Here's a little animation to summarize things and hopefully make it all clear:
Two pairs of electromagnet coils, shown here in red and blue, are energized in turn by an AC supply (not shown, but coming in to the leads on the right). The two red coils are wired in series and energized together and the two blue coils are wired the same way. Since it's AC, the current in each coil doesn't switch on and off abruptly (as this animation suggests), but rises and falls smoothly in the shape of a sine wave: when the red coils are at their most active, the blue coils are completely inactive, and vice-versa. In other words, their currents are out of step (90° out of phase).
As the coils are energized, the magnetic field they produce between them induces an electric current in the rotor. This current produces its own magnetic field that tries to oppose the thing that caused it (the magnetic field from the outer coils). The interaction between the two fields causes the rotor to turn.
As the magnetic field alternates between the red and blue coils, it effectively rotates around the motor. The rotating magnetic field makes the rotor spin in the same direction and (in theory) at almost the same speed.
Here's a little animation to summarize things and hopefully make it all clear:Two pairs of electromagnet coils, shown here in red and blue, are energized in turn by an AC supply (not shown, but coming in to the leads on the right). The two red coils are wired in series and energized together and the two blue coils are wired the same way. Since it's AC, the current in each coil doesn't switch on and off abruptly (as this animation suggests), but rises and falls smoothly in the shape of a sine wave: when the red coils are at their most active, the blue coils are completely inactive, and vice-versa. In other words, their currents are out of step (90° out of phase).As the coils are energized, the magnetic field they produce between them induces an electric current in the rotor. This current produces its own magnetic field that tries to oppose the thing that caused it (the magnetic field from the outer coils). The interaction between the two fields causes the rotor to turn.As the magnetic field alternates between the red and blue coils, it effectively rotates around the motor. The rotating magnetic field makes the rotor spin in the same direction and (in theory) at almost the same speed.
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