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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