Flux switching alternator
Encyclopedia
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. This makes them suitable for their only widespread use, in guided missile
s."Missile
" here is taken in its broad sense and could refer to any guided projectile
, potentially including torpedo
es as well as airborne missiles.
and fuzing
systems, possibly also the high-power loads of an active radar seeker (i.e. a transmitter) and rarely the missile's control surface
s. Control surface actuator
s for a high-speed missile require a high force and so these are usually powered by some non-electric means, such as tapping propellant exhaust gas from the missile's motor. Rare exceptions where electrically powered control surfaces are used are mostly medium-range subsonic naval missiles, e.g. Exocet
, Harpoon and Martel
. The total load varies for different missiles between around 100W to several kW.
The electrical supply for a missile must be reliable, particularly after long storage. Depending on the missile type, it may also be required to start delivering power almost immediately after start-up, or even before launch to allow gyroscopes to be accelerated to speed, and to provide power for varying lengths of time. Small anti-tank
or air-to-air missile
s may only require power for a few seconds of flight. Others, such as tactical missiles or ICBMs, may require power for several minutes. Turbojet-powered cruise missile
s have the longest flight times (being long-ranged, yet also slowest in flight) however these also have engines that are capable of driving a more conventional generator.
Two technologies are used in practice to power missiles: batteries and generators. The batteries used are usually esoteric types rarely found outside missiles, such as silver-zinc or thermal batteries. The generators used are simple high-speed generators, driven directly by a turbine rotor that is powered by either the rocket motor's exhaust, or else a dedicated gas generator
.
is still favoured for its mechanical robustness.
Unusually, both the field coil
s and the armature winding are carried on the fixed stator. The rotor is a simple toothed wheel, with no windings or electrical components.
In the simplest case, the stator has four poles and the field coils and armature windings are arranged alternately around the stator between the poles. The field magnets are arranged with their poles opposing each other, i.e. one armature is between the two North poles, one between the two South. The rotor is a simple bar of magnetic, but unmagnetized, iron. As it rotates between poles, it links the flux between a single pair of opposing poles. The magnetic circuit
of the stator is thus a pair of triangles, each containing a field, an armature and a shared path through the rotor. Flux passes in each circuit from one field and through one armature. As the rotor rotates, the other triangular path is formed, switching the flux from one pair of field and armature to the other and also reversing the direction of the flux in the armature coil. It is this reversal of flux that produces the alternating emf
.
The rotor must bridge the path between opposing pole pieces, but must never bridge all four simultaneously. It must thus have an even number of poles, but this must not be divisible by four. Practical rotors use six poles. As the rotation of one tooth pitch is sufficient to generate one AC cycle, the output frequency is thus the product of the rotation speed (in revs. per second) and the number of rotor teeth. Early AC systems used the standard frequency of 400 Hz, which limited alternators to two pole rotors and a maximum rotation speed of 24,000 rpm. The use of higher frequencies, from multi-pole rotors, was already recognised as a future means to achieve greater power for the same weight. The Sea Slug missile alternator used a speed of 24,000 rpm to produce 1.5 kVA of electricity at 2,400 Hz.
The field may be supplied by either permanent magnets or by field coils. Regulation of the output voltage is achieved by controlling the current through a winding, either the field coil, or a control winding around a permanent magnet.
Bleeding exhaust gas from the motor increases the amount of fuel required, but this effect is trivial, around 1%. The exhaust is hot, possibly as hot as 2,400°C, and at pressures varying from 2,600 psi at the boost phase
to 465 psi during sustain. A more serious drawback is the amount of sooty particulates in the exhaust, which requires a filter to keep them from the turbine. As such filters may themselves clog, this method is best suited for short flight durations.
is a chemical device that burns to provide a supply of gas under pressure. Although still hot, comparable to rocket motor exhaust, this gas can be cooler and cleaner of particulates than rocket efflux. Both solid and liquid-fuelled gas generators may be used.
Advantages of a gas generator drive, rather than motor exhaust are:
). Development of these began in the late 1940s, with air-to-air missiles such as Sparrow
. Sparrow was a relatively large missile with an airframe 8 inches in diameter. By the late 1950s, turbine-driven alternators were also being used in lightweight anti-tank missiles such as Vigilant
. Vigilant has a body diameter of 4½ inches, including a ¾ inch central jetpipe. The alternator and turbine was fitted into a remaining annular space of only 1.875 inches.
. Achieving the output needed depends on the use of modern rare earth magnets, such as samarium cobalt or neodymium
. The output coil is formed as a stator, with axial magnetic flux from a rotating multi-pole ring magnet., p. 58
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...
, an AC
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....
electrical generator, intended for direct drive by a turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
. They are simple in design, making them rugged and capable of high rotation speeds. This makes them suitable for their only widespread use, in guided missile
Guided 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."Missile
Missile
Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...
" here is taken in its broad sense and could refer to any guided projectile
Projectile
A projectile is any object projected into space by the exertion of a force. Although a thrown baseball is technically a projectile too, the term more commonly refers to a weapon....
, potentially including torpedo
Torpedo
The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...
es as well as airborne missiles.
Guided missiles
Guided missiles require a source of electrical power during flight. This is needed to power the guidanceMissile guidance
Missile guidance refers to a variety of methods of guiding a missile or a guided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness...
and fuzing
Fuze
Fuze Beverage, commercially referred to as just Fuze , is a manufacturer of teas and non-carbonated fruit drinks enriched with vitamins. Currently the brand consists of five vitamin-infused lines: Slenderize, Refresh, Tea, Defensify, and Vitalize...
systems, possibly also the high-power loads of an active radar seeker (i.e. a transmitter) and rarely the missile's control surface
Control surface
In the domain of digital audio, a control surface is a human interface device which allows the user to control a digital audio workstation or other digital audio application. Generally, a control surface will contain one or more controls that can be assigned to parameters in the software,...
s. Control surface actuator
Actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
s for a high-speed missile require a high force and so these are usually powered by some non-electric means, such as tapping propellant exhaust gas from the missile's motor. Rare exceptions where electrically powered control surfaces are used are mostly medium-range subsonic naval missiles, e.g. Exocet
Exocet
The Exocet is a French-built anti-ship missile whose various versions can be launched from surface vessels, submarines, helicopters and fixed wing aircraft. Hundreds were fired in combat during the 1980s.-Etymology:...
, Harpoon and Martel
Martel (missile)
The Martel is an Anglo-French anti-radiation missile . The name Martel is a contraction of Missile, Anti-Radiation, Television, referring to the guidance options...
. The total load varies for different missiles between around 100W to several kW.
The electrical supply for a missile must be reliable, particularly after long storage. Depending on the missile type, it may also be required to start delivering power almost immediately after start-up, or even before launch to allow gyroscopes to be accelerated to speed, and to provide power for varying lengths of time. Small anti-tank
Anti-tank guided missile
An anti-tank missile , anti-tank guided missile , anti-tank guided weapon or anti-armor guided weapon is a guided missile primarily designed to hit and destroy heavily-armored military vehicles....
or air-to-air missile
Air-to-air missile
An air-to-air missile is a missile fired from an aircraft for the purpose of destroying another aircraft. AAMs are typically powered by one or more rocket motors, usually solid fuelled but sometimes liquid fuelled...
s may only require power for a few seconds of flight. Others, such as tactical missiles or ICBMs, may require power for several minutes. Turbojet-powered cruise missile
Cruise missile
A cruise missile is a guided missile that carries an explosive payload and is propelled, usually by a jet engine, towards a land-based or sea-based target. Cruise missiles are designed to deliver a large warhead over long distances with high accuracy...
s have the longest flight times (being long-ranged, yet also slowest in flight) however these also have engines that are capable of driving a more conventional generator.
Two technologies are used in practice to power missiles: batteries and generators. The batteries used are usually esoteric types rarely found outside missiles, such as silver-zinc or thermal batteries. The generators used are simple high-speed generators, driven directly by a turbine rotor that is powered by either the rocket motor's exhaust, or else a dedicated gas generator
Gas generator
A gas generator usually refers to a device, often similar to a solid rocket or a liquid rocket that burns to produce large volumes of relatively cool gas, instead of maximizing the temperature and specific impulse. The low temperature allows the gas to be put to use more easily in many...
.
Alternator principles
The generator is required to be rugged and capable of very high speeds, as it is driven at the turbine's speed, without reduction gearing. The rotor must thus be simple in design and there can also be no sliding contacts to sliprings or other brushgear. Although the power requirement for the missile may be a largely DC supply, the AC alternator and its need for a rectifierRectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
is still favoured for its mechanical robustness.
Unusually, both the 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 and the armature winding are carried on the fixed stator. The rotor is a simple toothed wheel, with no windings or electrical components.
In the simplest case, the stator has four poles and the field coils and armature windings are arranged alternately around the stator between the poles. The field magnets are arranged with their poles opposing each other, i.e. one armature is between the two North poles, one between the two South. The rotor is a simple bar of magnetic, but unmagnetized, iron. As it rotates between poles, it links the flux between a single pair of opposing poles. The magnetic circuit
Magnetic circuit
A magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other...
of the stator is thus a pair of triangles, each containing a field, an armature and a shared path through the rotor. Flux passes in each circuit from one field and through one armature. As the rotor rotates, the other triangular path is formed, switching the flux from one pair of field and armature to the other and also reversing the direction of the flux in the armature coil. It is this reversal of flux that produces the alternating emf
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...
.
The rotor must bridge the path between opposing pole pieces, but must never bridge all four simultaneously. It must thus have an even number of poles, but this must not be divisible by four. Practical rotors use six poles. As the rotation of one tooth pitch is sufficient to generate one AC cycle, the output frequency is thus the product of the rotation speed (in revs. per second) and the number of rotor teeth. Early AC systems used the standard frequency of 400 Hz, which limited alternators to two pole rotors and a maximum rotation speed of 24,000 rpm. The use of higher frequencies, from multi-pole rotors, was already recognised as a future means to achieve greater power for the same weight. The Sea Slug missile alternator used a speed of 24,000 rpm to produce 1.5 kVA of electricity at 2,400 Hz.
The field may be supplied by either permanent magnets or by field coils. Regulation of the output voltage is achieved by controlling the current through a winding, either the field coil, or a control winding around a permanent magnet.
Propulsion motor
The simplest solution taps off some hot exhaust gas from the propulsion motor and routes it instead through the generator turbine. This gas may also be used to power the control surface actuators, as was used for Vigilant. This is one of the simplest and lightest electrical power supplies available for a missile.Bleeding exhaust gas from the motor increases the amount of fuel required, but this effect is trivial, around 1%. The exhaust is hot, possibly as hot as 2,400°C, and at pressures varying from 2,600 psi at the boost phase
Boost phase
The boost phase is the portion of the flight of a ballistic missile or space vehicle during which the booster and sustainer engines operate until it reaches peak velocity. This phase can take 3 to 4 minutes , the altitude at the end of this phase is 150–200 km, and the typical burn-out speed...
to 465 psi during sustain. A more serious drawback is the amount of sooty particulates in the exhaust, which requires a filter to keep them from the turbine. As such filters may themselves clog, this method is best suited for short flight durations.
Gas generator
A gas generatorGas generator
A gas generator usually refers to a device, often similar to a solid rocket or a liquid rocket that burns to produce large volumes of relatively cool gas, instead of maximizing the temperature and specific impulse. The low temperature allows the gas to be put to use more easily in many...
is a chemical device that burns to provide a supply of gas under pressure. Although still hot, comparable to rocket motor exhaust, this gas can be cooler and cleaner of particulates than rocket efflux. Both solid and liquid-fuelled gas generators may be used.
Advantages of a gas generator drive, rather than motor exhaust are:
- Cleaner, cooler exhaust, which is less likely to cause turbine problems.
- Ability to start the gas generator before launching, allowing time for gyroscopeGyroscopeA gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
s to be spun up to speed, power for control surfaces etc. - Ability to continue power generation after the motor has burned out, during a ballistic coast phase.
Development history
The first alternators of this type began with the first missiles requiring considerable electric power, those using radar seekers (initially semi-active radar homingSemi-active radar homing
Semi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external ...
). Development of these began in the late 1940s, with air-to-air missiles such as Sparrow
AIM-7 Sparrow
The AIM-7 Sparrow is an American, medium-range semi-active radar homing air-to-air missile operated by the United States Air Force, United States Navy and United States Marine Corps, as well as various allied air forces and navies. Sparrow and its derivatives were the West's principal beyond visual...
. Sparrow was a relatively large missile with an airframe 8 inches in diameter. By the late 1950s, turbine-driven alternators were also being used in lightweight anti-tank missiles such as Vigilant
Vickers Vigilant
The Vickers Vigilant was a British MCLOS wire guided anti-tank missile used by the British Army. It was licence-built for the US Marine Corps by Clevite in the USA.-History:...
. Vigilant has a body diameter of 4½ inches, including a ¾ inch central jetpipe. The alternator and turbine was fitted into a remaining annular space of only 1.875 inches.
Permanent magnet magnetos
An alternative high-speed generator is the permanent magnet magnetoMagneto (generator)
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...
. Achieving the output needed depends on the use of modern rare earth magnets, such as samarium cobalt or 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...
. The output coil is formed as a stator, with axial magnetic flux from a rotating multi-pole ring magnet., p. 58
See also
- Alexanderson alternatorAlexanderson alternatorAn Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high frequency alternating current up to 100 kHz, for use as a radio transmitter...
- Variable reluctance sensorVariable reluctance sensorA variable reluctance sensor , also called a monopole, is used to measure position and speed of moving metal components. This sensor consists of a permanent magnet, a ferromagnetic pole piece, a pickup coil, and a rotating toothed wheel....
- Switched reluctance motorSwitched Reluctance MotorThe switched reluctance motor is an electric motor which runs by reluctance torque. It has wound field coils as in a DC motor for the stator windings. The rotor however has no magnets or coils attached. The rotor of the motor becomes aligned as soon as the opposite poles of the stator become...