by Chris Woodford. Last updated: July 4, 2013.
Have you seen those extraordinary "maglev" (magnetic levitation) trains that float on air instead of rolling on wheels? They're entirely electric but, instead of using ordinary electric motors that spin around, they use a kind of "unwrapped" motor called a linear motor that causes them to move at high speed in a straight line. What are linear motors and how do they work? Let's take a closer look!
Photo: Linear motors have many peaceful uses, but they can also be used to accelerate projectiles in electromagnetic railguns like this one. The muzzle velocity here is a blistering 2520 meters per second (~9100 km/h or 5640 mph)! Picture courtesy of US Navy (follow this link for a hi-res version of the photo).
What are linear motors?
Linear motors are electric induction motors that produce motion in a straight line rather than rotational motion. In a traditional electric motor, the rotor (rotating part) spins inside the stator (static part); in a linear motor, the stator is unwrapped and laid out flat and the "rotor" moves past it in a straight line. Linear motors often use superconducting magnets, which are cooled to low temperatures to reduce power consumption.
Artwork: Top: Normal motor: The rotor spins inside the stator and the whole motor is fixed in place. Bottom: A linear motor is like a normal electric motor that has been unwrapped and laid in a straight line. Now the rotor moves past the stator as it turns.
The basic principle behind the linear motor was discovered in 1895, but practical devices were not developed until 1947. During the 1950s, British electrical engineer Eric Laithwaite started to consider whether linear motors could be used in electric weaving machines. Laithwaite's research at Imperial College, London attracted international recognition in the 1960s following a speech to the Royal Institution entitled "Electrical Machines of the Future."
Photo: NASA tests a linear motor on a prototype Maglev railroad, 1999. Tracks like this could be used to launch vehicles into space in future. According to NASA: "A full-scale, operational track would be about 1.5-miles long and capable of accelerating a vehicle to 600 mph in 9.5 seconds." Picture courtesy of NASA Marshall Space Flight Center (NASA-MSFC)
Linear motors are now used in all sorts of machines that require linear (as opposed to rotational) motion, including overhead traveling cranes and beltless conveyors for moving sheet metal. They are probably best known as the source of motive power in the latest generation of high-speed "maglev" (magnetic levitation) trains, which promise safe travel at very high speeds but are expensive and incompatible with existing railroads. Most research on maglev trains has been carried out in Japan and Germany.
How linear motors work
In a traditional DC electric motor, a central core of tightly wrapped magnetic material (known as the rotor) spins at high speed between the fixed poles of a magnet (known as the stator) when an electric current is applied. In an AC induction motor, electromagnets positioned around the edge of the motor are used to generate a rotating magnetic field in the central space between them. This "induces" (produces) electric currents in a rotor, causing it to spin. In an electric car, DC or AC motors like these are used to drive gears and wheels and convert rotational motion into motion in a straight line.
Photo: An ordinary electric motor is all about rotation: the rotor (the coils in the center) turns inside the stator (the outer magnetic case).
A linear motor is effectively an AC induction motor that has been cut open and unwrapped. The "stator" is laid out in the form of a track of flat coils made from aluminum or copper and is known as the "primary" of a linear motor. The "rotor" takes the form of a moving platform known as the "secondary." When the current is switched on, the secondary glides past the primary supported and propelled by a magnetic field.
Linear motors have a number of advantages over ordinary motors. Most obviously, there are no moving parts to go wrong. As the platform rides above the track on a cushion of air, there is no loss of energy to friction or vibration (but because the air-gap is greater in a linear motor, more power is required and the efficiency is lower). The lack of an intermediate gearbox to convert rotational motion into straight-line motion saves energy. Finally, as both acceleration and braking are achieved through electromagnetism, linear motors are much quieter than ordinary motors.
The main problem with linear motors has been the cost and difficulty of developing suitable electromagnets. Enormously powerful electromagnets are required to levitate (lift) and move something as big as a train, and these typically consume substantial amounts of electric power. Linear motors often now use superconducting magnets to solve this problem.
If electromagnets are cooled to low temperatures using liquid helium or nitrogen their electrical resistance disappears almost entirely, which reduces power consumption considerably. This helpful effect, known as superconductivity, has been the subject of intense research since the mid 1980s and makes large-scale linear motors that much more viable.