What is Electric Motor ?

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 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 commercial operation on the Moscow power system with a rating of 25

MW, the largest MHD plant rating in the world at that time.[19] MHD generators operated as a topping cycle are currently (2007) less efficient than combined cycle gas

turbines.

Alternating current (AC)

Induction generator

Main article: 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 moves back and forth through a solenoid - a spool of copper wire. An alternating current is induced in

the 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

Many renewable energy 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 turbines or other renewable energy technologies. These systems thus offer cost,