Characteristics of DC Generators

Characteristics of DC Generators

There are three most important characteristics of a dc generator. They are

(i) No load saturation characteristics

(ii) Internal (or) total characteristics

(iii) External characteristics

(i) No load saturation characteristics (E₀ Vs I_ƒ)

It is also known as magnetic characteristic or open circuit characteristic. It shows the relation between the no load generated emf in armature (E₀) and the field current (L_ƒ) at at a given fixed speed. Its shape is practically the same for all generators.

(ii) Internal (or) Total characteristics (EVs I_a)

It gives the relation between the emf actually induced in the armature (E) and the armature current (I_a).

(iii) External characteristic (V Vs I)

It is referred as performance characteristic or voltage regulating curve. It gives the relation between the terminal voltage V and the load current I. This curve lie below the internal characteristic curve.

Separately Excited Generator

Figure 3.17 shows the separately excited dc generator. Assume the armature is drive at constant speed and the field excitation is adjusted to give rated voltage at no load.

The terminal voltage V = E_g - I_aR_a

I_a = I_L

The characteristics of a separately excited generator is shown in fig 3.18.

Shunt Generator

A dc shunt generator is a self excited generator. Fig.3.19 shows the shunt generator with field resistance adjusted to a certain value by means of the regulating resistance, the desired no load voltage can be obtained.

The external characteristic of the generator can be obtained by a load test with total field resistance remaining fixed in the process fig.3.20 shows the external characteristic of dc shunt generator.

Predetermination of External Characteristic

The external characteristic of a shunt generator can be predetermined from OCC armature and field resistances without performing the load test. Assume the armature reaction voltage drop is negligible.

I_a = I_L + I_ƒ

 I_a ≈ I_L (shunt field current is neglected when compared to load current)

E_g = V + I_aR_a

Fig.3.21 shows the predetermination of external characteristic of de generator. For given I_aR_a is represented by a straight line parallel to the R_ƒ line. The intersection of this line with the OCC gives two solution points R and S for the induced emf. For these solution points the two terminal voltage values V₁ and V₂ can be read from the R_ƒ line corresponding points R' and S'. Maximum possible armature current corresponds to point Q at which a line parallel to the R_ƒ line is tangent to OCC. Corresponding to each solution point values of V and I_ƒ can be read and I_L = I_a + I_ƒ can be calculated. The short circuit point on the external characteristic corresponds to I_aR_a = V_r is the residual voltage.