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Magneto Theory

  1. May 3, 2015 #1
    My theory is rusty.
    A model for a magneto shows a magnet rotating in the "jaws" of what appears to be a closed soft iron frame. The resulting flux field is therefore contained in the frame, reversing at each revolution of the magnet.
    A primary coil is wound on this frame as is a secondary winding that feeds a spark plug. The primary coil is connected to a set of contacts that open once per revolution.
    The theory described states that the moving magnet creates a field in the iron frame that cuts and induces a current into the primary winding. The opening of the contacts causes a rapid drop in primary current and a subsequent change in primary flux which induces a high voltage into the secondary winding that feeds the spark.
    My question is if the rotating magnet produces a closed changing flux in the frame How can this flux cut the primary windings, to induce primary current. The flux is contained in the frame. What have I missed.
    Any Ideas
  2. jcsd
  3. May 3, 2015 #2


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    The flux contained within the iron frame still links to the coils through which the iron passes.
  4. May 3, 2015 #3
    Hi Merlin
    If the coil was on an open ended soft iron rod which was subjected to a rotating magnet then the rod would assume alternating magnetic properties. Have a north and south and the resulting flux although concentrated within the rod S-N would extend externally, N-S cutting the coil windings in the process, creating self inductance in the coil and producing EMF's.
    If the iron former is effectively closed then I am having trouble identifying an external field. I must be missing something.
  5. May 3, 2015 #4


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    I'm not sure about the flux cutting the coil. I think of it linking or passing through the coil.
    I think it's all about integrating flux through open surfaces and that gets messy with coils unless you simply look at the flux passing through the coil.
    If you believe a transformer works,
    Transformer3d_col3.svg.png then a magneto just uses a permanent magnet rotating instead of AC through the primary to produce an alternating field in the iron. The secondary doesn't know nor care how that flux was created, it just sees a changing flux through it.
    magneto.jpg magneto2.png
    PS. I see WikiP explains magneto operation differently than I do. I still believe myself, but perhaps you'd better be sceptical!
  6. May 3, 2015 #5
    Hi Merlin
    I learned my theory many years ago and just accepted the transformer explanation that the primary and secondary coils were liked by the flux in the common transformer core.
    It is only recently when I started to get interested in magnetos that I found that I did not know how the flux from a core actually influenced a coil mounted on the core. If those two magneto diagrams can be interpreted as not closed cores then they are just magnetic extensions of the rotor and then will influence the windings. But as for closed transformer cores, I have no problem seeing how the primary coil flux enters the core but its how the contained flux effects the secondary that is throwing me. I shall keep digging. Thanks for you interest
  7. May 3, 2015 #6

    jim hardy

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    Magneto is described in exruciating detail in Continental Motors publication IGN-51

    google finds it several places

    i had the good fortune to grow up next door to an aviation electrical specialist.
    He explained the outboard motor magneto to me , pointing out how the flux traversed the iron , and why it was important for the points to open at just the right time.

    it's same principle but mechanically a lot simpler.

    Synopsis for you:

    First: The wire doesn't have to be "cut" by flux, it only needs to encircle flux.
    Look up "right and rule" if it's not familiar. And "Faraday's Law" too.

    If that red flux is changing, voltage will be induced in the black wire.
    Current will flow in the black wire if it can.
    Interestingly, that current would make its own flux that tries to oppose any change in the red flux. Though It probably can't win it can briefly put up a good fight .
    That the "induced" current opposes changing flux is significant to magnetos. The effect is known as "Lenz's Law" and you'll encounter that term in your magneto studies.

    Next, two terms describing the position of the magnet need to be defined.
    1. "Full Register" position is the position of your second image.
    I'll use this one, it's smaller, from http://www.datwiki.net/page.php?id=...ating magnet (aircraft magneto)&searching=yes
    It's called that because the Magnetic poles on the rotor are aligned with the core. Flux in the core will be maximum then.

    2. Neutral position: it is just 90 degrees from full register position

    Aha they tried to slip one past us - changed from 2 pole to 4 pole rotor ....
    observe there'd be no flux in the core at neutral position.
    on your image that'd be with rotor poles vertical. Flux doesn't go through the coil it just takes the shortcut through those big circular shoes on the core , back to other pole of the rotor.
    So there'd be no flux through the coil core

    As you can see, spinning the rotor would give alternating flux through the coil core just as in an AC generator.
    But the voltage wouldn't be very high.

    Now we'll talk our way through a spark cycle using your magnet02 image. You'll have to rotate the magnetic rotor in your mind's eye.
    Starting condition: rotor at full register so flux is at or nearly at maximum traversing the core clockwise.
    breaker contact is open so there's no current anywhere

    Rotor turns clockwise.
    As rotor poles approach the edges of those laminated stator "shoes " that almost encircle it, , breaker contact closes.
    No or very little current flows because flux is not changing yet.

    Rotor continues clockwise. The poles start to no longer be covered by the laminated stator "shoes"
    Flux starts to decrease slightly.
    Current starts to flow in primary winding , opposing that change.

    Rotor continues clockwise, to neutral position.
    Flux doesn'tfall to zero - remember Lenz's Law? Primary current maintains magnetic flux.
    Primary current is now holding magnetic flux near maximum. But it's working to do so and can't keep it up for very long..

    Rotor continues a few degrees past neutral position. Let's just say eleven degrees.
    Rotor magnets are now beginning to push flux the other way, counterclockwise.
    Current in primary goes way up attempting to maintain flux against the force of the magnets, so it's really working now. .
    This is the instant you want the breaker points to open - the magnetic field now exists only because of primary current
    so interrupting primary current will cause a sudden collapse of the field , in fact it'll go a little past zero because the magnets are now pushing flux counterclockwise.
    that sudden collapse is what gives you the large induced voltage in both primary and secondary windings.

    Aha we got our spark.

    Rotor continues , and the cycle repeats every half revolution.

    Now - that small travel past neutral before the points open is called the "E GAP", for 'efficiency'. It's where you get best spark and i have no idea how to calculate it.
    But it's important to the magneto, and it's why point gap on a magneto is so much more critical than on battery and coil ignitions like cars.
    I chose eleven degrees because it's the number in that Continental link.

    I was in sixth grade and working on a 1951 Johnson 10hp outboard when my kindly neighbor explained the workings of the magneto, just about as i presented it here. Except we had the motor so it was easier to visualize.
    I learned to set E-GAP precisely. It is critical for good spark on a magneto. That's why the condition of points and the cam that operates them is so important.
    my test for optimum E-GAP was to hold the plug wire between thumb and forefinger with that hand resting on the motor, and turn the flywheel slowly with other hand. When E-GAP is right you feel a jolt even at that slow rate of rotation. I adjusted points for best jolt. .But don't try that with an electric starter, turn it slowly by hand.
    When the points cam is worn a couple thousandths you can still get proper E-GAP. When points close is not so critical as when they open.

    Aircraft magnetos have an "Impulse" mechanism that snaps the magnet through E-GAP to give you better spark when hand-propping the engine. I never set one of those. I think some tractors have them too.
    Turn your magneto by hand , if you feel and hear it "snap" it's an impulse type.

    To recap, the secret of magnetos is -
    Flux builds with points open
    Points close after flux reaches maximum - so primary current doesn't impede its rise (Lenz) .
    Magnets move out of position. leaving primary current in charge of flux,
    Flux is collapsed by opening points at same time as magnets begin trying to to reverse flux,,,,, because that's when current is maximum

    I hope this helps you guys. . I never had to analyze a magneto the goal was always to get them working.
    So i apologize for the hobbyist level of this post's content. Maybe with the basic operation in mind you can dig further.

    Search terms
    : magneto neutral full register E-Gap

    there exist small electronic modules that replace the points. If you find out how they work, please post a link. Lawnmower shops sell them.

    old jim

    ps that Continental link confuses me on figure 13 pdf page 16
    they're not consistent in numbering their degrees, i think they've mixed mechanical and electrical degrees on a 4 pole rotor...
    Last edited: May 3, 2015
  8. May 3, 2015 #7

    jim hardy

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    ps this is same model outboard i learned on. Mine was acquired as a bushel basket of parts for $6, lacked the green cover and looked much rattier

    a kid learns an awful lot with such a project.
    Mine served me well for two decades.
  9. May 4, 2015 #8


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    awesome post Jim
    not something I had ever or really needed to learn about, so learnt something new :smile:

    Its the second post of yours today that I have saved into a word file

  10. May 4, 2015 #9
    I see the conflict between your understanding and the replies. There are two different explanations in how current is generated in a conductor by a magnetic field.

    Your understanding is rule 'X'. There is some length of wire moving over a region of flux, B at some velocity. I think the equation is Voltage = B * length * velocity. The wire is cutting through the flux.

    The other one, rule 'Y' (it amounts to the same thing,actually) is that voltage induced on each loop of coil containing the magnetic flux is equal to the change of the magnetic flux passing through the loop per second.

    This is the one you need to use for a magneto. If I left out some constants or factors, I hope someone will correct me, but that's the basic idea.
    Last edited: May 4, 2015
  11. May 4, 2015 #10
    Thanks All, there are some good answers here. I haven't read them fully yet but I will.
    I was starting to think that it was "Fleming's" hand rule operating in reverse. All the books seem to deal with the current generating the fields, but it also works backwards. It is easy to draw the energy fields of a wire carrying current and how they expand outwards cutting each other (back EMF) and concentrate in the core. Not so easy to visualise the reverse when the physical position of the core doesn't seem to be getting cut by the core flux. I still wonder however if there is something else involved here like an electrostatic field that plays a part, but I'm getting out of my depth so I'll stay with electromagnetic effects..
  12. May 4, 2015 #11

    jim hardy

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    thanks dave

    those 1950's Johnsons were designed to run forever and are doing a good job of it.
    They're easy to modify for modern fuel pump(eliminates need for obsolete pressure tank) and ignition parts are readily available
    my local recycler gets one every couple months, i have rescued enough for all the grandkids.
    again this isn't a hobby forum
    but kids learn by doing
    and old machinery is simple enough they can learn the basics from it
  13. May 4, 2015 #12
    Just as a finisher, although I am interested in the flux behaviour in a magneto I probably should have directed my original question as to how does the flux in a torroidal transformer influence the output winding's current. All Lenz's description seem to focus around bar magnets or open ended flux sources that do exhibit an external field. As far as I know torroidals don't, just as a bar magnets formed in a circle don't. I can accept Jim's statement that the coil just has to encircle a field to be influenced by it but I still do not know how..............
  14. May 4, 2015 #13


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    found this on the net ......

    bottom of the page, diagram and text

    Toroidal transformer Poynting vector coupling from primary to secondary in the presence of total B field confinement

    In this figure, blue dots indicate where B flux from the primary current comes out of the picture and plus signs indicate where it goes into the picture.

    Explanation of the figure
    This figure shows the half section of a toroidal transformer. Quasi-static conditions are assumed, so the phase of each field is everywhere the same. The transformer, its windings and all things are distributed symmetrically about the axis of symmetry. The windings are such that there is no circumferential current. The requirements are met for full internal confinement of the B field due to the primary current. The core and primary winding are represented by the gray-brown torus. The primary winding is not shown, but the current in the winding at the cross section surface is shown as gold (or orange) ellipses. The B field caused by the primary current is entirely confined to the region enclosed by the primary winding (i.e. the core). Blue dots on the left hand cross section indicate that lines of B flux in the core come out of the left hand cross section. On the other cross section, blue plus signs indicate that the B flux enters there. The E field sourced from the primary currents is shown as green ellipses. The secondary winding is shown as a brown line coming directly down the axis of symmetry. In normal practice, the two ends of the secondary are connected together with a long wire that stays well away from the torus, but to maintain the absolute axial symmetry, the entire apparatus is envisioned as being inside a perfectly conductive sphere with the secondary wire "grounded" to the inside of the sphere at each end. The secondary is made of resistance wire, so there is no separate load. The E field along the secondary causes current in the secondary (yellow arrows) which causes a B field around the secondary (shown as blue ellipses). This B field fills space, including inside the transformer core, so in the end, there is continuous non-zero B field from the primary to the secondary, if the secondary is not open circuited. The cross product of the E field (sourced from primary currents) and the B field (sourced from the secondary currents) forms the Poynting vector which points from the primary toward the secondary.


    It basically answers the question, other than I am curious as to what the difference is when the secondary is windings on the toroid rather than a vertical wire down the centre axis of the toroid ??

    on a side note ... just using logic :rolleyes: :wink:

    if the varying current in the primary coil can create a totally confined varying magnetic field in the toroid core. The opposite is most likely probable .... hahaha sorry I couldn't resist :wink:

    Last edited: May 4, 2015
  15. May 5, 2015 #14

    jim hardy

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    that picture is interesting

    observe that one-turn secondary.
    They don't show it, but it makes a closed loop somewhere even if only by a voltmeter connected to its ends.
    So it wraps around(encircles) the flux that's enclosed inside that toroid.

    Note that toroid is the type that totally confines its flux to inside the primary windings. Looks to me like no flux gets out there to where the secondary is.

    So it's the flux enclosed by the loop.

    Induction, from Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html
  16. May 5, 2015 #15
    This drawing seems to introduces the E Field which exists outside the toroid...If my memory serves me right this is the "electric" field that is 90 degrees to a magnetic field. It also seems to be stating that it is the electric field that is inducing current into the secondary winding. Once the current is induced it produces its own magnetic field and electric field. The magnetic field becomes contained in the core but is in opposition to the primary becoming the "back emf" whilst the electric field interacts with all the other electric fields to presumably create the output energy. I don't think any external flux should exist in a perfect toroid.
  17. May 6, 2015 #16

    jim hardy

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    I think i got what you said, and if i did i agree.

    I have to assume they're representing AC excitation, else flux would have no rate of change so there'd be no E-field ? and no secondary current ?
    The way i think of it is quite mechanical. If one of those green ellipses were a length of tiny wire it'd have a voltage induced in itself, so there must be an E-field .

    The magnetic field must be changing (either in intensity or physical location) to make an electric field ?

    from Wiki on Faraday's law:

    i just bookmarked this site for more study: http://www.maxwells-equations.com/
    Last edited: May 6, 2015
  18. May 6, 2015 #17
    Sounds like action at a distance, doesn't it? Can anyone explain how it isn't?
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