Load Characteristics of D.C. Series Generator

 Load Characteristics of D.C. Series Generator

AU: May-10, Dec.-03, 09

Consider a series generator shown in the Fig. 3.26.1.

case of series generator,

I_a = I_se = I_L

• As load current increases, I_se increases. The flux is directly proportional to I_se. So flux also increases. The induced e.m.f. E is proportional to flux hence induced e.m.f. also increases. Thus the characteristics of E against i.e. internal characteristics is of increasing nature. As I_a increases, armature reaction increases but its effect is negligible compared to increase in E. But for high load current, saturation occurs and flux remains constant. In such case, due to the armature reaction E starts decreasing as shown by dotted line in the Fig. 3.26.2.

Now as I_L = I_a increase, thus the drop I_a (R_a + R_se ) increases.

V_t = E - I_a (R_a + R_se )

• Thus the external characteristics is also of rising nature as E increases but it will be below internal characteristics due to drop I_a (R_a + R_se). This is shown in the Fig. 3.26.2.

• In self excited series generator, open circuit characteristics cannot be obtained. In open circuit, I_a = I_L = 0 hence induced e.m.f. is zero. Thus open circuit characteristics is possible only by separately exciting the field winding. It is also shown in the Fig. 3.26.2.

• In practice when there is no load I_L = 0, then there exists certain induced e.m.f. due to residual flux retained by the field winding.

Key Point: Hence for zero load current, there is a voltage present and the internal and external characteristics do not pass through origin as shown in the Fig. 3.26.2.

Review Question

1. Draw and explain load characteristics of d.c. series generator.