The open circuit characteristics (O.C.C) or magnetization characteristics is the curve that shows the relationship between the generated EMF at no-load (E0) and the field current (If) at constant speed. It is also known as no-load saturation curve. Its shape practically the same for all types of DC generator whether separately excited or self-excited.
In order to determine the open circuit characteristics of a DC generator, the field winding is disconnected from the machine and is excited by an external DC source. The generator is run at its normal speed. The field current (If) is increased gradually from zero and the corresponding values of generated EMF at no-load (E0) is noted from a voltmeter connected across the armature terminals.
On plotting the relation between the E0 and If, we obtain the open circuit characteristics as shown in the figure.
Following points can be observed from the open circuit characteristics curve −