Linear Induction Motor

 Working Principle

• The working principle of the linear induction motor ( LIM ) is similar to that three phase induction motor but in the case of linear induction motor the stator is unrolled ( flat ) so it produces linear forces along its length.

Construction

• The single sides linear induction motor consists of primary coil assembly ( stator ) and secondary called as a reaction plate ( rotor ). 
• The reaction plate is made of either copper or aluminum in which current is induced. 
• The width of the reaction plate must be equal to the width of the coil assembly. 
• The magnetic attraction between the coil assembly and the reaction plate can be balanced by using a double sided primary construction.

Working

• When the three phase supply is given to coil assembly ( stator winding ), a travelling magnetic field is created. 
• This magnetic field moves at a liner synchronous speed. 
• The linear synchronous speed is given as

Vs = 2τf meter / second

Where

    τ = Pole pitch ( meter )

    f = Frequency ( Hz )

• It means that the linear synchronous speed depends on the pole pitch and the supply frequency not number of poles. 
• The linear magnetic field induced current in the reaction plate which produces a force in the direction of field. 
• As the magnetic field moves linearly, it drags the reaction that in that direction. 
• The reaction plate or coil assembly ( stator ) could be made movable.
•  The magnetic field at front end 11’ builds up and becomes zero at 22’.
•  The rise and decay of magnetic field induced current in the reaction plat which doesn’t contribute any force but produces ohmic losses in the motor. 
• A variable frequency inverter will provide velocity control of the linear induction motor.

Applications

• Hydraulic drives, ball screw drives, pneumatic drives as cam drives
• It is used in travelling cranes, electromagnetic pump, high speed rail traction, conveyors etc.
• Low acceleration LIM are suitable for Maglev ( Magnetic levitation ) trains and other ground based transportation.
• High acceleration LIM are used for studies of hypervelocity, collisions.
• Many amusement park roller coasters now use LIM to propel the train at high speed. 

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