Why do most electric traction motors use three-phase AC? The single-phase power in household use is continually varying in sine-wave fashion, rising above zero, peaking at ~ 155 volts, descending in voltage again, passing through zero, and producing a negative wave that is a mirror image of the preceding positive one 60 times a second. As the wave passes near and through zero, almost no power is being delivered (makes sense; near-zero voltage means little current equals little power). This means that much of the time, nothing, or close to it, is happening. So for higher power, three phases, separated by 120 degrees, are delivered separately by a three-wire system. This guarantees that power is always being delivered, for when one phase is crossing zero and delivering little power, the other two phases are either on their way up or down from peak voltage, and they are delivering power.