Characteristics of self-excited DC Generator
For self-excited DC generator, the performance is analyzed with three different characteristics. They are no-load characteristics, internal and external characteristics.
In this section, you will be learning about the characteristics of DC shunt generator, series generator and compound generator in detail. But before that, you should learn about the construction and working of DC Generator.
Characteristics of Shunt wound DC Generator
The open-circuit characteristics for a shunt-wound DC generator is obtained by conducting a no-load experiment. It is similar to the characteristics of separately excited DC Generator.
To obtain the characteristics, connect the circuit as shown below. Since it is a shunt DC generator, the field winding is connected across the armature conductors. To vary the field current, a variable resistant(rheostat) is connected in series with the field winding.
To measure the field current, an ammeter is connected to the field circuit. A voltmeter is connected across the armature conductor to measure the induced voltage.
As you know, the field current is varied from zero to maximum value. The armature voltage is measured for different values of field current. After saturation of the poles, the induced voltage becomes constant. The readings are plotted and a graph is drawn as shown below.
In shunt DC Generator, there is a residual magnetism in the poles. Because of this, some emf is induced in the generator even before field current If = 0. It is called Residual emf and is shown in the above graph as OA. Hence the curve starts little way up, instead of starting from the origin.
Critical resistance and critical speed
A tangent OB is drawn to the linear portion of the graph from the origin. It is shown as a blue color line in the above graph. The slope of this tangent(ΔEg / ΔIf) gives the value of critical resistance.
So, actually what is critical resistance mean?
It is the value of shunt field resistance, above which the voltage starts to build up in the armature conductors. The speed at which the generator rotates during this voltage build-up is known as critical speed. It is defined as the speed at which the shunt filed resistance is equal to the critical resistance.
For building up of voltage in a shunt generator, it should satisfy the following conditions.
- There must be some residual magnetism in the poles
- For the given direction of rotation, the field winding should be directly connected to the armature conductor. If not, the voltage will drop down due to the weakening of flux.
- For no-load or open circuit conditions, the shunt field resistance should be less than the critical resistance.
- For load conditions, the shunt field resistance should be more than the critical resistance.
Similarly, following factors that will affect the building up of voltage in shunt generator.
- Reversed residual magnetism – armature voltage reverses, which further reverses the field current.
- Reversed shunt field connection – causes the generator voltage to build down from its normal value.
- The direction of rotation of the rotor may get reversed – voltage reverses and so builds down from its original value.
The reverse polarity should be corrected to give its desired performance. It can be done by using an external DC source to remagnetize the field poles in correct direction. This process is said to be field flashing.
Let us conduct the load characteristics of the shunt DC Generator by applying a load. The following figure(a) shows the circuit connection for conducting the load characteristics. The graph(b) below shows the load characteristics of the self-excited DC shunt Generator. Both the internal and external characteristics are shown in the graph.
The field current is varied without applying load and the induced emf is measured. It gives the ideal characteristics as shown in the graph as horizontal line AB.
In practical cases, an increase in load will cause the terminal voltage to drop. Some voltage drops in the generated emf Eg due to armature reaction and so the characteristics also drop a little. It is called internal characteristics or total characteristics and shown as a curved line AC.
While plotting the terminal voltage against the load current, the curve drops further due to armature resistance. It is called as external characteristics and is shown as a curved line AD.
Characteristics of Series wound DC Generator
Here, to obtain the open circuit characteristics, disconnect the field winding from the circuit. Separate excitation is given to the machine to draw the characteristics, as shown below.
In the above characteristics, curve AB represents the open circuit characteristics. It is similar to the no-load characteristics. This is because, in a series generator, the field winding is connected in series with armature and load. Hence load current is similar to field current and OA represents residual emf.
The curves OC and OD represent internal and external characteristics respectively. Since load current and field current is the same, an increase in load current causes the terminal voltage to increase. Beyond a certain limit, the terminal voltage starts decreasing with an increase in load. It is shown as a dotted line in the above characteristics. This is due to excessive demagnetizing effects of the armature reaction.
Characteristics of Compound DC Generator
The compound generator is a self-excited DC Generator, whose characteristics depends on both shunt field winding and series field winding. As we studied, in the shunt generator, an increase in load causes the terminal voltage to decrease. Whereas in a series generator, an increase in load causes the terminal voltage to increase.
In the compound generator, the series field flux compensates the drop-in the shunt field flux. This will give a constant voltage characteristic. It is shown in the below graph as curve AC. The generator with such characteristics is called a flat compound generator.
If the series field excitation is more than the shunt field excitation, an increase in load current will cause the terminal voltage to increase. It is represented as curve AB and is known as Over compound generator.
The curve AD represents the characteristics of under compound generator. In this, the series field excitation is less than the shunt field excitation. Any increase in load current will cause the terminal voltage to decrease.