Electrical power generators are devices that convert mechanical, chemical, or other forms of energy into electrical energy. The most common type of electrical power generator, a bicycle dynamo, uses electromagnetic induction to convert mechanical energy into electrical energy. These simple devices are essentially reversed electric motors with a rotor that carries one or more coils surrounded by a magnetic field, typically supplied by a permanent magnet or electromagnet. In other electrical power generators, mechanical energy from steam turbines moves the rotor, which induces an electric current in the rotor coil. Electrical power generators that provide direct current (DC) typically include a mechanical switch or commutator that switches the current every half-rotation so that the rotor remains unidirectional. Large, modern generators or alternators in power stations provide alternating current (AC) for general distribution. Specialized electrostatic generators such as Wimshurst machines and, on a large scale, van de Graaff generators are principally used for special applications such as particle accelerators that require high voltages but low current.
Selecting electrical power generators requires an analysis of inputs, outputs, and portability. Typically, smaller units are movable while large devices are mounted or fixed in one location. Inputs usually include conventional fuel sources such as gasoline, diesel, propane, and natural gas; however, some electrical power generators use alternative forms of energy such as solar and wind power. In terms of outputs, some electrical power generators provide single phase or three phase AC voltage. Other devices output DC power. For AC electrical power generators, important specifications include AC prime power rating, which is expressed in voltamperes (VA). For DC devices, important specifications include DC power, which is expressed in watts (W). As a general principle, both AC and DC electrical power generators should not be operated at maximum power output for more than 30 minutes or for periods of time exceeding manufacturer recommendations.
Electrical power generators can be customized with a wide range of attachments, components, and performance options. These range from simple electromechanical devices to advanced digital microprocessor-based controls, remote communication capabilities, sound-attenuated and weather-protected enclosures, fuel tank bases, silencers, batteries, alternators, governors, air cleaners, starting aids, and cooling options Gasoline and diesel electrical power generators often feature reduced emissions, open and pre-chamber designs, fast-burn pistons, and low-overlap cams.