Three stationary windings placed at different spatial angles can produce one magnetic field whose direction rotates continuously when they carry balanced phase currents.

This is a rotating magnetic field.

Spatial and temporal phase shift

Suppose three windings are separated by $120^\circ$ in space and their currents are separated by $120^\circ$ in time. Each phase produces a magnetic-field contribution whose magnitude varies sinusoidally along its own fixed axis.

Vector addition of the three contributions produces a resultant field with approximately constant magnitude whose direction rotates.

Synchronous speed

For a machine with $P$ magnetic poles supplied at electrical frequency $f$, the rotating-field speed is

$$n_s=\frac{120f}{P}$$

in revolutions per minute.

More poles give a slower mechanical field speed at the same electrical frequency.

Phase sequence

Changing the order of the three phases reverses the direction of rotation. Interchanging any two supply phases therefore reverses the rotating magnetic field.

Why it matters

A rotating field can exert sustained electromagnetic torque on a rotor without mechanical commutation of the stator supply. This is the central field mechanism behind induction and synchronous AC machines.