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Thread: External Alternator Overvoltage Problems

  1. #11
    Senior Member jrevens's Avatar
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    Default Re: External Alternator Overvoltage Problems

    Having “no where for that power to go” does not damage the alternator, Josh.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE

  2. #12

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    Default Re: External Alternator Overvoltage Problems

    I am not sure why the regulator blew on the alternator. Perhaps it was the charge remaining inside, or it was disconnecting the field wire. Or something to do with the way it was wired into the bus and battery?

    Rewiring it has solved the problem so far. Leaving it permanently connected and not having a switch for it at all. I also upgraded the rectifier to the all aluminium Silent Hectic German rectifier that has a 37A rating, and have mounted that on the firewall where there is good air circulation. It looks a really solid piece of engineering and a definite upgrade to the standard Ducati one.

    Sorting the alternator created more problems however. The grounding of the alternator charge through the engine resulted in over reading of all the resistive senders by over 20 degrees C. So oil, CHT and EGT all went up while the engine was running and then would fall instantly by 20 C when the engine was off.

    This has been partially fixed by adding a large 30mm grounding cable directly from the alternator casing to the negative battery terminal.

  3. #13
    Senior Member Esser's Avatar
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    Default Re: External Alternator Overvoltage Problems

    Quote Originally Posted by jrevens View Post
    Having “no where for that power to go” does not damage the alternator, Josh.
    This isn’t my original thought so maybe I didn’t clarify myself. Here is an excerpt from a post at Rotax owners.com

    “When disabling an alternator from the system, you remove power from the field windings.
    The Output remains attached to the Battery.
    If you are removing the Output from the battery with the field still energized, The regulator loses its reference and the voltage can rise high enough to destroy the regulator.
    By removing the Field supply the Alternator output goes to zero.
    The regulator Diodes prevent any reverse currents to the alternator from the battery.
    It just quietly goes to sleep.”
    ------------------
    Josh Esser
    Flying SS7
    Rotax 914iS
    AirMaster Prop

    Edmonton, AB, CWL3

  4. #14
    Senior Member jrevens's Avatar
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    Default Re: External Alternator Overvoltage Problems

    OK, I don’t have that diagram at hand, but I understand now. Yes, the regulator/rectifier could be damaged if it was still connected to the output while the battery is disconnected, and while the engine is running and the field is still connected and being self-energized by the alternator.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE

  5. #15
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    Default Re: External Alternator Overvoltage Problems

    So, turning off the ignition switches first is the correct way to shut down?

  6. #16
    Super Moderator Av8r3400's Avatar
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    Default Re: External Alternator Overvoltage Problems

    My shut down checklist - MMMM in this order:

    Miscellaneous - All electrical loads, radios, lights, etc.
    Mixture - NA for Rotax, but that's part of the routine
    Mags - Ignition modules
    Master - Always last, after everything else

    Shutting off the master before the ignition will all but guarantee the failure of the voltage regulator.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
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