In a compound generator, both series and shunt excitation are combined as shown in the figure. The characteristics of DC compound generator is different for series and shunt generator characteristics.
The shunt winding can be connected either across the armature only (short-shunt connection S) or across armature plus series field (long-shunt connection G).
The compound generator can be cumulatively compounded or differentially compounded generator. More about them is explained in the Types of DC Generator.
The differentially compounded generator is rarely used in practice. Therefore, we shall discuss the characteristics of a cumulatively compounded generator in this article.
It may be noted that the external characteristics of long and short shunt compound generators are almost identical.
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Over Compounded Generator
If series winding turns are so adjusted that with the increase in load current the terminal voltage increases, it is called over-compounded generator.
In such a case, as the load current increases, the series field m.m.f. increases and tends to increase the flux and hence the generated voltage.
The increase in generated voltage is greater than the IaRa drop so that instead of decreasing, the terminal voltage increases as shown by curve A.
Flat Compounded Generator
If series winding turns are so adjusted that with the increase in load current, the terminal voltage substantially remains constant, it is called flat-compounded generator.
The series winding of such a machine has a lesser number of turns than the one in an over-compounded machine. Therefore, does not increase the flux as much for given load current. Consequently, the full-load voltage is nearly equal to the no-load voltage as indicated by curve B.
Under Compounded Generator
If the series field winding has a lesser number of turns than for a flat compounded machine, the terminal voltage falls with an increase in load current as indicated by curve C.
Such a machine is called under-compounded generator.