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I'm reading about battery systems on https://www.kitplanes.com/understanding-lithium-batteries/ and it says this:

"Once bus voltage is sustained above 60 volts, the internal MOSFETS (metal oxide semiconductor field effect transistor) on the EarthX BMS may no longer be capable of maintaining protection. Some engines like the Rotax 912 are only capable of putting out 40 volts in their worst-case overvoltage condition, so not all aircraft charging systems need an automatic overvoltage protection circuit."

 

As I understand the Rotax has a 14.2±0.3V AC generator provides with a permanent magnet and self-excited field coil in the rotor, and the generator coils (plus the "magneto" coils and a coil for the tach, in the carb engines) in the stator. I have three questions about that worst case 40v overvoltage condition if anyone can answer them:

1. What condition or type of failure could cause the Rotax generator to produce such a high voltage?

2. Would it have any effect on the mag coils (I don't mean ignition coils) or only the generator ones?

3. For iS models which have dual generators instead of one generator and two mags, how do the ECUs respond?

 

Thanks!

  • Re: Generator overvoltage scenarios

    by » one month ago


    This is mostly an educted guess based on 45 years of repairing electronic circuits to the component level and after Magic Blue Smoke Events.

    If the MOSFETs Regulators overheat due to a current overload they will initialy fail in a short,   passing the the full rectified (40v) to the output bus.
    The MOSfets are already over rated and behind 30 fuses so the fuses should theoretically blow before the FETs overheat.
    Assuming the worse, 40 v dumping into the ~14v battery is certainly going to "Crowbar" the circuit to blow the fuses within a second.
    I would not expect the bus voltage to get much above 18v in the interim.
    I would also expect the Regulator have a "Crowbar" circuit in place to intentionally blow the fuses in the event of a runaway and the Voltage gets much above 16v.  Crowabr circuits are common and inexpensive to implement.
    Assuming the Fuses didn't blow the FETs have a better than even chance of melting internally and  the SHORT becoming an OPEN Circuit quickly.
    Essentially self-fusing.

    The Generator coils should survive just fine as long as everything resolves ln under a few tens of seconds.
    They have a lot of thermal Mass and, in the Injected engines, are "Cooled" by the Engine Oil.

    The iS model ECUs control a set of 50 amp relays within the FUSEBOX  that are capable of selecting either of the generators as their power source.
    Of course, if Both GENs were to short at the same time and the everything fuses out; Zero voltage means the engine stops unless BACKUP Battery power is selected to power the ECU Buss.

    Keep in mind that this scenario is a Hundred year event!


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


    Thank you said by: Glenn Martin, Pilot Joe

  • Re: Generator overvoltage scenarios

    by » one month ago


    The Rotax systems have backups upon bacups upon backups as Bill describes.

    A Lead acid battery will also fail with too high a voltage actually Lithium Phos has a slightly higher tolerance to both high Volts and amps.

    If you are paranoid and add too many layers of backups the Aircraft willl be very safe because it will be too heavy to fly !

    Rotax aproves the Earth X battery so install one and relax it will be fine....


  • Re: Generator overvoltage scenarios

    by » one month ago


    You might consider installing it outside the cabin, or venting it to the outside, as specified by EarthX. The biggest danger would be all of the noxious smoke that is given off in the event of a "meltdown."


  • Re: Generator overvoltage scenarios

    by » one month ago


    Thanks Bill. I think I was misunderstanding it. So the overvoltage failure would be a problem with the regulator, not with the generator itself? So does the generator produce 40v (I guess only at max RPM?) and only if the MOSFETs fail closed will it overvoltage the rest of the system?


  • Re: Generator overvoltage scenarios

    by » one month ago


    Glenn Martin wrote:

    The Rotax systems have backups upon bacups upon backups as Bill describes.

    A Lead acid battery will also fail with too high a voltage actually Lithium Phos has a slightly higher tolerance to both high Volts and amps.

    If you are paranoid and add too many layers of backups the Aircraft willl be very safe because it will be too heavy to fly !

    Rotax aproves the Earth X battery so install one and relax it will be fine....

    It's not that I'm worried about it but just curious to understand more about the systems.

    Do LFP batteries really have higher tolerance, or is it just that it has a protection circuit that the lead acid battery does not? I've got experience with LCO batteries and I know that an unprotected cell is much more vulnerable to overvoltage (runaway exothermal) and undervoltage (dendrite build up).


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