The O.C.C. for a d.c. generator is set as follows. The fig above shows that the field winding of the d.c. generator is cut off from the machine and is separately excited from an external d.c. supply. The generator is allowed to run at constant speed. The Field current (If) is raised from zero in steps and therefore the corresponding values of generated e.m.f. (E0) reads off on a voltmeter connected through the armature winding terminals. The fig shows that the relation between E0 and If, we tend to get the open circuit characteristic.
The succeeding points could also be noted from O.C.C.:
- As soon as the field current is zero, there's some generated e.m.f. OA. This is often owing to the residual magnetism within the field poles.
- Over a reasonably wide selection of field current (up to point B within the curve), the curve is linear. It’s as a result of during this vary, reluctance of iron is insignificant as compared therewith of air gap. The air gap reluctance is constant and therefore linear relationship.
- After the curve reaches the point B, the reluctance of iron additionally comes into picture. It’s as a result of at higher flux densities, µr for iron reduces and reluctance of iron is not any longer tiny. Accordingly, the curve move away from linear relationship.
- After point C on the curve, the magnetic saturation of poles begins and E0 tends to level.
The reader could note that the O.C.C. of even self-excited generator is obtained by running it as a separately excited generator.