A typical alternator output vs RPM curve looks like:
So I designed my welder using standard A belt pulleys to increase the motor RPM from the 3450 range up to about 5000 or 6000 RPM. The alternator pulley I'm currently using is a 3.5" diameter and the motor pulley is a 6" diameter, a 1.7 ratio or approx 5900 RPM. I originally tried a 7" motor pulley, but found the noise level to be too high. When I went looking for a replacement for the 7", I was looking for a 5" (4900 RPM) but could not locate one at the time and settled on the 6". I have not noticed any appreciable belt slippage with this arrangement.
