AIRCRAFT ELECTRICAL SYSTEMS
Objectives Students will be able to:Describe the basic components of aircraft electrical systemExplain operation of electrical systemInterpret aircraft electrical diagramSelect proper size of wire for installationDescribe basic causes of electrical system malfunctions
Elements of aircraft electrical systemsAn aircraft electrical system is mainly composed of :Power sourcesComponents - Control devices - Conversion devices - Protection devices.Power distribution systemsElectrical loads
Electrical Power Sources Electricity power sources on an aircraft may be classified into two groups :Batteries - Lead-acid - Nickel-cadmiumGenerators - DC generators - AC generators
BatteryA battery is a device that converts chemical energy into electrical energy.It is a power reservoir that stores energy in chemical form.It does not produce energy.Its functions are: - To provide power when no other power source is available - To assist in damping transient loads in the dc system
- To provide a short term source during emergency conditionThe 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.Two types of batteries are used in aircraft - Lead-acid batteries - Nickel cadmium batteries
Lead-acid batteryIt is usually found in piston aircraft.It is made up of cells which have positive/negative plates of lead and filled with electrolyte of sulfuric acid and water.Each cell has app. 2.2v, but is simply rated as 2 v.It has corrosive effects.Frequent total discharge and remaining battery in discharged condition for a long time will shorten the life of the battery.
Nickel cadmium batteryThey 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 timeThey may be subject to thermal runaway condition caused by overheating, in which the battery destructs itself.
Nickel-cadmium batteryCellTerminalsContainerMain connectorVentpipe
GENERATORSA generator is a machine that converts mechanical energy into electrical energy by the process of electromagnetic induction They are driven by the engine(s), and sometimes by APU.A generator must be rated at adequate amperage to drive all the operating components on its circuit(s). The current required to operate each electrical component is known as its load.
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.
Basic dc generator
Operation of a basic dc generator
Producing steady dc
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 current to the circuits.Alternator generates three-phase current, and standard aircraft voltage is 115 vac with 400 Hz.
Basic ac generator
Modern electrical power generation types Mainly two types of electrical power generation currently in use on aircraft. - Constant frequency Integrated drive generators (IDG) - Variable speed constant frequency (VSCF) generators
Constant Speed IDGIt is essential that the alternator output have a frequency of 400 Hz with a very small tolerance. Thus rotational speed of alternator must remain absolutely constant.This is accomplished by using a Constant Speed Drive (CSD) unit between the engine and alternator.CSD provides a constant alternator rpm within a specified engine rpm range.
An alternator and constant speed drive unit combination is called integrated drive generator (IDG).CSD is hydromechanically operated and needs to be correctly maintained in terms of oil level and oil cleanliness.It is the most commonly used power generation method on today turbine-powered aircraft.
Integrated Drive Generator
Constant speed drive
Constant Speed Drive
VSCFIn this method, the variable frequency power produced by the alternator is converted to constant frequency 400 Hz, 115 vac by solid-state devices electronically.Thus there is no need for CSD unit.It is a new technique and limited in use.
Variable speed constant frequency generator
Power Generation ControlPower produced by generators is usually controlled by generator control unit (GCU).The main functions of this device are: - Act as a voltage regulator - Direct current to battery for recharging - Provide circuit and generator protection by disconnecting the generator from the system when electrical abnormalities occur.
Control DevicesThese devices are use to initiate and control the operation of the circuits.Control devices includeSwitchesRheostatsRelaysSolenoids
SwitchesA switch is used to start, to stop, or to change the direction of the current flow in the circuit.Toggle switches :They are on/off switches and extensively used in aircraft electrical system.Push Switches : They are used primarily for operations of short durationsRotary switches :When it is necessary to select several condition for a circuit, a rotary switch may be used.
Rheostats : control the amount of current that flows throughout the circuit and used as dimmer devices for instrument and cockpit lights.Relays :Electromagnetic switching devices which are used to remotely control electric circuits carrying large amount of current.Solenoids : Remote control devices quite similar to relays; but they are designed to move a shaft over a short distance. Thus, solenoids are used as mechanical control devices to operate hydraulic or pneumatic valves, locking pins, etc.
Schematic of a relay
Conversion Devices There are many occasions within an aircraft electrical system where it is required to convert power from one form to another.Typical examples of power conversion are :- Conversion from dc to ac power- Conversion from 115vac to 28 vdc- Conversion from one ac voltage level to another- Battery charging (from 115vac to 28vdc)
Following devices are used for these purposes :TransformersRectifiersTransformer-rectifier units (TRUs)InvertersTransistors
Transformers : Used to change ac voltage level.Rectifiers : Used to convert ac into high-amperage, low-voltage dc.TRUs: Combination of transformers and rectifiers and used as main unit to convert alternator output to dc in an aircraft Inverters : convert 28 vdc power to 26 vac power particularly for flight instruments or 115 vac.Transistors : Electronic devices that control electron flow and convert ac to dc.
Analogy to TRU (ac adapter)
Analogy to inverter (ups for computer)
Protection DevicesThese devices are used to protect circuits, cables and system components from damage due to failures.Fuses Circuit breakersDiodesGCUs
Fuses : They are designed to protect the cables against the flow of short-circuit and excessive current. They break the circuit and stop the current flow when the current exceeds a predetermined value. Circuit breakers :They have same function with the fuses.The difference from the fuse is that they are resettable, while a fuse must be replaced.Diodes : They are the electronic equivalent to the check valves in hydraulic system. They allow electricity to flow in one way only.
Primary Causes of Electrical Circuit FailuresOpen circuit : It is a circuit that is not complete or continuous. This is an uncommanded interruption of electrical power tosome components or systems. When an open occurs the affected component stop to operate, but the other components still remain in operative condition.
Common causes of open circuit
Short Circuit It occurs when electricity is allowed to take a shortcut through or around a component or system. This is the most serious problem. It has two effects : - Affected components have no power and fail to operate - Since current will not flow through affected components, the other components will be subjected to higher level of current causing them to burn out.
Common causes of a short
Power Distribution An electrical distribution system is required in order to convey the electrical power to the equipments and systems that need it.Busbar systems Wire and cables
Busbar SystemIn most types of aircraft, output from the generators is sent to one or more conductors before distribution throughout the system.These conductors are called busbars and they act as distribution centers f