Electric Genertor

8/13/2019 Electric Genertor

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Before the connection betweenmagnetismandelectricitywas discovered,electrostatic

generatorswere used. They operated onelectrostaticprinciples. Such generators generated very

highvoltageand lowcurrent.They operated by using movingelectrically chargedbelts, plates,and disks that carried charge to a high potential electrode. The charge was generated using either

of two mechanisms:

Electrostatic induction Thetriboelectric effect,where the contact between two insulators leaves them charged.

Because of their inefficiency and the difficulty ofinsulatingmachines that produced very high

voltages, electrostatic generators had low power ratings, and were never used for generation of

commercially significant quantities of electric power. TheWimshurst machineandVan deGraaff generatorare examples of these machines that have survived.

In 1827, HungarianAnyos Jedlikstarted experimenting with the electromagnetic rotating deviceswhich he called electromagnetic self-rotors, now called theJedlik's dynamo.In the prototype of

the single-pole electric starter (finished between 1852 and 1854) both the stationary and therevolving parts were electromagnetic. He formulated the concept of the dynamo at least 6 years

beforeSiemensandWheatstonebut didn't patent it as he thought he wasn't the first to realizethis. In essence the concept is that instead of permanent magnets, two electromagnets opposite to

each other induce the magnetic field around the rotor. It was also the discovery of the principle

ofself-excitation.[2]

Faraday disk, the first electric generator. The horseshoe-shaped magnet (A)created a magneticfield through the disk (D). When the disk was turned, this induced an electric current radially

outward from the center toward the rim. The current flowed out through the sliding spring

contact m, through the external circuit, and back into the center of the disk through the axle.

In the years of 1831

1832,Michael Faradaydiscovered the operating principle ofelectromagnetic generators. The principle, later calledFaraday's law,is that anelectromotive

forceis generated in an electrical conductor which encircles a varyingmagnetic flux.He alsobuilt the first electromagnetic generator, called theFaraday disk,a type ofhomopolar generator,

using acopperdisc rotating between the poles of a horseshoemagnet.It produced a small DC

voltage.
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ctrostatichttp://en.wikipedia.org/wiki/Electrostatic_generatorhttp://en.wikipedia.org/wiki/Electrostatic_generatorhttp://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Magnetism


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This design was inefficient, due to self-cancelling counterflows of current in regions that were

not under the influence of the magnetic field. While current was induced directly underneath the

magnet, the current would circulate backwards in regions that were outside the influence of themagnetic field. This counterflow limited the power output to the pickup wires, and induced 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 onecurrent-flow direction.

Another disadvantage was that the output voltage was very low, due to the single current paththrough 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.

Dynamos are no longer used for power generation due to the size and complexity of the

commutator needed for high power applications. This large belt-driven high-current dynamo

produced 310 amperes at 7 volts, or 2,170 watts, when spinning at 1400 RPM.

The dynamowas the first electrical generator capable of delivering power for industry. The

dynamo useselectromagnetic inductionto convert mechanical rotation intodirect currentthrough the use of acommutator.The first dynamo was built byHippolyte Pixiiin 1832.

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.

Through a series of accidental discoveries, the dynamo became the source of many laterinventions, including the DCelectric motor,the ACalternator,the ACsynchronous motor,and

therotary converter.
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Alternating current generating systems were known in simple forms from the discovery of the

magnetic induction of electric current.The early machines were developed by pioneers such as

Michael FaradayandHippolyte Pixii.

Faraday developed the "rotating rectangle", whose operation was heteropolar- each active

conductor passed successively through regions where the magnetic field was in oppositedirections.[3]

The first public demonstration of a more robust "alternator system" took place in

1886.[4]

Large two-phase alternating current generators were built by a British electrician,J.E.H.

Gordon,in 1882.Lord KelvinandSebastian Ferrantialso developed early alternators, producingfrequencies between 100 and 300Hz.In 1891,Nikola Teslapatented a practical "high-

frequency" alternator (which operated around 15 kHz).[5]

After 1891,polyphasealternators were

introduced to supply currents of multiple differing phases.[6]

Later alternators were designed for

varying alternating-current frequencies between sixteen and about one hundred hertz, for usewith arc lighting, incandescent lighting and electric motors.

[7]

Large power generation dynamos are now rarely seen due to the now nearly universal use of

alternating currentfor power distribution. Before the adoption of AC, very large direct-currentdynamos were the only means of power generation and distribution. AC has come to dominate

due to the ability of AC to be easilytransformedto and from very high voltages to permit lowlosses over large distances.

Electromagnetic generators

Dynamo

Main article:Dynamo

"Dynamo Electric Machine" (end view, partly section,U.S. Patent 284,110), 1883

A dynamo is an electrical generator that producesdirect currentwith the use of acommutator.Dynamos were the first electrical generators capable of delivering power for industry, and thefoundation upon which many other later electric-power conversion devices were based, including

theelectric motor,the alternating-currentalternator,and therotary converter.Today, the simpler

alternator dominates large scale power generation, for efficiency, reliability and cost reasons. Adynamo has the disadvantages of a mechanical commutator. Also, converting alternating to

direct current using power rectification devices (vacuum tube or more recentlysolid state)is

effective and usually economic.
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Alternator

Main article:Alternator

Without acommutator,a dynamo becomes analternator,which is asynchronous singly fed

generator.Alternators producealternating currentwith a frequency that is based on the rotationalspeed of the rotor and the number of magnetic poles.

Automotive alternators produce a varying frequency that changes with engine speed, which isthen converted by a rectifier to DC. By comparison, alternators used to feed anelectric power

gridare generally operated at a speed very close to a specific frequency, for the benefit of AC

devices that regulate their speed and performance based on grid frequency. Some devices such as

incandescent lampsand ballast-operatedfluorescent lampsdo not require a constant frequency,but synchronous motors such as in electric wall clocks do require a constant grid frequency.

When attached to a larger electric grid with other alternators, an alternator will dynamically

interact with the frequency already present on the grid, and operate at a speed that matches thegrid frequency. If no driving power is applied, the alternator will continue to spin at a constant

speed anyway, driven as a synchronous motor by the grid frequency. It is usually necessary foran alternator to be accelerated up to the correct speed and phase alignment before connecting to

the grid, as any mismatch in frequency will cause the alternator to act as a synchronous motor,

and suddenly leap to the correct phase alignment as it absorbs a large inrush current from thegrid, which may damage the rotor and other equipment.

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 carriesrectifiers to excite the main field winding.

Induction generator

Main article:induction generator

An induction generator or asynchronous generator is a type of ACelectrical generatorthat usesthe principles ofinduction motorsto produce power. Induction generators operate by

mechanically turning their rotor faster than the synchronous speed, giving negative slip. A

regular AC asynchronous motor usually can be used as a generator, without any internal

modifications. Induction generators are useful in applications such as minihydro power plants,wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can

recover energy with relatively simple controls.

To operate an induction generator must be excited with a leading voltage; this is usually done by

connection to an electrical grid, or sometimes they are self-excited by using phase correcting

capacitors.
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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. MHDgenerators were originally developed because the output of a plasma MHD generator is a flame,

well able to heat the boilers of asteampower plant.The first practical design was the AVCO

Mk. 25, developed in 1965. The U.S. government funded substantial development, culminating

in a 25 MW demonstration plant in 1987. In theSoviet Unionfrom 1972 until the late 1980s, theMHD 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.[8]

MHD generators

operated as atopping cycleare currently (2007) less efficient thancombined cyclegas turbines.

Other rotating electromagnetic generators

Other types of generators, such as theasynchronous or induction singly fed generator,thedoublyfed generator,or thebrushless wound-rotor doubly fed generator,do not incorporate permanent

magnets or field windings that establish a constant magnetic field, and as a result, are seeing

success in variable speed constant frequency applications, such aswind turbinesor otherrenewable energy technologies.

The full output performance of any generator can be optimized with electronic control but onlythedoubly fed generatorsor thebrushless wound-rotor doubly fed generatorincorporate

electronic control with power ratings that are substantially less than the power output of the

generator under control, a feature which, by itself, offers cost, reliability and efficiency benefits.

Homopolar generator

Main article:Homopolar generator

Faraday disk, the first homopolar generator

A homopolar generator is aDCelectrical generatorcomprising an electrically conductive disc orcylinder rotating in a plane perpendicular to a uniform static magnetic field. A potential

difference is created between the center of the disc and the rim (or ends of the cylinder), theelectrical polaritydepending on the direction of rotation and the orientation of the field. It is also
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known as a unipolar generator, acyclic generator, disk dynamo, or Faraday disc. The voltage

is typically low, on the order of a few volts in the case of small demonstration models, but large

research generators can produce hundreds of volts, and some systems have multiple generators inseries to produce an even larger voltage.

[9]They are unusual in that they can produce tremendous

electric current, some more than a millionamperes,because the homopolar generator can be

made to have very low internal resistance.

Excitation

A small early 1900s 75kVAdirect-driven power station AC alternator, with a separate belt-driven exciter generator.

Main article:Excitation (magnetic)

An 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 sometimesself-excited,which means the field coils are powered by thecurrent 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 ofremanent

magnetismpresent in the iron core provides a magnetic field to get it started, generating a smallcurrent in the armature. This flows through the field coils, creating a larger magnetic field which

generates a larger armature current. This "bootstrap" process continues until the magnetic field in

the core levels off due tosaturationand 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 widespreadpower outagewhereislandingof powerstations has occurred, the stations may need to perform ablack startto excite the fields of theirlargest generators, in order to restore customer power service.

[10]

Electrostatic generator

Main article:electrostatic generator
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AVan de Graaff generator,for class room demonstrations

Suppose that the conditions are as in the figure, with the segment A1 positive and the segmentB1 negative. Now, as A1 moves to the left and B1 to the right, their potentials will rise on

account of the work done in separating them against attraction. When A1 and neighboringsectors comes opposite the segment B2 of the B plate, which is now in contact with the brush Y,

they will cause a displacement of electricity along the conductor between Y and Y1 bringing a

negative charge, larger than the positive charge in A1 alone, on Y and sending a positive chargeto the segment touching Y1. As A1 moves on, it passes near the brush Z and is partially

discharged into the external circuit. It then passes on until, on touching the brush X, has a new
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charge, this time negative, driven into it by induction from B2 and neighboring sectors. As the

machine turns, the process causes exponential increases in the voltages on all positions, until

sparking occurs limiting the increase.

An electrostatic generator, or electrostatic machine, is a mechanical device that produces

static electricity,or electricity athigh voltageand lowcontinuous current.The knowledge ofstatic electricity dates back to the earliest civilizations, but for millennia it remained merely an

interesting and mystifyingphenomenon,without a theory to explain its behavior and often

confused with magnetism. By the end of the 17th Century, researchers had developed practicalmeans of generating electricity by friction, but the development of electrostatic machines did not

begin in earnest until the 18th century, when they became fundamental instruments in the studies

about the new science ofelectricity.Electrostatic generators operate by using manual (or other)

power to transformmechanical workintoelectric energy.Electrostatic generators developelectrostaticchargesof opposite signs rendered to two conductors, using only electric forces, and

work by using moving plates, drums, or belts to carry electric charge to a highpotential

electrode.The charge is generated by one of two methods: either thetriboelectric effect(friction)

orelectrostatic induction.

Wimshurst machine

Main article:Wimshurst machine

The Wimshurst influence machineis anelectrostatic generator,a machine for generating highvoltagesdeveloped between 1880 and 1883 by BritishinventorJames Wimshurst(18321903).

It has a distinctive appearance with two large contra-rotating discs mounted in a vertical plane,

two crossed bars with metallic brushes, and aspark gapformed by two metal spheres.
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Wimshurst machine with twoLeyden jars.

Van de Graaff generator

Main article:Van de Graaff generator

A Van de Graaff generatoris anelectrostatic generatorwhich uses a moving belt toaccumulate very highvoltageson a hollow metal globe on the top of the stand. It was invented

by American physicistRobert J. Van de Graaffin 1929. Thepotential differenceachieved inmodern Van de Graaff generators can reach 5 megavolts. The Van de Graaff generator can be

thought of as a constant-current source connected in parallel with acapacitorand a very large

electrical resistance, so it can produce a visible electrical discharge to a nearby grounding surface

which can potentially cause a "spark" depending on the voltage.

Terminology

The two main parts of a generator or motor can be described in either mechanical or electricalterms.

Mechanical:

Rotor:The rotating part of anelectrical 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 armaturecan be on either the rotor or the stator.

Field:The magnetic field component of an electrical machine. The magnetic field of thedynamo 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 armaturewinding. Only a small amount of field current must be transferred to the moving rotor, usingslip

rings.Direct current machines (dynamos) require acommutatoron the rotating shaft to convertthealternating currentproduced by the armature todirect current,so the armature winding is onthe rotor of the machine.

Equivalent circuit
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Equivalent circuit of generator and load.G = generator

VG=generator open-circuit voltage

RG=generator internal resistance

VL=generator on-load voltageRL=load resistance

The equivalent circuit of a generator and load is shown in the diagram to the right. The

generator's and parameters can be determined by measuring the winding resistance(corrected tooperating temperature), and measuring the open-circuit and loaded voltage for a

defined current load.

Vehicle-mounted generators

Early motor vehicles until about the 1960s tended to use DC generators with electromechanical

regulators. These have now been replaced byalternatorswith built-inrectifiercircuits, which areless 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 bemuch greater than that of a DC generator. Automotive alternators power the electrical systems on

the vehicle and recharge thebatteryafter 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 poweredsteering assistanceandair conditioning,which places a high load on theelectrical system. Large commercial vehicles are more likely to use 24 V to give sufficient power

at thestarter motorto turn over a largediesel engine.Vehicle alternators do not use permanent

magnets and are typically only 50-60% efficient over a wide speed range.[11]

Motorcyclealternators often use permanent magnetstatorsmade withrare earthmagnets, since they can be

made smaller and lighter than other types. See alsohybrid vehicle.

Amagneto,like a dynamo, uses permanent magnets but generates alternating current like analternator. Because of the limited field strength of permanent magnets,magneto generatorsare

not used for high-power production applications, but have had specialist uses, particularly in

lighthousesas they are simple and reliable. This reliability is part of why they are still used asignition magnetosin aviation piston engines.

Some of the smallest generators commonly found powerbicycle lights.Called abottle dynamo

these tend to be 0.5 ampere, permanent-magnet alternators supplying 3-6 W at 6 V or 12 V.
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Being powered by the rider, efficiency is at a premium, so these may incorporaterare-earth

magnetsand are designed and manufactured with great precision. The maximum efficiency is

around 80% for the best of these generators60% is more typicaldue in part to the rollingfriction at the tiregenerator interface, imperfect alignment, the small size of the generator, and

bearing losses. Cheaper designs tend to be less efficient. Due to the use of permanent magnets,

efficiency falls at high speeds because the magnetic field strength cannot be controlled in anyway.Hub dynamosremedy many of these flaws since they are internal to the bicycle hub and donot require an interface between the generator and tire. The increasing use ofLEDlights, more

efficientthan incandescent bulbs, reduces the power needed for cycle lighting.

Sailing boats may use a water- or wind-powered generator to trickle-charge the batteries. A small

propeller,wind turbineorimpelleris 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 inmicropowerapplications.

Engine-generator

Main article:Engine-generator

The Caterpillar 3512C Genset is an example of the engine-generator package. This unit produces

1225 kilowatts of electric power.

An engine-generatoris the combination of an electrical generator and anengine(prime mover)

mounted together to form a single piece of self-contained equipment. The engines used areusually piston engines, but gas turbines can also be used. And there are even hybrid diesel-gas

units, called dual-fuel units. Many different versions of engine-generators are available - ranging

from very small portablepetrolpowered sets to large turbine installations. The primary

advantage of engine-generators is the ability to independently supply electricity, allowing the

units to serve as backup power solutions.[12]

Human powered electrical generators

Main article:Self-powered equipment
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Protesters atOccupy Wall Streetusing bicycles connected to a motor and one-way diode to

charge batteries for their electronics[13]

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 projectof someDIYenthusiasts. Typically operated by means of pedal power, a converted bicycletrainer, 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 125-200watts on a pedal powered generator, but at a power of 200 W, a typical healthy human will reachcomplete exhaustion and fail to produce any more power after approximately 1.3 hours.

[14]

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 remotestations and towns.

Linear electric generator

Main article:Linear alternator

In the simplest form of linear electric generator, a slidingmagnetmoves back and forth through a

solenoid- a spool of copper wire. Analternating currentis induced in the loops of wire by

Faraday's law of inductioneach time the magnet slides through. This type of generator is used intheFaraday flashlight.Larger linear electricity generators are used inwave powerschemes

Template:Linear electric generator

TachogeneratorA tachogenerator is an electromechanical device which produce an output voltage proportional to

its shaft speed. It can be employed as an analogue speed indicator, velocity feedback device or asignal integrator. Two commonly used tachogenerators are DC and AC tachogenerators.

Tachogenerators are frequently used to powertachometersto measure the speeds of electricmotors, engines, and the equipment they power. Generators generate voltage roughly
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proportional to shaft speed. With precise construction and design, generators can be built to

produce very precise voltages for certain ranges of shaft speeds.

Electric Genertor

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