Linear induction motor
A linear induction motor (LIM) is an AC asynchronous linear motor that works by the same general principles as other induction motors 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.
Linear motors frequently run on a 3 phase power supply.
Their uses include magnetic levitation, linear propulsion, and linear actuators. They have also been used for pumping liquid metals. 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.
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, 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. 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 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.
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. Most linear motors in use 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.
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.
A linear electric motor's primary typically consists of a flat magnetic core (generally laminated) with transverse slots which are often straight cut with coils laid into the slots.
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.
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.
The primaries of transverse flux LIMs have a series of twin poles lying transversely side-by-side, with opposite winding directions.
Principle of operation 
Moving magnetic field 
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 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.
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.
End effect 
Unlike a circular induction motor, a linear induction motor shows end effects.
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.
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.
See also 
- "Charles Wheatstone - College History - King's College London". Kcl.ac.uk. Retrieved 2010-03-01.[dead link]
- Patent number 3585423, 1971 Laithwaite et al
- linear Electric Machines- A Personal View ERIC R. LAITHWAITE, FELLOW, IEEE, PROCEEDINGS OF THE IEEE, VOL. 63, NO. 2, FEBRUARY 1975
- Force Analysis of Linear Induction Motor for Magnetic Levitation System 14th International Power Electronics and Motion Control Conference, EPE-PEMC 2010