Performance of a Shunt Generator

Author: E.E. Kimberly

Not all of the voltage generated In a loaded generator is available at the terminals because of the potential drop in the resistance of the armature, brushes, leads, and contacts between the brushes and the commutator. The total armature current Ia is the sum of the load current and the field current, and

(10-2)

where V₁ = terminal voltage;

Eg = generated voltage; IaRa = voltage drop through the resistance of the armature circuit.

If the exciting current of the generator is obtained from any source other than its own armature, it is said to be separately excited. Separate excitation is necessary when the armature voltage is not suitable for excitation, as in high-voltage generators and arc-welding generators. Curve (a) of Fig. 10-16 shows a typical external-voltage characteristic curve of a separately excited generator. Curve (c) shows the external characteristic of the same generator plotted against external or load current, when self-excited. For curve (b) and total characteristic is plotted against Ia which includes both the field current and the load current.

Fig. 10-16. Shunt-Generator Characteristics

Curve (a) is not a straight line for the following reason: In addition to the' IaRa. voltage drop in the armature circuit, there is some voltage decrease because the magnetomotive force of the armature caused by its own current weakens somewhat the main field flux. Curves (6) and (c) are much lower than (a) because, as the terminal voltage drops because of IaRa drop, the voltage applied to the field decreases and reduces the excitation. Because of this cumulative action, a point is reached, as at y in Fig. 10-16, beyond which a further attempt to increase the load current will result in collapse of the terminal voltage. This action determines one of the limits of maximum momentary power output of any self-excited generator.