D.C. series generator is used

D.C. series generator has its field winding connected in series with the armature, so the field current increases with load current.

Because of this, the terminal voltage rises with increase in load, which makes it unsuitable for constant-voltage applications.

However, this characteristic is highly suitable for traction loads such as electric locomotives and trams, where heavy starting current and varying load conditions are required.

Hence, a D.C. series generator is mainly used to supply traction load.