When the field winding of a DC generator is connected in series with the armature, the generator is called a series-wound generator (Figure 10).

The excitation current in a series-wound generator is the same as the current the generator delivers to the load. If the load has a high resistance and only draws a small amount of current, the excitation current is also small. Therefore, the magnetic field of the series field winding is weak, making the generated voltage low. Conversely, if the load draws a large current, the excitation current is also high. Therefore, the magnetic field of the series field winding is very strong, and the generated voltage is high.

Figure 10 Series-Wound DC Generator

As you can see in Figure 11, in a series generator, changes in load current drastically affect the generator output voltage. A series generator has poor voltage regulation, and, as a result, series generators are not used for fluctuating loads. As is the case for the shunt-wound generator, a series-wound generator also exhibits some losses due to the resistance of the windings and armature reaction. These losses cause a lower terminal voltage than that for an ideal magnetization curve.

Figure 11 Output Voltage-vs-Load Current for Series-Wound DC Generator