Linear motor

A linear motor is an electric motor that has had its stator and rotor "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 (\vec F = q \cdot \vec v \times \vec 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 rather short, and are designed to accelerate an object to a very high speed, for example see the railgun.

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 AC linear induction motor (LIM) design with an active three-phase winding on one side of the air-gap and a passive conductor plate on the other side. However, the direct current homopolar linear motor railgun is another high acceleration linear motor design. The low-acceleration, high speed and high power motors are usually of the linear synchronous motor (LSM) design, 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.

Contents

Types

Induction motor

Main article: linear induction motor

In this design, 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, in accordance with Lenz's law. The two opposing fields will repel each other, thus creating motion as the magnetic field sweeps through the metal.

Synchronous motor

In this design the rate of movement of the magnetic field is controlled, usually electronically, to track the motion of the rotor. For cost reasons synchronous linear motors rarely use commutators, so the rotor often contains permanent magnets, or soft iron.

Homopolar

Main article: Railgun

In this design a large current is passed through a metal sabot across sliding contacts that are fed from two rails. The magnetic field this generates causes the metal to be projected along the rails.

Piezo electric

Main article: Piezoelectric_motor#Stepping_actions

Piezoelectric drive is often used to drive small linear motors.

History

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 782312 ( 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 a single sided version the magnetic repulsion forces the conductor away from the stator, levitating it, and carrying it along in the direction of the moving magnetic field. He called the later versions of it magnetic river.

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.

High acceleration

High-acceleration linear motors have been suggested for a number of uses. They have been considered for use as weapons, since current armour-piercing ammunition tends to consist of small rounds with very high kinetic energy, for which just such motors are suitable. 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. 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, though RLV launch assist like StarTram to low earth orbit has also been investigated.

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 [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.

Example : Maglev

Usage

Linear motors have been used for sliding doors and various similar actuators.

Linear motors are sometimes used to create rotary motion, for example, they have been used at observatories to deal with the large radius of curvature.

A linear motor has been used for accelerating cars for crash tests.[4]

Train propulsion

Conventional rails

All applications are in rapid transit.

Both the Kawasaki trains and Bombardier's ART have the active part of the motor in the cars and use overhead wires (Japanese Subways[6][7]) or a third rail (ART[8]) to transfer power to the train.

Monorail

Magnetic levitation

Main article: Maglev (transport)

See also

References

  1. ^ "Charles Wheatstone - College History - King's College London". Kcl.ac.uk. http://www.kcl.ac.uk/college/history/people/wheatstone.html. Retrieved 2010-03-01. 
  2. ^ http://cem.colorado.edu/archives/fl1997/thor.html
  3. ^ [1]
  4. ^ http://books.google.com/books?id=pgAAAAAAMBAJ&lpg=PA64&ots=h1ZnQLJny2&dq=crash%20testing%20linear%20motor&pg=PA64#v=onepage&q=crash%20testing%20linear%20motor&f=false
  5. ^ "> Asia > China > Guangzhou Metro". UrbanRail.Net. http://www.urbanrail.net/as/guan/guangzhou.htm. Retrieved 2010-03-01. 
  6. ^ "Adoption of Linear Motor Propulsion System for Subway". Home.inet-osaka.or.jp. http://home.inet-osaka.or.jp/~teraoka/old/tera98/ml98edit.htm. Retrieved 2010-03-01. 
  7. ^ [2]
  8. ^ "The Scarborough Rapid Transit Line - Transit Toronto - Content". Transit Toronto. November 10, 2006. http://transit.toronto.on.ca/subway/5107.shtml. Retrieved 2010-03-01. 
  9. ^ "The International Maglevboard". Maglev.de. http://maglev.de/index.php?en_korea. Retrieved 2010-03-01. 

External links

Broad motor categories
Conventional
electric motors
Unusual electric motors
Motor
controllers
See also