Characteristics of Compound DC Generator

There are mainly two types of dc compound generator. These are long shunt and short shunt. For more information for these two types of dc generator please check the post types of dc generator. Both of these generators can also be categorized as cumulatively compound and differentially compound dc generator. The differentially compound and cumulatively compound generators have their own typical characteristics. But the compound generators mostly affect the external characteristics of compound dc generator. That we shall discuss in this article later. But now we discuss the open circuit characteristic of dc compound generator.

Open Circuit Characteristic of DC Compound Generator

Like a separately excited dc generator, we use a separate source for the field for drawing OCC of a compound DC generator. In this case, we first disconnect the field winding from the machine. Then we excite the field from an external variable dc source. Now we slowly increase the field current from zero to its rated value. During this experiment, we run the generator with its rated speed without any connected load. Also, we gradually increase the field current and measure the output voltage at each step. Now we can draw the curve for open circuit characteristic with the measured voltages along with their corresponding field currents. The no-load or open circuit characteristic of a dc compound generator is similar to that of a separately excited dc generator. So you can study the characteristic from the article of characteristics of separately excited dc generator.

Internal Characteristics of Compound DC Generator

A shunt generator shows downward inclined internal characteristics. On the other hand, a series generator shows upward bend internal characteristics. The internal characteristics of a dc compound generator can be made flat for the range of field currents.

When the generator is differentially compounded, with the increase in load current, the voltage across the armature decreases more rapidly than a dc series generator.

Besides a cumulatively compound generator shows more upward internal characteristic than a standard series generator.

Since it is practically impossible to measure the actual armature emf in a dc machine. So, we use indirect methods of drawing the internal characteristic curve of a compound generator. We just add armature voltage drops with the external characteristic to draw the internal characteristic of a dc compound generator.

External Characteristics of Compound DC Generator

Here, we measure the load terminal voltages for different load currents. Then we plot the load terminal voltages and load currents. And we get the external characteristics of the dc compound generator. If the terminal voltage increases with the increasing load current, we call the machine as over compounded dc generator. Again, if the terminal voltage of the machine decreases with the increasing load current, we call this as under compounded dc generator. But by adjusting the field orientation we can make such a machine, which has almost constant terminal voltage for all the loading conditions. We call this generator as the flat compounded dc generator.

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