Everything about Linear Induction Motor totally explained
A
linear motor is essentially a multi-phase
alternating current (AC)
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
Lorentz-type actuator, in which the applied force is linearly proportional to the current and the magnetic field (
F = q
v ×
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, like
roller coasters. They are usually used for studies of
hypervelocity collisions, as
weapons, or as
mass drivers for
spacecraft propulsion. The high-acceleration motors are usually of the linear induction design (LIM) with an active
three-phase winding on one side of the air-gap and a passive conductor plate on the other side. The low-acceleration, high speed and high power motors are usually of the linear synchronous design (LSM), with an active winding on one side of the air-gap and an array of alternate-pole magnets on the other side. These magnets can be permanent magnets or energized magnets. The
Transrapid Shanghai motor is an LSM.
When a linear motor is used to accelerate beams of
ions or
subatomic particles, it's called a
particle accelerator. The design is usually rather different as the particles move close to the
speed of light and are generally
electrically charged.
Low acceleration
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
(External Link
), but Wheatstone's model was too inefficient to be practical. A feasible linear induction motor is described in the US patent 732312 (1905 - inventor Alfred Zehden of Frankfurt-am-Main ), for driving trains or lifts. The German engineer
Hermann Kemper built a working model in
1935 (External Link). 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's 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.Mostly used linear motors are LIM (Linear Induction Motor), LSM (linear Synchronous Motor). Linear DC motors are not used as it includes more cost and Linear SRM suffers from poor thrust. So for long run in traction LIM mostly preferred and for short run LSM mostly preferred.
From concept to industrial use
In the 1980s British engineer
Hugh-Peter Kelly designed the first tubular linear motor by enclosing the permanent magnets in a sealed stainless steel cylinder. It was brought to market by linear motor manufacturer Linear Drives (now
Copley Motion Systems) . The patented permanent magnet arrangement induces a sinusoidal response in the coils that are enclosed in a square profile body. This allowed machine builders to use the new linear motors with standard sinusoidal servo drives commonly used in motion control.
Tubular Linear Motors
Tubular linear motors are more rugged than early flat bed and u-channel linear motors allowing them to be used in dirty industrial environments such as food packaging and machine tools. The tubular construction protects the permanent magnets from the external environment and automatically balances attractive forces so that the motor is easier to integrate into machines. These motors operate at 5- 9 m/s with high acceleration for dynamic motion control.
A new type of linear motor, called the ServoTube (see
Eureka March 2005) has allowed linear motors to be used in industrial environments by integrating the position sensing electronics into the motor body (called a forcer).
Usages of a Linear Motor for Train Propulsion
Usage with conventional rails
All applications are in
rapid transit.
Several Subways in Japan and China, built by Kawasaki Heavy Industries:
Both the Kawasaki-Trains and Bombardier's ART have the active part of the motor in the cars and use overhead wires (Japanese Subways(External Link)(External Link)) or a third rail (ART(External Link)) to transfer power to the train.
Usage with magnetic levitation
High-speed trains:
Rapid transit:
- Birmingham Airport, UK (opened 1984, closed 1995)
- M-Bahn in Berlin, Germany (opened in 1989, closed in 1991)
- Daejeon EXPO, Korea (ran only 1993) (External Link)
- HSST: Linimo line in Aichi, Japan (opened 2005)
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 consist of small rounds with very high kinetic energy, for which just such motors are suitable. Many amusement park roller coaster now use linear induction motors to propel the train at a high speed, as an alternative to using a lift hill. The United States Navy is also using linear induction motors in the Electromagnetic Aircraft Launch System that will replace traditional steam catapults on future aircraft carriers. They have also been suggested for use in spacecraft propulsion. In this context they're 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 (External Link)) 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.
Further Information
Get more info on 'Linear Induction Motor'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://linear_motor.totallyexplained.com">Linear motor Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
