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Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529
Why not wind the layers separate from one another? You can choose the number of layers to connect in series then, its much more flexible. I did that with two coils a layer, several layers. You could tune to the projectile length too.
Yea im winding each layer seperately with ring connectors on either end, then epoxying and leaving to harden as i go. Will only be doing this for the first two coils though, as the last three will only be one layer. Im just hoping thicker wire will be easier to work with, this 16awg stuff is acting like a very strong spring at the moment, it's taking two clamps just to stop it unwinding itself.
Registered Member #1819
Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137
rp181 wrote ...
Im not sure, but I think the iron shell "confines" the magnetic field, increasing the strength in the middle of the coil. I think the shell will saturate, but it will not matter. The shell basically "stores" energy, and lengthens the pulse a little. There will be a little waste in magnetizing the the iron, but that will reduce after the first shot.
The effectiveness of external iron (the iron shell) is highly dependent on the size of the shell and the amount of magnetic flux that it is required to hold. If it saturates very early during the pulse, then it probably won't be very effective. With knowledge of the magnetic material saturation properties (which is easy to obtain for iron powder materials) and the use of Faraday's Law, the amount of external iron needed or the size of the coil and shell can be approximated. Optimization may require some empirical measurements to look for abnormal current increases for the duration of the pulse.
The shell will lengthen the pulse due to the increase in inductance, as you said, but any "stored" energy is due to inherent imperfections in any magnetic material (ideal magnetic materials store no energy at all). This stored energy ends up as loss in a transformer, but will add a very slight amount of inductance to an ordinary inductor (a.k.a. the coil). This will end up as energy returned to the coilgun LC circuit and must be dealt with as such, but it is usually inconsequential anyway.
I just ran a FEMM simulation with a large C shape of external iron, and it seems the iron does increase the strength of the field in the center of the coil. Also I dont think the iron will saturate at all, as the flux density is very low due to its large mass.
Also, I want to redesign my coils, I just cant think how I should go about making the optimal coil? The only definite parameters are 1. A certain amount of energy from the caps must be used in each coil, 2. the coil length must be 50mm. Is there an easier way to calculate the inductance and resistance of a coil that gives the perfect length pulse and perfect voltage drop in the caps?
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Thats difficult, especially because is constantly changes due to the projectile. Ignoring that, the wheeler formula gives a good aproximation. For resistance, look at wikipedia for the resistance of 1000' or 1km, and use that.
Iv successfully written a c# program to calculate the required R and L of a coil to reduce the voltage of a capacitor bank by X volts over a certain period of time :) (thanks to some help from barry)
Il upload it somewhere after a bit more tweaking and tidying up incase anybody else wants to use it (with barrys permission)
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9KJ coilgun construction
