Transcript 幻灯片 1 - Suranaree University of Technology

Chapter 2 Electrical Components
The satisfactory performance of any modern aircraft
depends on the continuing reliability of electrical
systems and subsystems. One of the most critical
factors for obtaining a high degree of reliability
involves the quality of workmanship that a
technician uses when installing electrical
connectors and wiring Improperly or carelessly
installed and maintained electrical wiring can be a
source of both immediate and potential danger.
Conductors are copper and may be either
solid or stranded. Although more expensive,
stranded wire is stronger, more flexible and
resistant to flexing failure than single
conductors. Stranded conductors are usually
plated in silver or nickel.
When choosing the wire for an electrical system, there
are several factors that must be considered. For
example, the wire selected must be large enough to
accommodate the required current without producing
excessive heat or causing an excessive voltage drop.
In addition, the insulation must prevent electrical
leakage and be strong enough to resist damage
caused by abrasion.
• Most wires used in a aircraft have a core
of pure copper.
• The majority of wiring in aircraft is made
from stranded copper.
• Now in some modern aircraft aluminum
wire was used as a engine crank circuit.
In most cases, the wiring is coated with tin,
silver, or nickel to help prevent oxidation.
In the mid-1990s it was discovered that
polyvinyl chloride insulation emits toxic
fumes when it burns. Therefore, a new
group of mil spec wires, MIL-W-22759, was
introduced which consists of stranded
copper wire with Teflon® insulation.
Cable
The simplest form of cable is two insulated conductors
twisted together to form a unit.
•
Conductors: Nickel plated annealed
flexible copper
•
Insulation: SIF (Silicone rubber) & GFB
(Glass fibre braid)
•
Voltage: 600V RMS (at a maximum
frequency 1600 c/s)
•
Operating temperature: Maximum:
190°C, minimum bending -55°C
Requirement for choosing wire
• Large enough to supply the required
current without producing excessive heat
or causing an excessive voltage drop.
• The insulation must prevent electrical
leakage and be strong enough to resist
damage caused by abrasion.
TYPES of AIRCRAFT WIRE
 The majority of wiring in aircraft is made
from stranded copper.
 In most cases, the wiring is coated with
tin, silver, or nickel to help prevent
oxidation.
 polyvinyl chloride (PVC), nylon, and glass
cloth braid for insulation purposes.
The smallest size wire normally used in
aircraft is 22-gauge wire, which has a
diameter of about .025 inch.
American Wire Gage (AWG)
Wire Marking
Wire Marking
Color
Meaning
Yellow
Power
Red
Power
Black
Return
Blue
Signal
Green
Signal
White
Signal
Shielded Wire
With the increase in number of highly sensitive
electronic devices found on modern aircraft, it has
become very important to ensure proper shielding
for many electric circuits. Shielding is the process
of applying a metallic covering to wiring and
equipment to eliminate interference caused by stray
electromagnetic energy.
Shielded wire
Shielded wire or cable is typically connected
to the aircraft’s ground at both ends of the
wire, or at connectors in the cable.
Make sure you connect the shield at
the ground end and cut it off flush at
the other end.
NOMINAL SYSTEM
VOLTAGE
ALLOWABLE VOLTAGE DROP
CONTINUOUS
OPERATION
INTERMITTEN
OPERATION
14
0.5
1.0
28
1.0
2.0
115
4.0
8.0
200
7.0
14.0
Before wiring a new component in an aircraft, the
system voltage, the allowable voltage drop, and the
component’s duty cycle must be known. All of these
factors are interrelated.
CIRCUIT
VOLTAGE
115 200 14
28
800
100 200
600
1
20
ELECTRIC WIRE CHART
18
12
14
16
700
630
560
490
420
350
50 100
45 90
40 80
35 70
30 60
25 50
160
280
20
8
6
4
2 1
1
2
0
0
3
0
4
0
AMPERES
75 150
400
360
320
280
240
200
10
3
40
2
120
100
80
72
64
56
48
40
36
32
28
24
20
210
175
140
120
112
98
84
70
63
56
49
42
35
15
12
10
9
8
7
6
5
4
3
2
30
25
20
18
16
14
12
10
9
8
7
6
5
4
7
.5
1
VOLTAGE DROP
CURES
1 CONTINOUS RATING-AMPERES
CABLES IN CONDUIT AND BUNDLES
2 CONTINUOUS RATING-AMPERES
SINGLE CABLE IN FREE AIR
3 INTERMITTENT RATING-AMPERES
MAXIMUM OF 2 MINUTES
4
20
18
14
16
12
10
8
5
WIRE SIZE
6
4
2 1
1
2
0
0
3
0
4
0
Wiring Installation
• Electrical wiring is installed in aircraft
either as open wiring or in conduit.
• With open wiring, individual wires, or wire
bundles, are routed inside the aircraft
structure without protective covering.
• On the other hand, when installed in
conduit, electrical wiring is put inside
either a rigid or flexible tubing that
provides a great deal of protection.
Several methods of identifying wire bundles include
pressure-sensitive tape or sleeve markers tied in place.
Regardless of the method of bundle assembly,
a plastic comb can be sued to keep individual
wires straight and parallel in a bundle.
Grounding and bonding
 One of the more important factors in the
design
and
maintenance
of
aircraft
electrical systems is proper bonding and
grounding.
 Inadequate bonding or grounding can lead
to unreliable operation of systems, e.g.,
EMI, electrostatic discharge damage to
sensitive electronics, personnel shock
hazard, or damage from lightning strike.
 This section provides an overview of the
principles involved in the design and
maintenance of electrical bonding and
grounding.
Grounding
Grounding is the process of electrically
connecting conductive objects to either a
conductive structure or some other
conductive return path for the purpose of
safely completing either a normal or fault
circuit.
The design of the ground return circuit should be
given as much attention as the other leads of a
circuit. A requirement for proper ground
connections is that they maintain an impedance that
is essentially constant. Ground return circuits
should have a current rating and voltage drop
adequate for satisfactory operation of the
connected electrical and electronic equipment.
EMI problems, that can be caused by a system’s
power wire, can be reduced substantially by
locating the associated ground return near the
origin of the power wiring (e.g. circuit breaker panel)
and routing the power wire and its ground return
in a twisted pair. Special care should be exercised
to ensure replacement on ground return leads.
Power ground connections, for generators,
transformer rectifiers, batteries, external
power receptacles, and other heavy-current,
loads must be attached to individual ground
ing brackets that are attached to aircraft
structure with a proper metal-to-metal
bonding attachment.
This attachment and the surrounding
structure must provide adequate
conductivity to accommodate normal and
fault currents of the system without creating
excessive voltage drop or damage to the
structure.
If the structure is fabricated of a material
such as carbon fiber composite (CFC),
which has a higher resistivity than
aluminum or copper, it will be necessary to
provide an alternative ground path(s) for
power return current. Special attention
should be considered for composite aircraft.
Bonding
Equipment Bonding
Low-impedance paths to aircraft structure
are normally required for electronic
equipment to provide radio frequency return
circuits and for most electrical equipment
to facilitate reduction in EMI. The cases of
components which produce electromagnetic
energy should be grounded to structure.
Metallic Surface Bonding
All conducting objects on the exterior of the
airframe must be electrically connected to
the airframe through mechanical joints,
conductive hinges, or bond straps capable
of conducting static charges and lightning
strikes.
Static Bonds
All isolated conducting parts inside and outside the
aircraft, having an area greater than 3 in2 and a linear
dimension over 3 inches, that are subjected to
appreciable electrostatic charging due to precipitation,
fluid, or air in motion, should have a mechanically secure
electrical connection to the aircraft structure of sufficient
conductivity to dissipate possible static charges. A
resistance of less than 1 ohm when clean and dry will
generally ensure such dissipation on larger objects.
CONNECTORS
Connectors
• Connectors are devices attached to the
ends of cables and sets of wires to make
them easier to connect and disconnect.
• Each connector consists of a plug
assembly and a receptacle assembly.
(a) Circular or Cylindrical
(b) Rectangular
(c) Coaxial
(d) Terminals and Splices
Figure 2-22. Four basic types of connector
• Contacts are typically male (pin) and female
(socket) which are installed in plug connectors
which are usually attached to the cable end or in
receptacle connectors which are usually attached
to a bulkhead or other fixed object.
•
When it is necessary to use an electrical
connector in an area where it may be exposed to
moisture, special moisture proof connectors
should be used.
The ground side of an electrical power conductor is
typically connected to a male connector while the
power side of the conductor is attached to the
female connector. This is done to reduce the
chance of an accidental short between the power
side of circuit and any conductive surface when the
mating connectors are separated.
Electrical connectors
BUSBAR
Busbar systems
• “Busbar systems” refers to conductors
that take the form of a bar or bars of
copper conductor.
• Bus bar is simply a copper strip acting as
a junction for the generator(s), battery and
the various loads.
Busbars
• Busbars are used in aircraft for power
distribution.
• The most commonly used materials for
busbars are plated copper.
• A bus bar is very simply a central point
where wires from electrical equipment are
grouped together and attached to a metal
bar that is then attached to a power source.
Without a bus bar we would have to connect
every electrical component directly to the
power source. This would be very
complicated and impractical.
In some general aircraft, avionics require
special attention. Avionic applications are
typically split off on to their own power bus.
It is very desirable for the power fed from
the main bus to the avionics bus to pass
through an avionics master switch. The
switch allows the pilot to turn off all
avionics before engine start up and
shutdown.
Applications which should be powered
from the avionics bus:
• Radio
• Intercom
• Transponder
• Altitude encoder
• GPS
• EFIS systems
• Autopilot
Applications which should NOT be
powered from the avionics bus:
• ETC (and other gyro instruments)
• EMS systems
• Strobes
• Lighting
• Fuel pumps
• Electrical flap drives
SWITCHES
Switch
• The purpose of a switch is to interrupt the
flow of current to the component it
controls.
• Each switch is rated with regard to the
voltage it can withstand and the current it
can carry.
Switches
•
Contacts
• Actuator
A pair of contacts is said to be ‘closed’
when
there is no space between them, allowing
electricity to flow from one to the other.
When the contacts are separated by a space,
they are said to be ‘open’, and no
electricity
can flow.
• Contacts touch to make a circuit, and
separate to break the circuit.
• Contact materials are also chosen on the
basis of electrical conductivity, hardness
(resistance to abrasive wear), mechanical
strength, low cost and low toxicity.
switch contacts
• Pole :a set of contacts that belong to a
single circuit
• Throw :one of two or more positions that
the switch can adopt
• single-pole, single-throw (SPST)
• single-pole, double-throw (SPDT)
• double-pole, single-throw (DPST)
• Double-pole, double-throw (DPDT)
TYPE OF SWITCHES
• Toggle switch
• Rocker switch
• Push-button
• Rotary switch
• Micro switch
• ….
TOGGLE AND ROCKER
SWITCHES
TOGGLE AND ROCKER SWITCHES
PULL-TO-UNLOCK OPTION
Guarded switch
ROCKER SWITCHES/MASTER SWITCHES
The battery switch which
connects battery power
to the bus bar (electrical
load distribution point or
bar).
the alternator switch, for
energizing the alternator.
It connects the alternator
field to the bus bar, thus
providing the alternator
with battery power for
field excitation.
Both switches must be ON for
normal operation of the
electrical system. If either
switch has to be turned OFF in
flight, then you should
consider terminating the flight
as soon as possible.
They can be switched on
separately, but only the
alternator can be switched off
separately – switching the
battery OFF will automatically
switch the alternator off as
well.
The master switch (or battery switch/
alternator switch) controls all of the airplane's
electrical systems, with one important
exception – the ignition system, which
receives electrical power directly from the
engine-driven magneto.
• The master switch needs to be ON for any
other electrical system to receive power,
or for the battery to be recharged when the
engine is running.
• It should be turned OFF after stopping the
engine, to avoid the battery discharging
via services that are connected to it.
Push-button switches
Push switches are used for momentary
actions when a circuit is to be completed or
interrupted for a finite time. An example of
this type of circuit is the start circuit of
many turbine aircraft.
Many push switches
incorporate illuminated
lens caps to indicate
that the specific circuit
has been selected.
ROTARY SWITCHES
• When it is necessary to select several
conditions for a circuit, a rotary switch may
be used.
• Rotary switches are manually operated and
are often used as selector switches such as
when selecting a single voltmeter to measure
voltage across different busbars or
generators.
ROTARY SWITCHES
PRECISION (MICRO) SWITCHES
• These switches require only a slight
movement of the operating plunger to cause
the internal spring to snap the contacts open
or closed.
• When precision switches are used to limit
the movement of a mechanism, they are
typically referred to as limit switches.
Some typical circuits using micro
switches include:
• Landing gear systems
• Door warning systems
• Power lever sequencing of system operation
(arming of power augmentation systems)
• Weight on wheels sensing, which isolates
circuits that should not operate on the ground
Rheostats
• Rheostats are used to alter the amount of
current in a circuit by varying the total
resistance (e.g. to vary the intensity of panel
or flight deck lighting).
• They normally also have an OFF position to
completely remove the current.
Proximity switches
• Proximity switches open or close an
electrical circuit when they make contact
with or come within a certain distance of
an object.
Types of proximity switches
• Infrared
• Acoustic
•
Capacitive
• Inductive.
RELAYS AND SOLENOIDS
When the bar magnet is at rest, the
current ceases to flow. If the bar magnet
is then removed, the current will again
flow.
An Electro Motive Force (EMF) is induced in
a conductor whenever the magnetic flux is
changed.
• One of the features of an electrical system is
the ability to remotely control components
which are located in some far corner of the
aircraft.
• By using a solenoid, a very small switch can
be used to control the current needed to
operate an aircraft engine starter or other
high-current device.
Figure 2-17. Electrical circuits can be
controlled remotely by use of relays (a), or
by solenoids (b). Both are sometimes
referred to as contactors.
• Relays and solenoids (contactor) are quite
similar, with only a mechanical difference.
• Both relays and solenoids are used for
electrical controls.
Normally a relay has a fixed soft-iron core
around which an electromagnetic coil is wound.
Movable contacts are closed by the magnetic
pull exerted by the core when the coil is
energized, and are opened by a spring when
the coil is de-energized (normally open relay).
A solenoid has a movable core that is pulled in
to the center of and electromagnetic coil when
the coil is energized. Due to the movable core,
solenoids respond quicker and are stronger
than relays.
• Solenoids are typically used for high current
applications and also find important use as
mechanical control devices;
• For example, to move locking pins into and
out of mechanically actuated devices.
SOLENOIDS
Relays
• A relay is an electrical switch that opens
and closes under the control of another
electrical circuit.
• Electromagnet is used to open or close
one or many sets of contacts.
Solenoids are relays also but the very large
types which carry huge amounts of current.
Types of Relays
• Electro mechanical
• Solid-state
• Hybrids
Relay
Armature relays are the
oldest , but first-rate.
Plenty turns of very fine
magnet-wire are wound
around an iron core to
form an electro-magnet.
A Polarized Relay placed the armature
between the poles of a permanent magnet to
increase sensitivity.
The more common magnetically polarized
relay uses the effect of a permanent magnet
introduced into the magnetic circuit or
circuits.
Solid-State Relay
A solid state relay (SSR) is a solid state
electronic component that provides a similar
function to an electromechanical relay but
does not have any moving components,
increasing long-term reliability.
• A solid state relay (SSR) is a solid state
electronic component that provides a similar
function to an electromechanical relay but
does not have any moving components,
increasing long-term reliability.
• Solid-state relays are available in AC and DC
versions.
Advantages of Solid State Relays
• low EMI/RFI
• long life
• no moving parts
• no contact bounce
• and fast response.
The drawback to using a solid state relay
it can only accomplish single pole switching.
Hybrid Relays
•
a solid-state
relay
• an electromagnet relay
Applications of relay
• To control a high-voltage circuit with a lowvoltage signal
• To control a high-current circuit with a lowcurrent signal
• To detect and isolate faults on transmission
and distribution lines by opening and closing
circuit breakers (protection relays),
Applications of relay(2)
• To isolate the controlling circuit from the
controlled circuit when the two are at
different potentials, for example when
controlling a mains-powered device from a
low-voltage switch.
• To perform logic functions.
Applications of relay(3)
• Early computing. Before vacuum tubes and
transistors, relays were used as logical
elements in digital computers.
• Safety-critical logic. Because relays are
much more resistant than semiconductors to
nuclear radiation, they are widely used in
safety-critical logic.
• To perform time delay functions.
Contactor
Contactor
• A contactor is an electrically controlled
switch (relay) used for switching a power
circuit.
• contactors are designed to be directly
connected to high-current load devices,
• includes Power Contacts , Auxiliary
Contacts, and Contact Springs.
CIRCUIT PROTECTIONS
Circuit Protections
• Fuses
• Circuit breakers
FUSES
Fuses are thermal devices whose primary
function is to protect the distribution cables
of a circuit against excess current flow due
to short-circuit or overload.
• The fuse is placed in series with the load
(component) it protects.
• The fuse wire is normally made of a zinc
alloy, which has the desired low melting
point.
• All fuses are rated in amperes (amps).
• In general fuses are selected on the basis
of the lowest rating which will ensure
reliable operation of the system
• When
replacing
a
blown
fuse
it
is
important that the new fuse be of the
correct rating.
Fuse
Heavy Duty fuses
fuse with an indicating lamp
When replacing fuses, ideally the power to the
circuit in question should be switched off.
Circuit Breakers
• A circuit breaker or thermal trip is designed
to isolate a circuit by means of a mechanical
trip that opens a switch whenever a surge of
current, or overload, occurs.
• The advantage of a circuit breaker over a fuse
is that a circuit breaker can be reset once the
overload situation has been remedied.
Types of circuit breaker
• Trip-free
• Non trip-free
trip-free Breakers
• Depressing the reset button will not
remake the circuit
• Cannot reset the circuit breaker, until the
overload condition has been cleared
Non trip-free Breakers
• it is possible to remake the circuit by holding
the button in
• The circuit breaker cannot be reset until the
overload has been cleared
• Some are equipped with manual trip buttons
so as to manually operated switching device.
• In the past, non trip-free circuit breakers
were installed in some essential service
circuits to permit emergency manual
reconnection of supply under overload
conditions, despite the fire hazard involved.
Circuit breakers make use of bimetallic
strips, which bend by an increasing
amount as the temperature of the strip
increases. At the temperature matched to
the rated current flow for that particular
circuit the bimetallic strip bends
sufficiently to break the circuit.
NOTES:
•
The circuit breaker must hold 100% of the rated
current, must trip at 150% and above, within the
limits shown in the curve. Trip times specified are at
25℃ ambient with no pre-loading.
•
To adjust the circuit breaker rating for ambient
temperature multiply the breaker rating by the factor.
For example, 5 Amp rating at 0℃:
5 ×0.67 = 3.3 Amp.
•
Therefore select 3 Amp rating.
END OF CHAPTER 2