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Lorentz force in a toroid coil

  1. Oct 12, 2013 #1

    I'm having a major issue trying to work out where the resultant lorentz force might act when I have a wire coiled around a doughnut (toroid) shape.

    Am I right in saying that there will be a resultant lorentz force acting to crush the doughnut because there will be a slightly denser magnetic field on the inside of the doughnut than on the outside?

    In addition, am I right in saying that the wire wrapped around the doughnut will be trying to escape from the doughnut in all directions?

    I don't know if this is a good enough description of what I am trying to understand, please let me know if you need more info.

    Thanks in advance

  2. jcsd
  3. Oct 12, 2013 #2


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    Yes your Lordship, I believe you are substantially correct.

    Consider two parallel wires. If a current in each is flowing in the same direction then the wires will be pulled together. If the currents are in opposition, then they will be pushed apart.

    Imagine a close wound toroidal coil without a magnetic core. The current running down the inside of one turn is in the opposite direction to the same current running up the outside of the same turn, so that turn of wire will try to expand away from the core. Now consider two adjacent turns, they have parallel currents so they will attract each other.

    The filamentary toroidal surface current will therefore try to make a toroid with a shorter magnetic path length, but a thicker section with a smaller hole in the centre.
    Magnetostriction will cause the magnetic core to reduce it's axial circumference while increasing it's cross section. That is in sympathy with the windings.

    Your humble servant, Sir Baluncore.
  4. Mar 11, 2014 #3
    Sir Baluncore, can we have multiple layers of coils in a Toroid or there is only single layer of coil allowed in Toroid? If we can have multiple layers, then does it matter they should run is same direction or it doesn't matter?
  5. Mar 11, 2014 #4


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    A toroid core can be wound like any other coil. You can scramble wind it backwards and forwards if you want. The only requirement is that the wire always passes through the hole in the same direction.

    The hole is a critical feature when winding a toroid. Keep the wire evenly distributed around the core so it can pass neatly through the hole and so use the shortest length of wire.
  6. Mar 11, 2014 #5
    What happens if it is not evenly distributed? I mean let's consider first layer is evenly distributed, but other layers are not. Would it still act as Toroid? I'm asking this because I'm trying to make a Toroid coil using a straight pipe. I was able to wind nice 1 layer, subsequent layers are not coming nice. Once finished, I'll take out that pipe and will try to form round toroid shape. But since I cannot make it evenly distributed, it was worrying me.
  7. Mar 11, 2014 #6


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    There is little use for an air cored toroid, apart from as a heating element.
    The core is a magnetic conductor just as wire is an electrical conductor.
    By providing a good magnetic path, a solenoid can be bent into a loop so as to eliminate the end effects that would otherwise generate stray fields.

    A toroidal core can be scramble wound because the magnetic path is strongly guided by the magnetic characteristics of the core. There is very little magnetic field that does not flow within the magnetic core, but instead passes between the wire turns.
  8. Mar 11, 2014 #7
    Thank you Sir, appreciate your response. By any chance, do you know any nice machine, which can help make Toroid coils? I'm not looking automatic ones, manual, hand driven, cheaper, used would work for me. I find it difficult to wind them by hand.
  9. Mar 11, 2014 #8


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    I do not know of any hand driven machine. If you are winding many cores by hand, you should make yourself a two part or cut reel that you assemble interlocked through the toroid. Then wind your wire onto the reel allowing the toroid to slide ahead of the wire as you load the reel. Next transfer the wire to the toroid from the loop on the reel, keeping the wire tight by taking up any slack on the reel. Use the smallest convenient reel diameter.

    Power assisted reeling on the wire can be done with a couple of pinch rollers that hold the reel.

    Now you know why the hole in the toroid is the critical limiting dimension.
  10. Mar 12, 2014 #9
    Alkemist did you consider using thicker wire? You could layer it more evenly and more closely approximate uniformity
  11. Mar 12, 2014 #10
    I'm using 14awg wire. It comes little Ok, but not near perfect. I can see the layers not evenly wound. e.g. if first layer is 100 turns then second may have 95 third may have 80 or so and then it gets messed up. But I guess it should matter if it still works as Toroid.
  12. Mar 12, 2014 #11
    Also what difference would it make if the core is Magnet or Iron.
    Secondly what if it is metal [aluminium or copper] but not Iron. Any advantages or disadvantages in each case?

    I'm trying to play with Toroid as Tesla Coil experiment.
  13. Mar 12, 2014 #12


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    The magnetic material must suit the frequency of operation.

    The skin effect is as important for the core material as it is for the wire used. The magnetic particles or laminations must be thin enough to be magnetised quickly. That is why powdered iron is used for low audio frequencies, then ferrite for RF frequencies. http://en.wikipedia.org/wiki/Skin_effect

    The core must be magnetic to guide the field. Al or Cu actually behave worse than air, they push the fields away. If the core is solid iron then it will be very slow to change and will saturate for sudden current changes. Permanent magnets are not good for cores because they saturate asymmetrically and cause distortion.
  14. Mar 12, 2014 #13
    Thank you Sir :)
  15. Mar 13, 2014 #14
    Keep in mind you can go to a construction site and get scrap materials
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