Editing Electric generator (section)

===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]].