PowerLabs Advanced Coil Gun Research!
PowerLabs Advanced Coil Gun Research! |
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Did you learn anything useful from this? Why not help me continue my research so I can continue to share my knowledge with you.
Project Introduction: |
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After my move from Brazil to study in the US I had to leave behind all
of my ongoing projects, including the 3kJ single stage coil gun and my 7kJ
multi stage coil gun prototype, both of which were incomplete (the multi
stage coil gun was crushing the glass coil forms I was using as barrels and
the 3kJ gun had barely been started). Not counting small prototypes I built
as a kid, this is my 4rth Coil Gun project (I have worked with 1.5, 3 and
2.6kJ systems before), and although this research is nothing new to me I
have come up with some theories over the years as my science and engineering
education progressed and am hoping to test these ideas on a new gun platform
with the objective of creating my most efficient and powerful gauss gun /
linear coaxial magnetic accelerator yet. The basic platform used here (busbars, barrel, SCR clamp and polycarbonate structure) was built in one afternoon when I arrived at the machine shop and found that the non metals center was closed so that the tables could be re-varnished. It is being built entirely from left over materials from the Rail Gun research and the Solid State Can Crusher. |
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Project Description and Goals: |
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Coil Guns, (also known as "gauss guns", or "coaxial accelerators",
or "linear electromagnetic accelerators") are extremely easy to make, a
fact that explains their popularity on the Internet and as a science
project in general. Basically, all one needs to build a coilgun is to wrap
a couple turns of wire around some type of tube and run electricity
through that wire with a piece of iron inside the tube. However, as with
most things, when it comes to making a coil gun that performs well, that
is, converts a significant portion of the energy stored in the capacitors
into kinetic energy, even the best designs -the ones on this web site
included- achieve at best a meager couple of % efficiency. Although a
comparison with conventional electric motors and transformers, which can
perform at efficiency levels as high as 90% and above, is not appropriate since they are completely
different devices, some governmental institution coil gun designs have
been quoted as achieving efficiencies as high as 26%, which makes it very
obvious to me that there is something seriously wrong with the current
amateur designs. It is clear that the problem lies with a very low degree of power coupling between the coil and the projectile, which can be remedied by a smaller coil/slug ratio, thinner barrel walls, and a tighter fitting projectile, however several other factors are probably playing a role, such as the permittivity and magnetic saturation of the projectile material and the general fine tuning of the device. Solving the first problem can be achieved through the use of a more optimized coil/projectile design, although this will bring with it a power switching problem. Solving the second issue will now be possible through the use of a chronograph, which will make it very easy for the efficiency to be calculated and changes in efficiency quantified. I also hope to gain access to specialized magnetic materials so as to maximize the energy transfer potential of my Coil Gun. |
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Parts: |
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Energy Storage: When Mr. Parler from Cornell-Dubilier
provided me with 40 capacitors for my Rail Gun
research I kept the 3 leftovers as possible replacements in case some
of the capacitors from the bank were destroyed from over current. These
capacitors have been sitting around for almost a year now and I decided that
it was time to put them to some good use. The exact specifications for this
3-capacitor bank are (as measured): 2019uF, 22.81mOhms ESL. The bank can be
charged to a peak voltage of 1300V, limited by the SCR being used. The
stored energy on the bank is 1706Joules at that voltage, which I have found
to be an appropriate value for a single stage (WARNING:
It only takes 16Joules to kill a human by electrocution: high energy capacitor
banks should only be handled by professionals!). The capacitors are
inter connected through 1/16inch thick oxygen free pure copper bus bars that
I had left over from the Rail Gun project. It is very interesting to note
that at only 2.7 kilograms and measuring 23x16x8cm (9 1/4x6 1/4x3in)
this capacitor bank stores 19% more energy than my first coil gun capacitor
bank, which contained 10capacitors the same size as these, 30% more energy
than one of the stages used in my 7kJ gun, which weighted just as much, and
38% less energy than the 3kJ coil gun capacitor bank, which weighted almost
7 times as much containing 20 non pulse rated capacitors! Coil Barrel:  Although
for a future multi stage gun it would be highly desirable to have
transparent barrels so that optical sensors could be easily moved along the
gun for optimization experience has thought me that glass and most plastics
are too soft to maintain good structural rigidity as a thin tube when a
solenoid wound on them is pulsed with more then a couple hundred joules.
Metals are ideal having a high strength to weight ratio but they suffer from
severe eddy current losses in coil gun duty. My solution was to utilize a
very thin (.001 inch wall thickness) brass tube (brass has the second lowest coefficient of friction of any
metal, second only to Bronze.), and slot it with a 1/16" mill bit so as to
eliminate eddy current losses. The current barrel has an inside diameter of
5/16" (.79cm), a common size for finding metal rods for, and also larger
than my previous 1/4" diameter barrels, in the hope that if magnetic
saturation is indeed one of the causes for poor performance this will help
alleviate it. In order to fully eliminate the eddy loss problem it may be
necessary to insulate the projectile.Coil:
I currently have the facilities to manufacture most types and sizes of high precision/high performance coils and coil barrels for Coil Guns and other applications. If you are looking for a small quantity of custom coils contact me and we may be able to work out a deal.
Power Switching, triggering and protection circuit:
Capacitor bank Charger: Again here I am using the same one as the Rail Gun (what do you expect from a device built in one afternoon!): Variac feeding a MOT with a full wave bridge rectifier. This crude prototype will have only one resistor which will serve as both current limiting for the PSU and as a discharge device. The temporary rail gun charger will eventually before a permanent charger for my coil gun experiments after the rail gun is switched over to a solid state high speed charger. |
Completed Device: |
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Here is a photo of the device are it currently stands, with the coil unconnected and minus charger, trigger and SCR protection circuit. Check this page back often as it will be continuously updated until the gun is ready, and click on the image for a higher resolution one.
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Results: |
 The gun has been completed, and it works! I finalized the design so that I could show it on my presentation on electromagnetic accelerators -which mainly focused on the Rail Gun- for "The Daily Planet", on The Discovery Channel. It works well, with only a few bugs to sort out; mainly the coil is moving backwards with the recoil of firing and this is disconnecting it and wasting energy. Also the back EMF is sufficient to blow all the diodes on the power supply and charge up the capacitor bank in reverse to over 100 volts, so a back-emf protection circuit is a must. I expect some more high power tests and videos soon, including chronograph results and optimization. For now, enjoy this low power test video (capacitors charged to +- 1kJ).
This chronograph will be used to measure the velocity of the projectiles to within 99.5% accuracy soon. |
Links: |
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You can see my older Coil Gun Research on the
PowerLabs Gauss Gun Page. For a more spectacular kind of linear electromagnetic accelerator be sure to check out the PowerLabs Rail Gun Research. |
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Comments? Mail me. Last updated 02/11/07
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Copyright
© 2002 -2003 by Sam Barros. All rights reserved. Removing any material from this site for display without consent from its author consists in an infringement of international copyright laws and can result in fines up to $50000 per infringement, plus legal costs. So ASK ME before you remove anything from here. |
The
coil was wound on a precision turned stainless steel form, slid out and then
inserted into the slotted brass tube. By winding the coil outside of the
coil form I can ensure that the turns are tight and accurate without
deforming the coilform. The coil is 8cm long (3 1/4") and 2cm dia (3/4") and
consists in 115 turns of AWG12 polyurethane insulated (Class A Type T-1
105C) magnet wire.
Since I am already foreseeing power switching problems with this gun as
higher and higher coupling coefficients are experimented with I decided to
use the largest solid state switch I could possibly get my hands on: a
1000Ampere, 1200V SCR. It will handle a 14kA, 1300V pulse, which may be just
what is needed for a projectile to be fully accelerated in a distance of
under one inch. I am currently looking for similar units; if anyone knows of
a good supplier please 
