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.