Fig. (3.6) shows the voltage build up in a series generator. Here R1, R2 etc. represent the total circuit resistance (load resistance and field winding resistance). If the total circuit resistance is R1, then series generator will build up a voltage OL. The line OC is tangent to O.C.C. and represents the critical resistance RC for a series generator. If the total resistance of the circuit is more than RC (say line OD), the generator will fail to build up voltage. Note that Fig. (3.6) is similar to Fig. (3.5) with the following differences:
(I) In Fig. (3.5), R1, R2 etc. represent the total field circuit resistance. However, R1, R2 etc. in Fig. (3.6) represent the total circuit resistance (load resistance and series field winding resistance etc.).
(II) In Fig (3.5), field current alone is represented along X-axis. However, in Fig. (3.6) load current IL is represented along Y-axis. Note that in a series generator, field current = load current IL.
