The fig above shows that the circuit diagram of series wound Generator. Since there's only one current that that flows through the full machine, the load current is that the same because the exciting current.
In the below fig the first curve shows that open circuit characteristic curve of a series generator. It will be obtained by experimentation by cut off the field winding from the machine and exciting it from a separate DC supply as mentioned in the O.C.C of DC Generator.
The second curve shows the internal characteristic of a series generator. It provides the relation amongst the generated e.m.f. E on load and armature current. As a result of armature reaction, the flux in the machine will be less than that the flux at no load. Hence, e.m.f. E generated underneath load conditions will be less than the e.m.f. E0 generated underneath no load conditions. Therefore, internal characteristic lies below the O.C.C. curve; the distinction between them representing the impact of armature reaction.
The external characteristic of a series generator is shown in curve of above fig. This curve shows the relation between terminal voltage and load current IL:
Therefore, external characteristic can lie below internal characteristic by an amount equal to resistance unit drop [i.e., Ia(Ra + Rse)] within the machine.
The internal and external characteristics of a DC series generator will be premeditated from each other as shown in the below Fig.
Suppose we have a tendency to given the interior characteristic of the generator. Let the graph OC represent the resistance of the full machine i.e. Ra + Rse. If the load current is OB, drop the machine is AB i.e. AB = Ohmic drop in the machine = OB (Ra+Rse).
Now draw a perpendicular line from point B and mark a point b on this line such ab = AB. Then point b can lie on the external characteristic of the generator. Following similar procedure, different points of external characteristic will be situated. It’s simple to examine that we will conjointly plot internal characteristic from the external characteristic.