In all types of generators, the field windings, the armature windings and the external load circuit are connected in series, as shown in the figure below.
Therefore, the same current flows through the armature winding, the field winding and the load.
Let, I a = I sc = I large
Here I a = armature current
I sc = current field field
I large = load current
There are generally three most important characteristics of a DC generator that show the relationship between different quantities, such as series field current or excitation current, generated voltage, terminal voltage and load current.
Magnetic or Open Circuit Characteristic of DC Series Wound Generator
The curve showing the relationship between the no-load voltage and the magnetic field or magnetic or open circular curve is displayed. As they are not charging, the load terminals are open and there will be no field current in the field as the armature, field and load are connected in series and these three make a closed circuit loop. Thus, this curve can be practically achieved to separate the field winding and excite the DC generator from an external source.
Here in the diagram below the AB curve shows the magnetic characteristic of the DC generator. The linearity of the curve will continue until the poles are saturated. After this, there will be no other significant change in the DC generator terminal voltage to increase the field current. Due to the residual magnetism, there will be a small initial tendency along with the reinforcement and therefore the curve started from a point A which is a little above the origin O.
Internal feature of DC series wound generator
the internal characteristic of the curve gives the relationship between the voltage produced in the armature and the load current. This curve is obtained by subtracting the drop due to the diamagnetic effect of the reinforcement reaction from the no-load voltage. Thus, the actual voltage produced (E sol ) will be less than the voltage without load (E 0 ). This is why the curve falls slightly from the typical open circuit curve. Here in the diagram below the OC curve shows the internal characteristic or the total characteristic of the DC generator.
External feature of the DC generator
The external characteristic curve shows the change in the terminal voltage (V) with the load current (I large ). The voltage of the terminal of this generator type was obtained by removing the ohmic drop due to the armature resistance (R a ) and the series field resistance (R sc ) from the actual output voltage (E sol ).
Terminal voltage V = E sol – I (R a + R sc )
The external characteristic curve is below the internal characteristic curve because the value of the terminal voltage is less than the output voltage. Here in the figure, the OD curve shows the external characteristic of the DC generator.
It can be observed from the characteristics of the DC generator, that as the load increases (load is increased when the load current increases) the voltage at the machine terminal increases. But once it reaches its maximum value, it begins to decrease due to the excessive demagnetizing effect of the armature reaction. This phenomenon is shown in the figure from the dashed line. The square part of the feature gives approximately constant current regardless of the external load resistance. This is because if the load increases, the field current increases as the field are connected in series with the load. Similarly, if the load increases, the armature current increases as the armature also connect to the load. But due to saturation, there will be no further increase in the magnetic field strength, hence any further increase in the induced voltage. But due to the increased armature current, the effect of the armature reaction increases significantly, which causes a significant drop in the load voltage. If the load voltage drops, the load current also decreases proportionally, as the current is proportional to the voltage according to Ohm’s law. Thus, by increasing the load, it tends to increase the load current, but by decreasing the load voltage, it tends to decrease the load current. Due to these two simultaneous results, there will be no significant change in the load current in a dashed portion of the outside Thus, increasing the load, tends to increase the load current, but decreasing the load voltage, it tends to decrease the load current. Due to these two simultaneous results, there will be no significant change in the load current in a dashed portion of the outside Thus, increasing the load, tends to increase the load current, but decreasing the load voltage, it tends to decrease the load current. Due to these two simultaneous results, there will be no significant change in the load current in a dashed portion of the outside characteristics of the DC generator. This is why the DC series generator is called a constant DC current generator.