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Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
After a couple years of pontificating about coilguns, it's time I actually made one. Have wanted to try, as a barrel, free aluminum tubing from a discarded TV antenna.
This material is pretty smooth, round, thin-walled, hard, and straight. Comes in a convenient diameter, and is already slotted along its entire length (because made by rolling up a flat strip of metal). The slot (approximately zero width) makes the tube notably elastic in torsion, as the two sides of the slot slip past each other. That can be fixed by gluing a sleeve, patch, or nonconductive winding across the slot, some place where we don't care about eddy currents.
I thought it would be instructional to start by finding a pre-wound coil of high impedance, such as from a mains-powered solenoid valve. With that as a sense coil, we should be able to electrically measure the changes in effective L and R when we insert a slotted metal tube, seamless metal tube, solid metal rod, ferromagnetic rod, wooden dowel, etc. Will probably start at 60 Hz; the fancy LCR meter in my lab doesn't do less than 30 kHz.
Does anyone else have experiences to report with antenna tubing, or coil inductance measurements?
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
I'm not sure here, but I think that any changes in inductance will be seen more at higher frequencies.
Also, it would be interesting to try and demonstrate the losses due to eddy currents that we all say is so critical to avoid. I would suggest you set up an RLC circuit with a large capacitance and a large inductance coil and capture the discharge on an 'o-scope. Then you can compare the time it takes for the waveform to go to zero for the different cores. The eddy current should add a damping factor so the longer the circuit rings the more efficient the barrel material would be. If the you can establish a procedure for testing barrel efficiency that the community can follow for themselves, we would all benefit.
It seems that wall thickness is the biggest factor in coilgun efficiency as far as the barrel goes. So I would imagine that your tubing would work great, especially at larger energies where the friction of the projectile sliding in the barrel wouldn't be as large of a factor.
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TV-antenna tubing (naturally slotted Al) for coilgun barrel?
