A linear motor, also known as a linear motor, is a type of motor that is created by cutting a traditional rotary motor along its axis. The primary winding of the rotary motor is unfolded and used as the stator of the linear motor. When the secondary winding is energized, it moves linearly along the primary winding under the action of electromagnetic force, thus becoming the mover of the linear motor.
The magnetic levitation we often talk about is closely related to linear motors. Magnetic levitation transportation systems typically use linear motors as their propulsion system.
The working principle of a linear motor
Imagine a linear induction motor with a moving electromagnet (acting as a stator) excited by three-phase AC power. It is mounted in two rows on either side of an aluminum plate (but not in contact). The magnetic lines of force intersect the aluminum plate perpendicularly, inducing a current and generating a driving force. Because the stator of a linear induction motor is mounted on the train and is shorter than the rail, it is also called a "short-stator linear motor." A linear synchronous motor, on the other hand, uses a superconducting electromagnet mounted on the train (acting as a rotor), while three-phase armature coils (acting as a stator) are mounted on the rail. When the coils on the rail are supplied with three-phase AC power of a variable frequency, the vehicle is driven. Because the vehicle's speed is proportional to the synchronous speed of the three-phase AC power, it is called a linear synchronous motor. Furthermore, because the stator of a linear synchronous motor is mounted on the rail and is the same length as the rail, it is also called a "long-stator linear motor."
Traditional rail transport systems use dedicated tracks and steel wheels for support and guidance. Therefore, as speed increases, running resistance increases while traction decreases. When the running resistance exceeds the traction, the train cannot accelerate further, thus preventing it from breaking the theoretical maximum speed limit of 375 kilometers per hour for ground transport systems. Although the French TGV once set a world record of 515.3 kilometers per hour for a traditional rail transport system, the high-speed rail in Germany, France, Spain, Japan, and other countries currently operates at speeds not exceeding 300 kilometers per hour due to overheating and fatigue issues with the wheel and rail materials.
Therefore, to further increase vehicle speed, it is necessary to abandon the traditional wheel-based method and adopt "magnetic levitation" (Magnetic Levitation, or Maglev for short), which allows the train to float off the track, reducing friction and significantly increasing vehicle speed. This method of floating off the track not only avoids noise and air pollution but also improves energy efficiency. Furthermore, using linear motors can further accelerate the speed of the maglev transportation system, thus leading to the development of maglev transportation systems using linear motors.
Maglev transportation systems utilize the principle of magnetic attraction or repulsion to levitate trains off the track. This magnetic force can be categorized into "permanent magnets" or "superconducting magnets" ( SCM). Permanent magnets are simply electromagnets, meaning they only possess magnetism when energized and lose it when the current is cut off. Because it's difficult to collect electricity at extremely high speeds, permanent magnets are only suitable for maglev trains that use the principle of magnetic repulsion and operate at relatively low speeds (around 300 kph). For maglev trains with speeds exceeding 500 kph (using the principle of magnetic attraction), superconducting magnets, which possess permanent magnetism after a single energization (thus eliminating the need for electricity collection), are essential.
Because maglev transportation systems utilize the principle of magnetic attraction or repulsion, they are divided into two types: "Electrodynamic Suspension" ( EDS) and "Electromagnetic Suspension" ( EMS). Electrodynamic levitation (EDS) utilizes the principle of like poles repelling. When the train moves under external force, a normally conductive magnet mounted on the train generates a moving magnetic field, inducing a current in a coil on the track. This current regenerates the magnetic field. Since these two magnetic fields are in the same direction, a repulsive force is generated between the train and the track, causing the train to levitate. Because the train's levitation is achieved by the balance of the forces of the two magnetic fields, its levitation height can remain constant (approximately 10-15 mm), thus giving the train considerable stability. Furthermore, the train must first be started in another way so that its magnetic field can generate induced current and magnetic field, allowing the vehicle to levitate. Therefore, the train must be equipped with wheels for "takeoff" and "landing." When the speed reaches 40 kph or higher, the train begins to levitate (i.e., "takeoff"), and the wheels automatically retract. Similarly, when the speed gradually decreases and levitation ceases, the wheels automatically extend for gliding (i.e., "landing"). Systems that typically use electric levitation (EDS) can only use linear synchronous motors ( LSMs) as their propulsion system, and their speed is relatively slow (approximately 300 kph).
The combination of an electric suspension system (EDS) and a linear synchronous motor (LSM)
Electromagnetic levitation (EMS) utilizes the principle of opposite poles attracting. The train is encircled by guide rails on both sides (similar to a straddle-type monorail system). Electromagnets are installed at the bottom of the train's enclosure, while steel plates replace coils at the bottom of the guide rails. In this configuration, the steel plates are on top of the guide rails, and the electromagnets are below. When energized, the magnetic field generated by the electromagnets attracts the train upwards, while gravity causes the train to sink. When the two forces are balanced, a gap is created between the train and the guide rails, thus leviting the train. The levitation height (approximately 10-15 mm) varies depending on the strength of the magnetic force; therefore, the excitation current must be in a closed loop to maintain magnetic stability. Furthermore, the train can levitate from the start (when its speed is zero), so wheels are not required. EMS systems typically use linear induction motors ( LIMs) or linear synchronous motors (LSMs) as propulsion systems, achieving speeds exceeding 500 kph.
Besides their use in maglev trains, linear motors are widely used in other fields, such as conveyor systems, electric hammers, and electromagnetic stirrers. In my country, linear motors are also gradually being promoted and applied. Although the principle of linear motors is not complicated, they have their own characteristics in design and manufacturing. The products are not as mature as rotary motors, so the price of linear motors has remained high. Further research and improvement of linear motors are needed.
