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# Re: Coil Dimensions

- To: femm@yahoogroups.com
- Subject: Re: Coil Dimensions
- From: jim271@hotmail.com
- Date: Sun, 18 Nov 2001 22:15:34 -0000
- In-reply-to: <000001c17048$3b544820$9dfc6f82@mrmosfet>
- User-agent: eGroups-EW/0.82

    Hi Samuel,

    You can find information on coils for coilgun at this site:

    http://web2.airmail.net/jd4usa/coildesign.htm
    [Magnetic Gun Club].

    There is also a coil calculator:

    http://web2.airmail.net/jd4usa/opticoil_trans_in.htm

    to help with the design parameters and it will give you a rough idea
    of relative performance for different combinations.

    I suppose your caps are 47 millifarads? An important consideration is
    the current pulse which has to be rather short (milliseconds at
    most). Therefore the L/R time constant of the coil is important.
    Shorter is generally better. The MGC site also shows a circuit for a
    dual timer that might be used to control the power switches.

    The starting distance of the projectile from the center of the coil
    is important. By adjusting the width of the current pulse, you can
    turn off the current when the projectile nears the center of the
    coil. Be sure to put a reverse diode across the coil to limit the
    switch voltage due to flyback.

    Email me if you need more info. I'll be interested in how your
    project turns out :)

    HTH

    -- Jim (MGC)

    ================================================================
    --- In femm@y..., "Samuel Ace Winchenbach" <swinchen@e...> wrote:
    > Hello, all
    >
    > A friend and I are currently building a high current coil gun for
    our
    > senior design project at the University of Maine. Recently we have
    been
    > trying to optimize the coil design, and I believe we are relatively
    > close. Here is what we have so far:
    >
    > We know that amp-turns (NI ? a.k.a. Magneto-Motive Force) is
    the
    most
    > important factor when designing the coil.
    > Our power supply is a bank of large electrolytic capacitors (47 mF)
    > charged to 340 volts. We are also setting a current limit of 100
    Amps,
    > even though the HVIGBT we have is rated to 400 amps continuous.
    Based on
    > these numbers our coil resistance has to be 3.4 Ohms. So basically
    what
    > we have found so far is to get the highest NI do the following
    steps:
    > 1) Select the largest wire you can afford
    > 2) Wind as many layers onto the barrel as needed to make the
    > necessary resistance.
    >
    > What we are not taking into account is the length of the barrel,
    and the
    > thickness of the coil winding. Right now we have a barrel (and
    > projectile) fixed at 1 foot long. And the coil windings have no
    limit.
    > At a certain point the distance from the center of the coil has to
    start
    > making a big difference correct? How do you determine the best
    length
    > and coil windings? By shrinking the length of the barrel you create
    > more force on the projectile, but that force is over a shorter
    distance.
    > Also how do you calculate inductance with J.A? I have looked at
    several
    > examples and it does not make sense to me. Included is the femm
    model of
    > our coil, and the matlab script I wrote to help us determine the
    optimal
    > number of layers. Thanks in advance for your help,
    >
    > Samuel Ace Winchenbach
    > University of Maine Electrical Engineering
    > swinchen@e...
    >
    >
    > /==== snip ====/
    >
    > Our barrel is 5/16" in diameter, a foot long piece of garolite.
    It
    has
    > an inside diameter of ¼". We are currently using 18 AWG magnet
    wire,
    > with 0 series resistance.
    >
    > EDU>> help CoilOpt
    >
    > function CoilOpt(
    > BarrelDiam (in),
    > CoilLength (ft),
    > WireDiam (mm),
    > ResPerkm (Ohms/km),
    > SeriesRes (Ohms),
    > Voltage (Volts),
    > CurrentLimit (A)
    > )
    >
    > EDU>> CoilOpt((5/16),1,1.0668,20.9,0,340,100)
    >
    > ----- Non-Optimized Coil Packing -----
    > |
    > | NI (Amps*Turns): 2.783180754224393e+005
    > | NI/L (Amps*Turns/CoilLength): 9.131170453492102e+005
    > | J (Amps/Meter^2): 8.559402374851990e+007
    > | CoilWidth (Meters): 0.010668000000000
    > |
    > | Resistance (Ohms): 3.481627984561275
    > | Current (Amps): 97.655465060504994
    > |
    > | WireUsed (ft): 546.538946954199220
    > | Number of Layers: 10
    > | Turns Per Layer: 285
    > | N (Total Turns): 2850
    > |
    > --------------------------------------
    >
    >
    > ----- Optimized Coil Packing -----
    > |
    > | NI (Amps*Turns): 2.838475142859972e+005
    > | NI/L (Amps*Turns/CoilLength): 9.312582489698071e+005
    > | J (Amps/Meter^2): 9.036465157159652e+007
    > | CoilWidth (Meters): 0.010305559007572
    > |
    > | Resistance (Ohms): 3.755185253889620
    > | Current (Amps): 90.541471861562101
    > |
    > | WireUsed (ft): 589.481415986892440
    > | Number of Layers: 11
    > | Turns Per Layer: 285
    > | N (Total Turns): 3135
    > |
    > ----------------------------------

- References:
  - Coil Dimensions
    - From: "Samuel Ace Winchenbach" <swinchen@eece.maine.edu>

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