator electrical output ⚫ Compares the generator/alternator voltage output to the battery voltage ⚫ Volt is a measure of Electrical Power ⚫ Voltage regulator automatically maintains constant voltage level Ammeter ⚫ Monitors the performance of the electrical system ⚫ Shows if the alternator/generator is producing an adequate supply of electrical power ⚫ Indicates whether or not the battery receives an electrical charge ⚫ Not all airplanes are equipped with an ammeter ⚫ Some airplanes have a warning light to alert the pilot of an electrical system problem VOLTMETER The voltmeter indicates the voltage of the battery which, when the alternator is not operating, should be just over 12 or 24 volts. Master Switch ⚫ Turns Electric system on or off ⚫ Controls entire electrical system except for Magneto Ignition system, including ⚫ interior/exterior lights, ⚫ radios, fuel gauges, pumps, -and – engine starter motor ⚫ Some airplanes equipped with battery switch and alternator switch (split switch) ⚫ Alternator switch (ALT) disconnects alternator from electrical system in case of alternator failure (ALT=OFF) Battery Solenoid/Contactor Starter Solenoid/Contactor ⚫Electrical Switch ⚫ElectroMechanical ⚫starter solenoid relays a large electrical current from origination source (Battery or Auxiliary Power Unit –APU) to rotate Starter Motor Starter Switch Starter ⚫ Electric Motor connected by special gearing to Engine Crankshaft ⚫ Rotates at very high speeds to rotate Engine Crankshaft ⚫ •Engine Crankshaft rotation Aircraft Engine Magnetos Magneto ⚫ Magneto is a Field Generator ⚫ –Rotating Magnet surrounded by loops of conducting wires ⚫ Rotating magnets generate electrical current flow through wires ⚫ •Aircraft engines have TWO Magnetos for Redundancy ⚫ –Safety feature –two spark sources are better than one Magneto Systems Basic Electrical/Magneto Aircraft System ⚫A busbar in electrical power in distributing refers to thick strips of copper and aluminum and conducts electricity within a switchboard, distributing board, or other electrical apparatus. ⚫is used as a terminal in the aircraft electrical system to connect the main electrical system to the equipment using electricity as a source of power. This simplifies the wiring system and provides a common point from which voltage can be distributed throughout the system. ⚫ The airplanes electrical system consists of a battery and a generator or alternator with associated voltage regulators, current limiters, or other protective devices. ⚫ The primary function of the battery is to provide power to the starter motor. ⚫ After the engine starts, the magnetos provide ignition, ⚫ generator or alternator takes on the ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ tasks of powering: •Radios •Instruments lights •Electrical landing gear •Flaps •Recharging the battery •Individual circuits are protected by circuit breakers or fuses. AIRCRAFT ELECTRICAL SYSTEM AIRCRAFT ELECTRICAL SYSTEM POWER SOURCE BATTERY •A battery is a device that converts chemical energy into electrical energy. • Provides electrical power when Alternator or Generator is not available. BATTERY FUNCTIONS • To provide a short term source during emergency condition • The capacity of battery is measured in ampere-hours. • Its normal rate is a little over 24vdc in a 28vdc system. • It is automatically recharged when the engine-driven generator is operational. • To supply power for short-term heavy loads when generator, or ground power, is not available: e.g. engine starting. Two types of batteries are used in aircraft: 1. Lead-acid batteries 2. Nickel cadmium batteries PRIMARY CELL/SECONDARY CELL • As a primary cell discharges, i.e. supplies electrical energy, the chemical action destroys the cell and it cannot be re-formed, i.e. charged. • As a secondary cell discharges, the chemical action converts the cell material into other forms and these can be converted into the original material, i.e. charged. • Therefore secondary cells can be discharged and charged during the 'life' of a battery. Lead-acid Battery • It is made up of cells which have positive/negative plates of lead and filled with electrolyte of sulfuric acid and water. CAPACITY OF BATTERIES • The capacity of a battery, or the total amount of energy available, depends upon the size and number of plates. • The capacity rating is measured in Ampere-Hours and is based on the maximum current, in amps, which it will deliver for a known time period, until it is discharged to a permissible minimum voltage of each cell. Battery Charging PREPARATION FOR CHARGE The procedure is as follows: • Unscrew the vent plugs but leave in the vent holes. This allows the cell to gas freely during the charge. • Adjust the level of electrolyte, if required, to the level specified in the manufacturer's instructions by adding distilled water. The plates must always be covered, do not over-fill. Record amount of distilled water added in ccs. • Connect to charging board. CHARGING THE BATTERY The charge must be monitored at frequent intervals to: 1. Adjust the charging current, as cell voltage will increase during the charge. 2. Ensure electrolyte remains above plates and cells are gassing. Adjust the level by adding distilled water. Record the quantity added; if the battery is always requiring distilled water, it must be rejected. 3. Record the terminal voltage to determine when the battery is fully charged. 4. The standard temperature is 15°C (60°F). COMPLETION OF CHARGE Completion of charge will be indicated as follows: 1. Constant terminal voltage, with charging current flowing, for three hours, 2. Constant RD and within the manufacturer's limit (after temperature correction). 3. Cells gassing freely. 4. If all three conditions are met, the battery is fully charged and charging should cease. CAPACITY TEST Reasons for a Capacity test are as follows: 1. After initial charge. 2. Routine maintenance at specified periods: e.g. 3 months. 3. If the capacity of the battery is in doubt. Nickel Cadmium Battery • They are the most common type of battery used in turboprop and turbojet aircraft. • They provide electrical discharge at a high rate without voltage drop and accept high charge rates that shorten recharge time • They may be subject to thermal runaway condition caused by overheating, in which the battery destructs itself. Nickel Cadmium Battery Vent Pipe Cell Terminal Container Main Connector Nickel/Cadmium Battery (NI/CD) Cell The Ni/Cd cell is one of three possible alkaline cells. The three are: 1. Nickel Cadmium (Ni/Cd). 2. Nickel Iron (Ni/Fe). 3. Silver Zinc. Of the three, the Ni/Cd cell has become that preferred for use in aircraft batteries. CELL VOLTAGE AND CAPACITANCE Each cell gives a voltage: 1. The nominal voltage of a L/A cell is 2 volts 2. The nominal voltage of a Ni/Cd is 1.2 volts. Each cell has capacity, a measure of current it is capable of delivering over 1 hour. The unit is AmpereHour (AH). If cells are connected in series, the total voltage across the arrangement is the sum of each cell voltage. The capacity is as for one cell. The plates are made from wire screens sintered with nickel powder. They are impregnated with the active plate material. 1. Positive Plate - Nickel. 2. Negative Plate - Cadmium. Aircraft that are outfitted with NiCd batteries typically have a fault protection system that monitors the condition of the battery. The battery charger is the unit that monitors the condition of the battery and the following conditions are monitored: Overheat condition Low temperature condition (below –40 °F) Cell imbalance Open circuit Shorted circuit If the battery charger finds a fault, it turns off and sends a fault signal to the Electrical Load Management System (ELMS). NiCd batteries have a ventilation system to control the temperature of the battery. A combination of high battery temperature (in excess of 160 °F) and overcharging can lead to a condition called thermal runaway. THERMAL RUNAWAY • Thermal runaway, perhaps more appropriately termed overcharge runaway, is a condition of overcharge instability. • It occurs in the later part of the charge cycle. • In a normal charge cycle, the heat generated by the charging current is dissipated within the battery and its temperature does not rise appreciably. • So thermal runaway takes place very rapidly and is a danger to aircraft. NI/CD BATTERY CHARGING CELL CAPS All batteries give off gas during charging. The cell caps of a Lead/Acid battery are open and the cell can vent at all times. In a Ni/Cd cell however, the cap is 'semi-open'. It is fitted with a non-return valve to allow gas to vent but not allow air to enter. This is because carbon dioxide in the atmosphere contaminates the electrolyte and reduces its RD. VOLTMETER Voltages are critical in the servicing of Ni/Cd batteries. The voltages we are required to measure are to two decimal places (1.24V, 1.55V, and 0.04V). To achieve this accuracy a Digital Voltmeter must be used. ACTION PRIOR TO CHARGE 1. Battery cover removed, (we require to measure the cell voltages). 2. Vent caps released but not removed from vent. 3. Check electrolyte is above the plates. If below the plates, high temperature and damaged gas barrier will be caused. Note: this is not looking for a set level. 4. Remove cell shorting links. BATTERY MAINTENANCE Battery inspection and maintenance procedures vary with the type of chemical technology and the type of physical construction. Always follow the battery manufacturer’s approved procedures. Battery performance at any time in a given application depends upon the battery’s age, state of health, state of charge, and mechanical integrity, which you can determine according to the following: • To determine the life and age of the battery, record the install date of the battery on the battery. During normal battery maintenance, battery age must be documented either in the aircraft maintenance log or in the shop maintenance log. • Battery state of charge is determined by the cumulative effect of charging and discharging the battery. In a normal electrical charging system, the aircraft generator or alternator restores a battery to full charge during a flight of 1 hour to 90 minutes. • Lead-acid battery state of health may be determined by duration of service interval (in the case of vented batteries), by environmental factors (such as excessive heat or cold), and by observed electrolyte leakage (as evidenced by corrosion of wiring and connectors or accumulation of powdered salts). • If the battery needs to be refilled often, with no evidence of external leakage, this may indicate a poor state of the battery, the battery charging system, or an overcharge condition.
