To say a word about the physics: The fundamental force at play is the electromagnetic force. If you run a current in a coil in the magnetic field, it will rotat

To say a word about the physics: The fundamental force at play is the electromagnetic force. If you run a current in a coil in the magnetic field, it will rotate so that the coil’s field comes into alignment with the outer field. Once it’s there it is happy to stay. You’ve got to change something to get the motor running continuously. AC is quite clever about it because it just changes the field electrically and this is much more reliable than changing it with moving parts.