Download Aircraft Electrical Power Supplies

Avionics and Aircraft
Systems
Chapter 1
Electrics
Aircraft Electrical Power Supplies
Electricity is the life blood of any modern aircraft.
Just about every action on an aircraft requires
electricity either to control the action, power the
action or monitor the action.
Aircraft Electrical Power Supplies
Alternating Current (AC)
3 Phase (Ø) 115 Volt 400Hz
Direct Current (DC)
28 Volt
First lets look at AC
Electrical Systems
Definitions
Electro Motive Force (EMF): A power source which causes the electrons to
move continuously and normally comes from
a generator or battery.
Direct Current (DC): A flow of electrons in an external circuit from the negative to
the positive terminals of the supply.
Alternating Current (AC): A flow of electrons which periodically reverses it’s
direction, reaching a maximum first in one direction
then in the other.
Resistance: Is the opposition to electron flow and is measured in Ohms.
Current: Is the flow of electrons through a circuit and is measured in Amperes.
Potential Difference: When an electron flow meets a resistance an EMF is
required to push them through. The same number of
electrons enter and leave a resistance but the EMF at the
input side will be greater than the EMF at the output side.
The difference between these two forces is the Potential
Difference or Volts Drop and is measured in Volts.
Busbar: A strip of metal which has a power supply attached to one side of it
and load circuits attached to the other side.
Electrical Systems
Definitions
Generator: A rotating device which produces either AC or DC voltage.
Battery:
A unit that stores an EMF and is a combination of cells, each of which
is a device for converting chemical to electrical energy.
Transformer: A static device that changes the amplitude or phase of an AC
current or voltage by electromagnetic induction.
Transformer Rectifier Unit (TRU): A device which changes AC electric into
DC electric.
Inverter: A device which changes DC electric into AC electric. It does this by
using a DC motor to drive an AC generator.
Relay: A device which when power is applied makes a set or sets of contacts
to connect parts of an electrical circuit. When power is removed the
relay relaxes.
Latched Relay: A relay which is powered instantaneously and held in position
mechanically. To release the relay a second instantaneous
application of power is required.
Electrical Systems
Alternating Current (AC)
By rotating a wire loop between
2 magnets of opposite poles
electricity is produced.
The polarity (+ or -) and strength
of the electricity produced is
dependent on the strength of
the magnets and the speed of
rotation of the loop of wire.
The strength of the magnets
can be increased by making
them electro-magnets :–
wrapping coils of wire around
them and passing a DC current
through the coil.
Also the rotating loop can be
multiplied to be a series of coils
and therefore produce more
power.
Electrical Systems
Alternating Current (AC)
There are two types of generator:
Brushed: The EMF is produced on the rotor with the magnetic field
in the stator. The EMF is taken from the rotor via carbon
brushes rubbing on copper slip rings.
Disadvantages:
Speed is limited due to forces acting on the heavy rotor.
Resistance between slip rings and brushes is high.
Cooling the rotor is difficult.
Brushless: The EMF is produced on the stator with the magnetic field
in the rotor. No connection to the rotor is required.
Advantages:
Greater power output for a given size machine.
Torque input to generator is uniform.
More efficient transmission of power.
Brushless is the most commonly used on aircraft.
Electrical Systems
Alternating Current (AC)
If we rotate the magnet surrounded by wire windings electricity is produced in
the coils.
This can be rectified to DC and applied to another set of coils wrapped around
magnets and will induce more powerful AC in the coils on the rotating shaft.
If this is repeated again on the rotating shaft a very strong power output is
achieved.
Output
Rectifier
This is known as a brushless generator.
This can be shown on a graph:
Coils
Electro
Magnet
Magnet
Coils
Coils
Electro
Magnet
Coils
Rectifier
Electro
Magnet
Coils
+
Voltage
Output
One Cycle
Frequency
Measured in Hertz (Hz)
and is the number of
cycles per second.
Electrical Systems
Alternating Current (AC)
AC Generator
8 Pole brush-less with
Permanent Magnet Exciter
Star Wound
Line to Earth - 115v
Star Point (Earth/Return)
Line to Line - 200v
Electrical Systems
Alternating Current (AC)
To produce 3 Ø electricity we have three sets of windings which produce
their electrical power at equally spaced intervals as follows:
No 1 Ø
120º
No 2 Ø
No 3 Ø
120º
Peak Value
1 Cycle
For aircraft electricity the Line to Earth value is 115V, with a Peak value of 200V.
The frequency is 400 Hz (Cycles per Second)
Electrical Systems
Generator Power
Generator power is stated in kVa.
A common generator used on aircraft produces 40kVa.
This is enough power to supply a small town.
Because of the power being produced the generator will also
produce heat and therefore a cooling system is required.
This is normally by ram air.
Electrical Systems
Generator Drive
If a generator has a mechanical fault it may cause damage to the engine, therefore
a protection system is used to disconnect the generator from the engine.
ENGINE
3
Disconnect Unit
LIFT
Operating the disconnect switch separates the CSDU from the engine.
To operate the disconnect system the engine must be running.
Once disconnected it can only be reconnected on the ground with the
engine shut down.
Electrical Systems
Constant Speed Drive Disconnect Unit
Operating the disconnect switch removes the latch and allows the disconnect
spring to separate the drive teeth.
Pivot
Latch
DISCONNECT
SPRING
CLUTCH
I/P
GEN DRIVE
O/P 6000 RPM
ENGINE
DRIVE
Non-destructive and can be re-connected
on the ground when engine shut-down
Electrical Systems
Other Types Of Disconnect System
Fusible Plug and Plunger
Drive Wasted Portion
Selecting disconnect, melts the
fusible plug and allows the plunger
to be pushed by the spring to
contact the paddles.
Paddles On Shaft
This causes the
wasted portion of
the shaft to shear.
Plunger
Drive and plunger assembly must
be replaced after being fired.
Fusible Plug
Spring
Electrical Systems
Other Types Of Disconnect System
Screw Thread And Running Nut
 Disengages the CSDU from the Engine Accessory
Gearbox
 Non-destructive and can be re-connected on the
ground when engine shut-down
Electrical Systems
16
Electrical Systems
17
Electrical Systems
Generator Drive
ENGINE
3
Disconnect Unit
LIFT
OIL
LP
CSDU
Electrical Systems
Constant Speed Drive Unit
CSDU
Electrical Systems
Constant Speed Drive Unit
INPUT
DRIVE TO
HYDRAULIC
MOTOR
ENGINE DRIVE FROM
GEARBOX
GENERATOR DRIVE
OUTPUT 6000 RPM
HYDRAULIC
MOTOR
SWASH PLATE CONTROL
By varying the angle of the swash plate, which can move positive or negative
to the vertical, the input RPM can be added to or subtracted from to give a
constant generator drive of 6000 RPM.
The swash plate is controlled by a governor which hydraulically controls the
angle of the swash plate.
Electrical Systems
Generator Drive
ENGINE
3
Disconnect Unit
LIFT
OIL
LP
CSDU
NORMAL
GCR
GEN 3
TRIP
Electrical Systems
AC Generator
Generator
Electrical Systems
Generator Drive
ENGINE
3
Disconnect Switch
Disconnect Unit
LIFT
OIL
LP
CSDU
NORMAL
GCR
GEN 3
TRIP
CLOSE
GEN CONTROL
TRIP
GEN
FAIL
LIGHT
GCB
Electrical Systems
Integrated Drive Generator (IDG)
This is a unit which combines the CSDU with the generator in one casing.
There are advantages and disadvantages to this system.
Advantages:
Less weight than separate CSDU and Generator.
Smaller in size.
Disadvantages:
More expensive to produce.
A malfunction means that both parts have to be replaced.
Harder to service.
Electrical Systems
4 Engine Generator Synchronised Busbar System
For two or more generators to synchronise on a busbar, the voltage. Frequency
and phase relationship must be within certain limits.
If they coupled outside these limits then one generator could take all the load
and drive the other generators like motors.
Therefore this type of system requires a connection, protection and control
system and a load sharing system to ensure each generator takes an equal
share of the load.
Electrical Systems
Real Load:
Generator Load
This is the amount of work carried out in the circuit and is
measured in Watts. It can be equated to physical work done.
Examples of Real Load are motors, actuators, heaters and lights.
Reactive Load: This is the amount of apparent work carried out in a circuit
and is measured in Volt Ampere Reactive(VAR). It can be
equated to mental work done.
Examples of Reactive loads are capacitors and coils.
Each busbar has different loads attached to it and the total load is evenly
distributed amongst all busbars.
Electrical Systems
4 Engine Generator Synchronised Busbar System
ENGINE
1
LIFT
ENGINE
2
LIFT
ENGINE
4
ENGINE
3
LIFT
LIFT
CSDU
CSDU
CSDU
CSDU
GEN 1
GEN 2
GEN 3
GEN 4
SUB 1
SUB 2
BUS 1
BTB
Bus
Tie
Breaker
SUB 3
BUS 2
Split
System
SUB 4
BUS 3
SSB
Breaker
Synchronising Busbar ”A”
BUS 4
Synchronising Busbar “B”
Electrical Systems
4 Engine Generator Independent Busbar System
ENGINE
1
LIFT
ENGINE
2
LIFT
ENGINE
4
ENGINE
3
LIFT
LIFT
CSDU
CSDU
CSDU
CSDU
GEN 1
GEN 2
GEN 3
GEN 4
SUB 1
SUB 2
Left Hand
SUB 3
Essential
SUB 4
Main
Right Hand
Each generator feeds it’s own Busbar, but if a generator/engine fails a Busbar
Transfer System allows another generator to take the failed Busbar and feed
two Busbars.
To prevent overloading the generator some loads may have to be switched off.
This system is similar to the Hercules K, but because the engines run at 100% RPM
constantly, there is no need for a CSDU.
Electrical Systems
System Control
To control the operation of the electrical system relays are used.
Switching high voltage AC electricity by using a normal switch is dangerous,
therefore by using DC switches to control a relay we can safely operate the
AC switching.
Relays can switch either one or many circuits together.
Relays can also be mechanically latched to a
specified position and therefore to break the circuit
a second coil is required to unlatch the relay.
Switched AC
1
2
3
4
Secondary switched
Circuits either AC or DC.
DC in
TRIP
CLOSE
DC in
GPB
Switched Circuits
DC in
1 contact
2 contact
4 contact
Electrical Systems
Emergency Power Generation
Some aircraft have the ability to select an emergency electrical power source.
This can be a mechanically released, propeller driven, unit which drops into the
airflow and the propeller drives a generator to give enough power to fly the
aircraft to the nearest airfield.
This system is known as an ELRAT unit(Electrical Ram Air Turbine).
Other aircraft have APUs (Auxiliary Power Units), which have a generator.
These APUs can be used airborne but usually only below a specified Altitude,
approximately 15,000Ft.
Electrical Systems
ELRAT Generator and Emergency Busbar
ENGINE
1
LIFT
ENGINE
2
LIFT
ENGINE
4
ENGINE
3
LIFT
CSDU
LIFT
CSDU
CSDU
CSDU
ELRAT
GEN 1
SUB 1
GEN 2
SUB 2
GEN 3
GEN 4
SUB 3
SUB 4
EMERG. BUS
BUS 1
BTB
Bus
Tie
Breaker
BUS 2
Split
System
BUS 3
SSB
Breaker
Synchronising Busbar ”A”
BUS 4
Synchronising Busbar “B”
Electrical Systems
Generator Control Devices
The voltage regulator adjusts
the magnetic field on the 2nd
stage of generation to keep
the output correct.
CSDU
CSDU drives generator
at constant speed.
Voltage
Regulator
To ensure a constant output
voltage a voltage regulator
is required.
Generator produces
200V, 3Ø, 400Hz output.
Electrical Systems
Generator Control Devices
Reactive load is sampled
on the output.
It is compared with the other
Generators on the busbar
And any adjustments are
made to the voltage regulator
Correction signal to equalise
the loads.
Reactive
Load
Control
Voltage
Regulator
CSDU
The adjustment signal
applies a magnetic force to
the bob weights of the
CSDU governor to achieve
load sharing by adjusting
the drive torque.
Real Load
Control
Real load is sampled on the output.
It is compared with the other
generators on the busbar and any
adjustments are fed to the CSDU.
Electrical Systems
Distribution
Each busbar has various circuits connected to it.
To ensure that a single busbar failure would not cause the aircraft systems to
stop operating completely, the power supplies for major systems are spread
across all the busbars.
Fuel Pumps
There are usually two pumps for each tank and these would be powered
by different busbars, to ensure a single busbar failure would leave one
pump operating.
Autopilots
Normally there are at least two autopilots and they would be powered from
separate busbars, to ensure a single busbar failure would leave one
Autopilot operating.
As well as coping with failures it also equalizes the loads on the busbars.
By placing the principle circuit loads on the main busbars, this allows certain
circuits to be prioritised to ensure continued power supply.
Electrical Systems
Generating System Protection
All these generating systems have automatic protection against faults and
some are listed below:
Protection Circuits
Over/Under Voltage
Over/Under Excitation
Over/Under Frequency
Earth Fault
Differential Protection
Negative Sequence Voltage
Stability
Most of these protection circuits will automatically disconnect the generator
from its busbar when a fault occurs.
These protection circuits are intricate and beyond the scope of this lesson.
Electrical Systems
Monitoring Systems
The output and load of the various production devices are monitored on gauges.
The outputs monitored are:
Generator Frequency.
Generator Voltage.
Battery Voltage.
Load Current/Power Used.
Warning lights are also used to indicate a
failure of a device.
ISOL DC
ON BAT
ESS
DC
OFF
MAIN
DC
OFF
MAIN
AC
OFF
RH
AC
OFF
Busbar Failure Lights.
LH
AC
0FF
ESS
AC
OFF
Monitoring Systems
Electrical Systems
AC POWER
DRIVE No1
NORMAL
DRIVE No2
NORMAL
DISCONNECT
DISCONNECT
LIFT
DISCONNECT
LIFT
50
OIL
LP
0
20
10
0
KW KVAR
20
10
40
0
50
30
KW KVAR
TRIP
KW KVAR
10
50
TRIP
GEN
FAIL
GEN CONTROL
CLOSE
GEN CONTROL
CLOSE
TRIP
TRIP
FREQ
FREQ
40
KVAR
KW KVAR
0
50
30
20
NORMAL
40
GEN
FAIL
GEN CONTROL
CLOSE
C 150
GCR
TRIP
TRIP
FREQ
KVAR
0
GEN
FAIL
100
50
150
30
20
10
50
0
CSDU
Warning
Light
GEN No4
NORMAL
40
KVAR
TRIP
GEN CONTROL
CLOSE
C
GCR
GEN
FAIL
OIL
LP
100
50
150
GCR
TRIP
LIFT
50
0
GEN No3
NORMAL
30
KVAR
GCR
Generator
Failure
Warning
Light
C
GEN No2
NORMAL
OIL
LP
100
50
GEN No1
Generator
Load
DISCONNECT
LIFT
50
0
C 150
DRIVE No4
NORMAL
50
OIL
LP
100
50
DRIVE No3
NORMAL
FREQ
AC EMERG BUS1
BUS 1
BUS 2
BUS 3
AUX BUS
BUS 4
AUTO
SPLIT
SYSTEM
BREAKER
375
375
MANUAL SPLIT
350
350
CLOSE
GRD
POWER
BREAKER
TRIP GRD SERVICE BUS
KW
GEN
GEN
22
GEN
GEN
11
GEN
CSD
OIL OUTLET STATOR
GRD
GRDPWR
PWR
KVAR
GEN
GEN
33
GEN
GEN
44
ELRAT
ELRAT
400
400
425
425
CYCLES
CYCLES
450
60
60
60
30
30
30
90
90
90
120
120
VOLTS
VOLTS
VOLTS
A.C.
A.C.
A.C.
Voltage
And
Frequency
Electrical Systems
Direct Current (DC)
The preferred voltage for an aircraft DC system is 28V as it provides
more power than the 12V system used in most cars.
This power can be provided from various units:
Batteries: Output measured in ampere hours.
The average battery used is 25 A/H
Which equates to 1Amp for 25 Hours or 25 Amps for 1 Hour.
Adverse weather can affect batteries and therefore when
aircraft are parked in extremely cold weather the battery
will have to be removed and stored somewhere warm.
Transformer Rectifier Units (TRU): Fed from the AC Busbars.
Electrical Systems
Battery Charging
The power from the TRUs can be used to keep the batteries fully charged.
Batteries should not be continually charged as they can be damaged by
over-charging.
Charging circuits are designed so that as the battery voltage drops below a
pre-determined level, the battery is connected to the generating system to
re-establish its output voltage.
This means the battery is charged cyclically.
Electrical Systems
Basic DC System
Emergency
Bus
Auxiliary
Bus
No1 Gen AC Bus
No2 Gen AC Bus
No3 Gen AC Bus
BTB
BTB
SSB
BTB
No 1
TRU
No 1 Essential
No 1
Battery
No4 Gen AC Bus
Standby
TRU
BTB
No 2
Battery
No 2
TRU
No 2 Essential
Electrical Systems
Secondary AC Supplies
Some instruments/gauges on the Flight Deck require different power supplies to
operate.
The most common of these are:- 26V AC single phase.
115V AC single phase.
To achieve these power supplies they can be taken from the main AC busbars
or by using transformers .
This power supply can also be produced from DC by using Inverters.
Inverters use more power than transformers, therefore the transformer supply
is normally used as the primary source and the inverter used as a backup.
Electrical Systems
Circuit Protection
Circuit protection can be supplied by use of fuses or circuit breakers.
Fuses
A device which when heated by an electrical current will break at a pre-determined
value and disconnect the circuit from the supply..
They can be the size of a normal domestic fuse (1 to 15 Amp) or a large bolt in
type (25 to 175 Amp).
When blown they are discarded and replaced with a new one.
To replace the bolt in type there must be no power on the aircraft.
Bolt in type fuses.
175 Amp
Electrical Systems
Circuit Protection
Circuit Breakers
These are devices which when the current in the circuit reaches the devices rated
value, it will trip and disconnect the circuit from the power supply.
These devices can be reset and used again.
Various Circuit Breakers.
Circuit Breakers have a quicker
reaction time than fuses.
Hercules Circuit Breaker Panels.