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I How does Lenz's law delay coil field collapse in magnetos?

  1. May 16, 2017 #1
    Perhaps the title of this post is not quite correct because I could not find a way to abbreviate what I am asking.

    Picture a magneto circuit, where, an alternator is used to generate an AC flux into a primary coil of a ignition coil by means of a magnetic core (the primary is wrapped around the alternator core). The flux in the primary coil increases as the AC current increases and thereby the field around it increases. The field in the secondary increases as a result. As the AC flux generated by the alternator reaches a maximum, I expect that the field in the primary would start to collapse as the current in the primary attempts to maintain the flux flow in it.

    In a magneto that is used for an ignition circuit, it is known that the primary of the coil attempts to maintain the field in the core as the alternator begins to change phase. We want the flux in the primary to be at maximum when we open the primary circuit in order to have the most flux in the secondary circuit, thereby generating the most spark when the secondary field collapses.

    Here is what I do not understand. As the primary tries to maintain the flow of flux in the magnetic core (the alternator core) does this decrease the flux in the primary? If it does, does the flux in the secondary reduce as well. Since we want as much flux as possible to "cut" through the secondary as the field collapses, why is the maximum spark achieved after the primary field starts to collapse.

    Clearly, I have made an incorrect assumption somewhere.

    Thank You
    Tom
     
  2. jcsd
  3. May 21, 2017 #2
    Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.
     
  4. May 24, 2017 #3

    Merlin3189

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    Gold Member

    I wonder whether this explanation might help you? High tension magneto operation.

    As I see it (not expert), the part you call "alternator" does not produce a big enough change of flux - not fast enough. So it is used to generate a current in a primary coil and then that is broken suddenly by the contact breaker to produce a very rapid change of flux.
    As to the timing, the current in the primary is generated by the *change* of flux from the "alternator". So we might expect this current to be maximum at the point where the "alternator" is reversing flux direction. At this point the current is in the direction which supports the existing "static" flux and opposes the way the "alternator" is trying to change the flux. If the break occurs exactly at the neutral point, the change of flux is simply that due to the current ceasing. A little later, the 'alternator" has started to try to build flux in the opposite direction. It seems (going by what they say) that there is an optimum point shortly after the neutral point where you get the maximum rate of change of flux and thus the biggest induced secondary voltage.

    I'm not sure if this helps with your specific questions. Some thoughts on those:-

    "We want the flux in the primary to be at maximum when we open the primary circuit. in order to have the most flux in the secondary circuit, thereby generating the most spark when the secondary field collapses. "
    I think maybe we don't want the maximum flux, so much as the maximum rate of change of flux. The flux is the resultant of the current in the primary and the position of the "alternator" magnet.

    "As the primary tries to maintain the ... flux in the magnetic core (the alternator core) does this decrease the flux in the primary? If it does, does the flux in the secondary reduce as well. Since we want as much flux as possible to "cut" through the secondary as the field collapses, why is the maximum spark achieved after the primary field starts to collapse."
    The alternator core, the primary core and the secondary core are the same thing (in the version of magneto shown in the link), so they all share the same flux and the answer to all the questions is yes. And again it is the maximum change of flux we want, not just the maximum flux.

    In a system where an alternator supplies current to a separate transformer, you would want to break the circuit at maximum current in the transformer primary, since that current is the only source of flux in the transformer core. That would occur a bit after the peak emf of the alternator, because the inductive reactance of the transformer primary means the current lags the emf. The peak emf of the alternator occurs at the point of minimum flux, ie. maximum rate of change of flux.

    In the single core system (what I understand by the term "magneto") the max change of flux seems to occur after the max flux: instead of changing from max flux to zero, it is changing from a (less than max) positive flux to a negative flux. Like the biggest difference between a sine and a cosine does not occur at 0, 90,etc., where one of them is at a maximum, rather at 45, 135, etc.
    Whether this point is also the actual max primary current, I'm not sure. It seems to me that since the change in flux is caused by breaking the primary current, the max change in flux should match the max change in current. This would imply that the bad link in your argument was that the max current occurred at max flux.

    Thinking about the primary current and baring in mind that the primary circuit is closed for only part of the cycle, it will not be a simple sine as in an alternator.
    A-The link shows a graph where the primary circuit is made when the rotating magnet has created max flux in the core.
    B- Then the magnet moves to reduce the flux, causing an emf and starting primary current.
    C- The primary current increases at an ever increasing rate, I assume reflecting an increasing emf caused by an increasing change in flux. The change in flux does not appear to be large: presumably a small emf can drive a large current in a short circuited primary.
    D- The flux is continuing to fall after the neutral point and the current continues, even appearing to increase, presumably as the flux reduces ever faster.
    E- Eventually a point is reached where the flux is reducing more slowly and the current is no longer increasing, then the circuit is broken near maximum current, not max flux, and produces the greatest change in flux to induce the max secondary voltage.
    F- The circuit remains open with zero primary current until the rotor has again caused max core flux, then it closes and restarts the cycle.
     
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