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External Alternator Over Voltage

in 912 / 914 Technical Questions 7 years ago

  • Re: External Alternator Over Voltage

    by » 7 years ago

    I am building a SuperSTOL with the 914 and I am doing as Rob suggests--using the integrated generator to run only the main fuel pump. Under normal circumstances, the generator will be connected to the main bus only for a couple of seconds to start up the regulator-rectifier (using a momentary contact switch on the C wire). That switch is actually a three position switch with up position being the momentary contact for starting the regulator-rectifier, center position "off" and bottom position "on" for a relay for connecting the generator to the main bus. So if I really needed to, I could connect the generator to the bus. This switch also allows me to run the main fuel pump with the engine off, should I need to do that for maintenance or trouble shooting.

    One nice thing about this is that it is impossible to turn off the main fuel pump by hitting the wrong switch. The only way to turn off the main fuel pump is to pull the 10 amp panel breaker.

    Although the Rotax diagram shows both the generator and the alternator being connected to the bus, others have reported problems and I see nothing to be gained by running them both connected to the bus at the same time.

    Since the main pump is powered by the generator, it naturally stops in the event of a crash when the engine stops turning. I have wired the auxiliary pump to a battery bus, going through a pressure switch activated by oil pressure. So, if the auxiliary pump is on during a crash, it too will stop when the engine stops producing oil pressure.

    On the panel, this translates to the following switches and breakers:

    Master switch ON - OFF
    Auxiliary fuel pump switch ON - OFF
    Generator activation switch (ON) - OFF - ON
    7.5 amp breaker for alternator field wire
    10 amp breaker for main fuel pump

    Start procedure would look something like this:

    1. Turn on master switch
    2. Turn on auxiliary fuel pump
    3. Start engine
    4. Observe auxiliary fuel pump light on
    5. Activate generator using momentary contact switch
    6. Observe main fuel pump light
    7. Turn off auxiliary fuel pump
    8. Observe auxiliary fuel pump light off
    9. Check bus voltage (reflects alternator output)

    I am attaching a photograph of my diagram. All comments welcomed.

    Rob, Bill -- regarding "alternator" vs "generator" Bob Nuckolls refers to it as a "dynamo."

    electrical-diagram.jpg (You do not have access to download this file.)
    electrical-diagram_2017-08-22.jpg (You do not have access to download this file.)
    Thank you said by: Amer Karim
      
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  • Re: External Alternator Over Voltage

    by » 7 years ago

    Many thanks for all the information, drawings and your time.

    I finally understand where things went wrong now and have learnt buckets about electrics and wiring in the last week.

    I managed to get the external alternator fixed for 10 bucks by changing the regulator plate at my local garage, and I have rewired the system so that I can disconnect the alternator and generator on demand using a set of relays as recommended. I am presently running with both the external alternator and internal generator on for now. The secondary pump is wired to the BUS and the first pump is wired to the generator as recommended.

    I don't think my load will ever exceed the 40A provided by the alternator, but there is no harm in having the generator connected as well as it is running the fuel pump and also in case the alternator develops a problem mid-flight. I also have 2 Li-Ion batteries installed, 1 x 36Ah and 1 x 6Ah for backup. I can also disconnect all the unnecessary electrical loads from switches so fingers crossed will be ok with some electrical endurance if there is ever a power problem.

    Thanks again for all your help

    Kindest regards

    Amer
      
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  • Re: External Alternator Over Voltage

    by » 7 years ago

    ... Bill, many, many years ago I asked a Rotax engineer to explain why the internal gen was not called an alternator. He had an explanation; although it could also be called an alternator it is technically a permanent magnet generator even though it does not have brushes and produces AC. I don't remember the finer details.


    I stand corrected.
    The permanent Magnets make it qualify as a Generator/Dynamo by my classification.
    The Early Automotive Generators used Commutators/Brushes to rectify the AC.
    Diodes are the norm today.

    The Alternator uses its Field Windings to replace the Permanent magnets.
    This gives the advantage that the field strength can be adjusted in real time to produce full output even at low rpms.

    The Generator with its fixed magnets tends to lose power at low RPMs.
    The generator will self start.
    The alternator often requires external power to produce the initial magnetic field.
    If there is a small amount of magnetism remaining in the frame of the alternator it will often self-start.
    Portable, battery-less, Emergency Generators use alternators that rely on this for initial startup.

    The Generator vs. Alternator argument is just semantics.
    All generators are technically (AC) Alternators at their core.
    Most portable Emergency Generators are motor driven Alternators, despite their common name.

    Potatoes, Tomatoes!

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

      
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  • Re: External Alternator Over Voltage

    by » 7 years ago

    Hi....i am anew user here. I will suggest you should use a meter and see what the internal output is to the VR and then switch and see what the external output is. That would be a good place to start.

    bittele
      
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  • Re: External Alternator Over Voltage

    by » 5 years ago

    Gentlemen,

    Where can I find a copy of the Rotax wiring diagram for the external alternator installation?  Several of you have referred to this document and John at Kitfox, in a phone conversation, also referred to it.  The current Rotax 914 Installation Manual, Ed 2, Rev 0, July 2008 doesn't have this diagram.  Is this elusive diagram in an earlier copy of the IM?  And if so could someone please provide me with a copy?  

    Jeffrey Fritts, USAF (ret.)

    www.flywwlsa.com

    "In aircraft maintenance, good enough is not good enough."

      
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