- 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 a specialized induction motor engineered for straight motion instead of the rotational movement typical of standard motors. These motors modify the traditional design to serve unique functions while maintaining a similar working principle, increasing their operational versatility. Next, we’ll explore the construction of a linear induction motor.
Linear Induction Motor Design
The design and construction of a linear induction motor resemble those of a three phase induction motor, albeit with a unique flat appearance. By cutting and flattening the stator of a polyphase induction motor, we form the system’s primary component. Similarly, flattening the rotor creates the system’s secondary component.
There is another variant of LIM also being used for increasing efficiency known as the Double Sided Linear Induction Motor or DLIM, as shown in the figure below. It has primary on either side of the secondary, for more effective utilization of the flux from both sides.
Working Principle of Linear Induction Motor
The primary of a LIM, when excited by a balanced three-phase power supply, generates a flux that travels its entire length. This flux, moving linearly, parallels the rotating magnetic field seen in conventional three phase induction motor or synchronous motors. The relative motion between this traveling flux and the secondary’s conductors induces an electric current, which interacts with the flux to produce linear thrust.
If the primary is fixed and the secondary is free to move, the force will pull the secondary in the direction of the force and will result in the required rectilinear motion. When we give supply to the system the developed field will result in a linear traveling field, the velocity of which is given by the equation,
Where fs is the supply frequency in Hz, Vs is the velocity of the linear traveling field in meter per second, and t is the linear pole pitch i.e. pole to pole linear distance in meter.
Much like in traditional induction motor, the secondary or runner in a LIM cannot match the magnetic field speed, resulting in a measurable slip. The speed of the linear induction motor, given a slip s, can be calculated accordingly.
Application of Linear Induction Motor
While not as common as conventional motors due to cost and specific design features, linear induction motors are crucial for certain specialized applications where their unique capabilities are required.
Few of such applications are listed below.
- Automatic sliding doors in electric trains.
- Mechanical handling equipment, such as propulsion of a train of tubs along a certain route.
- Metallic conveyor belts.
- Pumping of liquid metal, material handling in cranes, etc.
