Compound Generator Characteristics
In a compound generator, both series and shunt excitation are combined as shown in Fig. (3.13). 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. The latter is rarely used in practice. Therefore, we shall discuss the characteristics of cumulatively compounded generator. It may be noted that external characteristics of long and short shunt compound generators are almost identical.
Fig. (3.14) shows the external characteristics of a cumulatively compounded
generator. The series excitation aids the shunt excitation. The degree of compounding depends upon the increase in series excitation with the increase in load current.
(i) 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 in Fig. (3.14).
(ii) 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 lesser number of turns than the one in over-compounded machine and, therefore, does not increase the flux as much for a given load current. Consequently, the full-load voltage is nearly equal to the no-load voltage
as indicated by curve B in Fig (3.14).
(iii) If series field winding has lesser number of turns than for a flat compounded machine, the terminal voltage falls with increase in load
current as indicated by curve C m Fig. (3.14). Such a machine is called under-compounded generator.
The change in terminal voltage of a generator between full and no load (at constant speed) is called the voltage regulation, usually expressed as a percentage of the voltage at full-load.
% Voltage regulation= [ (VNL-VFL)/VFL ] × 100
where VNL = Terminal voltage of generator at no load
VFL = Terminal voltage of generator at full load
Note that voltage regulation of a generator is determined with field circuit and speed held constant. If the voltage regulation of a generator is 10%, it means that terminal voltage increases 10% as the load is changed from full load to no load