If we apply alternating current to the simple direct current motor, the obstacle of how to reverse the current, and thereby the polarity of the electromagnet, is no longer an issue. Since the current alternates direction at a certain frequency, the magnetic field of an electromagnet can be made to change polarity at the same frequency.
If the same device is put to use in reverse, that is, if an outside mechanical force is used to turn the armature, an alternating current will be produced:
Most AC motors in use are induction motors, the workings of which are significantly more complicated than the simple AC motor discussed above. As implied by the name, induction motors rely on induction of a current in the armature by the external magnetic field.
The armature is sometimes called a 'squirrel cage', a somewhat archaic reference to it's hamster wheel-like appearance. Wires run the length of the armature, evenly spaced around the circumference. All are connected by plates or rings at both ends of the armature. No current is supplied to the armature; the only magnetic field is the external field of the stator.
Since the current alternates direction, the electromagnetic external field changes polarity at the same frequency. The field induces a current on the windings of the armature. This induced current, in turn, creates a magnetic field. The result is a torque on the armature. Due to the sinusoidal nature of alternating current, the external field more or less rotates continuously around the armature. The armature in effect follows the rotating poles of the external field.
The further workings of alternating current induction motors get more complicated still, sometimes involving multiple poles and magnetic phases, but this simple explanation should suffice for the layman (and me).
