Power Sources - Power producing devices

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Power Sources - Power
producing devices
• Electro-mechanical
• Electro-chemical
• Alternate – solar, etc.
Power Sources - Electromechanical
• Primary unit is the electromagnetic generator
• Mechanical force rotates driveshaft which
uses magnetic flux to drive electrons through
circuit.
• Magnetic flux can come from a permanent
magnet (magneto) or from an electric magnet
(generator)
Power Sources - Electromechanical
• mechanical force can come from many
sources.
• Common ones are:
–
–
–
–
–
reciprocating engines
steam engine, geothermal, coal, nuclear, etc.
turbine engine
kinetic, hydro-dam, hydro-wave-pump, etc.
wind
Power Sources - Electromechanical
• In general unit produces alternating current,
then current is converted to DC if needed.
• One strategy uses AC phased in the armature
with magnetic poles using a commutator
switch to create DC.
• Armature = the output section, can be
stationary or rotary.
• Field = Electro-Magnetic Field = controls
output, can be stationary or rotary.
Power Sources - Electromechanical
• Although unit can be designed to maximize
efficiency at any RPM it will usually only be
efficient at that RPM.
• OK though because most users prefer stable
output frequency, which is established by
RPM.
• Two common frequencies are 60Hertz (full
cycles per second) or 400hz
Power Sources - Electromechanical
• USA AC electrical systems use 60hz, but
aircraft AC systems use 400hz.
• This saves on weight in several areas.
• Higher frequency means fewer coils of
copper and less core iron in generator.
• But higher Hz means greater line loss so
longer runs used on planet bound power
systems benefit from lower 60Hz.
Power Sources - Electromechanical
• Aircraft DC systems use 12 or 24 volts most
commonly.
• Aircraft power usually comes from engine
driven units, with bigger aircraft having a
dedicated unit for this – APU – Auxiliary
Power Unit.
• APU will use same type of fuel as main
engines.
• Common to marine applications of APUs.
Power Sources - Electromechanical
• Some older small aircraft use a wind driven
“generator”.
• These units are often an add on unit, by
major alteration process.
• Note: Once unit installed may nullify
qualification for Light Sport Aircraft.
Power Sources - Electromechanical
• Electromechanical generator may also be
used as signal generator for such things as
tachometers, wheel speed sensors and
synchronization components.
Power Sources - Electrochemical
• Electrochemical = same as battery.
• Primary = one time use
• Secondary means can be recharged and
used more than once.
Power Sources - Electrochemical
• Typically used for starting systems, and to run
basic equipment during non-engine run
phases or during emergencies.
• Will also act as a buffer to stabilize power
“grid” during various fluxuation events such
as the high load that occurs when starting.
Power Sources - Electrochemical
• Some very recent technologies use systems
that directly convert simple hydrocarbon fuels
into electricity.
• Fuel source must be critically clean,
technology has a long way to mature and
become reasonably priced.
• Upside is it can be very lightweight, clean,
and quiet.
Power Sources - Alternate –
solar, etc.
• Solar power sources use photovoltaic
qualities of some semiconductor
technologies.
• Essentially as photons strike the material
enough energy is added to it that the electron
is knocked free.
• The material is so arranged that the electrons
must go in a uniform direction thereby
becoming current.
Power Sources - Alternate –
solar, etc.
• The basic unit of a photovoltaic cell silicon
diode.
• by doping two layers of silicon with
phosphorus and boron, respectively, it will
then trade electrons to balance the atomic
need.
Power Sources - Alternate –
solar, etc.
• But this leaves the barrier very polarized so
that electrons easily jump the barrier one, but
won’t go the other way.
• This is because like poles repel, unlike poles
attract.
Power Sources - Alternate –
solar, etc.
• As electrons are added to the N side this will
tip the balance between chemical force and
electrostatic towards negative on the N side
so electrons will jump the barrier to the
positive since opposite polarity attracts.
• When electrons are added to the P side the
scale tip will be towards making the P side
more negative forcing the electron away
since like polarity repels.
Power Sources - Alternate –
solar, etc.
• This is called a P/N junction.
• If it is made in a manner that light can reach
the junction it can generate current flow.
• Then when photons hit an electron on the P
side it gets enough energy to jump the barrier
and will do so if a circuit exists to allow the
electrons to flow back around.
Power Sources - Alternate –
solar, etc.
• These tend to be fairly low power generators.
• In part this is because they can only really
use a narrow frequency band of of this
spectrum.
• Also, the substrate in which all this resides
must be a semi-conductor for it to work, so
therefore won’t be the best of current carriers.
Power Sources - Alternate –
solar, etc.
• END of Section
AC theory
• Alternating Current
• Electrons move back and forth in circuit.
• Works just like DC in a purely resistive
circuit.
• But no circuit is purely resistive.
AC theory
• Two primary reasons for using AC are;
– Is simple to cause alternating electron
motion via electro-mechanical means.
– Signal transmission via electromagnetic
spectrum.
AC theory
• From the resistive perspective Ohm’s
and Kirchhoff’s laws apply equally to AC
and DC circuits.
• But since the amount of current is
changing all the time and there is
always some magnetic output from a
conductor carrying current there will be
an “impedance” to the changing.
AC theory
• This is primarily due to the fact that as
current increases the magnetic field
builds, as it decreases the field
collapses.
• This since this field changing takes time
it will lag the current changes some.
AC theory
• Since current can be made by magnetic
fields passing through a conductor they
will pass through itself or other parts of
the circuit like the other loops of a coil.
• This will create an “Induced” voltage.
• This induced voltage is opposite of the
voltage causing the current change.
AC theory
• Therefore as the voltage increases, the
current increases, causing the magnetic
field to increase, creating an induced
voltage that is opposite in polarity to the
original voltage.
• No current change = no induced
voltage, high rate of change = high
induced voltage.
AC theory
• So as the rate of changing increases
the inductive impedance increases.
• Impedance is like resistance in an AC
circuit in that it causes a loss of energy.
• It can therefore be treated in a similar
mathematical manner with the “Laws”.
• Is properly called “Reactance”.
AC theory
• A circuit that has some capacitive
aspect will allow AC to pass through the
capacitor, but prevent DC from passing.
• DC is merely the absolute case of very
slow AC.
• So as the frequency of AC slows down
the impedance to AC will increase in a
capacitive circuit.
AC theory
• Called Capacitive Reactance this is the
opposite of inductive reactance with
respect to frequency.
• But it is similar in that it will impede the
“energy” of electron flow in an AC
circuit.
• Like inductive reactance, the “laws” can
be applied.
AC theory
• Like resistance, reactance exists in any
circuit where the current is changing.
• These effects can be reduced in design
by applying the “laws”.
• But before they can be applied we must
first understand how Alternating Current
is generated, and how we “model” that
generation mathematically.
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