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Electricity is produced when you pass a coil of wire through a magnetic field... or pass the magnetic field through the coil of wire.
If you wish to convert mechanical energy into electrical energy, if the input is rotary motion the output will be a sine wave -- alternating current.
The generic term for such devices is 'dynamo.'
The terms 'alternator' and 'generator' are used as a handy means of defining which type of dynamo is being used. In the generator the magnetic field is fixed and the coil(s) rotates through it. The output is an alternating current that is converted to pulsating direct current through the use of a mechanical switching system mounted on the shaft supporting the coil (ie, carbon brushes and a segmented commutator). In an alternator the magnetic field rotates while the coil(s) remains fixed. The alternating current that appears in the coil(s) is converted to direct current by a rectifier.
The output of any dynamo is a function of its rpm, the number of pick- up coils and the intensity of the magnetic field.
The most common means of controlling the output of your dynamo is to control the intensity of the magnetic field by regulating the amount of current that flows through the magnetic field winding.
If a variable output is not required you may replace the magnetic field winding with permanent magnets.
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The key points here are that 'alternators' and 'generators' are both dynamos. Each uses brushes. Each has a magnetic field and regulates its output by controlling that field. Since the advent of solid- state rectifiers, alternators have come into general use because they are less expensive to manufacture but they aren't really new. The Leese-Nevil alternator using copper oxide rectifiers has been available since 1921. Nor are permanent magnet dynamos 'new' in that they've been around in the form of magnetos since the 1890's. The reason we're seeing more of them nowadays is a reflection of economies of scale in the production of rare-earth magnets.
-R.S.Hoover
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