Linear motor
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. The most common mode of operation is as a Lorenz-type actuator, in which the applied force is linearly proportional to the current and the magnetic field (F = i x B).
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.
 |
ART trains propel themselves using an aluminium induction strip placed between the rails. |
The history of linear electric motors can be traced back at least as far as the
1840s, to the work of
Charles Wheatstone at
King's College in London [
1], but Wheatstone's model was too inefficient to be practical. The German engineer
Hermann Kemper built a working model in
1935 [
2]. In the late
1940s, professor
Eric Laithwaite of
Imperial College in
London developed the first full-size working model. 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 it 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, as in the Japanese
Linimo magnetic levitation train line near
Nagoya. However, linear motors have been used independently of magnetic levitation, as in
Bombardier's
Advanced Rapid Transit systems worldwide and a number of modern Japanese subways, including
Tokyo's
Toei Oedo Line.
Similar technology is also used in some
roller coasters with modifications, but at present is still impractical on street running
trams, although this in theory could be done, by burying it in a slotted conduit.
Outside of public transportation, vertical linear motors have been proposed as lifting mechanisms in deep
mines, and the use of linear motors is growing in
motion control applications. They are also often used on sliding doors, such as those of
low floor trams such as the
Citadis and the
Eurotram. Dual axis linear motors also exist. These specialized devices have been used to provide direct X-Y motion for precision laser cutting of cloth and sheet metal, automated
drafting, and cable forming.
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. Many amusement park roller coasters now use linear induction motors to propel the train at a high speed, as an alternative to using a lift hill. 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 [
3]) 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 convert the kinetic energy of a
flywheel into electric energy very rapidly. 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.
*
California Screamin' - roller coaster
*
capacitor*
coilgun*
compulsator*
flywheel*
homopolar generator*
linear actuator*
pulse transformer*
railgun*
Electromagnetic Guns - A page describing recent research at MIT.
*
The Pulsed Induction Motor Concept for High-Speed Trains SANDIA national laboratories
