What's the "phase" of an AC motor?
We don't necessarily have to drive the rotor with four coils (two opposing pairs), as illustrated here. It's possible to build induction motors with all kinds of other arrangements of coils. The more coils you have, the more smoothly the motor will run. The number of separate electric currents energizing the coils independently, out of step, is known as the phase of the motor, so the design shown above is a two-phase motor (with two currents energizing four coils that operate out of step in two pairs). In a three-phase motor, we could have three coils arranged around the stator in a triangle, six evenly spaced coils (three pairs), or even 12 coils (three sets of four coils), with either one, two, or four coils switched on and off together by three separate, out-of-phase currents.