What is an INDUCTION MOTOR (THREE-PHASE ASYNCHRONOUS MOTOR) - RMF Rotating Magnetic Field

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An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding.

Since 1885 Italian inventor Galileo Ferraris had shown that two orthogonal coils, in which alternating currents out of phase by 120 degrees flow, generate a rotating magnetic field.
After years of study, Ferraris was able to publish the results of his experiments in 1888.

But it was already in the autumn of 1887, that Serbian physicist Nikola Tesla, after mastering his engineering work on the induction motor, yelds the first patent application.

Today, almost the 90% of industrial engines is represented by induction motors.

Induction motors can be considered among the most reliable electric machines since they keep unaltered their perfomances for many years, with very low maintenance operations.

Let’s try to see in detail the inner working principle of these engines.

The induction motor consists of two main elements: the stator, and the rotor:
The stator is basically a three-coil winding powered by three-phase alternating current.
Each winding passes through the slots of the stator, which are made by stacking thin layers of steel with high magnetic permeability inside a steel or cast iron structure.

The flow of a three-phase current through these windings causes the formation of what Galileo Ferraris had already discovered in 1885, namely, a rotating magnetic field.

It is precisely this rotating magnetic field, R-M-F, that causes the rotation of the rotor.

To better understand how the rotating magnetic field as well as its properties are generated, let us take as an example a simplified version of a stator winding.

As we can see, this winding consists of three coils, connected 120 degrees apart.

A current-carrying wire, generates a magnetic field around it.
If we apply a three-phase electric power to this coil arrangement, the magnetic field will be generated at a specific moment, as shown.
Following the variations of the alternating current as shown, the magnetic field will assume different orientations and form.

When we compare these three results, we can see that it is like the rotation sequences of a magnetic field of uniform strenght.

We can call "synchronous speed" the rotational speed of the magnetic field.

Let’s try now to place a closed conductor inside the rotating magnetic field.
Since there is a variable electromagnetic field in a closed circuit, an induced current will be created on the conductor. We can therefore say that the rotating magnetic field will induce a current in the loop.

So, the situation obtained is a loop subject to induced current, located in the magnetic field.
Like we’ve already seen in our last experiment, the Lorentz force was responsible for the rotation of the tin wire placed over a battery and a magnet. The wire was immersed in a magnetic field and while the electricity flowed into it, a perpendicular force acted on it letting the rotation happen.
Also in this case an electromagnetic force will be produced on the loop, allowing the rotation of the connected rotor.

This same phenomenon happens inside the induction motor, with the only difference that the loop is replaced by a squirrel cage rotor.

Also in this case, the three-phase alternating current, passing through the stator produces a rotating magnetic field.

This is why this engine is called an "induction motor".
Electricity is induced on the rotor by electromagnetic induction, and NOT by direct electrical connection.

To facilitate this electromagnetic induction phenomenon, layers of iron sheets are enclosed within the rotor.

These thin ferromagnetic material layers favour the magnetic induction and minimize the eddy currents.

We can deduce that the induction motor is, by nature, self-starting. This means that: by varying its speed, It spontaneously and automatically develops a driving torque, able to counterbalance the resistant torque applied to the motor shaft, determining a stable operation.

As we can see in this animation, the magnetic field and the rotor are rotating, but how do we know the exact rotational speed of the rotor?

First of all, let’s suppose that the rotor is rotating at the same speed as that of the electromagnetic field.