Following are the three most important characteristics or curves of a dc generator:
1. No-load saturation Characteristic (E0/If):
It is also known as Magnetic Characteristic or Open-circuit Characteristic (O.C.C.). It shows the relation between the no-load generated MMF in armature, E0 and the field or exciting current If at a given fixed speed.
It is just the magnetization curve for the material of the electromagnets. Its shape is practically the same for all generators whether separately-excited or self-excited.
2. Internal or Total Characteristic (E/Ia):
It gives the relation between the MMF E actually induces in the armature (after allowing for the demagnetizing effect of armature reaction) and the armature current Ia. This characteristic is of interest mainly to the designer.
3. External Characteristic (V/I):
It is also referred to as performance characteristic or sometimes voltage-regulating curve. It gives relation between that terminal voltage V and the load current I. This curve lies below the internal characteristic because it takes into account the voltage drop over the armature circuit resistance. The values of V are obtained by subtracting IaRa from corresponding values of E. This characteristic is of great importance in judging the suitability of a generator for a particular purpose. It may be obtained in two ways
(i) By making simultaneous measurements with a suitable voltmeter and an ammeter on a loaded generator or
(ii) Graphically from the O.C.C.
Provided the armature and field resistances are known and also if the demagnetizing effect (under rated load conditions) or the armature reaction (from the short-circuit test) is known.