Avionics and Aircraft Systems

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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.

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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

Definitions

Electrical Systems

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.

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.

Potential Difference:

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.

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.

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.

Coils

Electro Magnet

Coils

This can be shown on a graph:

Magnet

Coils

Electro Magnet

Rectifier

+

Coils

Electro

Magnet

Coils

Voltage

Frequency

-

Measured in Hertz (Hz)

and is the number of

cycles per second.

Output

One Cycle

Line to Earth - 115v

Line to Line - 200v

Electrical Systems

Alternating Current (AC)

AC Generator

8 Pole brush-less with Permanent Magnet Exciter

Star Wound

Star Point (Earth/Return)

Electrical Systems

Alternating Current (AC)

To produce 3Ø electricity we have three sets of windings which produce

their electrical power at equally spaced intervals as follows:

No 1 Ø

No 2 Ø

No 3 Ø

120º

120º

1 Cycle

Peak Value

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.

ENGINE

3

LIFT

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.

Disconnect Unit

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

CLUTCH

DISCONNECT SPRING

I/P

ENGINE DRIVE

GEN DRIVE O/P 6000 RPM

• 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

Selecting disconnect, melts the fusible plug and allows the plunger to be pushed by the spring to contact the paddles.

Drive Wasted Portion

This causes the wasted portion of

the shaft to shear.

Plunger

Fusible Plug

Spring

Drive and plunger assembly must

be replaced after being fired.

Electrical Systems

Other Types Of Disconnect System

• Disengages the CSDU from the Engine Accessory Gearbox
• Non-destructive and can be re-connected on the ground when engine shut-down

ENGINE

3

LIFT

Electrical Systems

Generator Drive

OIL

LP

CSDU

Disconnect Unit

CSDU

Electrical Systems

Constant Speed Drive Unit

Electrical Systems

Constant Speed Drive Unit

INPUT DRIVE TO HYDRAULIC MOTOR

ENGINE DRIVE FROMGEARBOX

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.

ENGINE

3

LIFT

Electrical Systems

Generator Drive

OIL

LP

CSDU

Disconnect Unit

GEN 3

NORMAL

GCR

TRIP

Electrical Systems

AC Generator

Generator

ENGINE

3

LIFT

Electrical Systems

Generator Drive

OIL

LP

CSDU

Disconnect Switch

Disconnect Unit

GEN 3

NORMAL

GCR

TRIP

GEN

FAIL

LIGHT

CLOSE

GCB

GEN CONTROL

TRIP

Electrical Systems

Integrated Drive Generator (IDG)

This is a unit which combines the CSDU with the generator in one casing.

Less weight than separate CSDU and Generator.

Smaller in size.

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

Electrical Systems

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.

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.

ENGINE

4

ENGINE

1

ENGINE

2

ENGINE

3

LIFT

LIFT

LIFT

LIFT

CSDU

CSDU

CSDU

CSDU

GEN 1

GEN 2

GEN 3

GEN 4

SUB 1

SUB 2

SUB 3

SUB 4

BUS 1

BUS 2

BUS 3

BUS 4

BTB

Bus

Tie

Breaker

Electrical Systems

4 Engine Generator Synchronised Busbar System

Split

System

Breaker

Synchronising Busbar “B”

SSB

Synchronising Busbar ”A”

Electrical Systems

ENGINE

4

ENGINE

1

ENGINE

2

ENGINE

3

LIFT

LIFT

LIFT

LIFT

CSDU

CSDU

CSDU

CSDU

GEN 1

GEN 2

GEN 3

GEN 4

SUB 1

SUB 2

SUB 3

SUB 4

Left Hand

Essential

Main

Right Hand

4 Engine Generator Independent Busbar System

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.

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.

Switched Circuits

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.

1

2

3

4

DC in

TRIP

4 contact

1 contact

2 contact

Switched AC

CLOSE

GPB

Secondary switched

Circuits either AC or DC.

DC in

DC in

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.

ENGINE

4

ENGINE

1

ENGINE

2

ENGINE

3

LIFT

LIFT

LIFT

LIFT

CSDU

CSDU

CSDU

CSDU

ELRAT

GEN 1

GEN 2

GEN 3

GEN 4

SUB 1

SUB 2

SUB 3

SUB 4

EMERG. BUS

BUS 1

BUS 2

BUS 3

BUS 4

BTB

Bus

Tie

Breaker

Electrical Systems

ELRAT Generator and Emergency Busbar

Split

System

Breaker

Synchronising Busbar “B”

SSB

Synchronising Busbar ”A”

Electrical Systems

Generator Control Devices

the magnetic field on the 2nd

stage of generation to keep

the output correct.

CSDU drives generator

at constant speed.

CSDU

Voltage

Regulator

To ensure a constant output

voltage a voltage regulator

is required.

Generator produces

200V, 3Ø, 400Hz output.

Electrical Systems

Generator Control Devices

on the output.

CSDU

applies a magnetic force to

the bob weights of the

CSDU governor to achieve

the drive torque.

It is compared with the other

Generators on the busbar

Correction signal to equalise

Control

Voltage

Regulator

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.

Distribution

Electrical Systems

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.

ESS

DC

OFF

MAIN

DC

OFF

ISOL DC

ON BAT

LH

AC

0FF

MAIN

AC

OFF

RH

AC

OFF

ESS

AC

OFF

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.

Warning lights are also used to indicate a

failure of a device.

Busbar Failure Lights.

Monitoring Systems

AC POWER

DRIVE No1

DRIVE No2

DRIVE No3

DRIVE No4

NORMAL

NORMAL

NORMAL

NORMAL

DISCONNECT

DISCONNECT

DISCONNECT

DISCONNECT

LIFT

LIFT

LIFT

LIFT

50

50

50

50

OIL LP

OIL LP

OIL LP

OIL LP

0

0

0

0

100

100

100

100

C

C

C

C

50

50

50

50

150

150

150

150

GEN No1

GEN No2

GEN No3

GEN No4

NORMAL

NORMAL

30

NORMAL

NORMAL

20

30

30

20

20

30

20

40

40

40

10

40

10

10

10

KVAR

KVAR

KVAR

KVAR

KW KVAR

KW KVAR

KW KVAR

KW KVAR

0

0

0

0

50

50

50

50

GCR

GCR

GCR

GCR

TRIP

TRIP

TRIP

TRIP

GEN

GEN

GEN

GEN

FAIL

FAIL

FAIL

FAIL

GEN CONTROL CLOSE

GEN CONTROL CLOSE

GEN CONTROL CLOSE

GEN CONTROL CLOSE

TRIP

TRIP

TRIP

TRIP

FREQ

FREQ

FREQ

FREQ

AC EMERG BUS1

BUS 1

BUS 2

BUS 3

AUX BUS

BUS 4

AUTO

SPLIT

SYSTEM

400

375

BREAKER

425

MANUAL SPLIT

450

350

400

CYCLES

375

425

CLOSE

GRD

POWER

350

CYCLES

BREAKER

KW

60

KVAR

GEN3

90

GEN2

30

TRIP

60

GRD SERVICE BUS

GEN3

GEN2

120

GEN4

60

GEN1

VOLTS A.C.

30

90

90

30

GEN4

GEN1

VOLTS A.C.

GEN

CSD

ELRAT

GRD PWR

OIL OUTLET

STATOR

120

VOLTS A.C.

ELRAT

GRD PWR

Electrical Systems

CSDU

Warning

Light

Generator

Generator

Failure

Warning

Light

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.

Basic DC System

Emergency Bus

Auxiliary Bus

No1 Gen AC Bus

No2 Gen AC Bus

No4 Gen AC Bus

No3 Gen AC Bus

BTB

BTB

SSB

Standby TRU

No 1 TRU

No 2 TRU

No 2 Battery

No 1 Battery

No 2 Essential

No 1 Essential

BTB

BTB

Electrical Systems

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.

Circuit Protection

Electrical Systems

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

Circuit Protection

Electrical Systems

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.

Hercules Circuit Breaker Panels.

Various Circuit Breakers.

Circuit Breakers have a quicker

reaction time than fuses.