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In
electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delivery (Electric power transmiss ...
, a generator is a device that converts motive power (
mechanical energy In physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together called the "p ...
) into
electrical power Electric power is the rate, per unit time, at which electrical energy Electrical energy is energy derived from electric potential energy or kinetic energy. When used loosely, ''electrical energy'' refers to energy that has been converted ''f ...
for use in an external
circuitCircuit may refer to: Science and technology Electrical engineering * Electrical circuit, a complete electrical network with a closed-loop giving a return path for current ** Analog circuit, uses continuous signal levels ** Balanced circuit, p ...

circuit
. Sources of mechanical energy include
steam turbine A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do Work (physics), mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Algernon Parsons, Charles Parsons in 188 ...
s,
gas turbine A gas turbine, also called a combustion turbine, is a type of continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability di ...
s,
water turbine A water turbine is a rotary machine that converts kinetic energy and potential energy of water into mechanical work. Water turbines were developed in the 19th century and were widely used for industrial power prior to electrical grids. Now, the ...

water turbine
s,
internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and ...

internal combustion engine
s,
wind turbine A wind turbine is a device that converts Religious conversion is the adoption of a set of beliefs identified with one particular religious denomination A religious denomination is a subgroup within a religion Religion is a social s ...

wind turbine
s and even hand
crank Crank may refer to: Mechanisms * Crank (mechanism) A crank is an arm attached at a right angle to a rotating shaft by which circular motion is imparted to or received from the shaft. When combined with a connecting rod, it can be used to conv ...
s. The first electromagnetic generator, the
Faraday disk A homopolar generator is a DC electrical generator In electricity generation, a generator is a device that converts motive power ( mechanical energy) into electrical power for use in an external circuit. Sources of mechanical energy include st ...
, was invented in 1831 by British scientist
Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge ...

Michael Faraday
. Generators provide nearly all of the power for
electric power grid An electrical grid is an interconnected network for electricity delivery Electricity delivery is the process that starts after generation of electricity in the power station A power station, also referred to as a power plant and someti ...
s. The reverse conversion of electrical energy into mechanical energy is done by an
electric motor
electric motor
, and motors and generators have many similarities. Many motors can be mechanically driven to generate electricity; frequently they make acceptable manual generators.


Terminology

Electromagnetic Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagneti ...

Electromagnetic
generators fall into one of two broad categories, dynamos and alternators. *
Dynamo A dynamo is an that creates using a . Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later devices were based, including the , the , and the . Today, the simple ...

Dynamo
s generate pulsing
direct current Direct current (DC) is one-directional flow Flow may refer to: Science and technology * Flow (fluid) or fluid dynamics, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (math ...
through the use of a
commutator In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gene ...
. *
Alternator An alternator is an electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the s ...

Alternator
s generate
alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natu ...
. Mechanically a generator consists of a rotating part and a stationary part: *
Rotor ROTOR was an elaborate air defence radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An aircraft is a vehicle that is able to flight, fly ...
: The rotating part of an
electrical machine In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identif ...
. *
Stator The stator is the stationary part of a rotary system, found in electric generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities ...
: The stationary part of an electrical machine, which surrounds the rotor. One of these parts generates a magnetic field, the other has a wire winding in which the changing field induces an electric current: * Field winding or field (permanent) magnets: The
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

magnetic field
-producing component of an electrical machine. The magnetic field of the dynamo or alternator can be provided by either wire windings called
field coil A field coil is an electromagnet File:VFPt Solenoid correct2.svg, Magnetic field produced by a solenoid (coil of wire). This drawing shows a cross section through the center of the coil. The crosses are wires in which current is moving into ...
s or
permanent magnet A magnet is a material or object that produces a magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a ve ...
s. Electrically-excited generators include an excitation system to produce the field flux. A generator using
permanent magnets Magnetic field lines of a solenoid electromagnet, which are similar to a bar magnet as illustrated below with the iron filings">electromagnet.html" ;"title="solenoid electromagnet">solenoid electromagnet, which are similar to a bar magnet as ...

permanent magnets
(PMs) is sometimes called a
magneto A magneto is an electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage ...
, or a permanent magnet synchronous generator (PMSM). *
Armature Armature may refer to: * Armature (computer animation), kinematic chain used in computer animation to simulate the motions of virtual characters * Armature (electrical), one of the two principal electrical components of an electromechanical machine ...
: The power-producing component of an electrical machine. In a generator, alternator, or dynamo, the armature windings generate the electric current, which provides power to an external circuit. The armature can be on either the rotor or the stator, depending on the design, with the field coil or magnet on the other part.


History

Before the connection between
magnetism Magnetism is a class of physical attributes that are mediated by magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For in ...

magnetism
and
electricity Electricity is the set of physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' (album), a 1981 album by Olivia Newton-John **Physical (Olivia Newton-John song), "Physi ...

electricity
was discovered,
electrostatic generator 12" Quadruple Sector-less Wimshurst Machine (Bonetti Machine) An electrostatic generator, or electrostatic machine, is an electric generator, electromechanical generator that produces ''static electricity'', or electricity at high voltage and low ...
s were invented. They operated on
electrostatic Electrostatics is a branch of physics that studies electric charges at Rest (physics), rest (static electricity). Since classical antiquity, classical times, it has been known that some materials, such as amber, attract lightweight particles af ...
principles, by using moving
electrically charged Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respectively). Like c ...
belts, plates, and disks that carried charge to a high potential electrode. The charge was generated using either of two mechanisms:
electrostatic induction Electrostatic induction, also known as "electrostatic influence" or simply "influence" in Europe and Latin America, is a redistribution of electric charge Electric charge is the physical property of matter that causes it to experience a force w ...

electrostatic induction
or the
triboelectric effect The triboelectric effect (also known as triboelectric charging) is a type of contact electrification on which certain materials become electric charge, electrically charged after they are separated from a different material with which they were in ...
. Such generators generated very high
voltage Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the ...

voltage
and low
current Currents or The Current may refer to: Science and technology * Current (fluid) A current in a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. ...
. Because of their inefficiency and the difficulty of
insulating Insulation may refer to: Thermal * Thermal insulation, use of materials to reduce rates of heat transfer ** List of insulation materials ** Building insulation, thermal insulation added to buildings for comfort and energy efficiency *** Insulated s ...
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 tubeAn X-ray tube is a vacuum tube A vacuum tube, an electron tube, valve (British usage) or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference ...
s, and later in some atomic
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies, and to contain them in well-defined particle beam, beams. Large accelerators are used for funda ...
s.


Faraday disk generator

The operating principle of electromagnetic generators was discovered in the years of 1831–1832 by
Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge ...

Michael Faraday
. The principle, later called Faraday's law, is that an
electromotive force In electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is c ...
is generated in an electrical conductor which encircles a varying
magnetic flux In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. " ...

magnetic flux
. He also built the first electromagnetic generator, called the
Faraday disk A homopolar generator is a DC electrical generator In electricity generation, a generator is a device that converts motive power ( mechanical energy) into electrical power for use in an external circuit. Sources of mechanical energy include st ...
; a type of
homopolar generator A homopolar generator is a DC electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it i ...

homopolar generator
, using a
copper Copper is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

copper
disc rotating between the poles of a horseshoe
magnet A magnet is a material or object that produces a magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a ve ...

magnet
. It produced a small
DC voltage Direct current (DC) is the one directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor (material), conductor such as a wire, but can also flow through semiconductor ...
. This design was inefficient, due to self-cancelling counterflows of
current Currents or The Current may refer to: Science and technology * Current (fluid) A current in a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. ...
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 Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the ...

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 Ányos István Jedlik ( hu, Jedlik Ányos István; sk, Štefan Anián Jedlík; in older texts and publications: la, Stephanus Anianus Jedlik; 11 January 1800 – 13 December 1895) was a Kingdom of Hungary, Hungarian inventor, engineer, ph ...
started experimenting in 1827 with the electromagnetic rotating devices which he called 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, which replaced permanent magnet designs. He also may have formulated the concept of the
dynamo A dynamo is an that creates using a . Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later devices were based, including the , the , and the . Today, the simple ...

dynamo
in 1861 (before
Siemens Siemens AG ( ) is a German multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational state, ...

Siemens
and Wheatstone) but did not patent it as he thought he was not the first to realize this.


Direct current generators

A coil of wire rotating in a magnetic field produces a current which changes direction with each 180° rotation, an
alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natu ...
(AC). However many early uses of electricity required
direct current Direct current (DC) is one-directional flow Flow may refer to: Science and technology * Flow (fluid) or fluid dynamics, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (math ...
(DC). In the first practical electric generators, called ''
dynamo A dynamo is an that creates using a . Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later devices were based, including the , the , and the . Today, the simple ...

dynamo
s'', the AC was converted into DC with a ''
commutator In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gene ...
'', 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 An early form of an alternating current electrical generator built by Pixii Hippolyte Pixii (1808–1835) was an instrument maker from Paris, France. In 1832 he built an early form of alternating current Alternating current (AC) is an electric ...
in 1832. The
dynamo A dynamo is an that creates using a . Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later devices were based, including the , the , and the . Today, the simple ...

dynamo
was the first electrical generator capable of delivering power for industry. The Woolrich Electrical Generator of 1844, now in
Thinktank, Birmingham Science Museum Thinktank, Birmingham (formerly known as simply Thinktank) is a science museum in Birmingham, England. Opened in 2001, it is part of Birmingham Museums Trust and is located within the Millennium Point (Birmingham), Millennium Point complex on ...
, is the earliest electrical generator used in an industrial process. It was used by the firm of Elkingtons for commercial
electroplating Electroplating is a general name for processes that produce a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric current. The part to be coated acts as the cathode ...
. The modern dynamo, fit for use in industrial applications, was invented independently by
Sir Charles Wheatstone Sir Charles Wheatstone FRS FRSE DCL LLD (6 February 1802 – 19 October 1875), was an English scientist and inventor of many scientific breakthroughs of the Victorian era In the history of the United Kingdom, the Victorian era was ...
,
Werner von Siemens Ernst Werner Siemens (von Siemens from 1888; ; ; 13 December 1816 – 6 December 1892) was a German electrical engineer, inventor and industrialist. Siemens's name has been adopted as the SI unit of electrical conductance, the siemens (unit), si ...

Werner von Siemens
and
Samuel Alfred Varley Samuel Alfred Varley (1832–1921) was an English electrical engineer. He was one of ten children born to Cornelius Varley and Elizabeth Livermore Straker.http://www.gracesguide.co.uk/Samuel_Alfred_Varley Career In 1852, Samuel started work for ...
. Varley took out a patent on 24 December 1866, while Siemens and Wheatstone both announced their discoveries on 17 January 1867, the latter delivering a paper on his discovery to the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. Founded on 28 November 1660, it was granted a royal charter by Charles II of ...
. The "dynamo-electric machine" employed self-powering electromagnetic field coils rather than permanent magnets to create the stator field. 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. 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 An electric arc furnace (EAF) is a furnace A furnace is a structure in which heat is produced with the help of combustion. Furnace may also refer to: Appliances Buildings * Furnace (house heating): a furnace , or a heater or boiler , used to ...

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 Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natu ...
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
transformed
transformed
to and from very high voltages to permit low losses over large distances.


Synchronous generators (alternating current generators)

Through a series of discoveries, the dynamo was succeeded by many later inventions, especially the AC
alternator An alternator is an electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the s ...

alternator
, which was capable of generating
alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natu ...
. 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. Moreover, they are excited with special control to enhance the stability of the power system. Alternating current generating systems were known in simple forms from
Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge ...

Michael Faraday
's original discovery of the
magnetic induction of electric current
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. Large two-phase alternating current generators were built by a British electrician, 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 The Westinghouse Electric Corporation was an American manufacturing company founded in 1886 by George Westinghouse. It was originally named Westinghouse Electric & Manufacturing Company, and was renamed Westinghouse Electric Corporation in 1945. T ...
in 1886.
Sebastian Ziani de Ferranti Sebastian Pietro Innocenzo Adhemar Ziani de Ferranti (9 April 1864 – 13 January 1930) was a British electrical engineer and inventor. Personal life Sebastian Ziani de Ferranti was born in Liverpool Liverpool is a City status in the U ...

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 William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of ...

Lord Kelvin
.Ferranti Timeline
– '' Museum of Science and Industry (Accessed 22-02-2012)''
His early alternators produced frequencies between 100 and 300
Hz
Hz
. Ferranti went on to design the
Deptford Power Station Three distinct coal-fired power stations were built at Deptford Deptford is an area of south-east London London is the capital city, capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdo ...
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. After 1891, polyphase alternators were introduced to supply currents of multiple differing phases. 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.


Self-excitation

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 A field coil is an electromagnet File:VFPt Solenoid correct2.svg, Magnetic field produced by a solenoid (coil of wire). This drawing shows a cross section through the center of the coil. The crosses are wires in which current is moving into ...
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 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 A power outage (also called a powercut, a power out, a power blackout, a power failure, a power loss, or a blackout) is the loss of the electrical power Electric power is the rate, per unit time, at which electrical energy Electrical ene ...
where
islanding Islanding is the condition in which a distributed generator (DG) continues to power a location even though external electrical grid An electrical grid is an interconnected network for electricity delivery from producers to consumers. Electric ...
of power stations has occurred, the stations may need to perform a
black start A black start is the process of restoring an electric power stationPower Station or The Power Station may refer to: * Power station, a facility for the generation of electricity Music * The Power Station (band), a 1980s supergroup ** ''The Po ...
to excite the fields of their largest generators, in order to restore customer power service.


Specialized types of generator


Direct current (DC)

A
dynamo A dynamo is an that creates using a . Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later devices were based, including the , the , and the . Today, the simple ...

dynamo
uses commutators to produce direct current. It is self- excited, i.e. its field electromagnets are powered by the machine's own output. Other types of DC generators use a separate source of direct current to energize their field magnets.


Homopolar generator

A homopolar generator is a DC
electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delive ...
comprising an electrically conductive disc or cylinder 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), the
electrical polarity Electrical polarity is a term used throughout industries and fields that involve electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves ...
depending on the direction of rotation and the orientation of the field. It is also 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 in series to produce an even larger voltage. They are unusual in that they can produce tremendous electric current, some more than a million
amperes The ampere (, ; symbol: A), often Clipping (morphology), shortened to "amp",SI supports only the use of symbols and deprecates the use of abbreviations for units. is the SI base unit, base unit of electric current in the International System of ...
, because the homopolar generator can be made to have very low internal resistance.


Magnetohydrodynamic (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 Steam is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , ev ...
power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation A generation is "all of the people born and living Living or The Living may refer to: ...

power 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 the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a that spanned during its existence from 1922 to 1991. It was nominally a of multiple national ; in practice and were highly until its final years. The ...
from 1972 until the late 1980s, the MHD plant U 25 was in regular utility operation on the Moscow power system with a rating of 25 MW, the largest MHD plant rating in the world at that time. MHD generators operated as a topping cycle are currently (2007) less efficient than
combined cycle A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turbi ...
gas turbines A gas turbine, also called a combustion turbine, is a type of continuous and internal combustion engine An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combust ...
.


Alternating current (AC)


Induction generator

Induction AC motors may be used as generators, turning mechanical energy into electric current. 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. They do not require an exciter circuit because the rotating magnetic field is provided by induction from the stator circuit. They also do not require speed governor equipment as they inherently operate at the connected grid frequency. 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.


Linear electric generator

In the simplest form of linear electric generator, a sliding
magnet A magnet is a material or object that produces a magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a ve ...

magnet
moves back and forth through a
solenoid A solenoid (,) is a type of electromagnet Magnetic field produced by a solenoid (coil of wire). This drawing shows a cross section through the center of the coil. The crosses are wires in which current is moving into the page; the dots a ...

solenoid
- a spool of copper wire. An
alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natu ...
is induced in the loops of wire by
Faraday's law of induction Faraday's law of induction (briefly, Faraday's law) is a basic law of electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric ch ...
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 Wave power is the capture of energy of wind wave In fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdiscipline ...

wave power
schemes.


Variable-speed constant-frequency generators

Many
renewable energy Renewable energy is energy that is collected from renewable resource File:Global Vegetation.jpg, Global vegetation A renewable resource, also known as a flow resource, is a natural resource which will replenish to replace the portion resou ...
efforts attempt to harvest natural sources of mechanical energy (wind, tides, etc.) to produce electricity. Because these sources fluctuate in power applied, standard generators using permanent magnets and fixed windings would deliver unregulated voltage and frequency. The overhead of regulation (whether before the generator via gear reduction or after generation by electrical means) is high in proportion to the naturally-derived energy available. New generator designs such as the asynchronous or induction singly-fed generator, the doubly-fed generator, or the brushless wound-rotor doubly-fed generator are seeing success in variable speed constant frequency applications, such as
wind turbine A wind turbine is a device that converts Religious conversion is the adoption of a set of beliefs identified with one particular religious denomination A religious denomination is a subgroup within a religion Religion is a social s ...

wind turbine
s or other renewable energy technologies. These systems thus offer cost, reliability and efficiency benefits in certain use cases..


Common use cases


Power station

A ''power station'', also referred to as a ''power plant'' or ''powerhouse'' and sometimes ''generating station'' or ''generating plant'', is an industrial facility for the
generation A generation is "all of the people born and living Living or The Living may refer to: Common meanings *Life, a condition that distinguishes organisms from inorganic objects and dead organisms ** extant taxon, Living species, one that is not ex ...
of
electric power Electric power is the rate, per unit time, at which electrical energy Electrical energy is energy derived as a result of movement of electrically charged particles. When used loosely, ''electrical energy'' refers to energy that has been conve ...
. Most power stations contain one or more generators, a rotating machine that converts
mechanical power Mechanical power is a medical term which is a measure of the amount of energy imparted to a patient by a mechanical ventilator. While in many cases mechanical ventilation is a life-saving or life-preserving intervention, it also has the potential t ...
into
three-phase electric power Three-phase electric power (abbreviated 3φ) is a common type of alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct ...
. The
relative motion The ''relative velocity'' \vec_ (also \vec_ or \vec_) is the velocity of an object or observer B in the rest frame of another object or observer A. Classical mechanics In one dimension (non-relativistic) We begin with relative motion in the cla ...

relative motion
between a
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

magnetic field
and a
conductor Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble like, for example, an orchestra. * Conductor (album), ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of ...
creates an
electrical current Electricity is the set of physical phenomena A phenomenon (; plural phenomena) is an observable fact or event. The term came into its modern philosophical Philosophy (from , ) is the study of general and fundamental questions, ...
. The energy source harnessed to turn the generator varies widely. Most power stations in the world burn
fossil fuel A fossil fuel is a hydrocarbon In , a hydrocarbon is an consisting entirely of and . Hydrocarbons are examples of s. Hydrocarbons are generally colourless and hydrophobic with only weak odours. Because of their diverse molecular structure ...
s such as
coal Coal is a combustible , Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin , coordinates = , largest_city = capital , languages_type = Official language , languages = German language, German , ...

coal
,
oil An oil is any nonpolar In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound ...

oil
, and
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxid ...

natural gas
to generate electricity. Cleaner sources include
nuclear power Nuclear power is the use of nuclear reaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), ...

nuclear power
, and an increasing use of
renewables Renewable energy is energy that is collected from renewable resource File:Global Vegetation.jpg, Global vegetation A renewable resource, also known as a flow resource, is a natural resource which will replenish to replace the portion resour ...
such as
solar Solar may refer to: Astronomy * Of or relating to the Sun. ** A solar telescope 175px, The Swedish 1-m Solar Telescope at Roque de los Muchachos Observatory, La Palma in the Canary Islands. A solar telescope is a special purpose telescope used ...

solar
,
wind Wind is the natural movement of air or other gases relative to a planet's surface. Wind occurs on a range of scales, from thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by th ...

wind
,
wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

wave
and
hydroelectric Hydroelectricity, or hydroelectric power, is electricity produced from hydropower Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water Water (chemical formula H2O) is a ...
.


Vehicular generators


Roadway vehicles

Motor vehicles require electrical energy to power their instrumentation, keep the engine itself operating, and recharge their batteries. Until about the 1960s motor vehicles tended to use
DC generators (dynamos)
DC generators (dynamos)
with electromechanical regulators. Following the historical trend above and for many of the same reasons, these have now been replaced by
alternator An alternator is an electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the s ...
s with built-in
rectifier A rectifier is an electrical device that converts Religious conversion is the adoption of a set of beliefs identified with one particular religious denomination A religious denomination is a subgroup within a religion Religion is a ...

rectifier
circuits.


Bicycles

Bicycles require energy to power running lights and other equipment. There are two common kinds of generator in use on bicycles:
bottle dynamo upBottle dynamo mounted on a bicycle. A bottle dynamo or sidewall dynamo is a small electrical generator for bicycles employed to power a bicycle's bicycle lighting, lights. The traditional bottle dynamo (pictured) is not actually a dynamo at all ( ...
s which engage the bicycle's tire on an as-needed basis, and
hub dynamo Schmidt-Nabendynamo A hub dynamo is a small electrical generator In electricity generation, a generator is a device that converts motive power ( mechanical energy) into electrical power for use in an external circuit. Sources of mechanical en ...
s which are directly attached to the bicycle's drive train. The name is conventional as these they are small permanent-magnet alternators, not self-excited DC machines as are
dynamo A dynamo is an that creates using a . Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later devices were based, including the , the , and the . Today, the simple ...

dynamo
s. Some electric bicycles are capable of
regenerative braking Regenerative braking is an energy recovery Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the energy transfer, exchange of energy from one sub-system of the overall system with anoth ...
, where the drive motor is used as a generator to recover some energy during braking.


Sailboats

Sailing boats may use a water- or wind-powered generator to trickle-charge the batteries. A small
propeller . A propeller is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral, that, when rotated, exerts linear thrust upon a working fluid, such as water or air. Propellers are used to pump fluid through a ...

propeller
,
wind turbine A wind turbine is a device that converts Religious conversion is the adoption of a set of beliefs identified with one particular religious denomination A religious denomination is a subgroup within a religion Religion is a social s ...

wind turbine
or
impeller An impeller or impellor is a wikt:rotor#English, rotor used to increase the pressure and flow of a fluid. It is the opposite of a turbine, which extracts energy from, and reduces the pressure of, a flowing fluid. In pumps An impeller is a ...

impeller
is connected to a low-power generator to supply currents at typical wind or cruising speeds.


Electric scooters

Electric scooters with regenerative braking have become popular all over the world. Engineers use
kinetic energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...
recovery systems on the scooter to reduce energy consumption and increase its range up to 40-60% by simply recovering energy using the magnetic brake, which generates
electric energy Electrical energy is energy derived as a result of movement of electrically charged particles. When used loosely, ''electrical energy'' refers to energy that has been converted ''from'' electric potential energy. This energy is supplied by the comb ...
for further use. Modern vehicles reach speed up to 25–30 km/h and can run up to 35–40 km.


Genset

An ''engine-generator'' is the combination of an electrical generator and an
engine An engine or motor is a machine A machine is any physical system with ordered structural and functional properties. It may represent human-made or naturally occurring device molecular machine that uses Power (physics), power to apply For ...

engine
(
prime mover Prime mover in science and engineering is a machine or component that converts energy from an energy source into motive power. Prime mover may refer to: Engineering * Prime mover (engine), a component that transforms the flow or changes in pressu ...
) 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, and there are even hybrid diesel-gas units, called dual-fuel units. Many different versions of engine-generators are available - ranging from very small portable
petrol Gasoline () or petrol () (see the etymology Etymology ()The New Oxford Dictionary of English (1998) – p. 633 "Etymology /ˌɛtɪˈmɒlədʒi/ the study of the class in words and the way their meanings have changed throughout time". is ...
powered 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.


Human powered electrical generators

A generator can also be driven by human muscle power (for instance, in field radio station equipment). Human powered electric generators are commercially available, and have been the project of some
DIY "Do it yourself" ("DIY") is the method of building, modifying, or repair The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery, building infrastruc ...

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. An average "healthy human" can produce a steady 75 watts (0.1 horsepower) for a full eight hour period, while a "first class athlete" can produce approximately 298 watts (0.4 horsepower) for a similar period. At the end of which an undetermined period of rest and recovery will be required. At 298 watts the average "healthy human" becomes exhausted within 10 minutes. The net electrical power that can be produced will be less, due to the efficiency of the generator. Portable radio receivers with a crank are made to reduce battery purchase requirements, see
clockwork radio
clockwork radio
. During the mid 20th century, pedal powered radios were used throughout the
Australian outback The Outback is a vast, sparsely populated area of Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the isl ...

Australian outback
, to provide schooling (
School of the Air School of the Air is a generic term for correspondence school Distance education, also called distance learning, is the education of students who may not always be physically present at a school A school is an educational institution ...
), medical and other needs in remote stations and towns.


Mechanical measurement

A tachogenerator is an electromechanical device which produces an output voltage proportional to its shaft speed. It may be used for a speed indicator or in a feedback speed control system. Tachogenerators are frequently used to power
tachometer A tachometer (revolution-counter, tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft Shaft may refer to: Rotating machine elements * Shaft (mechanical engineering), a rotating machine element used to trans ...
s 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.


Equivalent circuit

An equivalent circuit of a generator and load is shown in the adjacent diagram. The generator is represented by an abstract generator consisting of an ideal
voltage source A voltage source is a two- terminal device which can maintain a fixed voltage Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ' ...

voltage source
and an internal impedance. The generator's V_\text and R_\text parameters can be determined by measuring the winding resistance (corrected to
operating temperature An operating temperature is the allowable temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat, a flow of ...
), and measuring the open-circuit and loaded voltage for a defined current load. This is the simplest model of a generator, further elements may need to be added for an accurate representation. In particular, inductance can be added to allow for the machine's windings and magnetic leakage flux, but a full representation can become much more complex than this.Yoshihide Hase, "10: Theory of generators", ''Handbook of Power System Engineering'', John Wiley & Sons, 2007 .


See also

*
Diesel generator . A diesel generator (DG) (also known as diesel genset) is the combination of a diesel engine The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of the diesel fuel, fuel is caused by ...

Diesel generator
*
Electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delivery (Electric power transmiss ...
*
Electric motor
Electric motor
*
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 equipment. This combination is also called an ''engine-generator set'' or a ''gen-set''. In many contexts, th ...
*
Faraday's law of induction Faraday's law of induction (briefly, Faraday's law) is a basic law of electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric ch ...
*
Gas turbine A gas turbine, also called a combustion turbine, is a type of continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability di ...
* Generation expansion planning * Goodness factor * Steam turbine * Superconducting electric machine * Thermogenerator


References

{{Authority control Electrical generators, * English inventions 19th-century inventions