foto della dinamo di Pacinotti

Galileo Ferraris (1847-1897), Piedmontese.
He introduced the concept of the rotating magnetic field. The consequent invention of the rotating magnetic field asynchronous motor dates back to 1886, but Ferraris published his idea only in 1888. Two months later, the physicist Tesla patented an identical motor, thus gaining fame and fortune. Most electrical motors in use today are of this type.

ritratto di Pacinotti con la sua prima 'Macchinetta'

This is one of the first rotating magnetic field motors built by Galileo Ferraris. It is evidently a two-phase motor.

schema di un indotto ad 'anello' con le spazzole commutatrici

It is quite easy to build a rotating magnetic field motor: here is an example:

To see it work, click on the photo.

The working principle in the case of the model illustrated.
Two alternating electric currents out of phase by 90 power two coils the poles of which form a 90 angle with respect to the axis of a copper disk free to rotate at a short distance from them. The sum of the two magnetic fields is equivalent to a vector magnetic field that rotates at an angular speed equal to the angular frequency of the alternating current.
The Foucault currents that appear on the disk produce the magnetic poles the sign of which is such as to oppose the sign of the rotating vector (Lenz's Law), and thus the disk is made to rotate. Ferraris said that the idea had come to him one day while strolling through the streets of Turin and musing over the circular polarization of light.


In the following two photographs we see the rotor and stator of a small 300W motor.

foto di un altoparlante

This is the rotor: A massive structure of iron and cast aluminium consisting of large aluminium short-circuited coils wound around laminated soft iron poles to avoid the parasite currents in the iron.

foto di un auricolare di una cuffia

This is the stator. We can see three couples of coils covered with yellow tape to isolate them, which thus generate a two-phase rotating magnetic field that makes two-thirds of a turn for each alternation of the power source.


The asynchronous motors in use today are always three-phase if they are above one or two horsepower. This system is composed of three conductors in which currents out of phase by 120 degrees flow. The motors thus have six or multiples of six poles.

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