Linear motor

A linear motor is an electric motor designed to produce a straight-line force. While most conventional motors are built to create rotational force, or torque, a linear motor is essentially an electric motor that has been "unrolled."

This design allows the motor to move along its length rather than spinning in a circle. These devices are essential for high-precision tasks and heavy transportation. In 2024, the market for these motors reached a value of USD 1.8 billion. They are used in millions of applications, ranging from tiny industrial robots to massive high-speed trains.

The fundamental mechanism often functions as a Lorentz-type actuator. In this process, the amount of force produced is linearly proportional to both the electric current and the magnetic field.

There are two primary ways these forces are generated. In a linear induction motor (LIM), a moving magnetic field acts on a conductor, such as an aluminum plate. This interaction induces eddy currents within the metal, which creates an opposing magnetic field according to Lenz's law. Because these two magnetic fields repel each other, the object begins to move as the field sweeps through the metal.

Another common mechanism is the linear synchronous motor (LSM). In an LSM design, an active winding is placed on one side of an air-gap, while an array of alternate-pole magnets sits on the other. These magnets can be permanent magnets or electromagnets. The rate of movement of the magnetic field is electronically controlled to track the motion of the rotor. This design is highly efficient for specific tasks. For example, high-precision industrial automation often uses a magnet stator paired with a moving coil. To ensure accuracy, a Hall effect sensor is often attached to the rotor to track the magnetic flux of the stator.

Linear motors are generally categorized by their acceleration capabilities. Low-acceleration motors are ideal for ground-based transportation, such as maglev trains. The Shanghai maglev train utilizes an LSM design for its propulsion.

These motors are often preferred for long-distance traction. Conversely, high-acceleration motors are designed to move objects to extremely high speeds over short distances. These include coilguns and the direct current homopolar linear motor railgun.

High-acceleration models are used in studies of hypervelocity collisions, as weapons, or as mass drivers for spacecraft propulsion.

The history of this technology spans nearly two centuries. The origins can be traced back to the 1840s with the work of Charles Wheatstone at King's College London. However, Wheatstone's early model was too inefficient for practical use. In 1905, Alfred Zehden of Frankfurt-am-Main described a feasible linear induction motor for lifts and trains. Later, the German engineer Hermann Kemper built a working model in 1935. The first full-size working model was developed in the late 1940s by Dr. Eric Laithwaite at Manchester University. His work eventually led to technologies used in the 1984 Air-Rail Link shuttle.

In industrial settings, linear motors offer significant advantages over traditional rotary or screw-driven systems. They provide direct-drive operation, which eliminates backlash and reduces the need for frequent maintenance.

They are used in CNC machining, semiconductor steppers, and high-speed robotics. These motors can achieve velocities of 2 m/s or more with micron-level positioning accuracy. Even in consumer infrastructure, they power sliding doors on low-floor trams and baggage handling systems. In massive scientific instruments, such as the European Extremely Large Telescope (ELT), hybrid actuators combine linear motors with piezoelectric elements. This allows for nanometer-level precision to keep mirror segments perfectly aligned.

Beyond industry and transport, these motors appear in unexpected places. Some modern roller coasters, such as Maverick at Cedar Point, use linear induction motors instead of traditional chain lifts to launch trains.

In the realm of space exploration, mass drivers are suggested as a way to accelerate cargo to escape velocity. However, high-acceleration motors face difficult design challenges. They require massive amounts of energy in very short periods. For example, one rocket launcher design requires 300 GJ of energy in less than a second. To meet these demands, engineers must use specialized energy storage like capacitors or homopolar generators.