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912 PM generator disconnect when engine running

in General Tech Discussion 9 years ago
Hi,

I am looking at implementing over voltage protection circuitry which would require an additional relay that effectively cuts off the Rotax 912 built in charging PM generator from the battery and the main bus.

This 'cut-off' relay can be inserted either on the wire which is the output of the rectifier-regulator or on the wire which feeds into the rectifier-regulator (carrying A/C current).

Any thoughts on which approach would be better?... what are the chances that the charging system and/or components are damaged if the battery is disconnected from the rectifier-regulator output, effectively "floating" the output of the charging system?

Any suggestions welcome.

Thanks,
M

  • Re: 912 PM generator disconnect when engine running

    by » 9 years ago

    It would be a big help if you had a drawing of your plans.
    -The regulator cannot store energy so it would fail if you removed the output only. The capacitor is installed for this reason; it acts like a battery and absorbs whatever is put out by the regulator when the master switch is shut off.
    -Bob Nuckolls of Aero electric has nice drawings and components for overvoltage protection. http://www.aeroelectric.com/
    As for his Crowbar overvoltage system.
      
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  • Re: 912 PM generator disconnect when engine running

    by » 9 years ago

    Ok,

    I've included two rudimentary drawings... not production quality so fair warning... but they do illustrate the placement before vs. after the regulator.

    I've looked over the schematics at aeroelectric ... some indication that it is best to place the relay before the rectifier/regulator otherwise a dummy load may be required when the battery is disconnected... I would not trust the capacitor alone to "save" the regulator... if it requires some type of load to be present...

    Thx
    Before.jpg (You do not have access to download this file.)
    After.jpg (You do not have access to download this file.)
      
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  • Re: 912 PM generator disconnect when engine running

    by » 9 years ago

    There is some ready for use over voltage protectors. Look for units from 14v piper or 14v Cessna.
    I'm not sure if this relay will not open accidentally when you'll use hi-load (starter for eg.).
      
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  • Re: 912 PM generator disconnect when engine running

    by » 9 years ago

    There is some ready for use over voltage protectors. Look for units from 14v piper or 14v Cessna.
    I'm not sure if this relay will not open accidentally when you'll use hi-load (starter for eg.).


    Relay only needs to handle 25A max... well within the range of the relay... starter circuit is separate... will be using Aerpoelectric's Crowbar OV gizmo... super lightweight :)

    ... as i understand the problem ... the decision to make is wether to place the relay on the wire feeding the rectifier/regulator or on the wire connecting the rectifier/regulator to the avionics bus/battery... i'll call these "before" and "after" placement from the stanpoint of the rectifier/regulator.

    In an "after" placement scenario, one has to overcome the effect of the disappearing battery, in case of a disconnect, efffectively the power generated has nowhere to go... this can damage the rectifier/regulator so a dummy load is required and the rectifier/regulator should then convert all excess power into heat (lots of heat).

    I was suggested a "before" placement, in which case the AC fed to the regulator/rectifier is cut off... I am wondering if this will result in the coils getting damaged eventually...

    Cheers.
      
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  • Re: 912 PM generator disconnect when engine running

    by » 9 years ago

    the coils will not be damaged at all. unless the yellow wires are connected together or grounded.
      
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