Compound Generators
Compound Generator
Series-wound and shunt-wound generators have a disadvantage in that
changes in load current cause changes in generator output voltage. Many
applications in which generators are used require a more stable output
voltage than can be supplied by a series-wound or shunt-wound generator.
One means of supplying a stable output voltage is by using a compound
generator.
The compound generator has a field winding in parallel with the
generator armature (the same as a shunt-wound generator) and a field
winding in series with the generator armature (the same as a series-wound generator) (Figure 1).
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| Figure 1 Compounded DC Generator |
The two windings of the compounded generator are made such that their magnetic fields will either aid or oppose one another.
If the two fields are wound so that their flux fields oppose one another, the generator is said to be differentially-compounded. Due to the nature of this type of generator, it is used only in special cases and will not be discussed further in this text.
If the two fields of a compound generator are wound so that their magnetic fields aid one another, the generator is said to be cumulatively-compounded.
As the load current increases, the current through the series field
winding increases, increasing the overall magnetic field strength and
causing an increase in the output voltage of the generator. With proper
design, the increase in the magnetic field strength of the series
winding will compensate for the decrease in shunt field strength.
Therefore, the overall strength of the combined magnetic fields remains
almost unchanged, so the output voltage will remain constant. In
reality, the two fields cannot be made so that their magnetic field
strengths compensate for each other completely. There will be some
change in output voltage from the no-load to full-load conditions.
In practical compounded generators, the change in output voltage from
no-load to full-load is less than 5 percent. A generator with this
characteristic is said to be flat-compounded (Figure 2).
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| Figure 2 Voltage-vs-Current for a Compounded DC Generator |
For some applications, the series winding is wound so that it
overcompensates for a change in the shunt field. The output gradually
rises with increasing load current over the normal operating range of
the machine. This type of generator is called an over-compounded
generator. The series winding can also be wound so that it
under-compensates for the change in shunt field strength. The output
voltage decreases gradually with an increase in load current. This type
of generator is called an under-compounded generator.
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