other power has members that can calculate how many turns you need just from how many lines of flux your crossing but sadly , i am not on of those. if your only looking for 5 volts and 1 amp, at 250 rpm it's quite a different device from a windmill alternator. you won't need many turns of copper in your coils . set up a steel rotor with mags and do a test coil. even though it is a temporary set up for testing , at 250 rpm you will want something to keep the magnets in place.