Linear induction motor
From the name linear; We can deduce that the linear induction motor is a rotary motor that has been cut and unrolled to provide us with linear (straight line) motion and force instead of rotational torque. In this motor, we have an unwrapped stator that spread out flat and it’s called the primary of the motor; The rotor consists of a flat aluminum conductor with ferromagnetic core and it moves in a straight line past the stator.
It’s important to know where we could find the linear motor before we know how it works; Let’s see.
Applications of linear induction motor:
Because of the economic aspects and versatility of usage of the linear motor, we may find it in quite applications that require rapid movement of a large payload as:
- With overhead traveling cranes for moving sheet metal.
- It’s used to drive conveyors, textile shuttles, sliding doors and machine tools.
- In electrical trains represented in the automatic sliding doors.
- In mechanical handling equipment.
- It’s used as electromagnetic Pumps as a liquid metal.
- Metallic conveyor belts.
- It’s also used in high voltage circuit breaker and in accelerators.
Working principle of a linear induction motor:
When we supply the primary of the linear motor with a three phase supply a flux produced and travels across the length of this primary; Due to the relative motion between the flux and the aluminum conductor a current will generate in this conductor and interacts with the traveling flux to produce a linear force. If we fix the secondary of the motor and make the primary free, the linear force causes the primary to move in the direction of the traveling wave to result in the required rectilinear motion.
I think some equations will make it easier to understand;
When we give a supply to the motor we will have a synchronous speed of the field:
ns: the synchronous speed of rotation of the magnetic field (revolutions/sec).
fs: the supply frequency (HZ).
P: number of poles.
And the velocity of the linear traveling field produced as a result of the field:
t: the pole pitch.
And for a slip (S) the speed of the linear motor will be:
From these equations we can understand that the speed of the linear motor depends on the frequency of the source, when we change the input frequency we control the speed of the motor.
It’s important to know that the linear induction motor requires a large air gap so it has a greater magnetizing current but at the same time the power factor and the efficiency are lower.