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[[File:Flux switching alternator.gif|thumb|right|Alternator with six-pole rotor]]
A '''flux switching alternator''' is a form of high-speed [[alternator]], an [[alternating current|AC]] electrical generator, intended for direct drive by a [[turbine]]. They are simple in design with the rotor containing no coils or magnets, making them rugged and capable of high rotation speeds. This makes them suitable for their only widespread use, in [[guided missile]]s.{{efn-lr|"[[Missile]]" here is taken in its broad sense and could refer to any guided [[projectile]], potentially including [[torpedo]]es as well as airborne missiles.}}

== Guided missiles ==
Guided missiles require a source of electrical power during flight. This is needed to power the [[missile guidance|guidance]] and [[fuze|fuzing]] systems, possibly also the high-power loads of an [[active radar seeker]] (i.e. a transmitter) and rarely the missile's [[Flight control surfaces|control surfaces]]. 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.<ref name="Forbat, Vigilant" >{{Cite book 
  |title=Vickers Guided Weapons
  |url=
  |last=Forbat  |first=John
  |year=2006
  |publisher=Tempus Publishing
  |isbn=0-7524-3769-0
  |pages=155–161
}}</ref> Rare exceptions where electrically powered control surfaces are used are mostly medium-range subsonic naval missiles, e.g. [[Exocet]], [[Harpoon (missile)|Harpoon]] and [[Martel (missile)|Martel]].<ref name="Brasseys, Requirements" >{{Cite book 
  |title=Guided Weapons
  |work=Land Warfare: Brassey's New Battlefield Weapons Systems & Technology Series
  |chapter=Electrical Power Supplies
  |chapter-url=
  |last1=Lee  |first1=Colonel R.G.
  |last2=Garland-Collins  |first2=T.K.
  |first3=D.E. | last3=Johnson
  |first4=E. | last4=Archer
  |first5=C. | last5=Sparkes
  |first6=G.M. | last6=Moss
  |first7=A.W. | last7=Mowat
  |volume=1
  |year=1988
  |publisher=[[Brassey's]]
  |isbn=0-08-035828-4
  |ref={{harvid|Brassey's, Guided Weapons|1988}}
  |page=43
}}</ref> The total load varies for different missiles between around 100W to several kW.<ref name="Brasseys, Requirements" />

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,<ref name="Brasseys, Requirements" /> and to provide power for varying lengths of time.<ref name="Brasseys, Requirements" /> Small [[anti-tank guided missile|anti-tank]] or [[air-to-air missile]]s may only require power for a few seconds of flight. Others, such as [[tactical missile]]s or [[ICBM]]s, 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 battery|silver-zinc]] or [[Molten salt battery|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]].<ref name="Brasseys, Alternator drives" >{{harvp|Brassey's, Guided Weapons|1988|p=55}}</ref>

== 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 [[slipring]]s or other [[brush (electric)|brushgear]].<ref name="Brasseys, Alternator drives" /><ref name="Rauch, 1955" >{{Cite journal
  |title=Design Principles of Flux-Switch Alternators 
  |last1=Rauch   |first1=S. E.
  |last2=Johnson   |first2=L. J.
  |journal=Power Apparatus and Systems
  |publisher=[[AIEE]]
  |volume=74 
  |issue=3 
  |date=Jan 1955
  |pages=1261–1268
  |doi=10.1109/AIEEPAS.1955.4499226  
  |s2cid=51633745 }}</ref> Although the power requirement for the missile may be a largely DC supply, the AC alternator and its need for a [[rectifier]] is still favoured for its mechanical robustness.{{sfnp|Mann|1957|pp=82–83}}

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.<ref name="Brasseys, Inductor Alternator" >{{harvp|Brassey's, Guided Weapons|1988|p=57}}</ref>

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 toothed disc 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 turns, 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 [[electromotive force|emf]].<ref name="Brasseys, Inductor Alternator" />

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.<ref name="Rauch, 1955" /> Practical rotors use six poles.<ref name="Brasseys, Inductor Alternator" /> 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.<ref name="Brasseys, Inductor Alternator" /> Early AC systems used the standard frequency of 400&nbsp;Hz, which limited alternators to two pole rotors and a maximum rotation speed of 24,000&nbsp;rpm.{{sfnp|Mann|1957|p=84}} The use of higher frequencies, from multi-pole rotors, was already recognised as a future means to achieve greater power for the same weight.{{sfnp|Mann|1957|pp=155–165}} The [[Seaslug (missile)|Seaslug]] missile alternator used a speed of 24,000&nbsp;rpm to produce 1.5&nbsp;kVA of electricity at 2,400&nbsp;Hz.<ref name="Brasseys, Inductor Alternator" />

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.<ref name="Brasseys, Inductor Alternator" />

== Alternator drive ==
=== Propulsion motor ===
The simplest solution taps off some hot exhaust gas from the propulsion motor and routes it instead through the generator turbine.<ref name="Brasseys, Alternator drives" /><ref name="MIT, 1957" >{{Cite journal
  |title=Missile Internal Power
  |last=Mann  |first=Robert Wellesley
  |journal=
  |publisher=[[MIT]]
  |volume=
  |issue= 
  |date=June 1957
  |pages=91
  |doi=
  |url=https://dspace.mit.edu/bitstream/handle/1721.1/33258/32145005-MIT.pdf?sequence=2
  |accessdate=14 May 2018
  }}</ref> This gas may also be used to power the control surface actuators, as was used for Vigilant.<ref name="Forbat, Vigilant" /> This is one of the simplest and lightest electrical power supplies available for a missile.<ref name="Brasseys, Alternator drives" />

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&nbsp;°C, and at pressures varying from 2,600&nbsp;psi at the [[boost phase]] to 465&nbsp;psi during [[sustain phase|sustain]].<ref name="Forbat, Vigilant" /> A more serious drawback is the amount of sooty particulates in the exhaust,{{sfnp|Mann|1957|p=35}} which requires a filter to keep them from the turbine.<ref name="Brasseys, Alternator drives" /> As such filters may themselves clog, this method is best suited for short flight durations.

=== Gas generator ===
A [[gas generator]] 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.<ref name="Brasseys, Alternator drives" /> Both solid and liquid-fuelled gas generators may be used.<ref name="Brasseys, Alternator drives" />

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 [[gyroscope]]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 homing]]). Development of these began in the late 1940s, with air-to-air missiles such as [[AIM-7 Sparrow|Sparrow]].<ref name="Rauch, 1955" /> 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 [[Vickers Vigilant|Vigilant]].<ref name="Forbat, Vigilant" /> Vigilant has a body diameter of 4{{frac|1|2}}&nbsp;inches, including a {{frac|3|4}}&nbsp;inch central jetpipe. The alternator and turbine were fitted into a remaining annular space of only 1{{frac|7|8}}&nbsp;inches.<ref name="Forbat, Vigilant" /><ref>{{Cite journal
  |url=http://www.flightglobal.com/pdfarchive/view/1959/1959%20-%201478.html 
  |title=Vickers Vigilant
  |journal=[[Flight (magazine)|Flight]]
  |date=22 May 1959
  |pages=716–717
}}, Cutaway drawing of Vigilant missile's main components</ref>

==== Permanent magnet magnetos ====
An alternative high-speed generator is the permanent magnet [[magneto (generator)|magneto]]. Achieving the output needed depends on the use of modern [[rare-earth magnet]]s, such as [[samarium cobalt]] or [[neodymium magnet|neodymium]]. The output coil is formed as a stator, with axial magnetic flux from a rotating multi-pole ring magnet.{{sfnp|Brassey's, Guided Weapons|1988|page=58}}

==See also==
*[[Alexanderson alternator]]
*[[Variable reluctance sensor]]
*[[Switched reluctance motor]]

== References ==
{{Notelist-lr}}

{{Reflist}}

[[Category:Alternators]]
[[Category:Electrical generators]]
[[Category:Missile technology]]