This
document is a brief for participants in an apprentice training scheme of
‘Collegiate Airways’. The main aim is to provide a document which highlights
and explains the main theory of operations; alternative current (AC) generator
theory, Boeing-type Fixed-frequency Main Engine Generators, Airbus-type
Frequency-Wild Main Engine Generators and power factor correction theory with examples.
Also, explain the practical implementation of the sources of electrical energy
typically found on an aircraft; batteries,
Auxiliary Power Unit (APU), Ram Air
Turbine (RAT) and ground power.
These topics are the basics an aircraft technician apprentice must know and
will be explained in a clear, concise manner.

Primary electrical Sources

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AC Generator Theory

An AC generator, converts
mechanical energy into electrical energy, by using various stations like;
power, thermal and hydroelectric power. It consists of four main parts; Stator,
Armature, Slip rings and Brushes. The Stator is a permanent magnet, in a
concave, cylinder shape, which produces a strong radio magnetic field. Instead
of a permeant magnet a stronger electromagnet can be used for a stator. The
armature is a loop of many turns of wound wire on a core of soft iron sheets, which
are tightly clamped together, these cut across the flux lines as it rotates and
the two slip rings, which are made of copper and are insulated from each other.
The rings rotate with the armature loop due to mechanical power applied to the
shaft where the rings are mounted on. The brushes are made of carbon, one end
is held stationary against the slip rings by the generator framework and the
other to the outer circuit. There is a galvanometer is connected to the outer
circuit, however, it is not part of the AC generator, but is used to show the
direction of the current which is induced by the rotating armature.

The loop rotates clockwise and
produces an electrical current as it rotates perpendicular to the flux lines.
If both legs are moving parallel, no flux lines are cut, therefore no current
is generated. If the loop is rotated 90°, one leg moves downward and the other
upwards, thus, both legs cutting across a maximum number of flux lines, producing
a maximum electrical current. At 180°, again the legs are parallel to the flux
lines and so no current is generated. Then the loop cuts the flux lines, as
above but in an opposite direction, and the emf induced causes the current to
flow in the opposite direction. Another 90° rotation brings the loop back to
the start and the cycle begins again.

Boeing-type Fixed-frequency Main
Engine Generators

A fixed-frequency engine,
similar to one found in a Boeing airplane is a mechanical gearbox which from a
variable speed input (created from the rotation of the wind turbine) creates a
constant speed. The constant speed shaft created will in turn drive the
generator and produce a fixed frequency supply of voltage of 400Hz.

They are usually found
supplying accessory equipment and are rarely needed for an engines main power
output. To produce the proper voltage at a constant alternating current (AC)
frequency, the generator needs to spin at a constant speed and three-phase electric
power is used.

Three-phase electric power is a
type of more polyphase system. Three conductors each carry an alternating current
of the same frequency and voltage amplitude relative to a common reference but
with a phase difference of one third the period. Connected to the ground is the
common reference. The voltage on the conductors reaches its peak at 1/3 of a
cycle after one of the other conductors, as there is a phase difference. Due to
the phase delay, there is a constant power transfer always.

The down fall of this type of
generator is that it is expensive to buy and maintain and only a single source
of power is used to power the generator.

Airbus-type Frequency-Wild Main
Engine Generators

The variable-frequency power supply is applied to the main electric generating
system and eliminates the need for constant speed shaft. It is less complex, more
environmentally friendly and is designed to produce power, by converting direct
current (DC) to alternating current (AC) at the frequency (400Hz) with the
engine shaft rotating at normal cruise speed. The frequency of the generated power is proportional
to the engine shaft speed. To convert the
current an inverter is used.

The inverter causes the electric current to be induced in a coil,
however, the changing magnetic field is produced by another coil, altering the
current flowing through it. Due to the direction of the current changing the
polarity of the magnetic field also changes.

Power Factor Correction

Theory and Benefits

Power Factor is the ratio between
the kW (actual load power) and the kVA (apparent load power) drawn by an electrical
load. The power factor correction is a technique used to increase the power
factor of supply. By switching power supplies, without power factor correction,
the drawn current is, in short, high-magnitude pulses. Using active or passive
techniques these pulses can be smoothed out. Normally with the addition of
capacitors to the electrical network, this is attained, therefore the relative
power demand of the inductive load is compensated for. Which reduces the
input RMS current and apparent input power, thus increasing the power factor
and maximises the real power from the AC supply. Typically, the accepted corrected
power factor will be between 0.92 to 0.95.

There are two different types of
power factor correction used for power supplies, which are; passive PFC and
active PFC.

Passive PFC is used more commonly
for small power supplies of approximately 100W or less, whereas an Active PFC for
power supplies over 100W. Passive PFC is inexpensive and efficient, due to
small components and do not generate EMI. An Active PFC provides more efficient
correction, operates over a wider range of input voltages (47Hz-63Hz), is
lighter and less bulky.

Overall the benefits of power factor
correction are they make the power factor as close to 1 as possible, which in
turn lowers the losses and generates efficient power. Due to the improved
efficiency, there is a reduction in power demand, hence a reduction in the load
on the switching gear and cables. Therefore, overall the capital and consumer
costs reduce and there is support for more load. Another benefit is that the
transmission and distribution equipment and systems runs cooler and last
longer. On an environmental level, there is a reduction in CO2 emissions.

Worked example

To
calculate power factor correction, we first must calculate the apparent power
in kVA;

2.308kVA is a much larger
figure than 1.5kW, which tells us the power factor in this circuit is rather
poor (substantially lower than 1). The power factor is found by;

Using the value above, we can
draw a power triangle, and from that determine the reactive power of this load;

Reactive
power can be calculated by using the Pythagorean Theorem;

With the reactive power,
1.754kVAR, we can calculate the size of the capacitor needed to counteract its
effects;

Using the true and new reactive power values, with the
Pythagorean Theorem, the new reactive power can be found:

Auxiliary Power Sources

Batteries

Lead-acid Battery

The lead-acid battery is the
oldest type of rechargeable battery and can supply high surge currents. It is
used in motor vehicles, hospitals, and marine industries. They usually consist
of two 6-volt batteries or a single 12-volt battery, which are constructed of
several single cells. Each of these cells is connected in series and each cell
can produce approximately 2.1 volts.

A battery cell is made up of two
lead plates; a positive plate, covered with a paste of lead dioxide and a
negative plate, made of sponge lead, with a separator in between, made of
insulating material. The plates are enclosed in a plastic battery case and then
submerged in an electrolyte consisting of water and sulfuric acid.

A discharge causes both
the positive and negative plate become lead (II) sulphate, and the electrolyte
loses its dissolved sulfuric acid and becomes primarily water. The discharge
process is driven by the conduction of electrons from the negative plate. The negative plates produce hydrogen, which is consumed
by the positive plates and both plates consume HSO-4, leading
to a net negative charge. While the battery is charging, the opposite happens
and this is driven by diffusion. Both reactions together are;

Pb(s)
+ PbO2(s) + 2H2SO4(aq) ? 2PbSO4(s) + 2H2O(l)

Lead-acid batteries do
not generate a voltage on their own; only stores a charge from another source,
known as amp hour (AH). Hence, they are known as storage batteries. A typical
12-volt battery has a rating of 125AH, which means it can supply 10amps of
current for 12.5 hours, to increase this they can be connected in parallel.

Nickel-cadmium Battery

The nickel-cadmium (Ni-Cd) battery
is a rechargeable battery which comes in a range of sizes and capacities. They have
a good life cycle (up to 2000 cycles), performs at various temperatures (+60°C to -20°C) and charges rapidly. But, the materials to
make this battery are costlier than lead-acid batteries has a negative
environmental impact and the cells have self-discharge rates. They are used in
emergency lighting, standby power, and uninterruptible power supplies.

A Ni-Cd
battery is made up of; cadmium in the negative electrode, in which usually
potassium hydroxide is used and nickel hydroxide in the positive electrode. Due
to a separator, the positive and negative electrodes plates are isolated from
each other. The selection of
the separator; nylon or polypropylene and the electrolyte; KOH, NaOH, influence
the voltage conditions in the case of a high current discharge, the service
life and the overcharging capability of the cell.

The net reaction during discharge is;

2NiO(OH)+Cd+2H2O?2Ni(OH)2+Cd(OH)2

The alkaline electrode (KOH) is unconsumed
in this reaction and therefore its specific gravity, unlike lead-acid
batteries, is not a guide to its state of charge. The maximum discharge rate for a Ni-Cd
battery varies depending on the size; for an AA-size cell, the discharge rate
is approximately 1.8amperes. During recharge, the
reaction goes from right to left. There is a higher risk of overcharging, which
can damage and lead the battery too quickly rise in pressure, thus they are
equipped with reversible safety valves.

Lithium-ion Battery

A lithium-ion battery (LIB) is a
rechargeable battery and more expensive than Ni-Cd batteries, however, they
operate over a wider temperature range and with a higher energy density.

A lithium battery is made up of
three primary components; the positive electrode, a metal oxide (e.g. lithium
cobalt oxide), the negative electrode, made of carbon (graphite) and the
electrolyte, a lithium salt in an organic solvent (e.g. ethylene carbonate). Depending
on the direction of current flow the electrochemical roles of the electrodes
reverses between anode and cathode. The voltage, energy density, life and
safety of the LIB is dependent on the materials were chosen.

Both the electrodes allow lithium
ions to move in and out of their structures with a process called insertion or
extraction, respectively. During, discharge, the positive lithium ions, and
electrons move from the negative to the positive electrode, forming a lithium
compound, the reverse happens during charging of the battery. Each gram of lithium gives 41.7KJ at 3V.

The full reaction, where the left
side is charged and right is discharged is;

LiC6+CoO2?C6+LiCoO2

LIBs are common in-home
electronics, for military battery electric vehicles and aerospace application
replacing heavier lead-acid batteries. There are different types of LIBs, which
use different lithium combinations, for example; lithium cobalt oxide, etc.
Each variation has different distinct characteristics, for example, lithium
cobalt oxide gives high-density energy, but have safety risks, especially when
damaged as lithium is a flammable electrolyte and may be pressurized and could
explode (Samsung Galaxy Note 7). Thus, when supplied as a battery pack for
notebooks and laptops, extra safety precautions such as temperature sensors,
and voltage taps are provided to minimize the risk of short-circuiting.

APU

An APU is commonly found on large
aircrafts and naval ships, with the primary purpose of providing power (electric,
hydraulic or pneumatic) to start the main engines. In large aircrafts, the
turbine engines must be accelerated to a high rotational speed to provide
sufficient air compression for self-sustaining operation, which an APU does,
thus, not requiring reliance on ground support equipment such as; an external
air-conditioning unit. Once the main engines are started, and as combustion
stabilizes, the engines only require fuel to run, and therefore, replace the
APU. Hence, the APU is unneeded when flying, unless, in the event of the loss
of an engine generator, it can be used to provide an additional source of
electrical power or to power the air-conditioning packs in the event the aircraft
must take off with the engine bleed turned off.

APU is a small jet engine which is normally
located at the tail cone of an aircraft, however, sometimes it is in the engine
nacelle or the wheel well.  An APU used
for commercial transport aircrafts is made up of three main sections; the power
section (gas generator, which provides all the shaft power), The load
compressor section (provide pneumatic power for the aircraft), and the gearbox
section (transfers the power from the main shaft to an oil-cooled generator for
electrical power). An APU generally produces 115 V AC at 400Hz, however, this
is open to variation depending on the design of the APU.

RAT

A Ram Air Turbine (RAT) is a
small wind turbine which is at the heart of an aircraft’s emergency power
system. It is connected to a hydraulic pump, or an electrical generator, and
used as an alternative/emergency hydraulic or electrical power source. If an
airplane loses it hydraulic systems or primary electrical generation, the RAT
deploys and rotates to extract sufficient power from the airstream to power
vital systems (communications equipment, navigation, etc) and land the
aircraft. There are different types of RATs, for instance, some only provide
hydraulic power which is used to power an electrical generator.

In normal, ideal conditions, the
RAT is stored away in a compartment in the wing or fuselage. It can be deployed
manually, or some installations will deploy it automatically following the
complete loss of AC power. The aircraft batteries are used to power the
essential instrumentation, during the interval between power loss and RAT
deployment.

A RAT is used commonly in
military aircrafts, as they must be able to withstand and survive a sudden and
complete loss of power, while in action. However, many modern aircrafts also
have them as an enhanced safety for the passengers on board.

Ground Power

When an aircraft is parked on the
ground, the normal 50Hz mains power is unable to move the plane, as it requires
400Hz power. Therefore, airports require specialist ground support equipment,
which is known as ground power units (GPUs), to allow the aircraft to function
while on the ground. The compact frequency converter units can be fixed to any
convenient place close to the aircraft. The ground power unit is very energy
efficient, as it is only switched on when required and is more flexible than
the earlier generation concepts, which drew continuous power from the mains.
Thus, the advantages of ground power units is it prevents the on-board turbo
generator APU being used, while on the ground, which causes tremendous noise
and air pollution.

The GPU is
connected to the normal 3 phase mains power source, through a special flexible
cable and can deliver an output of 200V 3 phase 400Hz power to the aircraft.
They have a built-in feature which allows the on-board aircraft power system to
synchronize with the converter, so the lights, air conditioning, and avionics
continue operating during the change over from aircraft power to the external
converter power.

Conclusion

Aircraft technician’s apprentices, with this document, should have
a better understanding of the theory of operations and practical implementations
of electrical energy power sources. The document gives an insight into how an
AC generator converts mechanical energy into electrical energy. On the
structure of the Boeing-type
Fixed-frequency Main Engine Generator and Airbus-type
Frequency-Wild Main Engine Generator and how they work. The importance of power
factor correction, when it comes to smoothing out impulses and a worked
example. The main features of the three rechargeable batteries, lithium,
nickel-cadmium and lead-acid. The emergency units, APU and RAT, where they are
stored and how they work and finally the use of the ground power unit. If extra
information is required the technicians are able to look into the references
provided.

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