What is the maximum voltage (RMS and pk-pk) of the AC generator in the 912 ULS? The voltage should be at maximum when the engine is at max RPM and no load is connected (not even the Ducati RR). I've heard some people say it's only 40V and some people say it's up to 100V at 800Hz.

What's correct? What is the RMS and what is the peak to peak voltage in that condition (max RPM and no load)?

  • Re: AC generator maximum voltage

    by » 3 months ago


    Open circuit voltage

    peak                     1/60 volt/RPM 

    effective (RMS)   1/120 volt/RPM

    Frequency             1/12 hz/RPM


    At 6000 RPM, 100 V peak, 50 V RMS, 500 hz

    Thank you said by: Joe

  • Re: AC generator maximum voltage

    by » 3 months ago

    Thanks! Where did you find that info? I couldn't find it in any of the manuals unless I missed something.

    So in other words if the regulator fails in the worst way possible (fail closed instead of fail open), the electrical bus would be subject to a maximum of 50 Vrms.

  • Re: AC generator maximum voltage

    by » 3 months ago

    This type of data is outside the scope of the Rotax manuals. It’s from the results of a French university study project of the stator.  You can see the English translation of the results here;



    Open circuit stator voltage is not a failure condition, it’s part of the normal operation. This is a switched bridge regulator, not a shunt regulator. Once the regulator reaches the set point it turns off, open circuit. 

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