Fig. (3.7) (i) shows the connections of a series wound generator. Since there is only one current (that which flows through the whole machine), the load currentis the same as the exciting current.

(i) O.C.C.
Curve 1 shows the open circuit characteristic (O.C.C.) of a series generator. It can be obtained experimentally by disconnecting the field winding from the machine and exciting it from a separate d.c. source as discussed in Sec. (3.2).
(ii) Internal characteristic

Curve 2 shows the total or internal characteristic of a series generator. It gives the relation between the generated e.m.f. E. on load and armature current. Due to armature reaction, the flux in the machine will be less than the flux at no load. Hence, e.m.f. E generated under load conditions will be less than the e.m.f.  E_O generated under no load conditions. Consequently, internal characteristic curve generated under no load conditions. Consequently, internal characteristic curve lies below the O.C.C. curve; the difference between them representing the effect of armature reaction [See Fig. 3.7 (ii)].

(iii) External characteristic
Curve 3 shows the external characteristic of a series generator. It gives the relation between terminal voltage and load current I_L.

V= E-I_a(R_a+R_se)

Therefore, external characteristic curve will lie below internal characteristic
curve by an amount equal to ohmic drop[i.e., I_a(R_a+R_se)] in the machine as
shown in Fig. (3.7) (ii).

The internal and external characteristics of a d.c. series generator can be plotted from one another as shown in Fig. (3.8). Suppose we are given the internal characteristic of the generator. Let the line OC represent the resistance of the whole machine i.e. R_a+R_se.If the load current is OB, drop in the machine is AB i.e.
AB = Ohmic drop in the machine = OB(R_a+R_se)

Now raise a perpendicular from point B and mark a point b on this line such that ab = AB. Then point b will lie on the external characteristic of the generator. Following similar procedure, other points of external characteristic can be located. It is easy to see that we can also plot internal characteristic from the external characteristic.

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