self-excited generators?

These are synchronous generators with a wound rotor, often two rotor
windings in parallel; each completeing it's circuit with a diode.
Common technology found in low-cost portable gasoline and deisel
generators 1-5kW range. There is always an excitation or auxilliary
winding within the stator and an capacitor attached (25uf is common
value). This provides for and sustains the self-excitation of the
rotor windings in a sort of L/C quasi-resonance at twice the AC
frequency (or second harmonic) coming from rotor diode rectification.
My question is in regards to the start-up characteristics of these
types of self-excitied generators. Some models and brands start (jump
into self-excitation) easily when there is no-load some start better
with some moderate load. The value of the auxilliary capacitor
affects self-voltage regulation and the threshold RPM where
self-excitation rapidly ramps up. Value of capacitor for optimal
voltage regulation does not inherently over-lap with value of
capacitor for best self-excitation start-up profile (a smooth linearly
rising Vout/RPM).
Some brands and models make use of cast aluminum shorting bars, much
like is found on the rotor of an induction motor. "Damper Windings"
is what this is often called when on a synchronous machine.
Does anyone have some experience with the damper winding design and
it's affects on Vout/RPM start-up profile as the generator goes from
non-rotating to full-speed rotating? Any speculations on effects and
relationships between all of these generator design features (damper
windings, auxillary cap/winding combo, rotor winding, air gap)?
I'm getting lots of production variations and I'm not sure what design
parameters strongly influence the whole self-excitation operation. It
seems it can be quite bi-stable, requiring the generator to virtually
be running at full RPM before it will produce any output voltage.
Who has, or where can be found...some practical design guidelines for
these small self-excited generators?
~Terry Moreau
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Terry Moreau
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