Electrical System Flashcards
What is Ohm’s Law?
What is the formula?
States that the current through a conductor is directly proportional to the voltage applied to that conductor and inversely proportional to the resistance of the conductor
E = I x R
E = Electromotive force in volts
I = Current flow in amps
R = Resistance measured in ohms
Describe a DC Generator
- transform mechanical energy into electrical
energy - produce direct current (DC)
- typically found on light aircraft
- requires a control circuit to generator
maintains correct voltage and current - normally maintains a nominal output voltage
of 14 volts or 28 volts
What are the 3 mains parts (assemblies) of DC generators
- Field frame
- Rotating armature
- Brush assembly
What are the 2 functions of the field frame in a DC generator
- to hold windings needed to produce a
magnetic field - to act as a mechanical support for the other
parts of the generator
Generators -
Things to Know
How do they work?
- Voltage is induced in the armature throughout
the entire 360 degree rotation of the
conductor - The armature is the rotating portion of a DC
generator - Slip rings & brushes can be used to transfer
electrical energy from the rotating loop to the
stationary aircraft loads - The slip rings are connected to the loop and
rotate / usually a copper material - The brushes are stationary and allow a
current path to the electrical loads / made of
a soft carbon substance - AC voltage is supplied to the slip rings
- A commutator is used to change the AC to DC
voltage - By replacing the slip rings of the basic AC
generator with two half cylinders ( the
commutator), a basic DC generator is
obtained - the red side coil is connected to the red
segment - the amber side of the coil is connected to the
amber segment - the rotating parts of a DC generator (coil and
commutator) are called an armature - The red brush is always in contact with the
coil side moving downward - The amber brush is always in contact with
the coil side moving upwards - Commutator action causes the current to
flow always in the same direction through
the external circuit or meter
The rotating parts of a DC generator (coil and commutator) are called what?
Armature
What parts make up the Armature in the DC generator?
The coil and the commutator
Describe the construction of the field frame
- The actual electromagnetic conductor is
wrapped around pieces of laminated metal
called field poles - The field poles are typically bolted to the
inside of the frame and laminated to reduce
eddy current losses; they also concentrate the
lines of force produced by the field coils - Field coils are made up of many turns of
insulated wire / usually wound on a form that
fits over the iron core of the pole - DC current is fed to the field coils to produce
an electromagnetic field.
Describe the armature of a generator
- consists of two primary elements
a. the wire coils (called windings) wound
around an iron core
b. the commutator assembly - Windings are evenly spaced around the
armature and mounted on a steel shaft - the armature rotates inside a magnetic field
produced by the field coils - the core of the armature acts as an iron
conductor and is laminated to prevent the
circulation of eddy currents
What are the 2 primary elements of the armature?
- wire coils (called windings) - wound around an
iron core - the commutator assembly
Describe the core of the armature
the core of the armature acts as an iron
conductor and is laminated to prevent the
circulation of eddy currents
Describe the commutator in a generator
- located at the end of an armature
- consists of copper segments divided by a thin
insulator - the insulator is made from mineral mica
- the brushes ride on the surface of the
commutator forming the electrical contact
between the armature coils and the external
circuit - A flexible, braided copper conductor called a
pigtail, connects each brush to the external
circuit - commutator surface is highly polished to
reduce friction - never use oil or grease on a commutator
What are the brushes made of and why?
- high grade carbon
- brushes wear out and need regular attention
or replacement - the brush must be quite large to provide a
current path for the armature windings
What are the 3 types of DC generators?
- series wound
- parallel (shunt) wound
- series parallel (compound wound)
What is the external circuit?
- the electrical load powered by the generator
- used for charging the aircraft battery and
supplying power to all electrical equipment
Describe Series Wound DC Generators
- contains a field winding connected in series
with the external circuit - have very poor voltage regulation under
changing load - when aircraft electrical load increases - the
voltage increases / when aircraft electrical
load decreased - voltage decreases - Since it has poor voltage and current
regulation - it is never employed as an
airplane generator
Describe Parallel (Shunt) Wound DC Generators
- a generator having a field winding connected
in parallel with the external circuit - In electrical terms, shunt means parallel
- Any increase in load causes a decrease in the
output voltage . any decrease in load causes a
increase in output voltage
4.
In a Parallel (Shunt) Wound DC Generator, any increase in load causes a decrease in the
output voltage and any decrease in load causes a increase in output voltage. Why ?
This occurs since the field winding is connected in parallel to the load and armature, and all the current flowing in the external circuit passes only through the armature winding (not the field).
How can the output voltage of a shunt generator be controlled?
a rheostat inserted in series with the field windings
OUTCOME:
Field circuit resistance increases - field current reduced and generated voltage reduced
Field current resistance decreases - field current increases and generated voltage increases
** In actual aircraft, the field rheostat would be
replaced with an automatic control device,
such as a voltage regulator**
Describe a Compound Wound DC Generator
- employs two field windings
a. one in series
b. one in parallel with the load - the output is relatively constant, even with
changes in load
Generator Ratings
1. how are they rated
2. things to know
- generators are typically rated for their voltage
and power output - each generator is designed to operate at a
specified voltage, approximately 14 or 28 volts
(aircraft electrical systems are designed to
operator at one of these two voltage values) - aircrafts voltage depends on which battery is
selected for the aircraft - batteries are either 12 or 24 volts when fully
charged - the generator selected must have a voltage
output slightly higher than the battery voltage- hence the 14 or 28 volt generator ratings
- the generator drive is usually geared to turn
the generator between 1 1/8 and 1 1/2 times
the engine crankshaft speed
What information is stamped on the generator nameplat?
- generator rating
- generator performance data
- direction of rotation
Why is it important to use a generator with the correct rotation?
the polarity of the output voltage will be reversed if a generator with the incorrect rotation is used
What is “coming in” speed?
- speed at which aircraft generators produce
their normal voltage - usually about 1500 rpm
In general, the inspection of the generator installed on an aircraft should include the following 6 items
- security of generator mounting
- condition of electrical connections
- dirt and oil in the generator
- if oil is present - check engine oil seals
- blow out any dirt with compressed air
- condition of generator brushes
- generator operation
- voltage regulator operation
always follow manufacturers instructions for
any given generator
What is the recommended procedure to seat brushes that do not make good contact with the slip rings or commutators?
- Lift the brush sufficiently to permit the
insertion of a strip of extra-fine 000(triple
aught) grit, or finer, sandpaper under the
brush, rough side towards the carbon brush - Pull sandpaper in direction of armature
rotation - Sand the brush only in the direction of
rotation - thoroughly clean any carbon dust from all
parts of generator
What is the Generator control system?
- designed to keep the generator output within
limits for all flight variables - often referred to as voltage regulators or
generator control units (GCU).
What are the 2 basic types of generator controls?
- electro-mechanical
- solid-state (transistorized)
Describe electro-mechanical general controls
- found on older aircraft
- tend to require regular inspection and
maintenance
Describe solid-state generator controls
- more modern
- typically considered more reliable
- more accurate generator output control
What are the six functions of the generator control system?
- Voltage regulation
- Overvoltage protection
- Parallel generator operations
- Overexcitation protection
- Differential voltage
- Reverse current sensing
Voltage Regulation is a function of the generator control system - what does this mean/ what does it do?
a. regulation unit takes sample of generator
output and compares to a known reference
b. makes adjustments to generator field
current when necessary
Overvoltage Protection is a function of the generator control system - what does this mean/ what does it do?
a. compares sampled voltage to a reference
voltage
b. used to open relay that controls the field
excitation current
c. typically found on more complex generator
control systems
Parallel Generator Functions is a function of the generator control system - what does this mean/ what does it do?
a. on multiengine aircraft
b. ensures all generators operate within limits
c. compares voltages between 2 or more
generators and adjusts voltage regulation
accordingly
Overexcitation Protection is a function of the generator control system - what does this mean/ what does it do?
- The overexcitation circuit often works in
conjunction with the overvoltage circuit to
control the generator
when one generator in a parallel system fails, one of the generators can become overexcited and tends to carry more if its share of the load - this causes the generator to produce too much current
Differential Voltage is a function of the generator control system - what does this mean/ what does it do?
this function is designed to ensure all generator voltage values are within a close tolerance before being connected to the bus.
Reverse Current Sensing is a function of the generator control system - what does this mean/ what does it do?
- Monitors the system for a reverse current
- Reverse current indicates that current is
flowing to the generator and not from the
generator - If this occurs, the system opens the generator
relay and disconnects the generator from the
bus
What is a starter generator
- high output generator found on turbine
powered corporate type aircraft - it is a generator and starter combined into
one unit - performs 2 tasks: engine starting and
generation of electrical power - unit contains two sets of field wingdings
one field is used to start the engine
one field is used for the generation of
electrical power
Wiring Diagrams
- what is it used for
- what information is included
- where are they found
- wiring diagrams are included in most aircraft
service manuals - information included:
a. size of wire
b. type of terminals to be used - identifies each component in a system by its
part number, serial number, and any changes
made during the production run of the aircraft
Wiring Diagrams are often used for
troubleshooting electrical malfunctions
Block Diagrams
- what is it used for?
- what is included?
- used as an aid for troubleshooting complex
electrical and electronic systems - consists of individual blocks that represent
several components, such as a printed circuit
board
Pictorial Diagrams
- what is it used for
- when is it used
- pictures of components are used instead of
conventional electrical symbols found in
schematic diagrams - helps the maintenance technician visualize
the operation of a system
Schematic Diagrams
- what is it used for
- when used
- used to illustrate a principle of operation
- it does not show parts as they actually appear
or function - it indicates the location of components with
respect to each other - best utilized for troubleshooting
What is a wire?
a single, solid conductor or a stranded conductor covered with an insulating material
The term “cable” as used in aircraft electrical installations includes
Hint: there are 4
- two or more separately insulated conductors
in the same jacket - two or more separately insulated conductors
twisted together (twisted pair) - One or more insulated conductors covered
with a metallic braided shield (shielded cable) - A single insulated center conductor with a
metallic braided outer conductor
(radio frequency cable)
What is a wire harness?
The term “wire harness” is used when a array of insulated conductors are bound together by lacing cord, metal bands, or other binding suitable for use only in specific equipment for which the harness was designed
What is the standard wire used in light aircraft?
MIL-W-5086A - uses a tin-coated copper conductor rated at 600 volts and temperatures of 105 degrees Celcius - then coated with various insulating coatings
What is the standard wire used in commercial and military aircraft?
use wire manufactured under MIL-W-22759 - which complies with military and FAA requirements
What is the most important consideration in the selection of aircraft wire?
matching the wire’s construction to the application environment
ADDITIONAL INFO:
a) wires are typically categorized as being
suitable for either open wiring or protecting
wiring systems
b) wire temperature rating is a measure of the
insulation’s ability to withstand the
combination of ambient temperature and
current-related conductor temperature rise
What is a conductor?
a material that allows electrons to move freely from one atom to another within the material
What are the 2 most commonly used conductors?
- copper
- aluminum
What are the advantages and disadvantages of using Copper as a conductor?
ADVANTAGES:
1. has a higher conductivity
2. it is more ductile
3. has relatively high tensile strength
4. can be easily soldered
DISADVANTAGES:
1. more expensive than aluminum
2. heavier than aluminum
What are the advantages and disadvantages of using Aluminum as a conductor?
ADVANTAGES:
1. lightness makes possible long spans
2. large diameter reduces corona
DISADVANTAGES:
1. has only about 60% of the conductivity of
copper
NOTE: Copper is used extensively
Plating
What is it?
Why is it done?
- Bare copper develops a surface oxide coating,
which is a poor conductor of electricity - So, all aircraft wiring has a coating of tin,
silver, or nickel that have slower oxidation
rates
Describe Tin-coated copper plating
- a very common plating material
- can be successfully soldered without highly
active fluxes - can be used up to the limiting temperature of
150 degrees Celcius
Describe Silver coated plating
- used where temperatures do not exceed 200
degrees C (392 degrees F)
Describe Nickel-coated plating
- nickel coated wire retains its properties up to
260 degrees Celcius - soldered terminations require the use of
different solder sleeves and or flux other than
those used with tin or silver plated conductor
What are the two fundamental properties of insulation materials?
- insulation resistance
- dielectric strength
Describe insulation resistance
- the resistance to current leakage through and
over the surface of insulation materials - can be measured with a
megohmmeter/insulation tester - data obtained this way may not show the true
condition of the insulation - clean, dry insulation with cracks or other
faults might show a high value of insulation
resistance, but would not be suitable for use
Describe Dielectric strength
- the ability of the insulator to withstand
potential difference - usually expressed in terms of the voltage at
which the insulation fails because of the
electrostatic stress - maximum dielectric strength values can be
measured by raising the voltage of a test
sample until the insulation breaks down
What characteristics are considered when choosing conductor insulation material?
Characteristics should be based on environment, such as:
1. abrasion resistance
2. arc resistance
3. corrosion resistance
4. cut-through strength
5. dielectric strength
6. flame resistant
7. mechanical strength
8. smoke emission
9. fluid resistance
10. heat distortion
What types of insulation materials are no longer used in aircraft?
- PVC/Nylon
- Kapton
- Teflon
can still be found on older aircraft
Describe wire shielding
- the process of applying a metallic covering to
wiring and equipment to eliminate
electromagnetic interference (EMI)
What caused Electromagnetic Interference (EMI)?
EMI is caused when electromagnetic fields (radio waves) induce high frequency (HF) voltages in a wire or component
Wire Size Selection - Things to know
- wire is manufactured in sizes according to a
standard known as the American wire gauge
(AWG) - wire diameter becomes smaller as the gauge
number becomes larger - typical wire sizes range from 40 to 0000
Wire Sizes - Things to know
** Go over Pg 9-69 in Vol 1
- wires containing less than 19 strands must
not be used - use high strength alloy conductors in small
gauge wires to increase mechanical strength - wires smaller than #20 should be provided
with additional clamps and grouped with at
least 3 wires. Should also have additional
support at terminations (connector grommets,
strain relief clamps, shrinkable sleeving, or
telescoping bushings) - insulated conductor does not easily dissipate
heat - too protect the insulation from too
much heat, the current through the conductor
must be maintained below a certain value -
allowance must be made for the influence of
external heating on the allowable conductor
current
What is the maximum allowable operating temperature of insulated conductors?
the maximum allowable operating temperature of insulated conductors varies with the type of conductor insulation being used
Current Carrying Capacity - Things to Know
- in some cases, the wire may be capable of
carrying more current than is recommended
for the contacts of the related connector - in
this case, it is the contact rating that dictates
the maximum current to be carried by a wire - wires of larger gauge may need to be used to
fit within the crimp range of connector
contacts - this gives a family of curves
whereby the bundle derating factor may be
obtained - first must determine the maximum current
that a given sized wire can carry without
exceeding the allowable temperature
difference
Describe the maximum operating temperature
- rated temperature may be based on the
ability of either the conductor or the insulation
to withstand continuous operation without
degradation
Describe Single Wire in Free Air
- first must determine the maximum current
that a given sized wire can carry without
exceeding the allowable temperature
difference - The curves are based upon a single copper
wire in free air
Describe Wires in a Harness
- when wires are bundled into harnesses, the
current derived for a single wire must be
reduced - the amount of current derating is a function of
the number of wires in the bundle and the
percentage of the total wire bundle capacity
that is being used
Describe Harness at Altitude
- since heat loss from the bundle is reduced
with increased altitude, the amount of current
should be derated - This gives a curve whereby the altitude-
derating factor may be obtained
Describe Aluminum Conductor Wires
- sizes should be selected on the basis of
current ratings - the use of sizes smaller than #8 is
discouraged - termination hardware specifically designed for
use with aluminum conductor wiring should
be used
WHEN NOT TO USE ALUMINUM WIRE - aluminum wire should not be attached to
engine mounted accessories - aluminum wire should not be used in areas
having corrosive fumes, severe vibration,
mechanical stresses or where frequent
disconnection is needed - do not use aluminum wire for runs less than 3
feet
Computing Current Carrying Capacity
See Volume 1 - Pgs 9-71 - 9-75
there are 2 examples of how this is computed
Allowable Voltage Drop = Things to know
- the voltage drop in the main power wires
from the generation source or the battery to
the bus should not exceed 2 percent of the
regulated voltage when the generator is
carrying rated current or the battery is being
discharged at the 5-minute rate - when checking a circuit, the input voltage
should be maintained at a constant value
What is the formula used to check voltage drop?
VD = resistance/ft x length x current
VD = voltage drop
What 2 major requirements must be met when selecting the correct size of electrical wire?
- the wire size should be sufficient to prevent
an excessive voltage drop while carrying the
required current over the required distance. - the size should be sufficient to prevent
overheating of the wire carrying the required
current
What information must be known in order to select the correct size electrical wire?
Hint: there are 7
- the wire length in feet
- the number of amperes of current to be
carried - The allowable voltage drop permitted
- the required continuous or intermittent
current - the estimated or measured conductor
temperature - is the wire to be installed in conduit and/or
bundle? - is the wire to be installed as a single wire in
free air?
Describe the placement of identification markings on electrical wires
- ID markings should be placed
a. at each end of the wire and
b. at 15 inch maximum intervals along the
length - wires less than 3 inches do not need to be
identified - wires 3-7 inches should be identified in center
of the wire
What are the 2 methods of marking wire or cable?
- Direct Marking
- Indirect Marking
Describe Direct Marking
See Vol 1 - Pg 9-77 for picture)
accomplished by printing the cable’s outer covering
Describe Indirect Marking
(See Vol 1 - Pg 9-77 for picture)
- accomplished by printing a heat-shrinkable
sleeve and installing the printed sleeve on the
wire or cables outer covering. - indirectly marked wire and cable should be
identified with printed sleeves at each end at
intervals not longer than 6 feet. - the individual wires inside a cable should be
identified within 3 inches of their termination
What methods are used for mark directly on the wire?
- hot stamp marking
a. can damage insulation of newer types of
wire using thin insulators - can cause arcing - ink jet printers
- laser jet printers
Identification sleeves are normally used for identifying which types of wire and cable?
- unjacketed shielded wire
- thermocouple wire
- coaxial cable
- multiconductor cable
- high temperature wire
Identification Sleeves - Additional Things to Know
- for sleeving exposed to temp over 400
degrees F - materials such as silicone
fiberglass should be used - Polyolefin sleeving should be used in areas
where resistance to solvent and synthetic
hydraulic fluids is necessary - in most cases, identification tape can be used
in place of sleeving
What is open wiring?
- electrical wire installed in aircraft without
special enclosing means
What is the advantage of open wiring?
ease of maintenance and reduced weight
Describe a wire harness
- wire are often installed in bundles
- wire bundles are called wire harnesses
- often made in the factory on a jig board so
wire bundles could be preformed to fit into
the aircraft, so each harness for a particular
aircraft installation is identical in shape and
length - wire harness can be covered by a shielding
(metal braid) to avoid EMI. - don’t group/bundle electrically unprotected
wiring - wire bundles should be less than 75 wires or 1
1/2 - 2 inches in diameter
Why should wiring be installed with sufficient slack?
So that the bundles and individual wires are not under tension
Give 3 examples when parallel wires must be twisted
- wiring in the vicinity of magnetic compass or
flux valve - three-phase distribution wiring
- certain other wires (usually radio wiring) as
specified on engineering drawings
Spliced Connections in Wire Bundles -
Things to know
- splicing of power wires, coaxial cables,
multiplex bus, and large gauge wire must have
approved data - splicing of electrical wire should be kept to a
minimum - splicing of electrical wire should be avoided in
locations subject to extreme vibrations - splicing of individual wires in a group/bundle
should have engineering approval and splices
should be located to allow periodic
maintenance - splicing individual wires - use a self insulated
splice connector - splicing in swamp areas - use environmentally
sealed splices that conform with MIL-T-7928 - there should be no more than one splice in
any one wire segment between 2 connectors - splices in bundles must be staggered
When is splicing wire permitted?
as long as it does not affect the reliability and the electromechanical characteristics of the wiring
As a rule there should only be one splice in any one wire segment between 2 connectors - what are the exceptions to this rule?
- when attaching to the spare pigtail lead of a
potted connector - when splicing multiple wires to a single wire
- when adjusting wire size to fit connector
contact crimp barrel size - when required to make an approved repair
The minimum radius of bends in wire groups/bundles must not be less than?
10 times the outside diameter of the largest wire or cable
Note: where the wire is suitably supported - the radius may be 3 times the diameter of the wire or cable.
RF cables (coaxial and triaxial) are bent at a radius of ?
No less than 6 times the outside diameter of the cable.
Wiring - Protection Against High Temperature
- Things to Know
- wiring must be routed away from high
temperature equipment and lines to prevent
deterioration of insulation - wires must be rated so the conductor
temperature remains within the wire
specification max - separate wires from high temperature
equipment, such as resistors, exhaust stacks,
and heating ducts to prevent insulation
breakdown - don’t use cables with soft plastic insulation,
such as polyethylene in high temperature
areas as these materials are subject to
deterioration and deformation at elevated
temperatures
Wiring - Protection Against Solvents and Fluids
- Things to Know
- must separate wire and lines from equipment
containing oxygen, oil, fuel, hydraulic fluid, or
alcohol - wiring must be routed above these lines and
equipment with a minimum separation of 6
inches or more
What is a drip loop?
- an area where the wire(s) are made to travel
downward and then up to the connector - fluids and moisture will flow along the wires to
the bottom of the loop and be trapped there
to drip or evaporate without affecting
electrical conductivity in the wire, junction, or
connected device
Protection of Wires in Wheel Well Areas
- Things to Know
- wires located on landing gear and in the wheel
well area can be exposed to many hazardous
conditions - wiring and protective tubing must be
inspected frequently - replace wiring at first sign of wear
- wires should be routed so that fluid flows
away from the connectors - wiring in wheel wells are other external areas
must be given extra protection in the form of
harness jacketing and connector strain relief - conduits or flexible sleeving used to protect
wiring must be equipped with drain holes
