Compound Generators

Series-wound and shunt-wound generators have a drawback in which changes in load current cause modifications in generator output voltage. Various applications in these generators are used to need a more stable output voltage than could be supplied through a series-wound or shunt- wound generator. A single means of supplying a stable output voltage is through using a compound generator.

A compound generator has a field winding in parallel along with the generator armature (the similar as a shunt-wound generator) and a field

                                                    Figure: Compounded DC Generator

winding in series along with the generator armature (the similar as a series-wound generator) which was shown in the figure.The two windings of the compounded generator are made like that their magnetic fields will either aid or oppose to each other.The generator is said to be differentially-compounded if the two fields are wound so that their flux fields oppose each other.  Due to the nature of this type of generator, it is used just in special cases and will not be declared follower in this text.

The generator is said to be cumulatively-compounded if the two fields of a compound generator are wound so which their magnetic fields aid one another. As the load current rise, the current by the series field winding increases, increasing the whole magnetic field strength and causing an increase in the output voltage of the generator. Along with proper design, an increase in the magnetic field strength of the series winding will compensate for the decrease in shunt field strength. Thus, the whole strength of the merged magnetic fields remains almost unchanged, so the output voltage will remain constant. Within reality, the two fields cannot be made so in which their magnetic field strengths compensate for every other completely. There will be a few modifications in outcome voltage from the no-load to full-load conditions.

Within practical compounded generators, a change in output voltage from no-load to full-load is less than 5 %. A generator along with this feature is said to be flat-compounded display in the Figure.

                                Figure: Voltage-vs-Current for a Compounded DC Generator

For a few applications, the series winding is wound so in which it overcompensates for a modification in the shunt field. The outcome gradually rises along with increasing load current over the normal operating range of the machine. This category of generator is known as an over-compounded generator. The series winding could also be wound so in which it under compensates for the change in shunt field strength. The output voltage decreases gradually within an increase in load current. This kind of generator is known as an under-compounded generator.