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Electrical supply system overcload condition

in 912 / 914 Technical Questions 7 years ago
Hi Everyone,

What would happen if the electrical supply system where to become overloaded? Would the 25A slow fuse blow? Or more simply, would only the system voltage be affected ?

Looking for details about the regulator/rectifier annunciator light. What does it exactly mean when it turns on ? Does it mean that battery voltage is higher than reg/rec voltage? Would it turn on during excessive demand of the supply system?

Also, the installation on the Pipistrel Virus has both of the AC alternator output leads passing through 20A fuses before going to the reg/rec. The Rotax installation manual does not mention the need of such fuses. Are they of the right capacity? Would they be expected to blow during excessive demand ? The installation also has the recommended 25A slow blowing fuse.

Blue skies

Denis Ranque

  • Re: Electrical supply system overcload condition

    by » 7 years ago

    As typical, Rotax does not release all the exact details but here is a reasonable explanation...

    Fuses are there to protect the wiring, not the components.
    It all depends on where the Overload comes from.

    If you are just using more current than the Alternator can generate, the battery supplies the rest.
    Ex. Everything Powered up, Fans running, Taxi Lights ON, Alternator output low because the engine is at ground Idle.

    The other extreme!
    The battery wire chafes through and you now have a catastrophic Short to ground.
    The Fuses blow, everything goes dead and an electrical fire is averted.

    Yes, the idiot light just means the Battery voltage is well above the Regulator output voltage.
    You will typically see this only when the engine is stopped or the Regulator has blown.
    The Fuses in the AC leads are just extra insurance against wire chaffing.
    Depending on the specific design, the two 20A fuse could be on different phases and are the equivalent of a 40A fuse.
    The 25Amp main fuse would blow first under normal overload conditions.

    A well-designed plane should be able to power everything all at once and nothing the pilot can do should ever blow any fuses.
    The electrical system should be Idiot Proof.
    The only overload condition should only be the expected temporary excess demand when at Engine Idle or OFF.
    And this should only drain the Battery a bit until you get moving.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

    Thank you said by: Denis Ranque
      
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