Transcript AIRCRAFT ELECTRICAL SYSTEMS
AIRCRAFT ELECTRICAL SYSTEMS
Objectives
Students will be able to: Describe the basic components of aircraft electrical system Explain operation of electrical system Interpret aircraft electrical diagram Select proper size of wire for installation Describe basic causes of electrical system malfunctions
Elements of aircraft electrical systems An aircraft electrical system is mainly composed of : Power sources Components - Control devices - Conversion devices - Protection devices.
Power distribution systems Electrical loads
Electrical Power Sources
Electricity power sources on an aircraft may be classified into two groups : Batteries - Lead-acid - Nickel-cadmium Generators - DC generators - AC generators
Battery
A 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 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.
Two types of batteries are used in aircraft - Lead-acid batteries - Nickel cadmium batteries
Lead-acid battery
It 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.
Battery Cell
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.
Vent pipe
Nickel-cadmium battery
Cell Terminals Container Main connector
GENERATORS
A 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 component is known as its
load
.
electrical
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
generator.
is used it will be an ac
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 IDG It 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 powered aircraft.
on today turbine-
Integrated Drive Generator
Constant speed drive
Constant Speed Drive
VSCF
In 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 Control
Power 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 Devices
These devices are use to initiate and control the operation of the circuits.Control devices include Switches Rheostats Relays Solenoids
Switches
A switch is used to start, to stop, or to change the direction of the current flow in the circuit.
Push Switches : They are used primarily for operations of short durations Toggle switches :They are on/off switches and extensively used in aircraft electrical system.
Rotary switches :When it is necessary to select several condition for a circuit, a rotary switch may be used.
Rotary Switch
Toggle switch
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.
Rheostat
Schematic of a relay
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 : Transformers Rectifiers Transformer-rectifier units (TRUs) Inverters Transistors
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.
Transformer
Analogy to TRU (ac adapter)
TRUs
Inverter
Analogy to inverter (ups for computer)
Protection Devices
These devices are used to protect circuits, cables and system components from damage due to failures.
Fuses Circuit breakers Diodes GCUs
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.
Circuit Breakers
Circuit Breakers
Primary Causes of Electrical Circuit Failures Open 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 System
These conductors are called busbars and they act as distribution centers for electric power.
In most types of aircraft, output from the generators is sent to one or more conductors before distribution throughout the system.
A busbar system is set up so that each power source supplies one or more specific buses.
Analogy to bus
A variety of electrical items are hooked up to each bus for power.
The buses are interconnected via circuit protection devices which are called bus ties.
Bus ties
are switches or relays used to connect or disconnect buses from one another.
They serve to isolate failed buses from working ones and/or send electrical power to buses that lost their normal power sources.
Each example, engine-driven normally generator, drives its for own generator bus. High-current drawn items are connected to these buses.
Items like fixed fire extinguishers and emergency lights are usually powered off battery bus. That way they are powered for use even when no generator power is available.
Typical 28vdc system
WIRES AND CABLES
Wires and cables conduct electrical power in its various forms and quantities to and between equipments.
There are various types of wires used in aircraft electrical system. The conductor is made of copper or aluminum. The insulation material may be nylon, PVC, or fiberglass.
Aircraft wires
Wire size selection
The wires installed in an aircraft electrical system are chosen on the basis of ability to carry the required current - without overheating and - without producing an excessive voltage drop.
Electrical wire charts may be used for this purposes.
Maximum voltage drop in load circuits
Wire chart
Electrical wire size measurement
Aircraft wiring diagrams
There are different types of electrical diagrams available to understand electrical systems. These diagrams may be in the following forms : - Block diagrams - Pictorial diagrams - Schematic diagrams Many symbols diagrams.
are used in these
Pictorial electric diagram
ELECTRICAL DIAGRAM
Aircraft electrical system diagram
Electrical loads
Once the aircraft electrical power has been generated and distributed then it is available to the aircraft services.
These electrical services are distributed throughout the aircraft and may be broadly subdivided into following categories.
Motors Lighting services Heating services Avionics
Aircraft Lighting System
Lighting system represent an important element of the aircraft electrical services.
External lighting systems Internal lighting systems
External lighting system
Provides illumination for such operation as landing at night, inspection of icing condition, and safety from midair collision.
Most common aircraft exterior lights : Position lights (navigation lights) Landing/taxi lights Anti-collision lights Inspection lights
Position lights - Red at left wing tip - Green at right wing tip - White at vertical stabilizer Anti-collision lights - Rotating beam lights - Usually at the top of fuselage or tail Landing and taxi lights - Landing lights at the leading edge of wings - Taxi lights at nose landing gear