Editing Electric generator (section)

==Specialised types of generator==

===Direct current (DC)===
A [[dynamo]] uses commutators to produce direct current. It is self-[[Excitation (magnetic)|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 energise their field magnets.

====Homopolar generator====
{{main|Homopolar generator}}
A homopolar generator is a [[Direct current|DC]] [[electrical generator]] 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]] 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.<ref>Losty, H.H.W & Lewis, D.L. (1973) "Homopolar Machines". Philosophical Transactions for the Royal Society of London. Series A, Mathematical and Physical Sciences. 275 (1248), 69–75</ref> They are unusual in that they can produce tremendous electric current, some more than a million [[amperes]], because the homopolar generator can be made to have very low internal resistance.

====Magnetohydrodynamic (MHD) generator====
{{main|Magnetohydrodynamic 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 [[Rankine cycle|steam]] [[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]] 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.<ref>Langdon Crane, ''Magnetohydrodynamic (MHD) Power Generator: More Energy from Less Fuel, Issue Brief Number IB74057'', Library of Congress Congressional Research Service, 1981, retrieved from [https://digital.library.unt.edu/govdocs/crs/permalink/meta-crs-8402:1 Digital.library.unt.edu] 18 July 2008.</ref> MHD generators operated as a [[topping cycle]] are currently (2007) less efficient than [[combined cycle]] [[gas turbines]].

===Alternating current (AC)===

====Induction generator====
{{main|Induction generator}}
[[Induction motor|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 simultaneous speed, giving negative slip. A regular AC non-simultaneous motor usually can be used as a generator, without any changes to its parts. Induction generators are useful in applications like 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 another circuit to start working because the turning magnetic field is provided by induction from the one they have. They also do not require speed governor equipment as they inherently operate at the connected grid frequency.

An induction generator must be powered with a leading voltage; this is usually done by connection to an electrical grid, or by powering themselves with phase correcting capacitors.

====Linear electric generator====
{{main|Linear alternator}}
In the simplest form of linear electric generator, a sliding [[magnet]] moves back and forth through a [[solenoid]], a copper wire or a coil. An [[alternating current]] is induced in the wire, or loops of wire, by [[Faraday's law of induction]] each time the magnet slides through. This type of generator is used in the [[Faraday flashlight]]. Larger linear electricity generators are used in [[wave power]] schemes.

====Variable-speed constant-frequency generators====
Grid-connected generators deliver power at a constant frequency. For generators of the synchronous or induction type, the prime mover speed turning the generator shaft must be at a particular speed (or narrow range of speed) to deliver power at the required utility frequency. Mechanical speed-regulating devices may waste a significant fraction of the input energy to maintain a required fixed frequency.

Where it is impractical or undesired to tightly regulate the speed of the prime mover, [[doubly fed electric machine]]s may be used as generators. With the assistance of power electronic devices, these can regulate the output frequency to a desired value over a wider range of generator shaft speeds. Alternatively, a standard generator can be used with no attempt to regulate frequency, and the resulting power converted to the desired output frequency with a rectifier and converter combination. Allowing a wider range of prime mover speeds can improve the overall energy production of an installation, at the cost of more complex generators and controls. For example, where a wind turbine operating at fixed frequency might be required to spill energy at high wind speeds, a variable speed system can allow recovery of energy contained during periods of high wind speed.