High Voltage Forum
Pulse power => Capacitor Banks => Topic started by: MRMILSTAR on October 20, 2021, 09:31:01 PM
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I have a pulsed power generator used for several things such as coin shrinking. I can shrink a quarter to the size of a dime with about 7000 joules.
Shrinking coins is a compromise between coupling, resonant frequency, and energy level. My current work coil consists of 10 turns of 13 AWG magnet wire. This was arrived at after a lot of experimentation. This combination works well for shrinking a quarter to dime-size. However adding more energy does not provide any further shrinkage due to the work coil exploding before the peak power level is reached.
I now want to make a "ball quarter". I have attached an image of one. I know that a more robust work coil that can hold together longer is needed, though it will still be sacrificial. And of coarse more energy is also needed. Its not as simple as just using larger AWG wire because the coil gets longer and reduces coupling.
I don't know the details of how the ball quarter in the picture was made. I heard that flat ribbon magnet wire was used wound "the hard way". Its unknown how much energy was used. I have available up to 24,000 joules which I think is plenty.
I don't want to make that difficult ribbon wire coil though. I had another idea which now brings me to the subject of this posting.
My idea is to closely surround one of my current 10-turn 13 AWG work coils with a solid block of G10. My thinking is that this may hold the work coil together long enough to make the ball quarter. G10 is the only non-conductive material that I can think of that has a chance of withstanding the expansive force of the work coil moving outwards.
Any thoughts are appreciated.
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Yes, those extremely-shrunk quarters are fascinating. I don't have the energy for that, only 14uF at 20kV, which I run at ~20.5kV for ~3kJ. You inspired me to post my old 2002 build:
https://highvoltageforum.net/index.php?topic=1793.msg13556#msg13556
I experimented a lot with coil casing. Nothing worked for me. That is likely due to my 2-layer coils. Any casing causes the two layers to munge together and short.
I gather that commercial magnetic forming coils are epoxy-encapsulated, likely with some steel or other such outer shell once far enough from the coil. Perhaps glass-wound too.
For single-shot coils, inertia might work as well as strength. Perhaps some dense ceramic. I can't recall if I gave up on casing before trying sand.
Glass-filled PEEK is probably stronger than FR4, but expensive. I just turned a new quarter coil form with unfilled PEEK, as my old delrin forms are worn out - abrasion and partial-melting from hot shrinking coins.
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Agree with Dave about inertia. I think the word is "tamping" in the blasting trade.
[edit] Maybe Dave already did the sand thing, which I wrote about below. [\edit]
Could either of you guys try an A-B test of quarter shrinking, where in one case the work coil is buried in dry sand to thickness of at least a few inches? Sand can be vacuumed out after the shot. Its conductivity will be negligible compared to copper work coil. You could get a bit more density, with lots more trouble, using mortar or concrete.
For higher density, how bout a wrap of flexible fiberglass sheet backed by lead shot?
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Could either of you guys try an A-B test of quarter shrinking, where in one case the work coil is buried in dry sand to thickness of at least a few inches?
My experimenting was all 18-19 years ago. Key result for my setup: confining the coil was always worse. Significant containment of any form caused the inside winding layer to smash into the outside layer and short. Cap discharge ended more abruptly at high tail-end current (not great for capacitor life), resulting in less coin shrinkage. Further containment experiments need to be on single-layer coils (driven by higher capacitance). Eventually (several years away) I may get to trying containment. I bought a physically-smaller 250uF 10kV PP energy-storage capacitor. Hoping it may survive for enough pulse discharges to use for portable quarter-shrinking at Maker Faire. At 250uF, it will definitely need a single-layer coil.