INTERACTIVE EXPERIMENTS AND EXHIBITS
THE ROTATING MAGNETIC FIELD
Galileo Ferraris (1847-1897), Piedmontese.
This is one of the first rotating magnetic field motors built by Galileo Ferraris. It is evidently a two-phase motor.
It is quite easy to build a rotating magnetic field motor: here is an example:
In the following two photographs we see the rotor and stator of a small 300W motor.
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.
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.