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 considered 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 engines without a low-voltage electrical system, such as lawnmowers and chainsaws, and to aircraft engines, in which keeping the ignition independent of the rest of the electrical system ensures that the engine continues running in the event of alternator or battery failure. For redundancy, virtually all piston engine aircraft are fitted with two magneto systems, each supplying power to one of two spark plugs in each cylinder.

Magnetos were used for specialized isolated power systems such as arc lamp systems or lighthouses, for which their simplicity was an advantage. They have never been widely applied for the purposes of bulk electricity generation, for the same purposes or to the same extent as either dynamos or alternators. Only in a few specialised cases have they been used for power generation.

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.

The first electrical machine used for an industrial process was a magneto, the Woolrich Electrical Generator. In 1842 John Stephen Woolrich was granted UK patent 9431 for the use of an electrical generator in electroplating, rather than batteries. A machine was built in 1844 and licensed to the use of the Elkington Works in Birmingham. Such electroplating expanded to become an important aspect of the Birmingham toy industry, the manufacture of buttons, buckles and similar small metal items.

The surviving machine has an applied field from four horseshoe magnets with axial fields. The rotor has ten axial bobbins. Electroplating requires DC and so the usual AC magneto is unworkable. Woolrich's machine, unusually, has a commutator to rectify its output to DC.

Most early dynamos were bipolar and so their output varied cyclically as the armature rotated past the two poles.

To achieve an adequate output power, magneto generators used many more poles; usually sixteen, from eight horseshoe magnets arranged in a ring. As the flux available was limited by the magnet metallurgy, the only option was to increase the field by using more magnets. As this was still an inadequate power, extra rotor disks were stacked axially, along the axle. This had the advantage that each rotor disk could at least share the flux of two expensive magnets. The machine illustrated here uses eight disks and nine rows of magnets: 72 magnets in all.