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.
