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I'm familiar with magnetos from aircraft engines, and alternators from car engines. I'm learning more about motorcycle systems now, and studying stator and rotor systems. This article has helped me.

My question is if these two systems are equivalent:

  • A magneto system like in a lawnmower, with a permanent magnet on a flywheel passing an induction coil and transformer, connected to the spark plug.

  • A stator with several sets of windings on it, and a flywheel rotor with permanent magnets on it, connected to the spark plug through a coil.

I know we then attach regulator and rectifier packs to then create useful DC power, but to just run the engine, that's not required, correct? That is to say I could start the engine with no battery at all, like a magneto?

I understand there's another form of stator and rotor, where power from the battery is passed through a coil to create an electromagnet, which then takes the role of the permanent magnet rotor in my previous example. Why would I do this? Is it so I don't need a regulator, because I can modulate the strength of the electromagnetic field? I would still need a rectifier in that system though, is that right?

Please feel free to point me to a good article covering this, I've failed to find one! Thank you.

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    Title sounds like the worst issue of X-Men ever.
    – Jason C
    Commented Aug 16, 2016 at 14:51

1 Answer 1

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A magneto is an alternator with a permanent magnet. Your two examples may differ in the way that they produce a higher voltage, through electromagnetic induction, but they appear to be functionally equivalent. An AC or DC source with sufficiently high voltage may be used to generate the arc gap required for your spark plug by way of coils and/or transformers.

Simple engine systems (i.e. lawnmowers) don't need regulated DC to power the spark plug, therefore don't need a rectifier/regulator or battery. See here for more info on magneto ignition theory.

As explained in this article, alternators require air cooling while magnetos have the advantage of being internally packaged due to their higher temperature tolerance. See here for more info on the history of thermal design challenges in modern alternators which typically integrate the rectifier/regulator.

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  • I'm not sure I understand your last question.
    – Jake
    Commented Aug 16, 2016 at 13:07
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    I think the OP's last question boils down to "Why would I choose a rotating field coil (alternator w/ rotor and stator) over a fixed coil (magneto)?" Check out gforceconsulting.com/A_primer_on_ATV_charging_systems.pdf - there they talk about a typical alternator needing to be air-cooled and open to the environment thus risk from damage from debris, vs. a magneto contained in the engine package. And some other differences too. Could be some good info there to add. This could also explain the use in a lawnmower where debris is flying everywhere.
    – Jason C
    Commented Aug 16, 2016 at 14:56
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    @JasonC - added details per your suggestion. thx
    – Jake
    Commented Aug 16, 2016 at 15:39
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    Nice answer +1. Thanks for contributing. Welcome to the site, cheers! Commented Aug 16, 2016 at 19:09

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