The three main characteristics of a series wound generator are used to analyze the operating behavior of the generator. After studying the generator characteristics, it is easy to establish the relationship between field or excitation current, load current, generator voltage, and terminal voltage.
A DC series wound Dc generator has a field and armature winding. The circuit diagram of the series generator is shown in the below figure.
In the series generator, the field winding and the armature winding are connected in the series and therefore both winding carries the same amount of electric current. The field winding is made with a thick wire conductor in order to have the least resistance. The least resistance of the series winding leads to a lesser voltage drop, and thus we get a better generator voltage regulation.
From the circuit diagram of the DC series generator ( Figure-1), it is clear that the same amount of electric current flow through the armature, field, and load.
Types of DC Series Generator Characteristics
There are three types of DC generator characteristics.
Open Circuit or Magnetic Characteristic
1. Open Circuit or Magnetic Characteristics
When the generator is open-circuited, no current flow through the load. In this condition. the generator draws a field excitation current and develops no load voltage. The terminal voltage is equal to the generator's generated EMF. For finding out the no-load characteristics, the field winding is disconnected from the generator circuit and the generator is separately excited from the external source. Thus, the series field winding remains disconnected from the circuit and the generator draws only the no-load current.
The plot between the no-load current and generator voltage is drawn to find the magnetic characteristics of the generator.
Here in the above diagram curve, A shows the magnetic characteristic of the series wound DC generator. The no-load voltage increases with an increase in the field current and the linearity of the curve remain up to a certain extent. After reaching the saturation point, the change in the no-load voltage is insignificant and no change in the no-load voltage. The graph starts above the origin point o shows that there exists some residual magnetism in the generator.
2. Internal Characteristics
The internal characteristics of the generator is a very important characteristic, and it shows the relationship between the load current, armature current, and the generated voltage. When the generator delivers current to the load, the terminal voltage(Eg) is always less than the no-load voltage (E0) because of the armature reaction. The armature reaction causes a voltage drop in the armature and thus we get a lower voltage at the output in comparison to the no-load voltage.
We can see the drop in the voltage in figure 2 curve B. The amount of voltage drop depends on the magnitude of the armature current. The voltage drop in the armature is equal to Ia * Ra. Curve B of figure 2 shows the internal characteristics of the series wound DC generator.
3. External Characteristics of Series Wound DC Generator
The external characteristics, as its name suggests, show the behavior of the generator with a change in the external factor, Here external factor is nothing but the load current (IL). The terminal voltage reduces with an increase in the load current. This happens because of an ohmic drop in resistances of the armature and field winding,
The terminal voltage can be obtained by subtracting the ohmic drop from the no-load voltage of the generator.
The external characteristic curve just lies below the internal characteristic curve. The voltage drop takes place in the winding on account of their resistance and this is the reason curve C lies below curve B.
It is very clear from the characteristics of a series wound DC generator, that the terminal voltage increase with an increase in the load current up to a certain point on the curve However, after reaching its maximum value, the voltage starts decreasing because of demagnetization effect caused by armature reaction. The armature reaction phenomenon is shown by the dotted line in the figure. The dotted portion of the generator characteristics gives approximately constant current irrespective of the load current. The reason is that the field current increases with an increase in load current. In the same way, the field current decrease with a decrease in the load current.
However, the saturation causes no further significant raise of magnetic field strength and consequently no further increase in induced voltage. On the other hand, the increased armature current causes a significant armature reaction that leads fall in load voltage. The fall in load voltage load causes load current to decrease proportionally because the current is proportional to voltage as per the ohms law. Therefore, when the load on the generator increase, the generator voltage drops, However, the load current decreases with decreasing load voltage. Because of these two simultaneous effects, we can see there is no significant change in the load current in a dotted portion of external characteristics of the series wound DC generator. That is why the series wound DC generator is also called a constant current DC generator.