The fig above shows that the compound generator with the combination of both series and shunt excitation. The shunt winding will be connected either across armature with series field (long-shunt connection G) or across the armature solely (short-shunt connection S). The compound generator will be cumulatively compound or differentially compound generator. The latter is never utilized in follow. Therefore, we have a tendency to shall discuss the characteristics of cumulatively- compound generator. It’s going to be noted that external characteristics of short and long shunt compound generators are nearly same.
The external characteristics curve of a cumulatively compound generator is shown in the above fig. The series excitation benefits the shunt excitation. The degree of compound be influenced by upon the rise series excitation with the rise in load current.
(i) If series winding turns are so adjusted that with the rise in load current the terminal voltage will increase, it's known as over-compounded generator. In such a case, because the load current will increase, the series field m.m.f. will increase and tends to extend the flux and therefore the generated voltage. The rise in generated voltage is larger than the IaRa drop in order that rather than decreasing, the terminal voltage will increase as shown by curve 1 in Fig.
(ii) If series winding turns are so adjusted that with the rise in load current, the terminal voltage considerably rests constant, it's known as flat-compounded generator. The series winding of such a machine has lesser amount of turns than the one in over-compounded machine and, therefore, doesn't increase the flux the maximum amount for a given load current. Accordingly, the full-load voltage is sort of adequate to the no-load voltage as shown by curve 2 in Fig.
(iii) If series field coil has lesser number of turns than for a flat- compound generator, the source voltage falls with increase in load current as shown by curve 3 Fig. Such a machine is termed under-compounded generator.