Thursday, August 1, 2019
InÃÂ Electricity Generation, anÃÂ Electric GeneratorÃÂ Is a Device
Electric generator Inà electricity generation, anà electric generatorà is a device that convertsà mechanical energyà toà electrical energy. A generator forcesà electric chargeà (usually carried byà electrons) to flow through an externalà electrical circuit. It is analogous to aà water pump, which causes water to flow (but does not create water). Theà source of mechanical energyà may be a reciprocating or turbineà steam engine, water falling through aà turbine or waterwheel, anà internal combustion engine, aà wind turbine, a handà crank,à compressed airà or any other source of mechanical energy.The reverse conversion of electrical energy into mechanical energy is done by anà electric motor, and motors and generators have many similarities. In fact many motors can be mechanically driven to generate electricity, and very frequently make acceptable generators. ââ¬âââ¬âââ¬â-Historical developments Before the connection betweenà magne tismà andà electricityà was discovered,à electrostatic generatorsà were invented that usedà electrostaticprinciples. These generated very highà voltagesà and lowà currents.They operated by using movingà electrically chargedà belts, plates and disks to carry charge to a high potential electrode. The charge was generated using either of two mechanisms: * Electrostatic induction * Theà triboelectric effect, where the contact between two insulators leaves them charged. Because of their inefficiency and the difficulty ofà insulatingà machines producing very high voltages, electrostatic generators had low power ratings and were never used for generation of commercially significant quantities of electric power.Theà Wimshurst machineà andà Van de Graaff generatorà are examples of these machines that have survived. Faraday's disk In the years of 1831ââ¬â1832,à Michael Faradayà discovered the operating principle of electromagnetic generators. The pr inciple, later calledFaraday's law, is that anà electromotive forceà is generated in an electrical conductor that encircles a varyingà magnetic flux. He also built the first electromagnetic generator, called theà Faraday disk, a type ofà homopolar generator, using aà copperà disc rotating between the poles of a horseshoeà magnet. It produced a small DC voltage.This design was inefficient due to self-cancelling counterflows of current in regions not under the influence of the magnetic field. While current was induced directly underneath the magnet, the current would circulate backwards in regions outside the influence of the magnetic field. This counterflow limits the power output to the pickup wires and induces waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.Another disadvantage was that the output volta ge was very low, due to the single current path through the magnetic flux. Experimenters found that using multiple turns of wire in a coil could produce higher more useful voltages. Since the output voltage is proportional to the number of turns, generators could be easily designed to produce any desired voltage by varying the number of turns. Wire windings became a basic feature of all subsequent generator designs. Dynamo Theà dynamoà was the first electrical generator capable of delivering power for industry.The dynamo usesà electromagneticà principles to convert mechanical rotation intopulsed DCà through the use of aà commutator. The first dynamo was built byà Hippolyte Pixiià in 1832. Through a series of accidental discoveries, the dynamo became the source of many later inventions, including the DCà electric motor, the ACà alternator, the ACà synchronous motor, and theà rotary converter. A dynamo machine consists of a stationary structure, which provides a constant magnetic field, and a set of rotating windings which turn within that field.On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils. Large power generation dynamos are now rarely seen due to the now nearly universal use ofà alternating currentà for power distribution andà solid stateà electronic AC to DC power conversion. But before the principles of AC were discovered, very large direct-current dynamos were the only means of power generation and distribution.Now power generation dynamos are mostly a curiosity. Alternator Without aà commutator, a dynamo becomes anà alternator, which is aà synchronous singly fed generator. When used to feed anelectric power grid, an alternator must always operate at a constant speed that is precisely synchronized to the electrical frequency of the power grid. A D C generator can operate at any speed within mechanical limits, but always outputs direct current. Typical alternators use a rotating field winding excited with direct current, and a stationary (stator) winding that produces alternating current.Since the rotor field only requires a tiny fraction of the power generated by the machine, the brushes for the field contact can be relatively small. In the case of a brushless exciter, no brushes are used at all and the rotor shaft carries rectifiers to excite the main field winding. MHD generator Main article:à MHD generator A magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of aà steamà power plant.The first practical design was the AVCO Mk. 25, developed in 1965. The U. S. government funded su bstantial development, culminating in a 25 MW demonstration plant in 1987. In theà Soviet Unionà from 1972 until the late 1980s, the MHD plant U 25 was in regular commercial operation on the Moscow power system with a rating of 25 MW, the largest MHD plant rating in the world at that time. [2]à MHD generators operated as aà topping cycleà are currently (2007) less efficient than combined-cycleà gas turbines. ââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬â- Terminology The two main parts of a generator or motor can be described in either echanical or electrical terms. Mechanical: * Rotor: The rotating part of anà electrical machine * Stator: The stationary part of an electrical machine Electrical: * Armature: The power-producing component of an electrical machine. In a generator, alternator, or dynamo the armature windings generate the electric current. The armature can be on either the rot or or the stator. * Field: The magnetic field component of an electrical machine. The magnetic field of the dynamo or alternator can be provided by either electromagnets or permanent magnets mounted on either the rotor or the stator.Because power transferred into the field circuit is much less than in the armature circuit, AC generators nearly always have the field winding on the rotor and the stator as the armature winding. Only a small amount of field current must be transferred to the moving rotor, usingà slip rings. Direct current machines (dynamos) require aà commutatorà on the rotating shaft to convert theà alternating currentà produced by the armature toà direct current, so the armature winding is on the rotor of the machine. ââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬â- ExcitationAn electric generator or electric motor that uses field coils rather than permanent magnets requires a current to be present in the field coils for the device to be able to work. If the field coils are not powered, the rotor in a generator can spin without producing any usable electrical energy, while the rotor of a motor may not spin at all. Smaller generators are sometimesà self-excited, which means the field coils are powered by the current produced by the generator itself. The field coils are connected in series or parallel with the armature winding.When the generator first starts to turn, the small amount ofà remanent magnetismà present in the iron core provides a magnetic field to get it started, generating a small current in the armature. This flows through the field coils, creating a larger magnetic field which generates a larger armature current. This ââ¬Å"bootstrapâ⬠process continues until the magnetic field in the core levels off due toà saturationà and the generator reaches a steady state power output. Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger.In the event of a severe widespreadà power outageà whereà islandingà of power stations has occurred, the stations may need to perform aà black startà to excite the fields of their largest generators, in order to restore customer power service. ââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬â- Equivalent circuit The equivalent circuit of a generator and load is shown in the diagram to the right. The generator'sà VGà andà RGà parameters can be determined by measuring the winding resistance (corrected to operating temperature), and measuring the open-circuit and loaded voltage for a defined current load. ââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬â [edit]Vehicle-mounted generators Early motor vehicles until about the 1960s tended to use D C generators with electromechanical regulators. These have now been replaced byalternatorsà with built-inà rectifierà circuits, which are less costly and lighter for equivalent output. Moreover, the power output of a DC generator is proportional to rotational speed, whereas the power output of an alternator is independent of rotational speed. As a result, the charging output of an alternator at engine idle speed can be much greater than that of a DC generator.Automotive alternators power the electrical systems on the vehicle and recharge theà batteryà after starting. Rated output will typically be in the range 50-100 A at 12 V, depending on the designed electrical load within the vehicle. Some cars now have electrically poweredà steering assistanceà andà air conditioning, which places a high load on the electrical system. Large commercial vehicles are more likely to use 24 V to give sufficient power at theà starter motorà to turn over a largediesel engine.Vehicle alternators do not use permanent magnets and are typically only 50-60% efficient over a wide speed range. [4]Motorcycle alternators often use permanent magnetà statorsà made withà rare earthà magnets, since they can be made smaller and lighter than other types. See alsoà hybrid vehicle. Some of the smallest generators commonly found powerà bicycle lights. These tend to be 0. 5 ampere, permanent-magnet alternators supplying 3-6 W at 6 V or 12 V. Being powered by the rider, efficiency is at a premium, so these may incorporateà rare-earth magnetsà and are designed and manufactured with great precision.Nevertheless, the maximum efficiency is only around 80% for the best of these generatorsââ¬â60% is more typicalââ¬âdue in part to the rolling friction at theà tyreââ¬âgeneratorà interface from poor alignment, the small size of the generator, bearing losses and cheap design. The use of permanent magnets means that efficiency falls even further at high speed s because the magnetic field strength cannot be controlled in any way. Hub dynamosà remedy many of these flaws since they are internal to the bicycle hub and do not require an interface between the generator and tyre. Until recently, these generators have been expensive and hard to find.Major bicycle component manufacturers like Shimano and SRAM have only just entered this market. However, significant gains can be expected in future as cycling becomes more mainstream transportation and LED technology allows brighter lighting at the reduced current these generators are capable of providing. Sailing yachts may use a water or wind powered generator to trickle-charge the batteries. A smallà propeller,à wind turbineà orà impellerà is connected to a low-power alternator and rectifier to supply currents of up to 12 A at typical cruising speeds. Still smaller generators are used inà micropowerà applications. ââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬ââ⠬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬â Engine-generator Anà engine-generatorà is the combination of an electrical generator and anà engineà (prime mover) mounted together to form a single piece of self-contained equipment. The engines used are usually piston engines, but gas turbines can also be used. Many different versions are available ââ¬â ranging from very small portableà petrolà powered sets to large turbine installations. ââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬â- Human powered electrical generators A generator can also be driven by human muscle power (for instance, in field radio station equipment).Human powered direct current generators are commercially available, and have been the project of someà DIYà enthusiasts. Typically operated by means of pedal power, a converted bicycle trainer, or a foot pump, such generators can be practically used to charge batteries, and in some cases are designed with an integral inverter. The average adult could generate about 125-200 watts on a pedal powered generator, but at a power of 200 W, a typical healthy human will reach complete exhaustion and fail to produce any more power after approximately 1. 3 hours. 6]Portable radio receivers with a crank are made to reduce battery purchase requirements, seeà clockwork radio. During the mid 20th century, pedal powered radios were used throughout the Australian outback, to provide schooling,(school of the air) medical and other needs in remote stations and towns. ââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬â- Linear electric generator In the simplest form of linear electric generator, a slidingà magnetà moves back and forth through aà solenoidà ââ¬â a spool of copper wire. Analternating currentà is induced in the loops of wire byà Faraday's law o f inductionà each time the magnet slides through.This type of generator is used in theà Faraday flashlight. Larger linear electricity generators are used inà wave powerà schemes. ââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬âââ¬â- Tachogenerator Tachogenerators are frequently used to powerà tachometersà to measure the speeds of electric motors, engines, and the equipment they power. Generators generate voltage roughly proportional to shaft speed. With precise construction and design, generators can be built to produce very precise voltages for certain ranges of shaft speeds
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