A flux switching alternator is a form of high-speed alternator, an 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 missiles. [lower-roman 1]
Guided missiles require a source of electrical power during flight. This is needed to power the guidance and fuzing systems, possibly also the high-power loads of an active radar seeker (i.e. a transmitter) and rarely the missile's control surfaces. Control surface actuators 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. [1] Rare exceptions where electrically powered control surfaces are used are mostly medium-range subsonic naval missiles, e.g. Exocet, Harpoon and Martel. [2] The total load varies for different missiles between around 100W to several kW. [2]
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, [2] and to provide power for varying lengths of time. [2] Small anti-tank or air-to-air missiles 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 missiles 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. [3]
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. [3] [4] 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. [5]
Unusually, both the field coils and the armature winding are carried on the fixed stator. The rotor is a simple toothed wheel, with no windings or electrical components. [6]
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 emf. [6]
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. [4] Practical rotors use six poles. [6] 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. [6] 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. [7] The use of higher frequencies, from multi-pole rotors, was already recognised as a future means to achieve greater power for the same weight. [8] The Seaslug missile alternator used a speed of 24,000 rpm to produce 1.5 kVA of electricity at 2,400 Hz. [6]
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. [6]
The simplest solution taps off some hot exhaust gas from the propulsion motor and routes it instead through the generator turbine. [3] [9] This gas may also be used to power the control surface actuators, as was used for Vigilant. [1] This is one of the simplest and lightest electrical power supplies available for a missile. [3]
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. [1] A more serious drawback is the amount of sooty particulates in the exhaust, [10] which requires a filter to keep them from the turbine. [3] As such filters may themselves clog, this method is best suited for short flight durations.
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. [3] Both solid and liquid-fuelled gas generators may be used. [3]
Advantages of a gas generator drive, rather than motor exhaust are:
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 Sparrow. [4] 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. [1] Vigilant has a body diameter of 41⁄2 inches, including a 3⁄4 inch central jetpipe. The alternator and turbine were fitted into a remaining annular space of only 17⁄8 inches. [1] [11]
An alternative high-speed generator is the permanent magnet magneto. 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. [12]
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. Electric motors can be powered by direct current (DC) sources, such as from batteries, or rectifiers, or by alternating current (AC) sources, such as a power grid, inverters or electrical generators. An electric generator is mechanically identical to an electric motor, but operates with a reversed flow of power, converting mechanical energy into electrical energy.
In electricity generation, a generator is a device that converts motive power into electrical power for use in an external circuit. Sources of mechanical energy include steam turbines, gas turbines, water turbines, internal combustion engines, wind turbines and even hand cranks. The first electromagnetic generator, the Faraday disk, was invented in 1831 by British scientist Michael Faraday. Generators provide nearly all of the power for electric power grids.
An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines.
The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of the system. In an electric motor, the stator provides a magnetic field that drives the rotating armature; in a generator, the stator converts the rotating magnetic field to electric current. In fluid powered devices, the stator guides the flow of fluid to or from the rotating part of the system.
A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors contain multiphase AC electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field of any AC motor. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.
A DC motor is any of a class of rotary electrical motors that converts direct current electrical energy into mechanical energy. The most common types rely on the forces produced by magnetic fields. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current in part of the motor.
In electrical engineering, an armature is the component of an electric machine which carries alternating current. The armature windings conduct AC even on DC machines, due to the commutator action or due to electronic commutation, as in brushless DC motors. The armature can be on either the rotor or the stator, depending on the type of electric machine.
A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows.
A Gramme machine, Gramme ring, Gramme magneto, or Gramme dynamo is an electrical generator that produces direct current, named for its Belgian inventor, Zénobe Gramme, and was built as either a dynamo or a magneto. It was the first generator to produce power on a commercial scale for industry. Inspired by a machine invented by Antonio Pacinotti in 1860, Gramme was the developer of a new induced rotor in form of a wire-wrapped ring and demonstrated this apparatus to the Academy of Sciences in Paris in 1871. Although popular in 19th century electrical machines, the Gramme winding principle is no longer used since it makes inefficient use of the conductors. The portion of the winding on the interior of the ring cuts no flux and does not contribute to energy conversion in the machine. The winding requires twice the number of turns and twice the number of commutator bars as an equivalent drum-wound armature.
An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. The rotor magnetic field may be produced by permanent magnets, reluctance saliency, or DC or AC electrical windings.
An induction generator or asynchronous generator is a type of alternating current (AC) electrical generator that uses the principles of induction motors to produce electric power. Induction generators operate by mechanically turning their rotors faster than synchronous speed. A regular AC induction motor usually can be used as a generator, without any internal modifications. Induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls.
The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor's axis.
A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source. Brushed motors were the first commercially important application of electric power to driving mechanical energy, and DC distribution systems were used for more than 100 years to operate motors in commercial and industrial buildings. Brushed DC motors can be varied in speed by changing the operating voltage or the strength of the magnetic field. Depending on the connections of the field to the power supply, the speed and torque characteristics of a brushed motor can be altered to provide steady speed or speed inversely proportional to the mechanical load. Brushed motors continue to be used for electrical propulsion, cranes, paper machines and steel rolling mills. Since the brushes wear down and require replacement, brushless DC motors using power electronic devices have displaced brushed motors from many applications.
A dynamo is an electrical generator that creates direct current using a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter.
In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity to mechanical power while an electric generator converts mechanical power to electricity. The moving parts in a machine can be rotating or linear. Besides motors and generators, a third category often included is transformers, which although they do not have any moving parts are also energy converters, changing the voltage level of an alternating current.
A permanent magnet synchronous generator is a generator where the excitation field is provided by a permanent magnet instead of a coil. The term synchronous refers here to the fact that the rotor and magnetic field rotate with the same speed, because the magnetic field is generated through a shaft mounted permanent magnet mechanism and current is induced into the stationary armature.
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.
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.
An alternator is a type of electric generator used in modern automobiles to charge the battery and to power the electrical system when its engine is running.
Single-phase generator is an alternating current electrical generator that produces a single, continuously alternating voltage. Single-phase generators can be used to generate power in single-phase electric power systems. However, polyphase generators are generally used to deliver power in three-phase distribution system and the current is converted to single-phase near the single-phase loads instead. Therefore, single-phase generators are found in applications that are most often used when the loads being driven are relatively light, and not connected to a three-phase distribution, for instance, portable engine-generators. Larger single-phase generators are also used in special applications such as single-phase traction power for railway electrification systems.
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