A linear motor or linear induction motor is essentially a multi-phase alternating current (AC) electric motor that has had its stator "unrolled" so that instead of producing a torque (rotation) it produces a linear force along its length. The most common mode of operation is as a Lorentz-type actuator, in which the applied force is linearly proportional to the current and the magnetic field (F = qv × B).
Many designs have been put forward for linear motors, falling into two major categories, low-acceleration and high-acceleration linear motors. Low-acceleration linear motors are suitable for maglev trains and other ground-based transportation applications. High-acceleration linear motors are normally quite short, and are designed to accelerate an object up to a very high speed and then release the object, like roller coasters. They are usually used for studies of hypervelocity collisions, as weapons, or as mass drivers for spacecraft propulsion. The high-acceleration motors are usually of the linear induction design (LIM) with an active three-phase winding on one side of the air-gap and a passive conductor plate on the other side. The low-acceleration, high speed and high power motors are usually of the linear synchronous design (LSM), with an active winding on one side of the air-gap and an array of alternate-pole magnets on the other side. These magnets can be permanent magnets or energized magnets. The Transrapid Shanghai motor is an LSM.
When a linear motor is used to accelerate beams of ions or subatomic particles, it is called a particle accelerator. The design is usually rather different as the particles move close to the speed of light and are generally electrically charged.
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 732312 (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 his design, and in most low-acceleration designs, the force is produced by a moving linear 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. The two opposing fields will repel each other, thus forcing the conductor away from the stator and carrying it along in the direction of the moving magnetic field.
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. Mostly used linear motors 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.
From concept to industrial use
In the 1980s British engineer Hugh-Peter Kelly designed the first tubular linear motor by enclosing the permanent magnets in a sealed stainless steel cylinder. It was brought to market by linear motor manufacturer Linear Drives (now Copley Motion Systems) . The patented permanent magnet arrangement induces a sinusoidal response in the coils that are enclosed in a square profile body. This allowed machine builders to use the new linear motors with standard sinusoidal servo drives commonly used in motion control.
Tubular Linear Motors
Tubular linear motors are more rugged than early flat bed and u-channel linear motors allowing them to be used in dirty industrial environments such as food packaging and machine tools. The tubular construction protects the permanent magnets from the external environment and automatically balances attractive forces so that the motor is easier to integrate into machines. These motors operate at 5- 9 m/s with high acceleration for dynamic motion control.
A new type of linear motor, called the ServoTube (see Eureka March 2005) has allowed linear motors to be used in industrial environments by integrating the position sensing electronics into the motor body (called a forcer).
High-acceleration linear motors are difficult to design for a number of reasons. They require large amounts of energy in very short periods of time. One rocket launcher design (see ) calls for 300 GJ for each launch in the space of less than a second. Normal electrical generators are not designed for this kind of load, but short-term electrical energy storage methods can be used. Capacitors are bulky and expensive but can supply large amounts of energy quickly. Homopolar generators can be used to convert the kinetic energy of a flywheel into electric energy very rapidly. High-acceleration linear motors also require very strong magnetic fields; in fact, the magnetic fields are often too strong to permit the use of superconductors. However, with careful design this need not be a major problem.
All applications are in rapid transit.