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[[File:Synchronous machine curves.png|thumb|upright=1.5|A diagram with multiple synchronous machine curves; ''open-circuit saturation curve'' is the leftmost one]]
The '''open-circuit saturation curve''' (also '''open-circuit characteristic''', OCC) of a [[synchronous generator]] is a plot of the output [[open circuit voltage]] as a function of the [[excitation current]] or field. The curve is typically plotted alongside the [[synchronous impedance curve]].{{sfn | Klempner | Kerszenbaum | 2004 | p=125}}
At the low field, the [[magnetic permeability|permeable]] iron in the [[magnetic circuit]] of the generator is not [[magnetic saturation|saturated]], therefore the [[reluctance]] almost entirely depends on the fixed contribution of the [[air gap (electric machine)|air gap]], so the part of the curve that starts at the [[Origin (mathematics)|point of origin]] is a linear "air-gap line" (output voltage is proportional to the excitation current). As the iron saturates with higher excitation and thus higher [[magnetic flux]], the reluctance increases, and the OCC deflects down from the air-gap line.{{sfn | Klempner | Kerszenbaum | 2004 | p=125}}
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