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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 [1], 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 [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 magnetic 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 magnetic 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 is mostly preferred and for short run LSM is 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).
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 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.
Usages of a Linear Motor for Train Propulsion
Usage with conventional rails
All applications are in rapid transit.
- Bombardier ART:
- AirTrain JFK in New York (opened 2003)
- Detroit People Mover in Detroit (opened 1987)
- EverLine Rapid Transit System in Yongin (under construction)
- Kelana Jaya Line in Kuala Lumpur (opened 1998)
- Scarborough RT in Toronto (opened 1985)
- SkyTrain in Vancouver (opened 1985)
- Beijing Capital Airport Link in Beijing (opened 2008)
- Several Subways in Japan and China, built by Kawasaki Heavy Industries:
- Limtrain in Saitama (short-lived demonstration track, 1988)
- Nagahori Tsurumi-ryokuchi Line in Osaka (opened 1990)
- Toei Ōedo Line in Tokyo (opened 2000)
- Kaigan Line in Kobe (opened 2001)
- Nanakuma Line in Fukuoka (opened 2005)
- Green Line in Yokohama (opened 2008)
- Tōzai Line in Sendai (under construction)
- Line 4 of Guangzhou Metro, China (opened 2005). Line 5 and 6 under construction. [4]
- Beijing Subway Capital Airport Track
Both the Kawasaki-Trains and Bombardier's ART have the active part of the motor in the cars and use overhead wires (Japanese Subways[5][6]) or a third rail (ART[7]) to transfer power to the train.
Usage with magnetic levitation
- High-speed trains:
- Transrapid: first commercial use in Shanghai (opened in 2004)
- JR-Maglev
- Rapid transit:
See also
- California Screamin' Roller coaster LIM application
- capacitor
- coilgun
- compulsator
- doubly-fed electric machine
- flywheel
- homopolar generator
- launch loop a proposed system for launching vehicles into space using a linear motor powered loop
- linear actuator
- Maverick - a roller coaster LSM application
- pulse transformer
- railgun
- SkyTrain (Vancouver), Canada - Rapid transit system
- Tether cable catapult system
- Tomorrowland Transit Authority Slow ride LIM application
- Dreamworld, Australia LSM reverse freefall roller coaster
- UniModal, for a futuristic Maglev personal rapid transit solution using LIM for forward propulsion.
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External links
- Design equations, spreadsheet, and drawings
- Electromagnetic Guns - A page describing recent research at MIT.
- Maglev
- Linear Motors for Nanopositioning Applications
- Linear Motors Application Guide
