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An Anti-Cogging Core??

Discussion in 'Electronic Basics' started by Randy Gross, Aug 18, 2005.

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  1. Randy Gross

    Randy Gross Guest

    This web site, , came up in another
    discussion. The author built a wooden alternator with air core coils to
    eliminate the cogging effect in the PMG.

    Could stainless steel, or other alloy, be used in this situation to
    boost inductance in the coil but not cog the magnet?

    The stainless I work with has a slight magnetic attraction. The magnet
    will not stick but, you can feel the tug when the magnet is close.

    This brings up another question:

    Is there a non-ferrous metal ( alloy ) that can be wrapped in a coil of
    wire and utilized as an electromagnet?

    Curiosity is a good thing!

  2. No. The magnetic attraction is caused by the sudden increase of flux
    as the magnet gets near. That same, sudden increase in flux is what
    makes the voltage generation so large and efficient.
    If it is attracted, it will help, but if it is attracted less than
    iron, it will work less well than iron.
    There are lots of ferromagnetic alloys, including some that contain
    little or no iron (especially those containing nickel), but you aren't
    likely to run across many of them in your basement or junkyard.
    You can retain much of the gain of an iron core without producing
    cogging, if the iron closes most of the flux path, but does not have
    what are called salient poles (protruding bits that concentrate the
    flux at specific angular locations).

    For instance, if you have a rotating two pole magnet structure and a
    pair of stationary coils on opposite sides of the rotating magnet, you
    have a very long path for the flux from one pole of the magnet,
    through the coil, through the air,around the outside to the back side
    of the other coil, through that coil and back to the other magnet
    pole. All that air really weakens the amount of flux taking that path.

    Now, if you place a smooth ring of iron around the coils (and make
    them as flat and thin as possible, to get that ring as close to the
    magnet as possible) you have provided a much easier path for the flux
    between the outsides of the coils, so that the only air in the path is
    from the magnet poles to the coils (the rotational air gap) and the
    thickness of the coils. So the peak flux passing through the coils
    will be much increased. But there are no chunks of iron sticking out
    toward the magnet (salient poles) at any particular angles, so there
    will be no cogging.
  3. Randy Gross

    Randy Gross Guest

    I think I understand. I read at

    where a process for magnetic shielding is described. The author placed
    short lengths of Iron rod, in a box configuration, around the object
    that he intended to "shield" from the flux. The Iron provided a
    positive path for the flux.

    Should the rings around the coils touch creating a complete path or,
    should they "halo" the coils?

  4. The shielding analogy is good, except that the magnet is inside the
    shield. The ring contains the flux to mostly inside the ring.
    The electrical analogy is that the iron represents a low resistance
    path for current. The permanent magnet produces some amount of
    magneto motive force that is analogous to DC voltage. The air gaps
    represent high resistance material. The less air the flux has to pass
    through and the more or the path that can be in iron, the higher the
    current (flux).
    The iron outside the coils should be one complete ring (a slice of
    iron pipe or a stack of washers). The coils are slipped between the
    ring and the rotating magnet. The space needed for the coils is what
    forces there to be a minimum magnetic gap at each end of the magnet.
    The ring collects the flux from one pole of the magnet, splits it in
    half, and brings each half to the other pole of the magnet through
    each half of the ring. Since this process is independent of the
    angular position of the magnet, there is no cogging. In the absence
    of any coil current, the total flux is constant, independent of position.
  5. Randy Gross

    Randy Gross Guest

    This is an interesting approach, it's like a venturi, a flux venturi! I
    almost wish I could "see" this process in operation. I see what you
    mean about efficiency too, the flux is gathered and then targeted.

  6. Jasen Betts

    Jasen Betts Guest

    as I understand it he's decribing a setup where there's a magnet rotating
    inside a freomagnetic ring and in the gaps between the ring asn the magnet
    is wound the output coils

    it seems to me that you could use a single continouas coil wound around the
    core like in a way similar to a toroidial transformer but with the ends
    shorted (to make it continuous) and tapped at equal spacing for however
    many phases you want.

  7. I agree.
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