Magneto

A magneto is an electrical generator that uses permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a commutator to produce direct current. It is categorized as a form of alternator, although it is usually regarded as distinct from most other alternators, which use field coils rather than permanent magnets.

Hand-cranked magneto generators were used to provide ringing current in telephone systems. Magnetos were also adapted to produce pulses of high voltage in the ignition systems of some gasoline-powered internal combustion engines to provide power to the spark plugs. Use of such ignition magnetos for ignition is now limited mainly to the following kinds of engines:

Magnetos were used for specialized isolated power systems such as arc lamp systems or lighthouses, for which their simplicity was an advantage. Generators connected to an electrical grid for central station power generation do not use the magneto principle.

Production of electric current from a moving magnetic field was demonstrated by Faraday in 1831. The first machines to produce electric current from magnetism used permanent magnets; the dynamo machine, which used an electromagnet to produce the magnetic field, was developed later. The machine built by Hippolyte Pixii in 1832 used a rotating permanent magnet to induce alternating voltage in two fixed coils.

Magnetos have advantages of simplicity and reliability, but are limited in size owing to the magnetic flux available from their permanent magnets. The fixed excitation of a magneto made it difficult to control its terminal voltage or reactive power production when operating on a synchronized grid. This restricted their use for high-power applications. Power generation magnetos were limited to narrow fields, such as powering arc lamps or lighthouses, where their particular features of output stability or simple reliability were most valued.

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Wikipedia