The most important characteristics of a DC generator are as follows:
- Open Circuit Characteristic
- Internal Characteristic
- External Characteristic
Open Circuit Characteristic:
The curve drawn between the generated e.m.f. at no-load (E0) and the field current (If) at continuous speed is called as OCC. It may also be referred as magnetic characteristic curve or no-load saturation curve. Its character is nearly the same for all generators either separately excited or self-excited. The readings for O.C.C. curve are achieved practically by working the generator at no load at continuous speed and recording the change in source voltage as the field current is varied.
Internal Characteristic (E/Ia) :
The curve represents the relation flanked by the generated e.m.f. on load (E) and the armature current (Ia) is called Internal characteristic. The e.m.f. E is fewer than E0 due to the demagnetizing effect of armature reaction. Therefore, this curve will lie under the open circuit characteristic (O.C.C.). This curve cannot be attained right away by an experiment. It is because a voltmeter cannot read the e.m.f. generated on load owing to the voltage drop in armature resistance. The internal characteristic can be attained from external characteristic if winding resistances are known because armature reaction effect is built-in both characteristics.
External Characteristic (V/IL) :
The curve drawn between the terminal voltage (V) and load current (IL) is known as External characteristic. The source voltage V will be less than E due to voltage drop in the armature circuit. Thus, this curve will lie under the internal characteristic. This characteristic curve is very significant in defining the aptness of a generator for a given purpose. It can be obtained by making concurrent data’s of source voltage and load current (with voltmeter and ammeter) of a loaded generator.