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Electrical Load Currents - Peak and Continuous

in 912 / 914 Technical Questions 8 years ago

  • Re: Electrical Load Currents - Peak and Continuous

    by » 8 years ago

    Hi Ian,

    The 22K is only a minimum requirement. Higher is okay and make sure it is rated to at least 105F temp. I know many people that have higher caps over 22K and they have had them many years.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Electrical Load Currents - Peak and Continuous

    by » 8 years ago

    Well, I have to withdraw what I said earlier (amended in post #14123 above) - I was just shown an schematic of the rectifier/regulator module and it's evident that the DC output is not driven by that 22A mosfet, but rather by a pair of SCRs. (I was misled earlier by a parts list... I didn't see the Ducati schematic before - as the module is not serviceable its schematic is not part of any standard ROTAX documentation. The Ducati module's 22A published rating exactly matching that mosfet's 22A pulsed output rating is purely coincidental! The mosfet only drives the fault lamp output...)

    I looked up the DC output's SCRs (they cross reference to 2N6504) and they are rated for 25A at 85C (and their "all conduction angles" rating is 16A)

    So Roger, maybe your estimate of 16A continuous could be a very good 'conservative real-world number'. (If the firing angle of the SCRs varies with RPM, perhaps??) But I can't say for sure - as I said earlier I'm not an electrical engineer, so must defer to the experts at ROTAX and Ducati : they provide a 22 Amp rating for the module. Incidentally, the schematic I saw goes back several years, so it might even have updated in the meantime.

    The modeling of this power generating system is surprisingly complex - there's a lot of induction, resistance and capacitance to account for.

    Accordingly, (Roger's anecdotal evidence notwithstanding!) I do not advise that anyone second-guess ROTAX on their capacitor sizing specs, because sometimes bigger isn't always better! (as Ian alludes to in the post above...) Over-specing the cap's voltage rating isn't going to hurt, but changing the actual capacitance value may have undesirable side-effects!
    Thank you said by: Roger Lee
      
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  • Re: Electrical Load Currents - Peak and Continuous

    by » 8 years ago

    But you know what... (double-checking) in the 912 Installation manual it DOES specify "A capacitor... of at least 22000 µF/25 V". So Roger's advice could be interpreted as "going by the book". However, that's kind of an ambiguous specification!

    Are they talking "at least 22,000 uF" - or "at least 25V"?
    Or both??

    I have put in a request to the factory to clarify if there is any reason to put a ceiling on the capacitance value...
      
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  • Re: Electrical Load Currents - Peak and Continuous

    by » 8 years ago

    It seems unlikely that Rotax is using a "rule of thumb" per amp for rating the capacitors. I say this because in the 914 installation manual they specify the same capacitor for the 40 amp alternator that they do for the 22 amp generator. Again, they say "at least 22,000 / 25 volt" but from what I have seen Rotax does not seem to take any great pains in having their German documents professionally translated, so I'm not sure we can reliably parse the finer points of their English translations.

    Speaking of the capacitor for the 40 amp external denso type alternator ... is it really necessary? Is it performing a useful function? Or is it just something sitting there waiting to fail?
      
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  • Re: Electrical Load Currents - Peak and Continuous

    by » 8 years ago

    I think that's a typo? the capacitor on the 40 amp external alternator is just a noise filter, just like all the other external alternators on GA aircraft.
    http://www.aircraftspruce.com/catalog/elpages/lsAlternatorfilter.php?clickkey=3061939
      
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