Linear Induction
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A linear induction motor (LIM) is an
alternating current Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in w ...
(AC), asynchronous
linear motor A linear motor is an electric motor that has had its stator and rotor (electric), rotor "unrolled", thus, instead of producing a torque (rotation), it produces a linear force along its length. However, linear motors are not necessarily straight. ...
that works by the same general principles as other
induction motor An induction motor or asynchronous motor is an AC motor, AC electric motor in which the electric current in the rotor (electric), rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding ...
s but is typically designed to directly produce motion in a straight line. Characteristically, linear induction motors have a finite primary or secondary length, which generates end-effects, whereas a conventional induction motor is arranged in an endless loop. 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. As with rotary motors, linear motors frequently run on a three-phase power supply and can support very high speeds. However, there are end-effects that reduce the motor's force, and it is 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. LIMs, unlike their rotary counterparts, can give a levitation effect. They are therefore 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 Magnetic levitation (maglev) or magnetic suspension is a method by which an object is levitation (physics), suspended with no support other than magnetic fields. Lorentz force, Magnetic force is used to counteract the effects of the gravitation ...
, linear propulsion, and linear actuators. They have also been used for pumping liquid metals.


History

The history of linear electric motors can be traced back at least as far as the 1840s to the work of
Charles Wheatstone Sir Charles Wheatstone (; 6 February 1802 – 19 October 1875) was an English physicist and inventor best known for his contributions to the development of the Wheatstone bridge, originally invented by Samuel Hunter Christie, which is used to m ...
at King's College in London, but Wheatstone's model was too inefficient to be practical. A feasible linear induction motor is described in US patent 782312 (1905; inventor Alfred Zehden of Frankfurt-am-Main), and is for driving trains or lifts. German engineer
Hermann Kemper Hermann Kemper (5 April 1892 in Nortrup – 13 July 1977) was a German engineer and pioneer in magnetic levitation sometimes known as the father of maglev. Herman Kemper began his research on magnetic levitation in 1922. In 1933, Kemper const ...
built a working model in 1935. In the late 1940s, professor
Eric Laithwaite Eric Roberts Laithwaite (14 June 1921 – 27 November 1997) was an English electrical engineer, known as the " Father of Maglev" for his development of the linear induction motor and maglev rail system after Hermann Kemper. Biography Er ...
of
Imperial College Imperial College London, also known as Imperial, is a public research university in London, England. Its history began with Prince Albert, husband of Queen Victoria, who envisioned a cultural district in South Kensington that included museums ...
in
London London is the Capital city, capital and List of urban areas in the United Kingdom, largest city of both England and the United Kingdom, with a population of in . London metropolitan area, Its wider metropolitan area is the largest in Wester ...
developed the first full-size working model. 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 the direction of the moving magnetic field. Laithwaite called the later versions a
magnetic river Magnetic river is an electrodynamic suspension, electrodynamic magnetic levitation (maglev) system designed by Fredrick Eastham and Eric Laithwaite in 1974. It consists of a thin conductive plate on an AC linear induction motor. Due to the transvers ...
. 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, and thus permits high speed and efficiency.


Construction

A linear induction motor's primary typically consists of a flat magnetic core (generally laminated) with transverse slots that are often straight cut
/ref> with coils laid into the slots, with each phase giving an alternating polarity so that the different phases physically overlap. The secondary is frequently a sheet of aluminium, often with an iron backing plate. Some LIMs are double sided with one primary on each side of the secondary, and, in this case, no iron backing is needed. Two types of linear motor exist: a ''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. The primaries of transverse flux LIMs have a series of twin poles lying transversely side-by-side with opposite winding directions. These poles are typically made either with a suitably cut laminated backing plate or a series of transverse U-cores.


Principles

In this electric motor design, the force is produced by a linearly moving
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
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 Lenz's law states that the direction of the electric current Electromagnetic induction, induced in a Electrical conductor, conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes changes in t ...
. The two opposing fields will repel each other, creating motion as the magnetic field sweeps through the metal. :n_s=2 f_s/p where is supply frequency in Hz, is the number of poles, and is the synchronous speed of the magnetic field in revolutions per second. The travelling field pattern has a velocity of: :v_s=2 t f_s where is velocity of the linear travelling field in m/s, and is the pole pitch. For a slip of , the speed of the secondary in a linear motor is given by :v_r=(1-s)v_s


Forces


Thrust

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.Force Analysis of Linear Induction Motor for Magnetic Levitation System 14th International Power Electronics and Motion Control Conference, EPE-PEMC 2010 Equations exist for calculating the thrust of a motor.


End effect

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. 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 that occurs at low slip (below about 0.3) until it is eight poles or longer.linear Electric Machines- A Personal View ERIC R. LAITHWAITE, FELLOW, IEEE, PROCEEDINGS OF THE IEEE, VOL. 63, NO. 2, FEBRUARY 1975 However, because of end effects, linear motors cannot 'run light' -- normal induction motors are able to run the motor with a near synchronous field under low load conditions. In contrast, end effects create much more significant losses with linear motors.


Levitation

In addition, unlike a rotary motor, an
electrodynamic levitation Electrodynamic suspension (EDS) is a form of magnetic levitation in which there are conductors which are exposed to time-varying magnetic fields. This induces eddy currents in the conductors that creates a repulsive magnetic field which holds th ...
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.


Performance

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. Similarly, the efficiency during generator operation (electric braking/recuperating) with a linear induction motor was reported as relatively low due to end effects. The larger air gap also increases the inductance of the motor which can require larger and more expensive capacitors. However, linear induction motors can avoid the need for gearboxes and similar drivetrains, and these have their own losses; and working knowledge of the importance of the
goodness factor The goodness factor is a metric developed by Eric Laithwaite to determine the 'goodness' of an electric motor. Using it he was able to develop efficient magnetic levitation induction motors. :G = \frac = \frac where : is the goodness factor ...
can minimise the effects of the larger air gap. 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. Also, using linear induction motors instead of rotating motors with rotary-to-linear transmissions in
motion control Motion control is a sub-field of automation, encompassing the systems or sub-systems involved in moving parts of machines in a controlled manner. Motion control systems are extensively used in a variety of fields for automation purposes, includi ...
systems, enables higher bandwidth and accuracy of the control system, because rotary-to-linear transmissions introduce backlash, static friction and/or mechanical compliance in the control system.


Uses

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. 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 Airport to the nearby Birmingham International railway station between 1984–1995. The length of the track was , and trains "flew" at an altitude of , levitated by electromagnets, and propelled with linear induction motors. 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. However, linear motors have been used independently of magnetic levitation, such as Tokyo's Toei Ōedo Line. The Bombardier Innovia Metro is an example of an automated system that utilizes LIM propulsion. The longest rapid transit system employing such technology is the Guangzhou Metro, with approximately of route using LIM propelled subway trains along Line 4 (Guangzhou Metro), Line 4, Line 5 (Guangzhou Metro), Line 5 and Line 6 (Guangzhou Metro), Line 6. They are also used by the Tomorrowland Transit Authority PeopleMover at Walt Disney World Resort in Bay Lake, Florida, and the Subway (George Bush Intercontinental Airport), Subway people mover at George Bush Intercontinental Airport in Houston, Texas, which uses the same design. Linear induction motor technology is also used in some launched roller coasters. At present it 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 mining, mines, and the use of linear motors is growing in
motion control Motion control is a sub-field of automation, encompassing the systems or sub-systems involved in moving parts of machines in a controlled manner. Motion control systems are extensively used in a variety of fields for automation purposes, includi ...
applications. They are also often used on sliding doors, such as those of low floor trams such as the Alstom Citadis and the Socimi Eurotram, 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. Also, linear induction motors with a cylindrical secondary have been used to provide simultaneous linear and rotating motion for mounting electronic devices on printed circuit boards. 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 Switched reluctance motor, SRM suffers from poor thrust. So for long run in traction LIM is mostly preferred and for short run LSM is mostly preferred. Linear induction motors have also been used for launching aircraft, the Westinghouse Electropult system in 1945 was an early example and the Electromagnetic Aircraft Launch System (EMALS) was due to be delivered in 2010. Linear induction motors are also used in looms, magnetic levitation enable bobbins to float between the fibers without direct contact. The first ropeless elevator invented by ThyssenKrupp uses a linear induction drive power.


See also

* Electromagnetic induction * Goodness factor * Maglev * Tracked Hovercraft * Linear Motor, Linear Synchronous Motor


References

{{Maglev Linear induction motors, Magnetic propulsion devices Maglev English inventions