Characteristic of Separately Excited Dc Generator

Characteristic of Separately Excited Dc Generator Points : Characteristic of Separately Excited Dc Generator In a separately excited dc generator the field coils are energized from an independent source. Since excited current is independent of load or armature current, if a curve is drawn between flux per pole and load current, keeping field current constant and neglecting armature reaction, a straight line parallel to X-axis is obtained as indicated by curve. But due to armature reaction, the curve of actual flux is slightly drooping as represented by curve . The second curve also gives to some other scale the emf generated in the armature and, therefore, second curve represents internal or total characteristic of the separately excited de generator. The terminal voltage on load is the emf generated less voltage drop in the armature i.e.. V = E I_a R_a, so if a line OA representing the armature resistance is drawn and its ordinates are deducted from curve II, curve III giving relation between terminal voltage and load current, known as external characteristic is obtained. From curve III. it is obvious that with the increase in load current, terminal voltage decreases light1y. This decrease can easily be neutralized by increasing field current slightly and, therefore,. constant terminal voltage can be maintained.