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Linear Induction Motor
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Linear Induction motor abbreviated as LIM, is basically a special purpose motor that is in use to achieve
rectilinear motion rather than rotational motion as in the case of conventional motors. This is quite an engineering
marvel, to convert a general motor for a special purpose with more or less similar working principle, thus
enhancing its versatility of operation. Let us first look into the construction of a LIM.
Construction of a Linear Induction Motor
Construction wise a LIM is similar to three phase induction motor in more ways than one as it has been depicted
in the figure below.
In LIM stator and rotor are called primary and secondary respectively. If the stator of the poly phase induction
motor shown in the figure is cut along the section aob and laid on a flat surface, then it forms the primary of the
LIM housing the field system, and consequently the rotor forms the secondary consisting of flat aluminum
conductors with ferromagnetic core for effective flux linkage.
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.
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Which has a primary winding on either side of the secondary, for more effective utilization of the induced flux from
both sides.
Working of a Linear Induction Motor
When the primary of an LIM is excited by a balanced three phase power supply, a traveling flux is induced in the
primary instead of rotating 3 φ flux, which will travel along the entire length of the primary. Electric current is
induced into the aluminum conductors or the secondary due to the relative motion between the traveling flux and
the conductors. This induced current interacts with the traveling flux wave to produce linear force or thrust F. If
the secondary is fixed and the primary is free to move, the force will move the primary in the direction of the
force, resulting in the required rectilinear motion.
When supply is given, the synchronous speed of the field is given by the equation :
Where, fs is supply frequency in Hz,
and p = number of poles,
ns is the synchronous speed of the rotation of magnetic field in revolutions per second.
The developed field will results in a linear traveling field, the velocity of which is given by the equation,
Where, vs is velocity of the linear traveling field, and t is the pole pitch.
For a slip of s, the speed of the LIM is given by
Application of Linear Induction Motor
A linear induction motor is not that widespread compared to a conventional motor, taking its economic aspects
and versatility of usage into consideration. But there are quite a few instances where the LIM is indeed necessary
for some specialized operations.
Few of the applications of a LIM have been listed below.
1. Automatic sliding doors in electric trains.
2. Mechanical handling equipment, such as propulsion of a train of tubs along a certain route.
3. Metallic conveyor belts.
4. Pumping of liquid metal, material handling in cranes, etc.
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