The O.C.C. for a d.c. generator is determined as follows. The field winding of the d.c. generator (series or shunt) is disconnected from the machine and is separately excited from an external d.c. source as shown in Fig. (3.1) (ii). The generator is run at fixed speed (i.e., normal speed). The field current ( I_f) is increased from zero in steps and the corresponding values of generated e.m.f.(E _o) read off on a voltmeter connected across the armature terminals. On plotting the relation between Eo and I_f, we get the open circuit characteristic as shown in Fig. (3.1) (i)

The following points may be noted from O.C.C.:
(i) When the field current is zero, there is some generated e.m.f. OA. This is due to the residual magnetism in the field poles.
(ii) Over a fairly wide range of field current (upto point B in the curve), the curve is linear. It is because in this range, reluctance of iron is negligible as compared with that of air gap. The air gap reluctance is constant and hence linear relationship.

(iii) After point B on the curve, the reluctance of iron also comes into picture. It is because at higher flux densities, μ_r for iron decreases and reluctance of iron is no longer negligible. Consequently, the curve deviates from linear relationship.

(iv) After point C on the curve, the magnetic saturation of poles begins and E_o tends to level off.The reader may note that the O.C.C. of even self-excited generator is obtained by running it as a separately excited generator.

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