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Characteristics of DC Compound Generator

Related Topics
Construction of DC Generator
Types of DC Generators
Losses in DC Machine
Armature Reaction in DC Machines
Compensating Windings and Interpoles
Commutation in DC Machines
DC Generator Characteristics
Characteristics of Separately Excited Generators
Characteristics of DC Series Generator
Characteristics of Shunt DC Generator
Characteristics of Compound DC Generators
Parallel Operation of DC Generators
Parallel Operation of DC Shunt Generators
Parallel Operation of DC Series Generators
Parallel Operation of DC Compound Generators

In a compound generator, both series and shunt excitation are combined as shown in figure.
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.

External characteristic

The external characteristics of a cumulatively compounded generator is shown in the figure. The series excitation aids the shunt excitation. The degree of compounding depends upon the increase in series excitation with the increase in load current.
  • 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.
  • 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.
  • 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. Such a machine is called under-compounded generator.