Magneto (generator)
Encyclopedia
A magneto
is an electrical generator
that uses permanent magnets to produce alternating current
. Unlike a dynamo
, there is no commutator
and so they cannot produce direct current
. They are categorised as a form of alternator
, although they are usually regarded as distinct from most other alternators, which use field coil
s rather than permanent magnets.
Magnetos date from the earliest days of electrical engineering
. Despite this, 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.
available from their permanent magnets. This restricted their use for high-power applications.
Some magnetos did find use as telephone magneto
s in early telephone
s, particularly for ringing.
The most common application for magnetos was as an ignition magneto, in spark-ignition petrol engine
s, ranging from early cars to aircraft (for reliability) and small engine
s such as chainsaw
s (for simplicity).
s as their generators. These had a two-pole stator with a field winding. It could generate considerable flux, and thus power, for the time. They also required a commutator to produce a direct current
(DC) output, which was complex to make and required regular maintenance. In contrast magnetos were not generally used, as they were inadequately powerful.
for a discussion of their development from bipolar to multipolar fields. 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.
The rotors first used were wound as sixteen axial bobbins, one per pole. Compared to the bipolar dynamo, this did have the advantage of more poles giving a smoother output per rotation,Actually a higher AC frequency. which was an advantage when driving arc lamps. Magnetos thus established a small niche for themselves as lighting generators.
The Belgian electrical engineer Floris Nollet
became particularly known for this type of arc lighing generator and founded the British-French company Société de l'Alliance to manufacture them.
The French engineer Auguste de Méritens
developed magnetos further for this purpose. His innovation was to replace the rotor coils previously wound on individual bobbins, with a 'ring wound' armature. These windings were wound on a segmented iron core, similar to a Gramme ring, so as to form a single continuous hoop. This gave a more even output current, which was still more advantageous for arc lamps.
is best remembered today for his production of magneto generators specifically for lighthouse
s. These were favoured for their simplicity and reliability, in particular their avoidance of commutators. In the sea air of a lighthouse, the commutator that had been used previously with dynamo generators was a continual source of trouble. The lighthouse keepers of the time, usually semi-retired sailors, were not mechanically or electrically skilled to maintain these more complex machines.
The de Méritens magneto generator illustrated shows the 'ring wound' armature. As there is now only a single rotor disk, the horseshoe magnets used are stacked together in groups, but act through a single pole piece
.
'.
Wilde, an electrical engineer of Manchester, developed a combination of magneto and electro-magnet generator, where the magneto was used only to supply the field to the larger alternator. These are illustrated in Rankin Kennedy's work Electrical Installations Kennedy himself developed a simpler version of this, intended for lighting use on ships, where a dynamo and magneto were assembled on the same shaft. Kennedy's innovation here was to avoid the need for brushgear altogether. The current generated in the magneto is transmitted by wires attached to the rotating shaft to the dynamo's rotating field coil. The output of the dynamo is then taken from the stator coils. This is 'inside-out' compared to the conventional dynamo, but avoids the need for brushgear.
The invention of the self-exciting field by Varley
, Siemens & Wheatstone
removed the need for a magneto exciter. A small residual field in the iron armature of the field coils acted as a weak permanent magnet, and thus a magneto. The shunt wiring
of the generator feeds some of its output current back into the field coils, which in turn increases output. By this means the field 'builds up' regeneratively, although this may take 20-30 seconds to do so fully.
Although the use of magnetos here is now obsolete, separate exciters are still used for high power generating sets, as they permit easier control of output power. These are particularly common with the transmissions of diesel-electric locomotives.
. Despite their name, these dynamos were usually A few late-model dynamos generated DC, usually by using a bridge rectifier rather than a commutator, so that they could charge batteries and thus maintain the lights when the bicycle was stopped at traffic junctions. AC magnetos.
Similar magnetos have also been used in hand torches.
s, such generators may replace the flux switching alternator
. These need to operate at high speeds, directly coupled to a turbine. Both types share the advantage of the output coils being part of the stator, thus avoiding the need for brushgear.
s, particularly self-build designs, are widely adopting magneto alternators for their generators. These use rotating neodymium
rare-earth magnet
s with a three-phase
stator
, then a bridge rectifier to produce DC. This is then used either directly for water-pumping, stored in batteries, or used to drive a mains inverter
which can supply the grid system for profit. These designs have been encouraged by one of the most popular self-build designs, described in Hugh Piggott's series of books and courses. A typical design here is an axial-flux generator recycled from a car brake disk and hub bearing. A Macpherson strut
provides the azimuth bearing to bring the turbine into the wind. The brake disk, and its attached rare-earth magnets, rotates to form the armature. A plywood disk carrying multiple axial coils is placed alongside this, with a further iron armature ring behind it.
Magneto
A magneto is a type of electrical generator.Magneto may also refer to:* Magneto , permanent magnetic alternating current rotary generator* ignition magneto, magnetos on internal combustion engines...
is an electrical generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
that uses permanent magnets to produce alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
. Unlike a dynamo
Dynamo
- Engineering :* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field- Software :...
, there is no commutator
Commutator (electric)
A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power...
and so they cannot produce direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
. They are categorised as a form of alternator
Alternator
An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...
, although they are usually regarded as distinct from most other alternators, which use field coil
Field coil
A field coil is a component of an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. A current-carrying coil is used to generate a magnetic field....
s rather than permanent magnets.
Magnetos date from the earliest days of electrical engineering
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
. Despite this, they have never been widely applied for the purposes of bulk electricity generation
Electricity generation
Electricity generation is the process of generating electric energy from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...
, for the same purposes or to the same extent as either dynamos or alternators.
Common uses of magnetos
Magnetos have advantages of simplicity and reliability, but are inefficient owing to the weak magnetic fluxMagnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
available from their permanent magnets. This restricted their use for high-power applications.
Some magnetos did find use as telephone magneto
Telephone magneto
A magneto is an electrical generator that uses permanent magnets to produce alternating current. Owing to their simplicity and reliability, they were widely used in early telephone systems.- Telegraphy :...
s in early telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
s, particularly for ringing.
The most common application for magnetos was as an ignition magneto, in spark-ignition petrol engine
Petrol engine
A petrol engine is an internal combustion engine with spark-ignition, designed to run on petrol and similar volatile fuels....
s, ranging from early cars to aircraft (for reliability) and small engine
Small engine
A small engine is the general term for a wide range of internal combustion engines used to power lawn mowers, generators, concrete mixers and many other machines that require independent power sources. Most small engines are single-cylinder, V-twin, or flat-twin units...
s such as chainsaw
Chainsaw
A chainsaw is a portable mechanical saw, powered by electricity, compressed air, hydraulic power, or most commonly a two-stroke engine...
s (for simplicity).
Magnetos for power generation
Early power generation systems adopted bipolar dynamoDynamo
- Engineering :* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field- Software :...
s as their generators. These had a two-pole stator with a field winding. It could generate considerable flux, and thus power, for the time. They also required a commutator to produce a direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC) output, which was complex to make and required regular maintenance. In contrast magnetos were not generally used, as they were inadequately powerful.
Arc lighting
Most early dynamos were bipolarSee the related bipolar motorBipolar electric motor
A bipolar electric motor is an electric motor with only two poles to its stationary field. They are an example of the simple brushed DC motor, with a commutator. This field may be generated by either a permanent magnet or a field coil....
for a discussion of their development from bipolar to multipolar fields. 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
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
available was limited by the magnet metallurgy, the only option was to increase the field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
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.
The rotors first used were wound as sixteen axial bobbins, one per pole. Compared to the bipolar dynamo, this did have the advantage of more poles giving a smoother output per rotation,Actually a higher AC frequency. which was an advantage when driving arc lamps. Magnetos thus established a small niche for themselves as lighting generators.
The Belgian electrical engineer Floris Nollet
Floris Nollet
Floris Nollet was a Belgian physicist, engineer, inventor.Nollet was a grandnephew of Jean-Antoine Nollet. He became professor of physics at the École Militaire in Brussels...
became particularly known for this type of arc lighing generator and founded the British-French company Société de l'Alliance to manufacture them.
The French engineer Auguste de Méritens
Auguste de Méritens
Baron Auguste de Méritens was a French electrical engineer of the 19th century.He was born in 1834.He is best known his work on magneto generators, particularly those used for arc lighting and lighthouses...
developed magnetos further for this purpose. His innovation was to replace the rotor coils previously wound on individual bobbins, with a 'ring wound' armature. These windings were wound on a segmented iron core, similar to a Gramme ring, so as to form a single continuous hoop. This gave a more even output current, which was still more advantageous for arc lamps.
Lighthouses
de MéritensAuguste de Méritens
Baron Auguste de Méritens was a French electrical engineer of the 19th century.He was born in 1834.He is best known his work on magneto generators, particularly those used for arc lighting and lighthouses...
is best remembered today for his production of magneto generators specifically for lighthouse
Lighthouse
A lighthouse is a tower, building, or other type of structure designed to emit light from a system of lamps and lenses or, in older times, from a fire, and used as an aid to navigation for maritime pilots at sea or on inland waterways....
s. These were favoured for their simplicity and reliability, in particular their avoidance of commutators. In the sea air of a lighthouse, the commutator that had been used previously with dynamo generators was a continual source of trouble. The lighthouse keepers of the time, usually semi-retired sailors, were not mechanically or electrically skilled to maintain these more complex machines.
The de Méritens magneto generator illustrated shows the 'ring wound' armature. As there is now only a single rotor disk, the horseshoe magnets used are stacked together in groups, but act through a single pole piece
Pole piece
A pole piece is a structure composed of material of high magnetic permeability that serves to direct the magnetic field produced by a magnet. A pole piece attaches to and in a sense extends a pole of the magnet, hence the name....
.
Self-exciting dynamos
Both dynamos and alternators required a source of power to drive their field coils. This could not be supplied by their own generator's output, without some process of 'bootstrappingBootstrapping
Bootstrapping or booting refers to a group of metaphors that share a common meaning: a self-sustaining process that proceeds without external help....
'.
Wilde, an electrical engineer of Manchester, developed a combination of magneto and electro-magnet generator, where the magneto was used only to supply the field to the larger alternator. These are illustrated in Rankin Kennedy's work Electrical Installations Kennedy himself developed a simpler version of this, intended for lighting use on ships, where a dynamo and magneto were assembled on the same shaft. Kennedy's innovation here was to avoid the need for brushgear altogether. The current generated in the magneto is transmitted by wires attached to the rotating shaft to the dynamo's rotating field coil. The output of the dynamo is then taken from the stator coils. This is 'inside-out' compared to the conventional dynamo, but avoids the need for brushgear.
The invention of the self-exciting field by Varley
C. F. Varley
Cromwell Fleetwood Varley was an English engineer, particularly associated with the development of the electric telegraph and the transatlantic telegraph cable.-Family:...
, Siemens & Wheatstone
Charles Wheatstone
Sir Charles Wheatstone FRS , was an English scientist and inventor of many scientific breakthroughs of the Victorian era, including the English concertina, the stereoscope , and the Playfair cipher...
removed the need for a magneto exciter. A small residual field in the iron armature of the field coils acted as a weak permanent magnet, and thus a magneto. The shunt wiring
Shunt generator
A shunt generator is a method of generating electricity in which field winding and armature winding are connected in parallel, and in which the armature supplies both the load current and the field current....
of the generator feeds some of its output current back into the field coils, which in turn increases output. By this means the field 'builds up' regeneratively, although this may take 20-30 seconds to do so fully.
Although the use of magnetos here is now obsolete, separate exciters are still used for high power generating sets, as they permit easier control of output power. These are particularly common with the transmissions of diesel-electric locomotives.
Low-power magnetos
For their simplicity, particularly their independence from a stored power source, magnetos have found some use for low-power electricity generation, usually human-powered. The best known of these is the cycle lamp bottle dynamoBottle dynamo
A bottle dynamo or sidewall dynamo is a small electrical generator for bicycles employed to power a bicycle's lights — or to charge batteries for electronic devices including cellphones or GPS receivers....
. Despite their name, these dynamos were usually A few late-model dynamos generated DC, usually by using a bridge rectifier rather than a commutator, so that they could charge batteries and thus maintain the lights when the bicycle was stopped at traffic junctions. AC magnetos.
Similar magnetos have also been used in hand torches.
Future possibilities
The modern development of rare earth magnets makes the simple magneto alternator a more practical proposition as a power generator, as these permit a greatly increased field strength. As the magnets are compact and of light weight, they generally form the rotor, allowing the output windings to be placed on the stator, avoiding the need for brushgear.Guided missiles
By the late 1980s, developments in magnetic materials such as samarium cobalt allowed permanent magnet alternators to be used where an extremely robust generator is required. In guided missileGuided Missile
Guided Missile is a London based independent record label set up by Paul Kearney in 1994.Guided Missile has always focused on 'the underground', preferring to put out a steady flow of releases and developing the numerous GM events around London and beyond....
s, such generators may replace the flux switching alternator
Flux switching alternator
A flux switching alternator or inductor alternator is a form of high-speed alternator, an AC electrical generator, intended for direct drive by a turbine. They are simple in design, making them rugged and capable of high rotation speeds...
. These need to operate at high speeds, directly coupled to a turbine. Both types share the advantage of the output coils being part of the stator, thus avoiding the need for brushgear.
Wind turbines
Small wind turbineWind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
s, particularly self-build designs, are widely adopting magneto alternators for their generators. These use rotating neodymium
Neodymium magnet
A neodymium magnet , the most widely-used type of rare-earth magnet, is a permanent magnet made from an alloy of neodymium, iron, and boron to form the Nd2Fe14B tetragonal crystalline structure. Developed in 1982 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest...
rare-earth magnet
Rare-earth magnet
Rare-earth magnets are strong permanent magnets made from alloys of rare earth elements. Developed in the 1970s and 80s, rare-earth magnets are the strongest type of permanent magnets made and have significant performance advantages over ferrite or alnico magnets...
s with a three-phase
Three-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...
stator
Stator
The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
, then a bridge rectifier to produce DC. This is then used either directly for water-pumping, stored in batteries, or used to drive a mains inverter
Inverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
which can supply the grid system for profit. These designs have been encouraged by one of the most popular self-build designs, described in Hugh Piggott's series of books and courses. A typical design here is an axial-flux generator recycled from a car brake disk and hub bearing. A Macpherson strut
MacPherson strut
The MacPherson strut is a type of car suspension system which uses the axis of a telescopic damper as the upper steering pivot. It is widely used in modern vehicles and named after Earle S. MacPherson, who developed the design.-History:...
provides the azimuth bearing to bring the turbine into the wind. The brake disk, and its attached rare-earth magnets, rotates to form the armature. A plywood disk carrying multiple axial coils is placed alongside this, with a further iron armature ring behind it.