Linear Induction Motor : Working, Application and Construction

Linear Induction motor is normally referred as Linear motor. It is a special purpose motor that is used for linear motors. This is interesting development of a conventional induction motor. It performs linear motion instead of rotary motion as in a conventional induction motor.

Construction of a Linear Motor or Linear Induction Motor

There are several ways and types of construction of a Linear Motor or Linear Induction Motor. The simplest form of construction of a Linear Motor is as simple as a three phase induction motor. It has three phase winding housed in slots in a field system. It is simply the primary winding on a stator in case of an induction motor. This is obtained if we cut the stator of an induction motor from middle.
In case of a moving object like in a train the primary winding is mounted on the body of vehicle. The rotor is made by aluminum or copper plates in parallel. In order to complete the flux path a ferromagnetic material is placed with the plates. As the primary is on vehicle or object and secondary is in form of plates so they will have unequal length. For larger distance primary is kept small and for very small and limited distance secondary is kept small. Normally two sided primary winding is used. In this configuration the two field system, one on either side of secondary are used.

Working of a Linear Induction Motor

When the primary is excited by a balanced three phase supply, a rotating electromagnetic flux is induced in primary. The synchronous speed of the field is given by the equation :
N_s=2 f_s/P,
Here, f_s is supply frequency in Hz,
P is the number of poles,
N_s 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,
V_s=2xtxf_s meter per second
here, V_s is velocity of the linear traveling field,
t is the pole pitch.
For a slip of s, the speed of conducting slave in a linear motor is given by
V_r=(1-s)V_s

Application of Linear Induction Motor or LIM

Although these motors are not frequently used. There are only a few instances where the linear motor is used or is utilized in a proper way. It seems that these motors are technically, feasible but due to economical point of view these motors are not frequently used.
However the possible applications of a Linear Induction Motor are listed below :

• Application for Stationary Field System
  • Automatic sliding doors in an electrical train,
  • Metallic belt conveyor,
  • Mechanical handling equipment, such as propulsion of a train of tubs along a certain route,
  • Shuttle-propelling application.
• Applications for the moving field system
  • High and medium speed applications have been tried with linear motor propulsion of vehicles with air cushion or magnetic suspension.
  • High speed application as a traveling crane motor where the field system is suspended from loist.

Problems Associated with LIM

The Linear Induction Motor (lim) is usually taken as an induction motor with three phase balanced excitation, but this comparison is just an approximation. There are certain effects, peculiar to a linear motor, which should be considered for accurate analysis. Of there, two important effects are :
1. Transverse edge effect and
2. End effect