Editing Electric generator (section)

==History==
Before the connection between [[magnetism]] and [[electricity]] was discovered, [[electrostatic generator]]s were invented. They operated on [[electrostatic]] principles, by using moving [[Electric charge|electrically charged]] belts, plates and disks that carried charge to a high potential electrode. The charge was generated using either of two mechanisms: [[electrostatic induction]] or the [[triboelectric effect]]. Such generators generated very high [[voltage]] and low [[Electric current|current]]. Because of their inefficiency and the difficulty of [[Electrical insulation|insulating]] machines that produced very high voltages, electrostatic generators had low power ratings, and were never used for generation of commercially significant quantities of electric power. Their only practical applications were to power early [[X-ray tube]]s, and later in some atomic [[particle accelerator]]s.

===Faraday disk generator===
[[File:Faraday disk generator.jpg|thumb|right|The [[Faraday disk]] was the first electric generator. The horseshoe-shaped magnet ''(A)'' created a magnetic field 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.]]
{{Main|Homopolar generator}}

The operating principle of electromagnetic generators was discovered in the years of 1831–1832 by [[Michael Faraday]]. The principle, later called [[Faraday's law of induction]], is that an [[electromotive force]] is generated in an electrical conductor which encircles a varying [[magnetic flux]].

Faraday 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 [[direct current|DC voltage]].

This design was inefficient, due to self-cancelling counterflows of [[electric current|current]] in regions of the disk 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 the magnetic 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 one current-flow direction.

Another disadvantage was that the output [[voltage]] 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.

===Jedlik and the self-excitation phenomenon===
Independently of Faraday, [[Ányos Jedlik]] started experimenting in 1827 with the electromagnetic rotating devices which he called [[Jedlik's dynamo|electromagnetic self-rotors]]. In the prototype of the single-pole electric starter (finished between 1852 and 1854) both the stationary and the revolving parts were electromagnetic. It was also the discovery of the principle of dynamo [[self-excitation]],<ref>{{cite journal|journal=Nature|title=Anianus Jedlik|author=Augustus Heller|publisher=Norman Lockyer|date=April 2, 1896|volume=53|issue=1379|page=516|url=https://books.google.com/books?id=nWojdmTmch0C&q=jedlik+dynamo+1827&pg=PA516|bibcode=1896Natur..53..516H|doi=10.1038/053516a0|doi-access=free}}</ref> which replaced permanent magnet designs. He also may have formulated the concept of the [[dynamo]] in 1861 (before [[Ernst Werner von Siemens|Siemens]] and [[Charles Wheatstone|Wheatstone]]) but did not patent it as he thought he was not the first to realize this.<ref>{{citation|journal=Nature|title=Anianus Jedlik|author=Augustus Heller|publisher=Norman Lockyer|date=2 April 1896|volume=53|issue=1379|page=516|url=https://books.google.com/books?id=nWojdmTmch0C&q=jedlik+dynamo+1827&pg=PA516|bibcode=1896Natur..53..516H|doi=10.1038/053516a0|doi-access=free}}</ref>

===Direct current generators===
{{Main|Dynamo}}
[[File:Wechselstromerzeuger Crop LevelAdj.jpg|thumb|upright=0.8|[[Hippolyte Pixii]]'s dynamo. The commutator is located on the shaft below the spinning magnet.]]
[[File:High-Current Copper-Brush Commutated Dynamo.jpg|thumb|upright=0.8|This large belt-driven high-current [[dynamo]] produced 310 amperes at 7 volts. Dynamos are no longer used due to the size and complexity of the [[commutator (electric)|commutator]] needed for high power applications.]]

A coil of wire rotating in a magnetic field produces a current which changes direction with each 180° rotation, an [[alternating current]] (AC). However many early uses of electricity required [[direct current]] (DC). In the first practical electric generators, called ''[[dynamo]]s'', the AC was converted into DC with a ''[[commutator (electric)|commutator]]'', a set of rotating switch contacts on the armature shaft. The commutator reversed the connection of the armature winding to the circuit every 180° rotation of the shaft, creating a pulsing DC current. One of the first dynamos was built by [[Hippolyte Pixii]] in 1832.

The [[dynamo]] was the first electrical generator capable of delivering power for industry. 
The [[Woolrich Electrical Generator]] of 1844, now in [[Thinktank, Birmingham|Thinktank, Birmingham Science Museum]], is the earliest electrical generator used in an industrial process.<ref>Birmingham Museums trust catalogue, accession number: 1889S00044</ref> It was used by the firm of [[Elkington Silver Electroplating Works|Elkingtons]] for commercial [[electroplating]].<ref name="thomas">{{cite book|last1=Thomas|first1=John Meurig|title=Michael Faraday and the Royal Institution: The Genius of Man and Place|date=1991|publisher=[[Hilger & Watts|Hilger]]|location=Bristol|isbn=978-0750301459|page=51}}</ref><ref>{{cite book|last1=Beauchamp|first1=K G|title=Exhibiting Electricity|date=1997|publisher=IET|isbn=9780852968956|page=90}}</ref><ref>{{cite journal|last1=Hunt|first1=L. B.|title=The early history of gold plating|journal=Gold Bulletin|date=March 1973|volume=6|issue=1|pages=16–27|doi=10.1007/BF03215178|doi-access=free}}</ref>

The modern dynamo, fit for use in industrial applications, was invented independently by [[Sir Charles Wheatstone]], [[Werner von Siemens]] and [[Samuel Alfred Varley]]. Varley took out a patent on 24 December 1866, while Siemens and Wheatstone both announced their discoveries on 17 January 1867 by delivering papers at the [[Royal Society]].<ref name=Siemens1867>{{cite journal |doi=10.1098/rspl.1866.0082 |title=II. On the conversion of dynamical into electrical force without the aid of permanent magnetism |journal=Proceedings of the Royal Society of London |last=Siemens |first=Charles William |date=1867 |volume=15 |pages=367–369 |url=https://zenodo.org/records/1432047/files/article.pdf}}</ref><ref name=Wheatstone1867>{{cite journal |doi=10.1098/rspl.1866.0083 |title=III. On the augmentation of the power of a magnet by the reaction thereon of currents induced by the magnet itself |last=Wheatstone |first=Charles |journal=Proceedings of the Royal Society of London |date=1867 |volume=15 |pages=369–372}}</ref>

The "dynamo-electric machine" employed self-powering electromagnetic field coils rather than permanent magnets to create the stator field.<ref>{{cite journal |journal=Berliner Berichte|date=January 1867}}</ref> Wheatstone's design was similar to Siemens', with the difference that in the Siemens design the stator electromagnets were in series with the rotor, but in Wheatstone's design they were in parallel.<ref name=Siemens1867/><ref name=Wheatstone1867/> The use of electromagnets rather than permanent magnets greatly increased the power output of a dynamo and enabled high power generation for the first time. This invention led directly to the first major industrial uses of electricity. For example, in the 1870s Siemens used electromagnetic dynamos to power [[electric arc furnace]]s for the production of metals and other materials.

The dynamo machine that was developed consisted of a stationary structure, which provides the magnetic field, and a set of rotating windings which turn within that field. On larger machines the constant magnetic field is 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. Before the adoption of AC, very large direct-current dynamos were the only means of power generation and distribution. AC has come to dominate due to the ability of AC to be easily [[transformer|transformed]] to and from very high voltages to permit low losses over large distances.

=== Synchronous generators (alternating current generators) ===
{{Main|Alternator}}
[[File:Ferranti two-phase generator set (Rankin Kennedy, Electrical Installations, Vol III, 1903).jpg|thumb|[[Sebastian Ziani de Ferranti|Ferranti]] [[alternator|alternating current generator]], {{circa|1900}}.]]
Through a series of discoveries, the dynamo was succeeded by many later inventions, especially the AC [[alternator]], which was capable of generating [[alternating current]]. It is commonly known to be the Synchronous Generators (SGs). The synchronous machines are directly connected to the grid and need to be properly synchronized during startup.<ref>{{Cite journal|last=Schaefer|first=Richard C.|date=Jan–Feb 2017|title=Art of Generator Synchronizing|journal=IEEE Transactions on Industry Applications|volume=53|issue=1|pages=751–757|doi=10.1109/tia.2016.2602215|s2cid=15682853|issn=0093-9994}}</ref> Moreover, they are excited with special control to enhance the stability of the power system.<ref>{{Cite journal|last1=Basler|first1=Michael J.|last2=Schaefer|first2=Richard C.|date=2008|title=Understanding Power-System Stability|journal=IEEE Transactions on Industry Applications|volume=44|issue=2|pages=463–474|doi=10.1109/tia.2008.916726|s2cid=62801526|issn=0093-9994}}</ref>

Alternating current generating systems were known in simple forms from [[Michael Faraday]]'s original discovery of the [[Electromagnetic induction|magnetic induction of electric current]]. Faraday himself built an early alternator. His machine was a "rotating rectangle", whose operation was ''heteropolar'': each active conductor passed successively through regions where the magnetic field was in opposite directions.<ref>Thompson, Sylvanus P., ''Dynamo-Electric Machinery''. p. 7</ref>

Large two-phase alternating current generators were built by a British electrician, [[James Edward Henry Gordon|J. E. H. Gordon]], in 1882. The first public demonstration of an "alternator system" was given by [[William Stanley Jr.]], an employee of [[Westinghouse Electric (1886)|Westinghouse Electric]] in 1886.<ref>Blalock, Thomas J., "''[https://web.archive.org/web/20040825140907/http://www.ieee.org/organizations/history_center/stanley.html Alternating Current Electrification, 1886]''". IEEE History Center, IEEE Milestone. (''ed''. first practical demonstration of a dc generator – ac transformer system.)</ref>

{{anchor|Ferranti steam generating plants}}[[Sebastian Ziani de Ferranti]] established ''Ferranti, Thompson and Ince'' in 1882, to market his ''Ferranti-Thompson Alternator'', invented with the help of renowned physicist [[Lord Kelvin]].<ref name=timeline>[http://www.mosi.org.uk/collections/explore-the-collections/ferranti-online/timeline.aspx Ferranti Timeline] {{webarchive |url=https://web.archive.org/web/20151003002335/http://www.mosi.org.uk/collections/explore-the-collections/ferranti-online/timeline.aspx |date=October 3, 2015 }} – ''[[Museum of Science and Industry (Manchester)|Museum of Science and Industry]] (Accessed 22-02-2012)''</ref> His early alternators produced frequencies between 100 and 300 [[hertz|Hz]]. Ferranti went on to design the [[Deptford Power Station]] for the London Electric Supply Corporation in 1887 using an alternating current system. On its completion in 1891, it was the first truly modern power station, supplying high-voltage AC power that was then "stepped down" for consumer use on each street. This basic system remains in use today around the world.

[[File:Murray Alternator with Belt-Driven Exciter.jpg|thumb|200px|A small early 1900s 75 [[kilovolt-ampere|kVA]] direct-driven power station AC alternator, with a separate belt-driven exciter generator.]]
After 1891, [[Polyphase system|polyphase]] alternators were introduced to supply currents of multiple differing phases.<ref>Thompson, Sylvanus P., ''Dynamo-Electric Machinery''. p. 17</ref> Later alternators were designed for varying alternating-current frequencies between sixteen and about one hundred hertz, for use with arc lighting, incandescent lighting and electric motors.<ref>Thompson, Sylvanus P., ''Dynamo-Electric Machinery''. p. 16</ref>

===Self-excitation===
{{main|Excitation (magnetic)}}
As the requirements for larger scale power generation increased, a new limitation rose: the magnetic fields available from permanent magnets. Diverting a small amount of the power generated by the generator to an electromagnetic [[field coil]] mounted on the rotor allowed the generator to produce substantially more power. This concept was dubbed [[self-excitation]].

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 "bootstrap" process continues until the magnetic field in the core levels off due to [[saturation (magnetic)|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.