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Self–Excited GeneratorsNew Microsoft PowerPoint Presentation.jpg
The excitation system of a self–excited generator is powered, via the automatic voltage
regulator (AVR), by tapping (shunting) power from the generator power output. The
voltage regulator senses generator output voltage and frequency, compares them to
reference values and then supplies a regulated DC output to the exciter field windings.
The exciter field induces an AC output in the exciter rotor, which is on the rotating,
engine–driven generator shaft. Exciter output is rectified by the rotating diodes, also on
the generator shaft, to supply DC for the main rotor (generator field). The voltage
regulator increases or decreases exciter current as it senses changes in output voltage
and frequency due to changes in load, thus increasing or decreasing the generator field
strength. Generator output is directly proportional to field strength. Refer to Figure 4–2.
Typically, a self–excited generator excitation system is the least expensive system
available from a manufacturer. It provides good service under all operating conditions
when the generator set is sized properly for the application. The advantage of a
self–excited system over a separately–excited system is that the self–excited system is
inherently self protecting under symmetrical short circuit conditions because the field
“collapses”. Because of this, a main line circuit breaker for protecting the generator and
the conductors to the first level of distribution may not be considered necessary, further
reducing the installed cost of the system.