I think it's the coil, but I can't remember why. I think it has to do with the more surface for current to travel, but I can''t remember. Any help?
The simple motor tries to reach an output speed proportional to applied voltage. So the battery is the no. 1 influence on speed. The motor actually generates a back EMF when running (that's how it approaches a speed proportional to voltage; the lower the speed the lower the opposing EMF so the more current flows). Interestingly, a weaker magnet, or fewer windings in the coil, will also make the motor run faster; it has to in order to make the same back EMF. So both could be considered secondary influences on speed. EDIT: The AC requirement changes things. First, there's no battery. The simplest AC motor I can think of is an eddy-current motor, with an AC stator and a simple single-conductor rotor (e.g., a solid cylinder), a reduced version of the single-phase induction motor, whose rotor has either conducting coils or a squirrel-cage design with several conducting bars. The primary no-load speed influence is the AC frequency; the field strength and coil area affect torque. The next simplest AC motor I can think of is a synchronous type; AC on the stator coil and a permanent-magnet rotor. As long as it runs at all, it runs at synchronous speed determined by the AC frequency, e.g., 3600 rpm for 60 Hz and a dipole rotor. Assuming it's running with no load, changes in magnet strength or coil turns or current either stop it or don't affect its speed.
