Alinear motor is anelectric motor that has had itsstator androtor "unrolled", thus, instead of producing atorque (rotation), it produces a linearforce along its length. However, linear motors are not necessarily straight. Characteristically, a linear motor's active section has ends, whereas more conventional motors are arranged as a continuous loop.
A typical mode of operation is as aLorentz-type actuator, in which the applied force islinearly proportional to thecurrent and themagnetic field.
Linear motors are most commonly found in high accuracy engineering[1] applications.
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 formaglev 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 thecoilgun.
High-acceleration linear motors are typically used in studies ofhypervelocity collisions, asweapons, or asmass drivers forspacecraft propulsion.[citation needed] They are usually of the AClinear induction motor (LIM) design with an activethree-phase winding on one side of theair-gap and a passive conductor plate on the other side. However, the direct currenthomopolar linear motorrailgun is another high acceleration linear motor design. The low-acceleration, high speed and high power motors are usually of thelinear 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 bepermanent magnets orelectromagnets. The motor for theShanghai maglev train, for instance, is an LSM.
Brushless linear motors are members of the Synchronous motor family. They are typically used in standardlinear stages or integrated into custom,high performance positioning systems. Invented in the late 1980s byAnwar Chitayat at Anorad Corporation, nowRockwell Automation, and helped improve the throughput and quality of industrial manufacturing processes.[2]
Brushed linear motors were used in industrial automation applications prior to the invention of Brushless linear motors. Compared withthree phase brushless motors, which are typically being used today, brush motors operate on a single phase.[3] Brush linear motors have a lower cost since they do not need moving cables or three phase servo drives. However, they require higher maintenance since their brushes wear out.
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 usecommutators, so the rotor often contains permanent magnets, orsoft iron. Examples includecoilguns and the motors used on somemaglev systems, as well as many other linear motors. In high precision industrial automation linear motors are typically configured with a magnet stator and a moving coil. AHall effect sensor is attached to the rotor to track themagnetic flux of the stator. The electric current is typically provided from a stationaryservo drive to the moving coil by a moving cable inside acable carrier.
In this design, the force is produced by a moving linearmagnetic 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 haveeddy currentsinduced in it thus creating an opposing magnetic field, in accordance withLenz's law.[4] The two opposing fields will repel each other, thus creating motion as the magnetic field sweeps through the metal.
In this design a large current is passed through a metal sabot across sliding contacts that are fed by two rails. The magnetic field this generates causes the metal to be projected along the rails.
Efficient and compact design applicable to the replacement ofpneumatic cylinders.
Piezoelectric drive is often used to drive small linear motors.
The history of linear electric motors can be traced back at least as far as the 1840s, to the work ofCharles Wheatstone atKing's College London,[5] but Wheatstone's model was too inefficient to be practical. A feasible linear induction motor is described inU.S. patent 782,312 (1905 - inventor Alfred Zehden of Frankfurt-am-Main), for driving trains or lifts. The German engineerHermann Kemper built a working model in 1935.[6] In the late 1940s, Dr.Eric Laithwaite ofManchester University, later Professor of Heavy Electrical Engineering atImperial College inLondon 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 itmagnetic river. The technologies would later be applied, in the 1984,Air-Rail Link shuttle, between Birmingham's airport and an adjacent train station.
Because of these properties, linear motors are often used inmaglev propulsion, as in the JapaneseLinimomagnetic levitation train line nearNagoya. However, linear motors have been used independently of magnetic levitation, as in theBombardier Innovia Metro systems worldwide and a number of modern Japanese subways, includingTokyo'sToei Ōedo Line.
Similar technology is also used in someroller coasters with modifications but, at present, is still impractical on street runningtrams, 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 deepmines, and the use of linear motors is growing inmotion control applications. They are also often used on sliding doors, such as those oflow floor trams such as theAlstom Citadis and theSocimi Eurotram. Dual axis linear motors also exist. These specialized devices have been used to provide directX-Y motion for precision laser cutting of cloth and sheet metal, automateddrafting, and cable forming. Most linear motors in use are LIM (linear induction motor), or LSM (linear synchronous motor). Linear DC motors are not used due to their higher cost and linear SRM suffers from poor thrust. So for long runs in traction LIM is mostly preferred and for short runs LSM is mostly preferred.
High-acceleration linear motors have been suggested for a number of uses.They have been considered for use asweapons, since currentarmour-piercing ammunition tends to consist of small rounds with very highkinetic energy, for which just such motors are suitable. Many amusement parklaunched roller coasters now use linear induction motors to propel the train at a high speed, as an alternative to using alift hill.
The United States Navy is also using linear induction motors in theElectromagnetic Aircraft Launch System that will replace traditionalsteam catapults on future aircraft carriers. They have also been suggested for use inspacecraft propulsion. In this context they are usually calledmass drivers. The simplest way to use mass drivers for spacecraft propulsion would be to build a large mass driver that can accelerate cargo up toescape velocity, thoughRLV launch assist likeStarTram tolow Earth orbit has also been investigated.
High-acceleration linear motors are difficult to design for a number of reasons. They require large amounts ofenergy in very short periods of time. One rocket launcher design[7] calls for 300 GJ for each launch in the space of less than a second. Normalelectrical 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 aflywheel 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 ofsuperconductors. However, with careful design, this need not be a major problem.[8]
Two different basic designs have been invented for high-acceleration linear motors:railguns andcoilguns.
Linear motors are commonly used for actuating high performance industrial automation equipment. Their advantage, unlike any other commonly used actuator, such as aball screw,timing belt, orrack and pinion, is that they provide any combination of high precision, high velocity, high force and long travel.
Linear motors are widely used. One of the major uses of linear motors is for propelling the shuttle inlooms.
A linear motor has been used for sliding doors and various similar actuators. They have been used for baggage handling and even large-scale bulk materials transport.
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.
Linear motors may also be used as an alternative to conventional chain-run lift hills for roller coasters. The coasterMaverick at Cedar Point uses one such linear motor in place of a chain lift.
A linear motor has been used to accelerate cars forcrash tests.[9]
The combination of high precision, high velocity, high force, and long travel makes brushless linear motors attractive for driving industrial automations equipment. They serve industries and applications such as semiconductorsteppers, electronicssurface-mount technology, automotivecartesian coordinate robots, aerospacechemical milling, opticselectron microscope, healthcarelaboratory automation, food and beveragepick and place.
Synchronous linear motoractuators, used in machine tools, provide high force, high velocity, high precision and high dynamic stiffness, resulting in high smoothness of motion and low settling time. They may reach velocities of 2 m/s and micron-level accuracies, with short cycle times and a smooth surface finish.[10]
All of the following applications are inrapid transit and have the active part of the motor in the cars.[11][12]
Originally developed in the late 1970s byUTDC in Canada as theIntermediate Capacity Transit System (ICTS). A test track was constructed inMillhaven, Ontario, for extensive testing of prototype cars, after which three lines were constructed:
ICTS was sold toBombardier Transportation in 1991 and later known asAdvanced Rapid Transit (ART) before adopting its current branding in 2011. Since then, several more installations have been made:
All Innovia Metro systems usethird rail electrification.
One of the biggest challenges faced by Japanese railway engineers in the 1970s to the 1980s was the ever increasing construction costs of subways. In response, the Japan Subway Association began studying on the feasibility of the "mini-metro" for meeting urban traffic demand in 1979. In 1981, the Japan Railway Engineering Association studied on the use oflinear induction motors for such small-profile subways and by 1984 was investigating on the practical applications of linear motors for urban rail with the JapaneseMinistry of Land, Infrastructure, Transport and Tourism. In 1988, a successful demonstration was made with the Limtrain atSaitama and influenced the eventual adoption of the linear motor for theNagahori Tsurumi-ryokuchi Line inOsaka and Toei Line 12 (present-dayToei Oedo Line) inTokyo.[14]
To date, the following subway lines in Japan use linear motors and useoverhead lines for power collection:
In addition,Kawasaki Heavy Industries has also exported the Linear Metro to theGuangzhou Metro in China;[15] all of the Linear Metro lines in Guangzhou use third rail electrification:
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There are many roller coasters throughout the world that use LIMs to accelerate the ride vehicles. The first beingFlight of Fear atKings Island andKings Dominion, both opening in 1996.Battlestar Galactica: Human VS Cylon &Revenge of the Mummy atUniversal Studios Singapore opened in 2010. They both use LIMs to accelerate from certain point in the rides.
Revenge of the Mummy (located at bothUniversal Studios Hollywood andUniversal Studios Florida),The Incredible Hulk Coaster, andVelociCoaster atUniversal Islands of Adventure use linear motors. AtWalt Disney World,Rock 'n' Roller Coaster Starring Aerosmith atDisney's Hollywood Studios andGuardians of the Galaxy: Cosmic Rewind atEpcot both use LSM to launch their ride vehicles into their indoor ride enclosures.
In 2023 ahydraulic launch roller coaster,Top Thrill Dragster atCedar Point in Ohio, USA, was renovated and the hydraulic launch replaced with a weaker multi-launch system using LSM, that creates lessg-force.
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