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generator
Also called a dynamo, a device for converting mechanical energy into electrical energy. Traditional forms are based on inducing electric fields by changing the magnetic field lines through a circuit (see electromagnetic induction). All generators can be, and sometimes are, run in reverse as electric motors.
The simplest generator consists of a permanent magnet (the rotor) spun inside a coil of wire (the stator); the magnetic field is thus reversed twice each revolution, and an AC voltage is generated at the frequency of rotation (see also magneto. In practical designs, the rotor is usually an electromagnet driven by a direct current obtained by rectification of a part of the voltage generated, and passed to the rotor through a pair of carbon brush/slip ring contacts. The use of three sets of stator coils 120° apart allows generation of a three-phase supply (see also armature).
Simple DC generators consist of a coil rotating in the field of a permanent magnet: the voltage induced in the coil alternates at the frequency of rotation, but it is collected through a commutator – a slip ring broken into two two semicircular parts, to each of which one end of the coil is connected, so that the connection between the coil and the brushes is reversed twice each revolution – resulting in a rapidly pulsating direct voltage. A steadier voltage can be achieved through the use of multiple coil/commutator arrangements, and except in very small generators, the permanent magnet is again replaced by an electromagnet driven by part of the generated voltage.
For large-scale generation, the mechanical power is usually derived from steam turbines, or from dam-fed water turbines, and the process is only moderately efficient. The magnetohydrodynamic generator avoids this step and has no moving parts either. A hot conducting fluid (treated coal gas, or reactor-heated liquid) passes through the field of an electromagnet, so that the charges are forced in opposite directions producing a DC voltage. In another device, the electrogasdynamic generator, the voltage is produced by by using a high speed gas stream to pump charge from an electric discharge, against the electric field, to a collector.
Generators originated with the discovery of induction by Michael Faraday in 1831; the considerable advantages of electromagnets over permanent magnets were first exploited by E. W. von Siemens in 1866.
Related category
• ELECTRICITY AND MAGNETISM
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