A typical 3 phase linear induction motor. The "primary" core (grey) has grooves, and the windings are laid into them on top of each other. An aluminium plate above (not shown) serves as "secondary" and will move relative to the primary if a 3 phase AC is applied.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)
A linear induction motor (LIM) is an alternating current (AC), asynchronous linear motor that works by the same general principles as other induction motors 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.[1]
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, linear propulsion, and linear actuators. They have also been used for pumping liquid metals.[2]
History
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]
Construction
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.
Principles
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 currentsinduced 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.
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.[9]
Equations exist for calculating the thrust of a motor.[10]
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.[7]
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.[7]
Levitation
Levitation and thrust force curves of a linear motor
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.[9]
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.[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.
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 (2,000ft), and trains "flew" at an altitude of 15 millimetres (0.59in), 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]
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 floortrams 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.
A linear motor is an electric motor that has had its stator and rotor "unrolled", thus, instead of producing a torque (rotation), it produces a linear force 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.
An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. An electric generator is mechanically identical to an electric motor, but operates in reverse, converting mechanical energy into electrical energy.
An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor therefore needs no electrical connections to the rotor. An induction motor's rotor can be either wound type or squirrel-cage type.
Eric Roberts Laithwaite was a British electrical engineer, known as the "Father of Maglev" for his development of the linear induction motor and maglev rail system after Hermann Kemper.
A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integer number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. A synchronous motor is termed doubly fed if it uses independently-excited multiphase AC electromagnets for both rotor and stator.
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 the two objects apart.
Electromagnetic propulsion (EMP) is the principle of accelerating an object by the utilization of a flowing electrical current and magnetic fields. The electrical current is used to either create an opposing magnetic field, or to charge a field, which can then be repelled. When a current flows through a conductor in a magnetic field, an electromagnetic force known as a Lorentz force, pushes the conductor in a direction perpendicular to the conductor and the magnetic field. This repulsing force is what causes propulsion in a system designed to take advantage of the phenomenon. The term electromagnetic propulsion (EMP) can be described by its individual components: electromagnetic – using electricity to create a magnetic field, and propulsion – the process of propelling something. When a fluid is employed as the moving conductor, the propulsion may be termed magnetohydrodynamic drive. One key difference between EMP and propulsion achieved by electric motors is that the electrical energy used for EMP is not used to produce rotational energy for motion; though both use magnetic fields and a flowing electrical current.
Maglev is a system of rail transport whose rolling stock is levitated by electromagnets rather than rolled on wheels, eliminating rolling resistance.
A squirrel-cage rotor is the rotating part of the common squirrel-cage induction motor. It consists of a cylinder of steel laminations, with aluminium or copper conductors embedded in its surface. In operation, the non-rotating stator winding is connected to an alternating current power source; the alternating current in the stator produces a rotating magnetic field. The rotor winding has current induced in it by the stator field, like a transformer except that the current in the rotor is varying at the stator field rotation rate minus the physical rotation rate. The interaction of the magnetic fields in the stator and the currents in the rotor produce a torque on the rotor.
Electromagnetic suspension (EMS) is the magnetic levitation of an object achieved by constantly altering the strength of a magnetic field produced by electromagnets using a feedback loop. In most cases the levitation effect is mostly due to permanent magnets as they have no power dissipation, with electromagnets only used to stabilise the effect.
The SCMaglev is a magnetic levitation (maglev) railway system developed by Central Japan Railway Company and the Railway Technical Research Institute.
An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. The rotor magnetic field may be produced by permanent magnets, reluctance saliency, or DC or AC electrical windings.
Magnetic propulsion may refer to:
The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor's axis.
The switched reluctance motor (SRM) is a type of reluctance motor. Unlike brushed DC motors, power is delivered to windings in the stator (case) rather than the rotor. This simplifies mechanical design because power does not have to be delivered to the moving rotor, which eliminates the need for a commutator. However it complicates the electrical design, because a switching system must deliver power to the different windings and limit torque ripple. Sources disagree on whether it is a type of stepper motor.
Tracked Hovercraft was an experimental high-speed train developed in the United Kingdom during the 1960s. It combined two British inventions, the hovercraft and linear induction motor, in an effort to produce a train system that would provide 250 mph (400 km/h) inter-city service with lowered capital costs compared to other high-speed solutions. Substantially similar to the French Aérotrain and other hovertrain systems of the 1960s, Tracked Hovercraft suffered a similar fate to these projects when it was cancelled as a part of wide budget cuts in 1973.
Magnetic river is an 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 transverse flux and the geometry, this gives it lift, stability and propulsion as well as being relatively efficient. The name refers to the action that provides stability along the longitudinal axis, which acts similar to the flow of water in a river.
Magnetic levitation (maglev) or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields. Magnetic force is used to counteract the effects of the gravitational force and any other forces.
Electromagnetically induced acoustic noise, electromagnetically excited acoustic noise, or more commonly known as coil whine, is audible sound directly produced by materials vibrating under the excitation of electromagnetic forces. Some examples of this noise include the mains hum, hum of transformers, the whine of some rotating electric machines, or the buzz of fluorescent lamps. The hissing of high voltage transmission lines is due to corona discharge, not magnetism.
Switched reluctance linear motors (SRLMs) are a type of electric machines called linear motors which work based on the principle of a varying magnetic reluctance for force generation. The system can be used in reversed mode and then is called Switched Reluctance Linear Generator. The SRLMs consist of two parts: the active part or primary part and the passive or secondary. The active part contains the windings and defines two main types of LSRMs: transverse and longitudinal. It is longitudinal when the plane that contains the flux lines is parallel to the line of movement and transverse when it is perpendicular. Other classifications are considering the windings totally concentrated in one coil per phase or partially concentrated in two poles per phase or four poles per phase (double-sided). Switched Reluctance motors have been used extensively in clocks and phonograph turntables before, but nowadays, with the rising emphasis on energy efficiency, SR motors are taking more prominent roles in appliances, industrial uses, and commercial and vehicular applications and they are getting traction in the linear applications due to their simplicity, robustness, economic rationality, and high fault tolerance ability as compared with the Linear Synchronous and Linear Induction motors. The SRLM has been researched widely and there are applications of SRLMs and generators for example in wave energy conversion or hyperloop ultra high speed transportation system. One of the main advantages of the SRLM is that it does not require the use of permanent magnets, which are considered a scarce material, so it enables it to be deployed over long distances.
1 2 Force Analysis of Linear Induction Motor for Magnetic Levitation System 14th International Power Electronics and Motion Control Conference, EPE-PEMC 2010
↑ Journal of Modern Transportation June 2012, Volume 20, Issue 2, pp 76–81 A novel method to calculate the thrust of linear induction motor based on instantaneous current value
↑ A Maglev unit for Railworld Rail issue 425 26 December 2001 page 65
↑ Mechatronic design of a z-φ induction actuator, P. de Wit, J. van Dijk, T. Blomer, and P. Rutgers, proc. of IEE EMD '97 Conference. Cambridge 1997. pp. 279–283, 1-3 Sept. 1997
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