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Inductive Coupling through a Steel Enclosure

Discussion in 'Electronic Design' started by Ms.Malamu Habavir, Nov 22, 2006.

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  1. I am trying to inductively-couple, for the purpose of charging a battery
    (capacitor bank), across a 5mm space. On each side of the gap is mild steel
    channel stock of approximately 10mm thick. The inductive coupling frequency
    is around 1 MHz. While the coupling works on the bench, the steel greatly
    affects the field. Is there any technique for lessening the affects? Any
    links or hints would be greatly appreciated.

  2. Henry Kiefer

    Henry Kiefer Guest

    As steel is a ferromagnetic material I don't see any change other than to
    change the frequency up and use a grid of metal.
    You should think of another mechanic system here.

    What is the reason for the steel plate?

    - Henry
  3. Thank you for your comments.

    The Steel can be thought of as two steel boxes. Each box has a 20mm hole
    which are aligned. The coils are set in to the steel and made flush with the
    surface. The steel is unavoidable. A higher frequency you say?

  4. Henry Kiefer

    Henry Kiefer Guest

    Hm. I don't really realize what you mean in mechanical sense. Is there a NDA

    If you have 20mm holes where you can transmit thru the power, then it would
    be possible to send the electromagnetic wave if the frequency is high
    enough. Think of a microwave oven, where you see a fine metallic grid in the
    front door. The wavelength of the microwave (2.45GHz) cannot pass the small
    holes in the grid. If you make the holes bigger, the wave will pass!

    What is the power requirement? mW, W or kW?

    For high-power rf amplifier Class E is interesting. I've done work with it.

    If you only want to electrostatic shield the capacitor bank, there would be
    an Al-foil enough to do. With Al in-between you can even couple magnetic
    without great problems! Shielding is different in terms of magnetic and

    - Henry
  5. J.A. Legris

    J.A. Legris Guest

    You need to minimize the reluctance of the magnetic circuit that
    includes just your cores while maximizing the the reluctance of the
    magnetic circuit through the steel channels.

    First step, shape your cores to maximize the cross sectional area and
    minimize the magnetic path length that passes through air. One
    possibility is a horseshoe shape, but better still might be a pot core
    such as this:

    Second step, maximize the magnetic path length through air to the steel
    by making as large a hole as possible, deeply chamfering the inside,
    and filling it in with a plastic insert (which might also be used to
    mount your coil).
  6. Yes, I'm trying not to disclose too much information to protect myself and
    the company.

    I have tried an aluminum shield around the coils with some success. I am
    using a class E amplifier. I was trying to stay below 1 Mhz for FCC reasons.
    I came across some literature that the FCC allows inductive coupling in the
    13 MHz region.

    So I should get better performance at the higher frequency?
  7. Henry Kiefer

    Henry Kiefer Guest

    The exact frequency is 13.56MHz wordwide. It is almost free to use for
    There is another at 6.8MHz. At the moment I don't know if this is worldwide.
    13.56MHz is a common frequency for power, even class E.

    If the mechanic is as I wrote it, then YES, you get more power.
    If you place a ferrite rod in the hole, or two narrow fronted, then you
    always get good coupling.
    The primary ferrite broken in two part with an air-gap and the same as
    secondary will give even more couping. But I think this is a little tiny in
    20mm hole.

    Are you primary power limited on the transmitter side?

    You can reach me by

    - Henry
  8. Zak

    Zak Guest

    I'd say using a split pot core for the coil would work. That would keep
    the field out of the surrounding steel and would fit a round hole.

  9. Any time a magnetic field passes through a hole in a
    conductor, a potential is generated around the hole (1
    turn's worth) that causes eddy current to encircle the hole.
    If you could pass two equal and opposite fields (or any
    even number of pairs) through the same hole, the voltages
    they generate would add up to zero, eliminating the
    circulating current.
  10. Is it possible to cut a thin radial slot in the steel and fill it with a non
    conductive material?
  11. Picture two hollow stell boxes each with a 20mm hole drilled in them at the
    same location. The thickness of the steel is aprox. 5mm. The boxes are 9cm
    cubed. The coils, facing each other, sit flush with the surface of the
    steel. The boxes are aprox. 5mm apart.

    What do you mean by a radial slot?
  12. Thank you for your comments. Is there a paper or website that might discribe
    the properties of the split pot core in terms of the field control that you
    might know of?
  13. So basically, two coils with signals 180° out of phase. One coil generating
    a negative pulse when the other is generating a positive pulse?
  14. That is the idea, though you can accomplish the same thing
    by bending the flux from the inside end of the coil, and
    sending that back through the hole beside, or around the
    original flux. A common way to do this is to add a high
    permeability core to the coil, that has both a center post,
    and a surrounding shell. When the post is the north pole,
    the shell is the south pole, and vice versa. A matching
    core on the receiving side captures the flux from these two
    poles and wraps it around and through the second coil.

    Google [pot core].
  15. 5 mm is an exceptionally long distance for even an unshielded inductive
    coupling. Especially if large amounts of energy are to be efficienty

    This is called the "electric toothbrush problem" and it has yet to be
    satisfactorily solved.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  16. You will have as much luck inductive coupling through steel as you would

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  17. From the center of each 20 mm hole, saw as fine a slot as you wish in one
    direction away from the coil. Fill with epoxy etc. I expect this would cut a
    lot of the eddy currents which reduce the coupling between the coils.
  18. Yes. It's damn near 1/4" and that's a big gap. Even in a big electric motor
    that would be excessive, and their magnetic system is designed to work
  19. The usual reason for inductively coupled charging is to pass energy
    through a container is to maintain the sealed integrity of that
    container. If you can have holes in it, I suggest putting a plug in one
    and a socket in the other.
  20. The distance you can transmit energy with a pair of poles
    depends on the spacing between them. The larger the gap
    between the opposite poles, the further out the field
    fringes. This means that the pot core may not give the best
    performance. See this page for some pictures of different
    core shapes that are commonly available.

    If you could find a U or C core whose poles just fit in the
    hole, you would have a pair of magnetic poles further apart
    than you would get with a pot core that would produce
    slightly further reaching fringes. If you could actually
    place the core in the hole, so that the air gap between
    halves was very much smaller, then the pot core would be
    better, because of its symmetry and ease of winding.
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