If the drive pulley is larger than the alternator pulley then the alternator will be spinning faster than the engine.For example if the drive is 10 inches and the alternator is 5 inches and the engine is turning 1800 RPM then the alternator is turning 3600 RPM.most 60 HZ generators are either 1800 or 3600 RPM units,you need to get the correct RPM to have the right frequency which determines the RPM of the electric motors that you are trying to power.