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Why is external DC power needed for Alternators with Permanent Magnets on start

in 912iS Technical Questions 6 years ago

  • Re: Why is external DC power needed for Alternators with Permanent Magnets on start

    by » 6 years ago

    Hi Allen,
    I don't know, I will have to pass this question to someone with a better knowledge of the EMU.
    I was told that voltage can be present, despite a failure, so it is not a reliable indication.
    The best solution would be a shunt on both gen outputs but this gets complicated and increases the parts/connections/failure points. It is also unnecessary as we already get a warning of discharge when the gens switch over.
    Thank you said by: Allen Inks
      
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  • Re: Why is external DC power needed for Alternators with Permanent Magnets on start

    by » 6 years ago

    The Lane/Reg/Gen"A" Voltage is normally the voltage on the EMS internal Bus.
    The Lane/Reg/Gen"B" Voltage is normally the voltage on the Airframe Systems. Battery, Panel, etc.
    The Two Generators are not in parallel and will show separate but similar voltage.
    If one generator fails the two voltage will differ significantly.
    ~ ~ ~

    The X3 connector on the Fuse Box has three pins.

    The BACKUP Power Pin(#1) ties directly to the EMS internal power Bus for emergency use only.
    It bypasses everything.

    The START Power Pin(#2) connects to the EMS internal power bus through a relay.
    When the engine is stopped the relay closes and allow START Power to energize the EMS.
    Once the engine starts and the Reg"A" comes online, The START Power relay Opens and isolates the EMS from the Airframe Main Bus (the big 12v Battery.) and Reg"A" takes over powering the EMS.

    The Main Power Pin(#3) connects internally to Reg"B" when the engine is runnings normally. This is the power source to charge the battery and operate all the airframe loads.
    When the Engine Stops or the Reg"A" fails, Reg"B"s Output is removed from the X3 Main Power Pin and transfers to the EMS internal bus to maintain the ignition.
    The Airframe power is supplied by the Big 12v battery for as long as it will last. 30 minutes???


    [quote]What indication is there of an Alternator A failure?[/quote]

    The Lane"A" Lamp will illuminate and the airframe power buss Amp Meter will show a DISCHARGE from the battery.

    In the double rare event that BOTH REGs fail simultaneously, the engine will stop.
    The emergency procedure is to activate the Emergency Bypass circuit that connects the big 12v Battery directly to the EMS internal buss and attempt a restart.
    In this case, the main battery is the only source of power for everything and draining fast. Land Soon!
    Leave the START Power Switch ON for extra confidence.

    This is the reason that a lower capacity, higher current, Lithium Battery is NOT a good option for the 912is engine. It will start the engine just fine but if ever need, unlike the 912ULS engine that can run forever on its internal magnetos, the 912is engine will only continue to operate for as long as the battery lasts.
    ~ ~ ~

    A seldom documented option in the case that the Reg "A" FAILS and Reg"B" takes over the EMS Bus leaving the Airframe loads to the Main Battery and you are in a position where the battery is not expected to last until touchdown, Or you need IFR Instruments, or it is Night, etc., ... try this...

    The Reg"B" generator has much more capacity than the Reg"A" Generator that it is replacing internally.
    Before the Main Battery gets too far discharged, turn OFF all electrical Airframe loads.
    Then Activate The Emergency BACKUP Switch.
    This will allow you to "Steal" some of the excess power from the EMS internal bus.

    Watch the Amp Meter closely for a battery CHARGE indication.
    One by One, turn ON any critically need equipment while watching the Amp Meter.
    As long as it stays on the CHARGE Side of the indicator, The Generator is holding its own.
    This will allow you to at least get some flight instruments and a radio working long enough to make an airport, and not be restricted by the battery's limited capacity.

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

    Thank you said by: Allen Inks, Paul Simmonds-Short
      
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  • Re: Why is external DC power needed for Alternators with Permanent Magnets on start

    by » 6 years ago

    THIS is good and valuable information. Thanks a bunch!
      
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