- Linear Induction Motor Defined: A Linear Induction Motor is a specialized type of induction motor designed to produce linear rather than rotational movement.
- Working Principle: The induction motor working principle in LIMs involves a moving magnetic field created by a three-phase supply, which induces a current that generates force.
- Design Features: The primary component of a LIM is formed by laying a conventional motor’s stator flat, while the secondary comes from flattening the rotor.
- Applications: LIMs are essential in systems requiring linear motion, such as train doors, conveyor belts, and material handling devices.
- Velocity and Slip: The velocity of LIM’s traveling field is determined by its supply frequency and pole pitch, with performance affected by slip similar to traditional motors.
What is Linear Induction Motor
A linear induction motor, or LIM, is an induction motor that produces thrust along a straight path. Its primary winding creates a travelling magnetic field. That field induces current in a conductive secondary, and their electromagnetic interaction produces linear force without a rotary-to-linear transmission.
Linear Induction Motor Design
A flat LIM can be understood as a three phase induction motor opened along its circumference and laid flat. The wound member is called the primary. The reaction member is called the secondary and commonly uses a conducting sheet with a ferromagnetic backing. Either member can be the moving part.
A Double Sided Linear Induction Motor, or DLIM, places the secondary between two primaries. This geometry uses both faces of the reaction plate and can balance the attractive normal forces produced by the two sides. Its efficiency and thrust still depend on the air gaps, winding design, materials and operating speed.
Working Principle of Linear Induction Motor
A balanced three-phase supply to the primary produces a magnetic wave that travels along the air gap. It is the linear counterpart of the rotating magnetic field in a conventional three phase induction motor. Relative motion between the travelling field and conductive secondary induces an electric current. The induced current interacts with the air-gap field to create thrust.
If the primary is fixed and the secondary can move, the electromagnetic thrust moves the secondary along the field direction. Fixing the secondary instead produces an equal opposing force on the primary. For a sinusoidally distributed winding, the travelling-field speed is given by the equation below.
Here fs is supply frequency in hertz, Vs is travelling-field speed in metres per second and τ is pole pitch in metres. Pole pitch is the linear distance from one pole centre to the next.
As with a rotary induction motor, thrust requires relative speed between the secondary and the travelling magnetic field. In motoring operation, the secondary therefore moves below travelling-field speed. Linear slip is the speed difference divided by travelling-field speed, and the equation above gives secondary speed for slip s.
Application of Linear Induction Motor
Linear induction motors suit systems that need direct thrust, a simple reaction rail or contactless force on a conductor. Their open magnetic circuit creates end effects and larger air gaps than many rotary machines, so each application must balance thrust, efficiency, control and installation cost.
Examples include:
- Rail and guided-transport propulsion.
- Direct-drive material-handling vehicles on a fixed route.
- Conveyors and transfer systems with a conductive reaction plate.
- Electromagnetic pumping of electrically conductive liquid metals.
