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# RE: [femm] Coil with multiple turns

- To: "'femm@yahoogroups.com'" <femm@yahoogroups.com>
- Subject: RE: [femm] Coil with multiple turns
- From: Finlay Evans <finlay.evans@renishaw.com>
- Date: Tue, 9 Apr 2002 08:36:27 +0100

    Hi,

    Just a practical note:
    If you intend to actually build this model then you should take account of
    the packing factor you think you can achieve when winding the coils. This
    will not actually change the results of the Femm model, but will determine
    the maximum current density you could realistically use in the single block
    of current in the model. For example, if you want to use wire that has a
    maximum current rating of say 2 amps, then you need to convert that maximum
    rating into a maximum current density for the coil block. Divide the coil
    block area by the product of the wire section area * packing factor to get
    the number of turns you can expect to wind in the coil.

    I know that I cannot design a coil to have a packing factor of more than
    about 0.78 and expect to actually build it reliably, and this is true only
    if I leave a small clearance on the outside surface of the coil to allow for
    messy overlaps. This number actually is very close to a theoretical
    winding that has a square lay pattern (i.e. one turn directly on top of
    another = (pi/4)). In reality a coil tends to form in a hexagonal
    close-packed lay which has a theoretical packing factor of about 0.9,
    however the lead-in turn messes things up and the coil generally begins to
    get messy towards the outer turn, hence the 0.78 figure. This 0.78 figure
    is pretty handy though since you can easily guess how many turns you can
    expect to get in a rectangular enclosed space (e.g. a coil using 0.5mm
    diameter wire will have 10 x 20 turns in a space of 5mm x 10mm).

    All the above applies to round section wire and coils with more than about
    50-100 turns. With just a few turns you can do better, and with square
    section wire you can do a little better (but no where near the theoretical
    1.0).

    Anyway I just thaught you should be aware of this before you try and build
    something impossible.

    Regards,

    Finlay Evans

    -----Original Message-----
    From: Adrian [mailto:adrianw@solid-state-logic.com]
    Sent: 08 April 2002 16:58
    To: femm@yahoogroups.com
    Subject: RE: [femm] Coil with multiple turns

    Hi,

    Multiply the current per turn by the number of turns and this will give you
    the total current.

    Divide by the area to give Amps per square meter.

    Create a block to encompass the area of the coil and give it the current
    density calculated above.

    Adrian

    -----Original Message-----
    From:
    sentto-1333690-1067-1018280579-adrianwadey=compuserve.com@returns.groups.yah
    oo.com
    [mailto:sentto-1333690-1067-1018280579-adrianwadey=compuserve.com@returns.gr
    oups.yahoo.com]On Behalf Of milanhampl
    Sent: 08 April 2002 14:43
    To: femm@yahoogroups.com
    Subject: [femm] Coil with multiple turns

    Hi,
    I'm working on my diploma theses and using FEMM for some computations.
    I model a field of gap in toroidal magnetic soft ferrit. The only
    thing I need, is how to model coil with multiple turns. I've tried to
    make the coil like a block with a circuit property, but I read about
    this and it should model only one turn. We know the curent in coil
    and a number of turns, but don't how to make a model. I'm sending an
    example, if you would like to make more accurate idea about this
    problem.
    Thanks

    Milan Hampl

    [Format] = 3.0
    [Frequency] = 0
    [Precision] = 1e-008
    [LengthUnits] = millimeters
    [ProblemType] = planar
    [Coordinates] = cartesian
    [Comment] = "Add comments here."
    [PointProps] = 0
    [BdryProps] = 1
    <BeginBdry>
    <BdryName> = "Nula"
    <BdryType> = 0
    <A_0> = 0
    <A_1> = 0
    <A_2> = 0
    <Phi> = 0
    <c0> = 0
    <c1> = 0
    <Mu_ssd> = 0
    <Sigma_ssd> = 0
    <EndBdry>
    [BlockProps] = 4
    <BeginBlock>
    <BlockName> = "Air"
    <Mu_x> = 1
    <Mu_y> = 1
    <H_c> = 0
    <H_cAngle> = 0
    <J_re> = 0
    <J_im> = 0
    <Sigma> = 0
    <d_lam> = 0
    <Phi_h> = 0
    <LamType> = 0
    <LamFill> = 1
    <BHPoints> = 0
    <EndBlock>
    <BeginBlock>
    <BlockName> = "Fonox H20"
    <Mu_x> = 2000
    <Mu_y> = 2000
    <H_c> = 20
    <H_cAngle> = 0
    <J_re> = 0
    <J_im> = 0
    <Sigma> = 1.9999999999999999e-006
    <d_lam> = 0
    <Phi_h> = 0
    <LamType> = 0
    <LamFill> = 1
    <BHPoints> = 8
    0 0
    0.10000000000000001 100
    0.32500000000000001 200
    0.375 300
    0.40000000000000002 400
    0.41499999999999998 500
    0.41999999999999998 1000
    0.42499999999999999 2000
    <EndBlock>
    <BeginBlock>
    <BlockName> = "Copper"
    <Mu_x> = 1
    <Mu_y> = 1
    <H_c> = 0
    <H_cAngle> = 0
    <J_re> = 0
    <J_im> = 0
    <Sigma> = 58
    <d_lam> = 0
    <Phi_h> = 0
    <LamType> = 0
    <LamFill> = 1
    <BHPoints> = 0
    <EndBlock>
    <BeginBlock>
    <BlockName> = "Alnico 6"
    <Mu_x> = 3.2999999999999998
    <Mu_y> = 3.2999999999999998
    <H_c> = 59907
    <H_cAngle> = 0
    <J_re> = 0
    <J_im> = 0
    <Sigma> = 2.25
    <d_lam> = 0
    <Phi_h> = 0
    <LamType> = 0
    <LamFill> = 1
    <BHPoints> = 26
    0 0
    0.10402 1131.5899999999999
    0.18934000000000001 2098.46
    0.26712000000000002 3207.77
    0.33743000000000001 4310.71
    0.40393000000000001 5560.0799999999999
    0.47026000000000001 7106.2700000000004
    0.52539000000000002 8643.7000000000007
    0.57662999999999998 10475.6
    0.63507999999999998 12760.200000000001
    0.69674999999999998 15942.6
    0.74002000000000001 18661.700000000001
    0.78312000000000004 21679.299999999999
    0.81535999999999997 24090.5
    0.84031 26198.5
    0.87238000000000004 28908.099999999999
    0.90412000000000003 32213.799999999999
    0.92864999999999998 35065.800000000003
    0.95318000000000003 37917.900000000001
    0.97397999999999996 40767.5
    0.99451999999999996 44062.800000000003
    1.0149999999999999 47507.800000000003
    1.0354000000000001 51102.300000000003
    1.0487 53944.800000000003
    1.0622 56489.599999999999
    1.0751999999999999 59928.199999999997
    <EndBlock>
    [CircuitProps] = 6
    <BeginCircuit>
    <CircuitName> = "Proud+100mA"
    <VoltGradient_re> = 0
    <VoltGradient_im> = 0
    <TotalAmps_re> = 0.10000000000000001
    <TotalAmps_im> = 0.10000000000000001
    <CircuitType> = 0
    <EndCircuit>
    <BeginCircuit>
    <CircuitName> = "Proud-100mA"
    <VoltGradient_re> = 0
    <VoltGradient_im> = 0
    <TotalAmps_re> = -0.10000000000000001
    <TotalAmps_im> = -0.10000000000000001
    <CircuitType> = 0
    <EndCircuit>
    <BeginCircuit>
    <CircuitName> = "Proud+500mA"
    <VoltGradient_re> = 0
    <VoltGradient_im> = 0
    <TotalAmps_re> = 0.5
    <TotalAmps_im> = 0.5
    <CircuitType> = 0
    <EndCircuit>
    <BeginCircuit>
    <CircuitName> = "Proud-500mA"
    <VoltGradient_re> = 0
    <VoltGradient_im> = 0
    <TotalAmps_re> = -0.5
    <TotalAmps_im> = -0.5
    <CircuitType> = 0
    <EndCircuit>
    <BeginCircuit>
    <CircuitName> = "Proud +100A"
    <VoltGradient_re> = 0
    <VoltGradient_im> = 0
    <TotalAmps_re> = 100
    <TotalAmps_im> = 100
    <CircuitType> = 0
    <EndCircuit>
    <BeginCircuit>
    <CircuitName> = "Proud -100A"
    <VoltGradient_re> = 0
    <VoltGradient_im> = 0
    <TotalAmps_re> = -100
    <TotalAmps_im> = -100
    <CircuitType> = 0
    <EndCircuit>
    [NumPoints] = 33
    8.0525260000000003 8.5216619999999992 0 0
    12.31427 9.0191409999999994 0 0
    7.4058029999999997 7.8417729999999999 0 0
    12.59618 7.8583550000000004 0 0
    7.438968 4.1604210000000004 0 0
    12.57512 4.1703840000000003 0 0
    11.13842 5.3035569999999996 0 0
    11.898349764642123 8.5552310071369675 0 0
    11.635530407705382 7.0372087335478177 0 0
    8.4285736610594597 4.91710671754955 0 0
    11.599382983445485 4.9578380845006009 0 0
    8.8713821589270836 6.6801028742424267 0 0
    12.310494788789148 9.0149301969252793 0 0
    13.087914927418108 8.2025694491926746 0 0
    13.056369754522434 3.8046412261331786 0 0
    8.3499999999999996 7.0300000000000002 0 0
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    8.9100000000000001 5.2599999999999998 0 0
    7.6799999999999997 9 0 0
    8.8100000000000005 7.5599999999999996 0 0
    9.1899999999999995 7.0300000000000002 0 0
    11.15 7.5700000000000003 0 0
    10.73 7.0599999999999996 0 0
    9.75 4.1200000000000001 0 0
    10.25 4.1200000000000001 0 0
    9.75 2.8700000000000001 0 0
    10.25 2.8700000000000001 0 0
    0.5 11.5 0 0
    19.5 11.5 0 0
    0.5 -1.3 0 0
    19.5 -1.3 0 0
    6.9900000000000002 3.7799999999999998 0 0
    6.9000000000000004 8.1199999999999992 0 0
    [NumSegments] = 19
    1 12 -1 0 0 0
    6 10 -1 0 0 0
    12 7 -1 0 0 0
    13 3 -1 0 0 0
    14 5 -1 0 0 0
    11 15 -1 0 0 0
    8 16 -1 0 0 0
    9 17 -1 0 0 0
    18 0 -1 0 0 0
    19 20 -1 0 0 0
    22 21 -1 0 0 0
    23 25 -1 0 0 0
    26 24 -1 0 0 0
    27 29 -1 0 0 0
    29 30 -1 0 0 0
    30 28 -1 0 0 0
    28 27 -1 0 0 0
    32 2 -1 0 0 0
    4 31 -1 0 0 0
    [NumArcSegments] = 20
    8 21 21.497898698184784 5 0 0 0
    12 18 75.653121930250592 5 0 0 0
    11 17 70 10 0 0 0
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    19 15 20.914543452182755 5 0 0 0
    10 8 65 10 0 0 0
    6 16 70 10 0 0 0
    22 20 70 10 0 0 0
    15 9 70 10 0 0 0
    21 19 70 10 0 0 0
    2 4 70 10 0 0 0
    7 0 70 10 0 0 0
    5 3 70 10 0 0 0
    0 2 20 10 0 0 0
    3 7 20 10 0 0 0
    9 23 50 10 0 0 0
    24 10 50 10 0 0 0
    4 25 50 10 0 0 0
    26 5 50 10 0 0 0
    32 31 75 10 0 0 0
    [NumHoles] = 0
    [NumBlockLabels] = 8
    8.4000000000000004 6.0999999999999996 3
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    6.5 6.0999999999999996 3 0.29999999999999999 4
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    9.9000000000000004 8.4000000000000004 2
    0.10000000000000001 0 90 0

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