Or, they could have used electromagnets for the stator and the rotor. The stator would reverse polarity at the same moment the rotor reversed polarity (modulo

Or, they could have used electromagnets for the stator and the rotor. The stator would reverse polarity at the same moment the rotor reversed polarity (modulo differences the transfer functions), so the force would still be in the same direction, despite the alternating current. One could make A/C motors this way, it's just that A/C induction motors are much more efficient.