Saturation means no increase in flux density for increase in current. The general relationship is E=BLV where E is generator output voltage, B is the flux density, L is length of conductor in field and V is the velocity of conductor in field. When field is saturated their is no change in output voltage.
Comment: The magnetic material magnetization theory may include very small magnetic domains (south and north pole magnetically polarized). Once these little domains are aligned, even periodically by the alternating current, the further alignment by increasing the magnetization (field) current is producing negligible effect, if any. Therefore, one will not obtain more shaft torque (or horsepower) when the perfect alignment is in effect. It means that the better magnetic material (higher relative permeability) and/or larger cross-sectional area of magnetic poles is needed to obtain more powerful rotating machine and possibly avoid the magnetic saturation.
