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A bit of perhaps helpful clarity ...
 
The built-in power electrical power source is an alternator, though Rotax itself often refers to it in most places as a generator as do many others in discussions.
 
A generator rotates the power coils (armature) in a stationary fixed magnetic field.
 
An alternator has stationary power coils (stator) with rotating magnetic field creator. This is the case for the Rotax! The crankshaft rotates a disk containing permanent magnets inside stationary power coils. But since it uses permanent magnets rather than an excited field coil, it's more-accurately called a MAGNETO! One place Rotax labels it so is in the Illustrated Parts Manual where they call this rotating disk a MAGNETO!
 
Compared to a typical excited-field alternator, magnetos can't produce nearly the power because the magnetic field of (cheap) permanent magnets is quite low. Hence the 250W rating on this one, just like my 1998 Ducati.
 
On the other hand, without excited fields, a magneto will charge a completely dead battery, which a normal alternator has a problem doing. A magneto however, acts like a generator in that output is very rpm dependent, so  you need the ~5k rpm to get full output of 18A. (Though more than 12A continuous is a hazard to Italian stator windings in the poorly cooled 912/914 setup).
 
This led to a couple of other realizations. One is that my alternator on/off switch is  killing the power from the stator coil circuit, not the field coils, because a MAGNETO doesn't have them! Another is that MAG A & B switches for the 912/914 really are what they say. Since we have CD ignition modules, I'd assumed that the labels came from quaint airplane tradition. But no, the electronic ignition modules really are powered by a magneto after all.
 
  • Re: Generator, no. Alternator, close. It's a MAGNETO!

    by » 6 months ago


    Hi Jeffry

    An interesting point of view, however I think you missed the point of why Rotax prefers to call it a generator.  First, we don't want to confuse people with aircraft typical "magnetos", they are so different in function and design and require constant maintenance as they employ parts that wear.  In the version used by Rotax the only moving part is the crankshaft driven flywheel with the permanent magnets.  The ignition function is charging the SMD modules for the ignition side of things, it is not part of the 250 W electrical part of the stator.  The ignition has 2 separate coils, one for each module.  

    So is the generator a magneto?  yes for sure but it is not at all like a magneto used in a Lycoming or Conti engine as we know it.  Within the manuals you will see Rotax uses both terms, magneto and generator.  The design is a classical description of a magneto where the function is to generate AC power...I don't understand how that bit is hard to follow? I will still call it  generator because that is also correct.

    And by the way, the 250W part of the stator rarely fails in my experience.  Most of the failures are with regulators (another lovely Ducati part)  Last thing, the kill wires go into the modules, it is a normally hot system.  To shut off the engine we ground the ignition output via the modules.  Indirectly I guess that is the charge coil for each side.  Some very good detail in the whole system, with system description, can be found in the maintenance manual heavy, MMH.  

    Cheers


    Thank you said by: Sean Griffin

  • Re: Generator, no. Alternator, close. It's a MAGNETO!

    by » 6 months ago


    I’ve always referred to it as a Permanent Magnet Alternator (PMA).  Same thing, different name.  Its also important to know that the Stator has eight coils to provide power to the battery/airframe, but those don’t power the ignition modules.  In addition to those eight coils, each ignition module has its own dedicated charging coil and two trigger coils on the Stator.  So if something happens to the alternator Stator coils, the ignition charging coils and trigger coils are not affected. In other words, the ignition coils are isolated and redundant. The mag switches act upon the charging coils only.  There is also on extra coil to drive the tach.  

    Sorry to be redundant Rotax Wizard, we must have been typing at the same time!  


    Thank you said by: Rotax Wizard

  • Re: Generator, no. Alternator, close. It's a MAGNETO!

    by » 6 months ago


    The word "generator" (when used in an electrical context) refers to all devices that produce/generate an electrical current, ie electrical generator.

    Then there are the specific types eg alternator, dynamo, etc. - all generators!

    It seems to me that we should try to be very much more precise (disciplined?) in our us of technical language, when discussing engines.

     


  • Re: Generator, no. Alternator, close. It's a MAGNETO!

    by » 6 months ago


    Hi again all

    Well this is fun....

    In checking with my electrical references within the Rotax world here is what they state.  The system is indeed a magneto and it is a generator.  No question there.  The references in the manuals are correct in that the reasoning is more engineering and not intended to mislead anyone.  

    If we look in the description of the MMH for the 912/914 we find a number of references to magneto, we find a lot more on generator.  Generator as Sean pointed out is more general and almost anything is a generator.  When Rotax references the ignition we find it is consistent with its description.  The stator is the part that has the windings and it is mounted on the block, fixed in place.  The rotating flywheel that has the permanent magnets are referred to as...wait for it...Ignition magneto generator. 

    It gets better, here is the technical description"

    "INTERNAL GENERATOR
    Consisting of stator with 8 generator coils and the in.
    dependent ignition charging coils and the 10-pole
    magneto ring
    The fly wheel hub (ignition triggering) is attached to
    the magneto ring
    The 4 trigger coils are fitted externally on the
    alternator."

    So depending on which viewpoint you take it is generator, magneto, take your pick.  For the factory the flywheel has the magnets so they feel their description is technically accurate.  By the way the section this is found in is called 

    Chapter: 24–20–00

    INTERNAL GENERATOR

    all good fun, Cheers

    37621_2_MMH sec 24.jpg (You do not have access to download this file.)
    37621_2_MMH sec 24 dis.jpg (You do not have access to download this file.)

    Thank you said by: Sean Griffin

  • Re: Generator, no. Alternator, close. It's a MAGNETO!

    by » 6 months ago


    Ok, we can keep this going a bit longer!?   Sean wants precision so….

    A generator is a device that converts mechanical energy into electrical energy, either alternating current (AC) or direct current (DC).  An alternator is the same except that it specifically refers to a device the produces AC voltage, which is what the Rotax generator/alternator does.   Soooo, “Generator” is correct, but “Alternator” is more precise.  “Magneto” is a type of generator/alternator and infers that permanent magnets are used to create the rotating magnetic field; so yes this is correct but not precise because the term Magneto does not specify an AC waveform output.  In all other industries these devices, such as used in the Rotax Engines, are referred to as Permanent Magnet Alternators (PMA), which is very precise.  


    Thank you said by: Sean Griffin

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