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| [[File:Linear induction motor.gif|thumb|right|A transverse flux linear induction motor (here the primary is at top of picture) and has two sets of opposite poles side by side. (Picture from US Patent 3824414 by Eric Laithwaite)]]
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| A '''linear induction motor''' (LIM) is an AC asynchronous [[linear motor]] that works by the same general principles as other [[induction motor]]s but is very typically designed to directly produce motion in a straight line. Characteristically, linear induction motors have a finite length primary, which generates end-effects, whereas with a conventional induction motor the primary is arranged in an endless loop.
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| Despite their name, not all linear induction motors produce linear motion, some linear induction motors are employed for generating rotations of large diameters where the use of a continuous primary would be very expensive.
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| As with rotary motors, linear motors frequently run on a 3 phase power supply and can support very high speeds. However, there are end-effects which reduce the force, and it's often not possible to fit a gearbox to trade off force and speed. Linear induction motors are thus frequently less energy efficient than normal rotary motors for any given required force output.
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| LIMs are often used where contactless force is required, where low maintenance is desirable, or where the duty cycle is low. Their practical uses include [[magnetic levitation]], linear propulsion, and linear actuators. They have also been used for pumping liquid metals.<ref>http://books.google.co.uk/books?id=fgsAAAAAMBAJ&lpg=PA52&ots=NfAng_7A27&dq=einstein%20Linear%20induction%20motor&pg=PA52#v=onepage&q=einstein%20Linear%20induction%20motor&f=false</ref>
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| ==History==
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| 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 London|King's College]] in London,<ref>{{cite web|url=http://www.kcl.ac.uk/college/history/people/wheatstone.html |title=Charles Wheatstone - College History - King's College London |publisher=Kcl.ac.uk |date= |accessdate=2010-03-01}} {{Dead link|date=April 2012|bot=H3llBot}}</ref> 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.<ref>http://cem.colorado.edu/archives/fl1997/thor.html</ref> In the late 1940s, professor [[Eric Laithwaite]] of [[Imperial College]] in [[London]] developed the first full-size working model.
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| In a single sided version the magnetic field can create repulsion forces that push the conductor away from the stator, levitating it, and carrying it along in the direction of the moving magnetic field. Laithewaite called the later versions of it [[magnetic river]]. These versions of the linear induction motor use a principle called ''transverse flux'' where two opposite poles are placed side by side. This permits very long poles to be used, which permits high speed and efficiency.<ref>Patent number 3585423, 1971 Laithwaite et al</ref>
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| ==Construction==
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| A linear electric motor's primary typically consists of a flat magnetic core (generally laminated) with transverse slots which are often straight cut<ref name=inexpensive>www.manchesteruniversitypress.co.uk/uploads/docs/380117.pdf</ref> with coils laid into the slots, with each phase giving an alternating polarity and so that the different phases physically overlap.
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| The secondary is frequently a sheet of aluminum, often with an iron backing plate. Some LIMs are double sided, with one primary either side of the secondary, and in this case no iron backing is needed.
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| Two sorts of linear motor exist, ''short primary'', where the coils are truncated shorter than the secondary, and a ''short secondary'' where the conductive plate is smaller. Short secondary LIMs are often wound as parallel connections between coils of the same phase, whereas short primaries are usually wound in series.<ref name=personalView/>
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| The primaries of transverse flux LIMs have a series of twin poles lying transversely side-by-side, with opposite winding directions. These are typically made either with a suitably cut laminated backing plate or a series of transverse U-cores.
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| ==Principles==
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| [[File:Linear motor field.gif|thumb|The magnetic field of a linear motor sweeping to the left, past an aluminium block. Coloured by induced electric current.]]
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| In this design of electric motor, the force is produced by a linearly moving [[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 current]]s [[electromagnetic induction|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.
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| :<math>n_s=2 f_s/p</math><ref name=sunilsaharan>http://electrical.sunilsaharan.in/2010/07/linear-induction-motor-working.html</ref>
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| where, <math>f_s</math> is supply frequency in Hz, p is the number of poles, and <math>n_s</math> is the synchronous speed of the magnetic field in revolutions per second.
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| The travelling field pattern has a velocity of:
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| :<math>v_s=2 t f_s</math><ref name=sunilsaharan/>
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| <math>v_s</math> is velocity of the linear travelling field in m/s, t is the pole pitch.
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| For a slip of s, the speed of the secondary in a linear motor is given by
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| :<math>v_r=(1-s)v_s</math><ref name=sunilsaharan/> | |
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| {{clear}}
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| ==Forces==
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| ===Thrust===
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| [[File:Couple glissement MAs.svg|thumb|right|Thrust generated as a function of slip]]
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| The drive generated by linear induction motors is somewhat similar to conventional induction motors; the drive forces show a roughly similar characteristic shape relative to slip, albeit modulated by end effects.<ref name=force>Force Analysis of Linear Induction Motor for Magnetic Levitation System 14th International Power Electronics and Motion Control Conference, EPE-PEMC 2010</ref>
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| ===End effect===
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| Unlike a circular induction motor, a linear induction motor shows 'end effects'. These end effects include losses in performance and efficiency that are believed to be caused by magnetic energy being carried away and lost at the end of the primary by the relative movement of the primary and secondary.
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| With a short secondary, the behaviour is almost identical to a rotary machine, provided it is at least two poles long, but with a short primary reduction in thrust occurs at low slip (below about 0.3) until it is eight poles or longer.<ref name=personalView>linear Electric Machines- A Personal View ERIC R. LAITHWAITE, FELLOW, IEEE, PROCEEDINGS OF THE IEEE, VOL. 63, NO. 2, FEBRUARY 1975</ref>
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| However, because of end effect, linear motors cannot 'run light'- normal induction motors are able to run the motor with a near synchronous field under low load conditions. Due to end effect this creates much more significant losses with linear motors.<ref name=personalView/>
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| ===Levitation===
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| [[File:Linear motor torque curve.png|thumb|Levitation and thrust force curves of a linear motor]]
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| In addition, unlike a rotary motor, an [[electrodynamic levitation]] force is shown, this is zero at zero slip, and gives a roughly constant amount of force/gap as slip increases in either direction. This occurs in single sided motors, and levitation will not usually occur when an iron backing plate is used on the secondary, since this causes an attraction that overwhelms the lifting force.<ref name=force/>
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| {{clear}}
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| ==Performance==
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| Linear induction motors are often less efficient than conventional rotary induction motors; the end effects and the relatively large air gap that is often present will typically reduce the forces produced for the same electrical power. However, linear induction motors can avoid the need for gearboxes and similar drivetrains, and these have their own losses; and in any case power use is not always the most important consideration. For example, in many cases linear induction motors have far fewer moving parts, and have very low maintenance.
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| ==Uses==
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| [[File:LIMTV testing at DoT Pueblo test facility.tiff|thumb|right|LIMTV Linear Induction Motor Test Vehicle in 1973]]
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| Because of these properties, linear motors are often used in [[magnetic levitation|maglev]] propulsion, as in the Japanese [[Linimo]] [[magnetic levitation train]] line near [[Nagoya]].
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| [[File:Birmingham International Maglev.jpg|thumb|The Birmingham International Maglev shuttle]]
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| The world's first commercial automated maglev system was a [[AirRail_Link#Maglev|low-speed maglev shuttle]] that ran from the airport terminal of [[Birmingham International Airport (United Kingdom)|Birmingham International Airport]] to the nearby [[Birmingham International railway station]] between 1984–1995.<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/sci/tech/488394.stm |title=The magnetic attraction of trains |publisher=BBC News |date=9 November 1999}}</ref> The length of the track was {{convert|600|m|ft|sp=uk}}, and trains "flew" at an altitude of {{convert|15|mm|in|sp=uk}}, levitated by electromagnets, and propelled with linear induction motors.<ref>Maglev, A film for The People Mover Group</ref> It was in operation for nearly eleven years, but [[obsolescence]] problems with the electronic systems made it unreliable in its later years. One of the original cars is now on display at [[Railworld]] in Peterborough, together with the [[Research Test Vehicle 31|RTV31]] hover train vehicle.
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| However, linear motors have been used independently of magnetic levitation, as in [[Bombardier Transportation|Bombardier]]'s [[Bombardier Advanced Rapid Transit|Advanced Rapid Transit]] systems worldwide and a number of modern Japanese subways, including [[Tokyo]]'s [[Toei Oedo Line]].
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| Linear induction motor technology is also used in some [[launched roller coaster|roller coaster]]s. At present it is still impractical on street running [[tram]]s, although this, in theory, could be done by burying it in a slotted conduit.
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| [[Image:JFK AirTrain.agr.jpg|thumb|left|250px|[[Bombardier Advanced Rapid Transit|ART]] trains propel themselves using an aluminium induction strip placed between the rails.]]
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| Outside of public transportation, vertical linear motors have been proposed as lifting mechanisms in deep [[mining|mine]]s, 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]] [[tram]]s 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 [[Technical drawing|drafting]], and cable forming. Most linear motors in use are LIM (linear induction motors) or LSM (linear synchronous motors). 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.
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| [[File:EMALS.JPG|thumb|A diagram of EMALS' induction motor]]
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| Linear induction motors have also been used for launching aircraft, the Westinghouse Electropult<ref name=personalView/> system in 1945 was an early example and the [[Electromagnetic Aircraft Launch System]] (EMALS) was due to be delivered in 2010.
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| Linear induction motors are also used in looms, magnetic levitation permit allows bobbins to float between the fibers without direct contact.
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| ==See also==
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| *[[Goodness factor]]
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| * [[Maglev]]
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| * [[Tracked Hovercraft]]
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| {{clear}}
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| ==References==
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| {{reflist}}
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| [[Category:Electric motors]]
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| [[Category:Electrical engineering]]
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| [[Category:Electromagnetic components]]
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| [[Category:Magnetic propulsion devices]]
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| [[Category:Magnetic levitation]]
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| [[Category:AC motors]]
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| [[he:מנוע השראה ישר]]
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| {{electric motor}}
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