Linear motor

From Academic Kids

A linear motor is essentially an electric motor that has had its stator "unrolled" so that instead of producing a torque (rotation), it produces a linear force along its length.

Many designs have been put forward for linear motors, falling into two major categories, low-acceleration and high-acceleration linear motors. Low-acceleration linear motors are suitable for maglev trains and other ground-based transportation applications. High-acceleration linear motors are normally quite short, and are designed to accelerate an object up to a very high speed and then release the object. They are usually used for studies of hypervelocity collisions, as weapons, or as mass drivers for spacecraft propulsion.

When a linear motor is used to accelerate beams of ions or subatomic particles, it is called a particle accelerator. The design is usually rather different as the particles move close to the speed of light and are usually electrically charged.

Contents

Low acceleration

AirTrain at JFK. Note aluminum induction strip between rails.
Enlarge
AirTrain at JFK. Note aluminum induction strip between rails.

The concept of a linear motor was pioneered by professor Eric Laithwaite of Imperial College in London. In his design, and in most low-acceleration designs, the force is produced by a moving linear electromagnetic field acting on conductors in the field. Any conductor, be it a loop, a coil or simply a piece of plate metal, that is placed in this field will have eddy currents induced in the loop thus creating an opposing electromagnetic field. The two opposing fields will repel each other, thus forcing the conductor away from the stator and carrying it along in the direction of the moving magnetic field.

Because of these properties, linear motors are often used in maglev propulsion, although they can also be used independently of magnetic levitation, as in the advanced light rapid transit technology used in Vancouver's SkyTrain system, Toronto's Scarborough RT, New York City's JFK Airport AirTrain and Kuala Lumpur's Putra LRT. This technology is also used in some roller coasters with modifications.

Vertical linear motors have been proposed as lifting mechanisms in deep mines.

High acceleration

High-acceleration linear motors have been suggested for a number of uses. They have been considered for use as weapons, since current armor-piercing ammunition tends to be small rounds with very high kinetic energy, just what such motors supply. They have also been suggested for use in spacecraft propulsion. In this context they are usually called mass drivers. The simplest way to use mass drivers for spacecraft propulsion would be to build a large mass driver that can accelerate cargo up to escape velocity.

High-acceleration linear motors are difficult to design for a number of reasons. They require large amounts of energy in very short periods of time. One rocket launcher design (see [1] (http://www.oz.net/~coilgun/theory/electroguns.htm)) calls for 300 GJ for each launch in the space of less than a second. Normal electrical generators are not designed for this kind of load, but short-term electrical energy storage methods can be used. Capacitors are bulky and expensive but can supply large amounts of energy quickly. Homopolar generators can be used to rapidly convert the kinetic energy of a flywheel into electric energy. High-acceleration linear motors also require very strong magnetic fields; in fact, the magnetic fields are often too strong to permit the use of superconductors. However, with careful design this need not be a major problem.

Two different basic designs have been invented for high-acceleration linear motors: railguns and coilguns.

See also

External links

de:Linearmotor fr:Moteur lectrique#Moteurs Linaire it:Motore lineare ja:リニアモーター

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