Load Characteristics of D.C. Shunt Generator

       Consider the d.c. shunt generator shown in the Fig. 1. The internal characteristics is E V_s I_L while the external characteristics is V_t  against I_L.

Fig. 1 Internal characteristics

       Let us see the nature of these two characteristics.

       Ideally the induced e.m.f. is not dependent on the load current I_L or armature current I_a. But as load current increases, the armature current I_a increases to supply load demand. As I_a increases, armature flux increases.

Note : The effect of flux produced by armature on the main flux produced by the field winding is called an armature reaction.

       Due to the armature reaction, main flux pattern gets distorted. Hence lesser flux gets linked with the armature conductors. This reduces the induced e.m.f.

Note : Thus the armature conductors. This reduces the induced e.m.f.

       This is shown in the Fig. 2.

Fig. 2  Internal characteristics

1.2 External Characteristics 

       For d.c. shunt generator we know that, E = V_t + I_a R_a neglecting other drops. So as load current increases I_L, I_a increases. Thus the drop I_a R_a increases and terminal voltage V_t  = E - I_a R_a decreases. But the value of armature resistance is very small, the drop in terminal voltage as I_L changes from no load to full load is very small. This is shown in the Fig. 3. Hence d.c. shunt generator is called constant voltage generator.

Fig. 3  External characteristics

Why shunt generator load characteristics turns back when overloaded?

       Consider the external characteristics of d.c. shunt generator as shown in the Fig. 3(a). From to point P, the load resistance decreases hence load current increases and due to I_a R_a and armature reaction, the voltage decreases by small amount. Hence characteristics is drooping in nature.

       But if the load resistance is reduced beyond point P i.e. load I_L is increased beyond P then it increases momentarilly. This is very large current and generator gets overloaded. Due to such a large current the armature reaction is severe and drop I_a R_a is also large due to which the voltage V_t drastically reduces. This causes the current to decreases from P to Q, rather than increasing. Thus on the curve pqr, the voltage goes on reducing rapidly and at point r becomes zero. Thus beyond point p, if the generator is loaded, the load characteristics turns back till the generator gets short circuited and the curve meets x axis at point r where voltage is zero. At this point, small E is present due to residual magnetism.

Note : In portion to p, the effect of decrease in load resistance dominates the effect of change in V_t as change in V_t is very small. While in portion 'pqr', the drastic reduction in V_t dominates the effect of decrease in load resistance hence curve turns back, reducing the load current.