Re: Coil Dimensions

[jim271@xxxxxxxxxxx]

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@xxxx, "Samuel Ace Winchenbach" <swinchen@xxxx> 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@xxxx
>  
>  
> /==== 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
> |
> ----------------------------------