The characteristics of a DC series generator describe how its generated EMF, terminal voltage, field current, and load current vary under different operating conditions. These characteristic curves are widely used to analyze generator performance, voltage regulation, and load behavior.
In this article, we will discuss the characteristics of a DC series generator (Series wound DC generator) with diagram, including its open circuit, internal, and external characteristics.
What is a DC Series Generator?
A DC series wound DC generator has a field and armature winding. In the series generator, the field and armature winding are connected in series, and therefore, both windings carry the same current.
Therefore,
Ia = Ise = IL
Where:
- Ia = Armature current
- Ise = Series field current
- IL = Load 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; thus, we get a better generator voltage regulation.
Circuit Diagram of Series Wound DC Generator
The circuit diagram of the dc series generator is shown in the figure below.
In this type of DC generator, the armature winding, field winding, and external load are connected in series with each other. Therefore, the same current flows through all three components.
This means:
- Armature current (Ia) = Field current (Ise) = Load current (IL)
Hence, the current produced by the armature directly flows through the field winding and then to the load, which increases the field strength as the load current increases.
Types of DC Series Generator Characteristics
There are three types of DC generator characteristics.
- Open Circuit or Magnetic Characteristic
- Internal Characteristics
- External Characteristics
Each characteristic provides different information about generator performance. The open-circuit characteristic represents the magnetization behavior, the internal characteristic shows the effect of armature reaction, and the external characteristic represents the actual terminal voltage available to the load.
1. Open Circuit Characteristcics (OCC) of DC Series Generator
When the generator is open-circuited, no current flows 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.
The field winding is disconnected from the generator circuit to find out the no-load characteristics, 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 remains up to a certain extent.
After magnetic saturation, further increases in field current produce only a very small increase in the generated voltage. The graph starts above the origin point “O” and shows that there exists some residual magnetism in the generator.
2. Internal Characteristics of DC Series Generator
The internal characteristics of the DC generator are 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 drops the voltage in the armature, lowering the output voltage compared 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. As the armature current increases, the armature reaction becomes stronger, resulting in a greater reduction in the generated EMF. Curve B of Figure 2 shows the internal characteristics of the series wound DC generator.
3. External Characteristics of DC Series Generators
The external characteristic represents the relationship between the terminal voltage (V) and the load current (IL) of a DC series generator. It shows how the output voltage available at the generator terminals changes as the electrical load increases.
The terminal voltage is lower than the generated EMF because part of the generated voltage is lost across the internal resistance of the armature and the series field winding. It is expressed as:
Where:
- V = Terminal voltage
- Eg = Generated EMF
- Ia = Armature current
- Ra = Armature resistance
- Rse = Series field resistance
Since these internal voltage drops increase with current, the external characteristic curve always lies below the internal characteristic curve.
Variation of Terminal Voltage with Load
When the generator is lightly loaded, an increase in load current also increases the series field current. This strengthens the magnetic flux, causing the generated voltage and terminal voltage to rise.
As the load continues to increase, the magnetic circuit approaches saturation, so additional field current produces only a small increase in magnetic flux. At the same time, higher armature current causes larger I²R losses and a stronger armature reaction, both of which reduce the terminal voltage.
Consequently, the external characteristic curve rises initially, reaches a maximum value, and then gradually falls as the load current increases further.
In the high-load region, the reduction in terminal voltage offsets the increase in current, so the output current remains nearly constant over a range of load conditions. Because of this characteristic, a DC series generator is often referred to as a constant-current generator, making it suitable for applications where maintaining nearly constant current is more important than maintaining constant voltage.
Understanding these characteristics helps engineers select the appropriate generator for specific applications and predict its performance under varying load conditions.
Conclusion
It is very clear from the characteristics of a series-wound DC generator that the terminal voltage increases 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 the demagnetization effect caused by the armature reaction. The dotted line in the figure shows the armature reaction phenomenon.
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 decreases with a decrease in the load current.
However, the saturation causes no further significant rise in 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 to a reduction in the load voltage.
The reduction in load voltage causes load current to decrease proportionally because the current is proportional to voltage according to ohms law. Therefore, when the load on the generator increases, the generator voltage drops; however, the load current decreases with decreasing load voltage.
These two effects keep the load current nearly constant in the dotted portion of the external characteristics, which is why we call the series wound DC generator a constant current DC generator.
Frequently Asked Questions (FAQs)
A DC series generator shows how its voltage and current behave under different conditions. These characteristics help predict performance, stability, and current flow in practical applications.
The terminal voltage changes depending on the load. As the load increases, the voltage initially rises slightly and then may decrease due to internal resistance and magnetic effects.
Internal characteristics focus on voltage drops within the generator itself, while external characteristics show how the generator behaves under load, reflecting the voltage available at the terminals.
Armature reaction alters the magnetic field in the generator, which can reduce output voltage and slightly affect current flow, especially under heavy load conditions.
A DC series generator is called a constant-current generator because, over a certain operating range, the load current remains nearly constant despite changes in load resistance. This occurs due to the combined effects of magnetic saturation and armature reaction, which limit changes in the output current.
Related Articles: