Linear induction motor

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,[3] 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 built a working model in 1935.[4] In the late 1940s, professor Eric Laithwaite of Imperial College in London developed the first full-size working model.

FEMM simulation of a Cross-section of Magnetic River, coloured by electric current density

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. 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.[5]

A linear induction motor's primary typically consists of a flat magnetic core (generally laminated) with transverse slots that are often straight cut[6] 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.[7]

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.

The magnetic field of a linear motor sweeping to the left, past an aluminium block. Coloured by induced electric current.

In this electric motor design, 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, creating motion as the magnetic field sweeps through the metal.

${\displaystyle n_{s}=2f_{s}/p}$[8]

where f_s is supply frequency in Hz, p is the number of poles, and n_s is the synchronous speed of the magnetic field in revolutions per second.

The travelling field pattern has a velocity of:

${\displaystyle v_{s}=2tf_{s}}$[8]

where v_s is velocity of the linear travelling field in m/s, and t is the pole pitch.

For a slip of s, the speed of the secondary in a linear motor is given by

${\displaystyle v_{r}=(1-s)v_{s}}$[8]

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.[1] Similarly, the efficiency during generator operation (electric braking/recuperating) with a linear induction motor was reported as relatively low due to end effects.[11] 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 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 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.

LIMTV Linear Induction Motor Test Vehicle in 1973

Because of these properties, linear motors are often used in maglev propulsion, as in the Japanese Linimo magnetic levitation train line near Nagoya.

The Birmingham Airport Maglev shuttle

The world's first commercial automated maglev system was a low-speed maglev shuttle that ran from the airport terminal of Birmingham Airport to the nearby Birmingham International railway station between 1984–1995.[12] The length of the track was 600 metres (2,000 ft), and trains "flew" at an altitude of 15 millimetres (0.59 in), levitated by electromagnets, and propelled with linear induction motors.[13] 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 RTV31 hover train vehicle.[14]

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 130 km (81 mi) of route using LIM propelled subway trains along Line 4, Line 5 and Line 6. They are also used by the Tomorrowland Transit Authority PeopleMover at Walt Disney World Resort in Bay Lake, Florida, and the 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.

AirTrain JFK trains propel themselves using an aluminium induction strip placed between the rails.

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 Alstom 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. 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.[15]

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.

A diagram of EMALS' induction motor

Linear induction motors have also been used for launching aircraft, the Westinghouse Electropult[7] 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.[16]

• Electromagnetic induction
• Goodness factor
• Maglev
• Tracked Hovercraft
• Linear Synchronous Motor