A linear motor is an electric motor that has had
its stator and rotor "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. 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 rather short, and are designed to
accelerate an object to a very high speed, for example see the
coilgun.
High-acceleration motors are usually used for studies of
hypervelocity collisions, as weapons, or as mass drivers for spacecraft
propulsion. They are usually of the AC linear induction motor (LIM) design with
an active three-phase winding on one side of the air-gap and a passive conductor
plate on the other side. However, the direct current homopolar linear motor
railgun is another high acceleration linear motor design. The low-acceleration,
high speed and high power motors are usually of the linear synchronous motor
(LSM) design, 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.
