Electrical systems

Question 1

What parts in an aircraft require electrical supplies?

Answer 1

o All avionics
o Electric motors - pumps, actuators, back-up to
hydraulics, starting
o Heating (e.g. anti-ice) - LE, intakes, pitot probes
o Instruments
o Lighting

Question 2

How will the electrical energy be supplied?

Answer 2

it will have to be generated during the flight, (It is not possible to store enough energy in the batteries)

Question 3

What are 2 simple facts of any electrical machine?

Answer 3

1. An electric current flowing through a
   conductor produces a magnetic field.
2. A conductor moving through a magnetic field
   produces an electric current.

Question 4

The fundamental principle upon which electrical generation is
based on is?

Answer 4

An electron experiences a force when it
passes through a magnetic field. That
force is called the Electromotive Force and
is usually denoted by “E”.

Question 5

What is the equation for the electromotive force (E)?

Answer 5

E = BLV
B = magnetic field strength
L = length of the wire
V = velocity of the wire

Question 6

How is an alternating current generated?

Answer 6

As the wire rotates in the magnetic field, the changing strength of the magnetic field through the wire produces a force which drives the electric charges around the wire. The force initially generates an electric current in one direction along the wire. Then as the loop rotates through 180 degrees the force reverses to give an electric current in the opposite direction along the wire. Every time the loop rotates through 180 degrees the direction of the force and therefore the current changes. The changing direction of the force after every 180 degrees of rotation gives the alternating current.

Question 7

How is direct current generated?

Answer 7

Essentially the same process as
an AC generator, but this time
the continuous “slip rings” are
replaced by sectioned
“commutators”.

Question 8

How does the commutator convert AC current to DC current?

Answer 8

By switching polarity at the same moment the AC signal reverses direction.

Question 9

An aircraft requires large amount of power to be produced by

the smallest, lightest machine possible. How?

Answer 9

You can increase the strength of the magnetic field, increase the length of the wire or increase the velocity

Question 10

Characteristics of AC generation in an aircraft:

Answer 10

• More complex than DC
• 400Hz, 115V/200V three-phase
• Voltage & current (amplitude) varies
continuously with time
• Frequency of voltage & current vary with speed of
the engine

Question 11

Characteristics of DC generation in an aircraft:

Answer 11

• Voltage limits +/-10%
• Generator produces AC internally
• Commutator rectifies the voltage

Question 12

What would 200V 3-phase AC usually be used for?

Answer 12

Radar, heating, starting, other major drains
(some equipment may use single-phase power
provided phases are kept balanced)

Question 13

What would 28V DC usually be used for?

Answer 13

Instruments and avionics (Some equipment may need different supplies, usually derived locally from these)

Question 14

How is the power generated for these electrical supplies?

Answer 14

The main source of power is usually the engines

Question 15

In engine-driven generators, what do ac alternators require?

Answer 15

Require phase synchronisation and

usually constant-speed drive

Question 16

What are some reasons that the AC and DC Functions may be combined?

Answer 16

• Combination of functions can save weight,
e.g. Starter-generators
• AC and DC can be converted

Question 17

AC may be fixed frequency (400 Hz) or ‘frequency-wild’

(~350-700 Hz), what are the differences between the two?

Answer 17

• Fixed frequency
o inductive & capacitive loads have fixed impedance
o easy conversion for other frequencies, e.g. radar
o requires constant-speed drive unit (CSDU)
• Frequency-wild
o lighter and cheaper
o improved reliability

Question 18

Usually, Either AC or DC is generated at the engine and then converted into the other one after, why is this the case instead of generating both AC and DC at the engine?

Answer 18

Often more convenient not to generate both
supplies at engine
• AC – DC using transformer-rectifier
• DC – AC using Dc motor driving an alternator,
or a static inverter
Provides back-up in case of failure

Question 19

What are the emergency supplies of an aircraft?

Answer 19

o APU
o Batteries
o Hydraulically driven generators
o RAT