What is Linear Induction Motor (LIM)? How it's work? ~ Electrical & Electronics Knowledge

What is Linear Induction Motor (LIM)? How it's work ?

A linear induction motor (LIM) is a motor which given linear or translational motion instead of rotational motion as in the case of a conventional induction motor.

Linear induction motor

Fig (a) shows a polyphase rotary induction motor. Let the stator by cut along the line ab and spread out flat as shown fig (b). This forms primary of the linear induction motor. In a linear induction motor, stator and rotor are called primary and secondary respectively. Secondary of the linear induction motor consists of a flat aluminium conductor with a ferromagnetic core.

Fig.(a) Rotary induction motor

If a 3-phase supply is connected to the stator of a conventional rotary induction motor, a rotating flux is produced. This flux rotates at a synchronous speed in the air gap. Similarly, if primary of the linear induction motor is connected to a 3-phase supply, a travelling flux wave is produced that travels along the length of the primary.

Fig.(b) Linear Induction Motor

Current is induced in the aluminium conductor due to the relative motion between the travelling flux wave and aluminium conductor. The induced current interacts with travelling flux wave and aluminium conductor. The induced current interacts with travelling flux wave to produce a linear force (or thrust) F. If secondary is fixed and primary is free to move , the force will move the primary in the direction of the travelling wave. The LIM shown in fig (1) is called single-sided linear induction motor (SLIM). The LIM shown in fig (2) is known as double-sided linear induction motor (DLIM). It has primary on both the sides of the secondary.

Fig. (1) single - sided linear induction motor (SLIM)

Fig. (2) Double - sided linear induction motor (DLIM)

All Basic Electrical Laws

Performance of LIM :

The linear synchronous speed Vs of the travelling wave is given by

Vs = 2f (pole pitch) m/s

Where f is the supply frequency in Hz.

As in a rotary induction motor the speed of the secondary in LIM is less than the synchronous speed Vs.

Vr = Vs (1 - s)

Where s is the slip of the linear induction motor given by

s = Vs - Vr/ Vs

The thrust or linear force is given by

F = air gap power ÷ linear synchronous velocity, Vs.

The equivalent circuit of a linear induction motor is of the same form as that of a rotary induction motor. The thrust-velocity curve of the linear induction motor is similar to that of the torque-speed curve of a rotary induction motor. It is shown in fig below.

Thrust-speed curve of a linear induction motor

As compared to rotary induction motor, the linear induction motor requires a larger air gap. Therefore the magnetizing current is larger and Power factor and efficiency are lower than those in corresponding rotary induction motors of similar rating.

In order to facilitate operational explanation, a linear induction motor has been assumed as a developed version of rotary induction motor where the stator is cut axially and spread out flat. There is, however, a very important difference between the linear development of a rotary induction motor and an actual linear induction motor.

In a rotary induction motor, the stator and rotor developments are of the same length (because of the smaller air gap) and after one revolution the rotor and stator are back in the same position with respect to each other. At a constant speed steady state conditions exist in such a machine. In the linear motor one member will be continuously passing over a new part of the other member. At a steady speed in linear induction motor, the performance is greatly affected by transient conditions which exist at. the entry and trailing edges of the shorter member.

Application :

The main application of linear induction motor is in transportation, including electric traction. Primary is mounted on the vehicle and secondary laid along the track. It is used in cranes for material handling, pumping of liquid metal, actuators for door movement and HV circuit breakers. It is also used in accelerators for rigs for testing vehicle performance under impact conditions.

About Admin MC3

This is dummy text. It is not meant to be read. Accordingly, it is difficult to figure out when to end it. But then, this is dummy text. It is not meant to be read. Period.