The performance of a separately excited DC generator is analyzed with the help of three different characteristics: Magnetic or no-load or open circuit characteristics, internal or total characteristics and external characteristics.
Now, Let us look at the characteristics of separately excited DC generators in detail.
Open circuit characteristics(OCC)
It is also known as magnetic characteristics or no-load saturation characteristics. It shows the relation between the induced emf E0 at the no-load condition and the field current If at a constant speed.
For separately excited DC generator, the open circuit characteristics is obtained by conducting an experiment under no-load conditions. An ammeter is connected to the field winding and a voltmeter is connected to the generator to measure the induced voltage.
The circuit is connected as shown in the above diagram. The field current is varied by connecting an additional resistance(Rheostat) and is measured by an ammeter.
At a constant speed, when the field current is increased from zero, the flux and hence the induced emf increases. The values of induced emf corresponding to the field current is measured and tabulated. From the tabulation, a graph is drawn with field current as the x-axis and generated emf as the y-axis.
The below graph shows the open circuit characteristics of a separately excited DC generator
From the above graph, it is observed that the increase in field current increases the emf induced. When the poles get saturated, the increase in field current does not increase the flux and thus the emf induced also remains constant.
Different curves can be obtained for different speeds. For higher speeds, the emf induced will be more as you can see from the above graph.
Let us consider a separately excited DC generator, operated at no-load condition. At this condition, if there is no armature reaction and armature resistance drop, then the obtained no-load voltage E0 for a certain field current remains constant. It is shown in the below graph as a horizontal line AB.
Now, when a generator is loaded from no load to full load, the armature current or load current increases steadily and reaches the rated current. At this loaded condition, the armature reaction and armature resistance will come into effect, thereby causing the voltage to drop.
The induced emf will drop slightly due to the armature reaction. It is obtained by subtracting the drops due to armature reaction from no-load voltage as shown in the graph below. It is called as internal characteristics or total characteristics.
Internal characteristics is drawn between the induced emf Eg and the armature current Ia. It lies below the ideal characteristics represented by a curved line AC.
It gives the performance behavior of the DC Generator and so-called performance characteristics or external characteristics. The terminal voltage is measured for every load that is applied to the generator and tabulated. A graph is drawn between the terminal voltage and load current, which is known as external characteristics.
As we know, as load increases, the load current increases causing a drop in voltage. It can be seen in the above graph as a curved line AD, which lies below the internal characteristics. It is obtained by subtracting the drop due to armature resistance from the internal characteristics.