In Electricity Generation, an Electric Generator Is a Device

Electric generator In  electricity generation, an  electric generator  is a device that converts  mechanical energy  to  electrical energy. A generator forces  electric charge  (usually carried by  electrons) to flow through an external  electrical circuit. It is analogous to a  water pump, which causes water to flow (but does not create water). The  source of mechanical energy  may be a reciprocating or turbine  steam engine, water falling through a  turbine or waterwheel, an  internal combustion engine, a  wind turbine, a hand  crank,  compressed air  or any other source of mechanical energy.The reverse conversion of electrical energy into mechanical energy is done by an  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. ———-Historical developments Before the connection between  magne tism  and  electricity  was discovered,  electrostatic generators  were invented that used  electrostaticprinciples. These generated very high  voltages  and low  currents.They operated by using moving  electrically charged  belts, plates and disks to carry charge to a high potential electrode. The charge was generated using either of two mechanisms: * Electrostatic induction * The  triboelectric effect, where the contact between two insulators leaves them charged. Because of their inefficiency and the difficulty of  insulating  machines producing very high voltages, electrostatic generators had low power ratings and were never used for generation of commercially significant quantities of electric power.The  Wimshurst machine  and  Van de Graaff generator  are examples of these machines that have survived. Faraday's disk In the years of 1831–1832,  Michael Faraday  discovered the operating principle of electromagnetic generators. The pr inciple, later calledFaraday's law, is that an  electromotive force  is generated in an electrical conductor that encircles a varying  magnetic flux. He also built the first electromagnetic generator, called the  Faraday disk, a type of  homopolar generator, using a  copper  disc rotating between the poles of a horseshoe  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 The  dynamo  was the first electrical generator capable of delivering power for industry.The dynamo uses  electromagnetic  principles to convert mechanical rotation intopulsed DC  through the use of a  commutator. The first dynamo was built by  Hippolyte Pixii  in 1832. Through a series of accidental discoveries, the dynamo became the source of many later inventions, including the DC  electric motor, the AC  alternator, the AC  synchronous motor, and the  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 of  alternating current  for power distribution and  solid state  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 a  commutator, a dynamo becomes an  alternator, which is a  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:  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 a  steam  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 the  Soviet Union  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]  MHD generators operated as a  topping cycle  are currently (2007) less efficient than combined-cycle  gas turbines. ————————————————- 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 an  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, using  slip rings. Direct current machines (dynamos) require a  commutator  on the rotating shaft to convert the  alternating current  produced by the armature to  direct current, so the armature winding is on the rotor of the machine. ————————————————- 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 sometimes  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 of  remanent magnetism  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 â€Å"bootstrap† process continues until the magnetic field in the core levels off due to  saturation  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 widespread  power outage  where  islanding  of power stations has occurred, the stations may need to perform a  black start  to excite the fields of their largest generators, in order to restore customer power service. ————————————————- Equivalent circuit The equivalent circuit of a generator and load is shown in the diagram to the right. The generator's  VG  and  RG  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. ———————————————— [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 byalternators  with built-in  rectifier  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 the  battery  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 powered  steering assistance  and  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 the  starter motor  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 magnet  stators  made with  rare earth  magnets, since they can be made smaller and lighter than other types. See also  hybrid vehicle. Some of the smallest generators commonly found power  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 incorporate  rare-earth magnets  and are designed and manufactured with great precision.Nevertheless, the maximum efficiency is only around 80% for the best of these generators—60% is more typical—due in part to the rolling friction at the  tyre–generator  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 dynamos  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 small  propeller,  wind turbine  or  impeller  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 in  micropower  applications. ————————â⠂¬â€Ã¢â‚¬â€Ã¢â‚¬â€Ã¢â‚¬â€Ã¢â‚¬â€Ã¢â‚¬â€Ã¢â‚¬â€Ã¢â‚¬â€ Engine-generator An  engine-generator  is the combination of an electrical generator and an  engine  (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 – ranging from very small portable  petrol  powered sets to large turbine installations. ————————————————- 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 some  DIY  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, see  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. ————————————————- Linear electric generator In the simplest form of linear electric generator, a sliding  magnet  moves back and forth through a  solenoid  Ã¢â‚¬â€œ a spool of copper wire. Analternating current  is induced in the loops of wire by  Faraday's law o f induction  each time the magnet slides through.This type of generator is used in the  Faraday flashlight. Larger linear electricity generators are used in  wave power  schemes. ————————————————- Tachogenerator Tachogenerators are frequently used to power  tachometers  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