Open circuit characteristic
The O.C.C. of a separately excited generator is determined in a manner described in the previous section.
It shows the variation of generated e.m f. on no load with field current for various fixed speeds.
Note that if the value of constant speed is increased, the steepness of the curve also increases.
When the field current is zero, the residual magnetism in the poles will give rise to the small initial e.m.f. as shown.
Internal and External Characteristics
(a) The armature reaction weakens the main flux so that actual e.m.f. generated E on load is less than that generated (E) on no load.
- (b) There is a voltage drop across armature resistance (= IRa = IR).
Due to these reasons, the external characteristic is a drooping curve [curve 3 in Fig (ii)]. Note that in the absence of armature reaction and armature drop, the generated e.m.f. would have been E (Curve 1).
Advantages and Disadvantages