Electrical Systems

1
Electrical Systems

• All avionics systems and several other services
  require electrical power.
• Electrical power is generated by the engines and
  the auxiliary power unit (APU) using generators
  of some sort.
• Before it can be used, it must be transmitted,
  modified and controlled.
• This section describes the various means by which
  the electrical power is changed into a usable
  form.

2
Electrical Systems

• Basic Electrical Laws (refresher)
• Power (P) Voltage(V) x Current (I)
• Voltage Drop (loss) in a wire Current x Wire
  Resistance
• Power dissipated in a wire Current2 x Wire
  Resistance
• or Voltage2/ Wire Resistance

R
R
3
Electrical Systems

• Basic Electrical Laws (refresher)
• Thus the voltage at the load is less than the
  voltage at the generator by 2 x Current x Wire
  Resistance
• VLOAD VGEN 2xIxR
• Since the objective is to deliver as much power
  as possible to the load, it is advantageous to
  keep the losses in the wires as low as possible

4
Electrical Systems

• Types of Aircraft Power
• DC (Direct Current) current flows only in one
  direction
• On aircraft the DC Voltage is standardized at a
  nominal 28 Volts
• It is easy to generate but, because of the low
  voltage, large amounts of power require large
  currents
• e.g. 1 kW _at_ 28V requires 35 Amps of current
• To reduce the amount of power loss in the wires,
  the wire resistance has to be reduced.
• This can only be done by increasing the diameter
  of the wire and hence its weight.
• One advantage is that there is little or no shock
  hazard

5
Electrical Systems

• Aircraft Power
• AC (Alternating Current) current flows in both
  directions in a sinusoidal fashion
• The standard AC frequency on aircraft is 400 Hz
  rather than the 60 Hz used in Terrestrial systems
• This is because with higher frequencies, smaller
  (and lighter) transformers are required
• Nominal voltages are 115V for power transmission
  and 26V for synchro use.
• .

6
Electrical Systems

• Aircraft Power
• AC (Alternating Current).
• Advantages
• Voltages can be changed easily using transformers
• Disadvantages
• More danger of electrical shock
• Generates alternating magnetic fields which can
  interfere with the operation of systems

7
Electrical Systems

• Aircraft Power Specifications
• Unlike land-based power systems, aircraft power
  systems are subject to a wide variety of
  operating conditions such as low engine power
  settings and engine failures.
• Thus equipment must be able to operate with
  widely varying voltage inputs
• Two specifications which specify these conditions
  are MIL STD 704 (military ) and DO 160 the RTCA
  document which covers avionics environmental
  requirements

8
Electrical Systems

• Aircraft Power Specifications

9
Electrical Systems

• Aircraft Power Generation
• A changing magnetic field passing through a loop
  of wire induces a voltage around that wire
• The voltage is proportional to the rate of change
  of magnetic flux through the loop.
• Flux is the product of the magnetic field
  strength and the area of the loop perpendicular
  to the field
• Thus a voltage is induced if either (or both) the
  field strength or the area changes

10
Electrical Systems

• Aircraft Power Generation

?
?
Area perpendicular to field is proportional to
cos(?) Or, if the loop is rotating, proportional
to cos(?t) where ? is the angular velocity Thus
the rate of change of area and hence flux is d/dt
(cos ?t) ?sin(?t)
11
Electrical Systems

• Aircraft Power Generation (DC)
• Thus the output voltage is sinusoidal with a
  magnitude proportional to ? (and flux density)

To generate DC, the output terminals are reversed
every half cycle by use of a commutator
Average
VLOOP
VOUT
Thus the output has a non-zero average
12
Electrical Systems

• Aircraft Power Generation (DC)
• The output is filtered so that the voltage is
  (nearly) constant.
• In practice there are several loops regularly
  spaced around the rotor so that the raw output is
  much smoother than in the example

The variation in voltage is called ripple
13
Electrical Systems

• Aircraft Power Generation
• AC Power Generation
• This is much simpler to generate since
  commutators are not necessary
• The problem is that the frequency of the output
  voltage is entirely dependent on the angular
  velocity of the motor which is driving the
  generator
• In an aircraft, this is usually the engine.
• The engine speed varies widely during a given
  flight and thus so does the frequency of the AC
  voltage
• This is called wild AC

14
Electrical Systems

• Aircraft Power Generation
• AC Power Generation
• Most avionics systems are designed to operate
  with 400Hz AC
• Recall that the specification for AC frequency is
  400 /- 20 Hz
• Thus the wild AC has to be converted to tame AC
• This requires two steps
• Conversion to DC (rectification)
• Conversion back to AC (inversion)

15
Electrical Systems

• Aircraft Power Generation - AC
• Rectification
• Transformer/Rectifier

Rectifier
1
2
Full wave rectifier Provides output on both
positive and negative cycles
3
4
Filter
Transformer
Load
16
Electrical Systems

• Aircraft Power Generation - AC
• Inverter
• The inverter converts 28V DC to 400Hz 115V/25V AC
• Original inverters were simply DC motors and AC
  generators using a common shaft. Rotation speed
  and hence frequency was controlled by the
  excitation current in the DC motor.
• Modern inverters are called static in that
  there are no moving parts. They are essentially
  400Hz oscillators.
• Typical static inverters can produce up to 4 kW
  of power and weigh about 50 lbs.

17
Electrical Systems

• Aircraft Power Generation - AC
• Inverter

18
Electrical Systems

• Aircraft Power Generation - AC
• Auxiliary Power Units (APUs)
• Most airports provide ground power units (GPUs)
  which can supply the aircrafts electrical needs
  while the main engines are not running. These are
  connected through a standardized plug.
• These power units can be fixed (at the airport
  gate) or mobile, usually designed as a trailer
  which is towed by a mule
• Where GPUs are not available, and it is
  necessary to run aircraft systems on the ground,
  the APU is required.

19
Electrical Systems

• Aircraft Power Generation
• Auxiliary Power Units (APUs)
• An APU is simply a small engine, usually a jet
  turbine, but, in older aircraft a four stroke
  gasoline engines are used
• The engine drives the same kind of generator that
  the main engines do
• They can usually provide enough power for
  lighting, INS alignment and possibly some air
  conditioning
• It is essential to provide some sort of reverse
  current protection to avoid the situation where
  the engine power drives the APU generator as a
  motor.

20
Electrical Systems

• Aircraft Power
• Batteries
• Prime power batteries are required for two main
  reasons
• To provide power to start the APU or engines if
  there is no APU
• To provide emergency power in case of a complete
  failure of the engine driven power system
• Other batteries are also used in various computer
  systems to keep data memories operating.
• Note most computer memory is dynamic which
  means that it has to be refreshed at frequent
  intervals. If power is lost the data is lost very
  quickly.

21
Electrical Systems

• Aircraft Power
• Batteries
• Batteries are collections of individual cells in
  series.
• Each cell has the capacity to store electrical
  energy in chemical form
• Note There are many types of chemical
  combinations which are used in cells but, in most
  cases, one individual cell produces only about
  1.5V, Thus several cells must be connected
  together to produce usable voltages.
• Characteristics of cells
• Cell voltage a function of
• the chemical reaction used
• The state of charge
• the

22
Electrical Systems

• Aircraft Power
• Batteries
• Characteristics of cells
• Capacity the amount of energy that can be
  stored
• Cell voltage a function of
• the chemical reaction used (lead-acid,
  nickel-cadmium)
• the state of charge
• The temperature
• Internal Resistance limits the amount of
  current available

23
Electrical Systems

• Aircraft Power
• Batteries
• Capacity
• Charge state is determined by measuring the open
  circuit (no load) voltage.
• The fully discharged state is defined by a
  minimum voltage which is a function of the type
  of battery
• The one hour discharge rate is the current which
  will cause the battery to go from fully charged
  to discharged in one hour
• The capacity of a battery is given in
  ampere-hours
• If it can provide 10 amperes for 10 hours, its
  capacity is 100 ampere-hours

24
Electrical Systems

• Aircraft Power
• Batteries types
• Lead-acid
• Nickel-Cadmium
• Nickel-Metal Hydride
• Lead-acid
• Although lead-acid (automobile type) batteries
  are used in some smaller aircaft, they have the
  disadvantage of venting hydrogen when charging
  which could produce a fire hazard

25
Electrical Systems

• Aircraft Power
• Batteries types
• Nickel Cadmium
• By far the most popular aircraft battery is the
  Nickel Cadmium (Ni-CD pronounced Ni-Cad)
• Advantages
• Long life
• Typically 1000 charge/discharge cycles
• High Current Output (low resistance)
• Need protection from overloads
• Potential for explosion if overloaded

26
Electrical Systems

• Aircraft Power
• Batteries types
• Nickel Cadmium
• Advantages
• Wide temperature operation
• Provides 60 capacity at -30ºC
• Beneficial in a clod climate
• Capable of being overcharged without damage
• Cell voltage remains (almost) constant during
  discharge
• Makes it more difficult to determine state of
  charge
• Capable of fast charging
• Good Energy Density (high energy, small volume)

27
Electrical Systems

• Aircraft Power
• Batteries types
• Nickel Cadmium
• Charging
• When the battery is disharged, such as after the
  engine start it need to be reharged.
• This is done through the aircraft electrical
  system via the DC bus through a voltage regulator.

Battery
DC Bus
Voltage Regulator
28
Electrical Systems

• Aircraft Power
• Batteries types
• Nickel Cadmium
• Charging
• When the battery is discharged, its output
  voltage is lower than the output of the voltage
  regulator. The resistance is low, so the charge
  rate is high.
• As the battery charges, its output voltage
  increases which decreases the amount of current
  provided by the regulator
• Full charge is hard to detect by measuring just
  the output voltage.
• Rate of change of voltage, temperature and
  charging current can all be used to determine
  full charge and hence the point to disconnect the
  charging circuit

29
Electrical Systems

• Aircraft Power
• Batteries types
• Nickel Cadmium
• Memory Effect
• If a NiCd battery is repeatedly recharged after
  being discharged only partially, it will seem to
  take the attitude that it needs to supply only
  the partial discharge amaount of energy. Thus the
  apparent capacity is reduced
• This is remedied by conditioning the battery
  which means discharging it completely and
  recharging it fully several times.
• This can be done in the avionics shop or
  automatcally by some battery monitor circuits

30
Electrical Systems

• Aircraft Power
• Batteries types
• Nickel Cadmium
• Thermal Runaway
• In certain cases e.g. large demand on the battery
  for engine starts, the battery will be discharged
  more than normal
• The charging current will then be high and the
  battery temperature will increase
• As the temperature increases, the cell resistance
  decreases which means that the charging current
  will increase further
• This cycle can continue until the battery fails
  catastrophically, (possibly explode)

31
Electrical Systems

• Aircraft Power
• Batteries types
• Nickel Cadmium
• Thermal Runaway
• This can be avoided by monitoring the battery
  temperature after starting. In both the videos
  you may notice that part of the after start check
  was a battery temperature check.
• FAR 23.1335 requires that the charging system be
  designed to prevent overheating. Also a
  temperature warning device and a charging
  disconnect must be provided

32
Electrical Systems

• Aircraft Power
• Batteries types
• Nickel Metal Hydride
• Developed because Cadmium is extremely toxic.
• Characteristics very similar to those of NiCd
• Advantages (over NiCd)
• Up to 50 higher energy density
• Disdvantages
• Inferior cold temperature performance

33
Electrical Systems

• Aircraft Power
• Wiring
• Until we get fibre optics operational we are
  going to have to keep using copper wire to
  connect avionics systems together
• Even after FO becomes available, we will still
  need copper wire to provide power to all of these
  systems
• What is wire?

Copper core
Insulation
Protective Layer
34
Electrical Systems

• Aircraft Power
• What is wire?
• Copper core
• The main component
• Composition depends on application temperature
• lt105ºC ordinary copper
• 105ºCltTlt200ºC silver coated copper
• 200ºCltTlt260ºC nickel coated copper

35
Electrical Systems

• Aircraft Power
• What is wire?
• Insulation
• To prevent contact with other wire or airframe
• Must
• NOT SUPPORT COMBUSTION
• NOT PRODUCE TOXIC FUMES WHEN HEATED
• Be flexible
• Withstand abrasion (from vibration)
• Be resistant to hydraulic and other fluids
• Not crack

36
Electrical Systems

• Aircraft Power
• What is wire?
• Insulation

37
Electrical Systems

• Aircraft Power
• What is wire?
• Insulation
• Wire insulation has been either confirmed or
  suspected as being the cause of several aircraft
  fires most notably Swissair 111
• Wire Sizes
• Designated by the American Wire Gauge (AWG)
  system
• Smaller diameter wires have higher gauge numbers
• AWG22 has a diameter of 0.0254 in.
• AWG16 has a diameter of 0.0508 in
• NOTE for an increase of 6 in the wire size, the
  diameter doubles
• This is a logarithmic scale 6dB 4x, an
  decrease of 6 AWG indicates an increase of AREA
  by 4

38
Electrical Systems

• Aircraft Power - Wire
• The size of the wire determines how much current
  it can carry safely (without overheating)
• The resistance per unit length increases with
  decreasing diameter
• Power dissipated in the wire I2R. The wire
  temperature increases until its radiated power
  equals the input power.
• The ability of the wire to dissipate the heat
  determines the maximum current it is allowed to
  carry
• Wire in free air can carry more current than a
  wire in a bundle

39
Electrical Systems

• Aircraft Power - Wire
• Selection of wire
• What current must the wire carry?
• What is the length of the wire run?
• What is the allowable voltage drop?
• 28V system 1 V
• 115V system 4 volts
• From the voltage drop and the current, the
  maximum resistance of the wire can be determined.
• From the length of run, the resistance/unit
  length can be calculated
• The gauge of the wire can then be determined from
  tables
• e.g. AWG22 wire is 16 ohms/1000Ft.

40
Electrical Systems

• Aircraft Power Wire
• Selection of wire
• Once the size has been determined, the conductor
  and insulation material can be chosen according
  to the expected temperatures and abrasion in the
  area of use.

41
Electrical Systems

• Aircraft Power Wire
• Identification of wire
• All aircraft wires over 3 inches in length must
  carry identification numbers
• These are to permit maintenance technicians to
  identify wires quickly
• Normally a standard designation convention is
  followed
• Identity of the system
• A wire serial number within that system
• An alphabetical segment identifier
• The wire size

42
Electrical Systems

• Aircraft Power Wire
• Identification of wire
• Example
• 2RN152B22
• 2 indicates the second system of type RN
• RN indicates it is a radio navigation system
• 152 means it is the 152nd wire in the system
• B indicates that it is the second segment of the
  cable run
• 22 indicates that it is AWG 22 wire

43
Electrical Systems

• Aircraft Power Wire
• Identification of wire
• Wiring diagram example

Single Wire
Flight Management System
VOR 2
11 12 13
3 4 5
6 7 8
6 7 8
Twisted Shielded Pair
44
Electrical Systems

• Aircraft Power
• Circuit Protection
• The purpose of circuit protection is to prevent
  fires resulting from overheated wiring
• If a piece of equipment develops a short circuit,
  the input current increases and causes the power
  wire to heat up.
• Two devices are used for protection
• Fuse
• Circuit Breaker

45
Electrical Systems

• Aircraft Power Circuit Protection
• Fuse
• A fuse is primarily a short section of wire made
  from a material with a low melting point
• This fuse is inserted in series with the power
  line and thus carries the same current as the
  device being powered
• If the current in the line exceeds the fuse
  value, the metal melts and interrupts the flow of
  current
• Two types of fuses are available
• Fast acting
• Slow acting

46
Electrical Systems

• Aircraft Power Circuit Protection
• Fuses
• In some circuits there is an initial surge in
  current when the device is turned on. A light
  bulb is a good example. If you measure the
  resistance of an unlit light bulb you will find
  it is very low.
• When power is applied, the current is very high
  for the short period it takes for the filament to
  heat up. When it reaches operating temperature
  the resistance is high and the current decreases
  to a low value.
• A fast acting fuse would blow every time you
  turned the light on
• In such circuits a slow acting fuse is installed

47
Electrical Systems

• Aircraft Power Circuit Protection
• Fuses
• The main drawback with fuses is that, once they
  have done their job, they have to be replaced.
• In an aircraft this is impractical since it would
  require carrying a stock of replacement fuses
• Thus fuses are mainly used in Line Replaceable
  Units (LRUs) because the LRUs are usually not
  repairable in flight and have to be taken out
  anyway

48
Electrical Systems

• Aircraft Power Circuit Protection
• Circuit Breakers
• A circuit breaker is a spring-loaded switch
  which is held in the ON position by a bimetallic
  strip.
• When the strip is heated it releases the switch
• And the current is cut off.
• When the circuit breaker has cooled, it can be
  reset.

49
Electrical Systems

• Aircraft Power Circuit Protection
• Circuit Breakers
• Because the tripping of the circuit breaker
  depends on a rise in its temperature, the
  external temperature of the breaker has an effect
  on the activation current and time.
• If it is cold, the breaker will take longer to
  trip or it will take a higher current.
• The opposite is true if the ambient temperature
  is high

50
Electrical Systems

• Aircraft Power Circuit Protection
• Circuit Breakers

51
Electrical Systems

• Aircraft Power
• Connectors
• Connectors are a necessary evil of avionics life
• They are used
• To connect cables to equipment
• To carry electrical power/signals through
  pressure bulkheads
• To allow sections of the airframe to be
  disconnected for maintenance/inspection

52
Electrical Systems

• Aircraft Power
• Connectors
• Unfortunately for the designer there is a vast
  array of connector types used in aircraft today
• Aircraft wiring usually uses MIL SPEC connectors
  e.g.
• MIL-C-5015
• MIL-C-38999

53
Electrical Systems

• Aircraft Power
• Connectors ANATOMY

ALIGNMENT KEY
SHELL
INSERT
PINS
SOCKETS
54
Electrical Systems

• Aircraft Power Connectors
• The example shown was a MIL-C-5015 connector
  which uses a threaded shell which is secure but
  time-consuming.
• Others use a bayonet shell which is easily
  connected and disconnected

Locking Slots
Locking Pins
Shape of Locking Slot
55
Electrical Systems

• Aircraft Power Connectors
• Variables in selection of connectors
• Series (MIL-C-5015,MIL-C-38999,MIL-C-38723 etc.
  etc
• Size (diameter)
• Shell (male/female)
• Pins/Sockets (solder or crimp)
• Number and size of pins/sockets
• Cable or bukhead
• Environmental (temperature/waterproof/EMI)

56
Electrical Systems

• Aircraft Power Connectors
• Reliability
• Connectors remain the weak link in aircraft
  wiring systems
• A USAF study showed that about 40 of all
  avionics faults could be traced to connectors

57
Electrical Systems

• Aircraft Power Distribution Airbus A320