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Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Hi,
I'm creating molded end caps for the coils on my next coilgun out of potting epoxy and iron powder, in the style of Coilgun Systems. The only thing I've done differently is that I used a finer iron powder and applied more compression to the mold.
Now that I have one complete end cap produced, I notice that it's fully conductive (as tested by my multimeter, which reads it as a short from one side to the next). I expected it to be less than fully conductive (not a short) since the point of making the cap out of powdered iron is to prevent eddy currents from forming in the cap while the coil is discharged.
My question is, since my end-cap is conductive, will I still have problems with eddy currents? Or will fact that the cap is made from powdered iron mitigate the effects? The author of the aforementioned website doesn't have any data on the conductivity of his end caps but is clear that a powdered or laminated material must be used to reduce eddy currents.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
That's interesting. Is the resistance ~0 or just <100 ohms? my guesses: perhaps your compression forced the epoxy out and the iron together, so maybe try less compression.
did you mix the iron powder very thoroughly into one of the epoxy parts before mixing the two parts together, resulting in the particles not being coated completely?
could the iron of had any slight magnetization to it that may have caused the particles to link together while in suspension?
if you don't want to remake it then you could cut a slot through it to help some with the eddy currents.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Good news everyone,
I think I solved the problem. Turns out my piece isn't conductive! I noticed that it only measured as a short when the multimeter leads were placed across the top surface. It appears that there was a thin conductive layer right on the top. My theory is that the resin got squeezed out of the upper surface during the compression, leaving a conductive iron-rich layer at the top. This is consistent with my observation that some resin oozed out of the top of the mold while I compressed it.
Oh, and Forty, to answer your question, it read it as a dead short, R~=0.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
looks like an emi suppresion ring now. could one of those be used instead or would ferrite be a poor choice? I'm working on a single slotted mu-metal (hard drive magnet shielding) washer for the same purpose.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Yes I believe a soft ferrite part would be ideally suited to this application, since ferrite is non-conductive and highly ferromagnetic (hence its usage for transformer cores).
The iron powder approach is advantageous when you need custom molded parts.
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Question about enhanced flux linkage
