Separately excited dc generator

Separately excited dc generator

 As the name suggests, a separately excited dc generator requires an independent dc external source for the field winding. The equivalent circuit representation under steady state condition is show in Fig.12. The equations under steady-state are:

 Vf = If Rt  Rt = Rf + Rext  Va = E + IL Ra  IL = Ia

When the field current is held constant and the armature is rotating at a constant speed, the induced emf in an ideal generator is independent of the armature current, as shown by the dotted line in Fig.13. As the load current IL increases, the terminal voltage decreases as indicated by solid line. If the armature reaction is neglected, decrease in Va should be linear and equal to the voltage drop across Ra and carbon brushes. However, if the generator is operated at the knee point in the magnetization curve, the armature reaction causes a further drop in terminal voltage.