The linear induction motors use for traction work. It works on electromagnetic induction principle. Similar to the rotational induction motor, it has two windings; primary winding and a secondary winding. Generally, primary winding makes from copper and secondary winding make from aluminum. The general energy flow diagram of the linear electric motor is as shown in the below figure.
The principle of operation of Linear Induction Motor (LIM)
Three-phase supply is given to the primary winding. Traveling flux produce and travels along the length of the primary part of the linear induction motor. The EMF induces in the secondary winding. Because of this EMF, the current circulates in the secondary winding. This induced current interacts with flux and produces thrust. If one-part fix and others can move, then the force will produce on the moving part.
Generally, the primary winding is kept fix and secondary is moving. The secondary winding will move in the direction of the traveling wave. This is not compulsory to make the primary fix and secondary moving.
Let’s understand the principle of LIM with an example.
The above figure shows a traction locomotive using a linear induction motor. In the above system, the primary is moving part and secondary is stationary. By using pantograph, the single-phase supply is collected from the overhead conductor. This supply is given to the converter. The converter uses to convert single-phase supply into the three-phase supply.
Three-phase supply is given to the primary winding of the three-phase linear induction motor. The primary winding produces the traveling flux that links with the secondary winding. The EMF induces in the secondary winding. By this EMF, current circulates in the secondary winding which interacts with the primary traveling flux and a thrust produce on the moving part (primary winding). Therefore, The locomotive propels by this force or thrust.
Classification of Linear Induction Motor:
The linear induction motor classified into two types;
- single-sided LIM
- double-sided LIM
Single-sided Linear Induction motor
Single sided LIM divides according to which winding is moving and which winding is stationary. By this way, this motor divides into two parts;
- Moving primary and fixed secondary
- Moving secondary and fixed primary
Let’s explain types of LIM in detail.
Moving primary and fixed secondary
Below figure shows the construction of moving primary and fixed secondary single-sided LIM. As discussed above, it has two winding; primary winding and a secondary winding. Primary consists of slots embedded in the iron core. Three-phase winding arranges in these slots. So, here in this type of LIM, primary winding is moving and the secondary winding is stationary. The working principle is the same as discussed above.
Primary winding is moving in the opposite direction of the traveling flux. In this type of LIM, the magnetic circuit performance is poor. Therefore, the ferromagnetic sheet use.
Moving secondary and fixed primary
Below figure shows the construction of moving secondary and fixed primary single-sided LIM. The primary consists of slots. Three-phase winding arranges in these slots. Secondary winding is moving and the primary winding is stationary. The working principle is the same as discussed above.
The secondary winding moves in the same direction as the traveling wave. Hence, secondary winding is in the form of a thin aluminum sheet.
Double-sided Linear Induction motor
For traction purpose, double-sided LIM with the moving primary and fixed secondary construction is popular. Below figure shows the construction of double-sided LIM. According to the construction there are two types of LIM.
When the movement required is similar then the primary is made longer than secondary. This type of LIM uses when the distance between the two limits is shorter as shown in the below figure.
When the distance is large in this condition, the primary is made shorter than secondary. So, This type of construction is useful for the transportation. Hence, for traction work this type of LIM use.
Advantage of LIM
- Low maintenance
- Low initial cost
- No overheating
- Rotation of parts avoided. Hence mechanical loss is low.
- No limitations to maximum speed
Disadvantage of LIM
- Poor efficiency
- Power factor is low
- Magnetizing current is large due to large air-gap
- End effect and Edge effect are present. Which affect the performance of the motor.
- Difficulties at the crossing and hence other provisions required
Application of LIM
- Traction motors
- Metallic belt conveyors
- Electromagnetic pumps
- Sliding door operation
- Medium and high-speed vehicles
- Crane moments