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Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
While working with inductor design for the critical current damping of very large Xe flashlamps, the subject came up of the strength of the magnetic field generated inside a coil by a large current pulse.
Here is a sample of the discussion of what form the B-field would take under increasingly large parameters (current, coil thickness, etc.)
...clip...from the 'other' guy... "" And yes, I agree with you that the maximum will be on a ring around the center, and this ring is in a plane that the axis is normal (perpendicular in 2 dimensions) to. Except, I give some chance that the maximum is at the center point. But if the layer length of the coil is decreased, and the inside diameter is not decreased, and especially if also the outside diameter is decreased, then at some point the maximum will definitely become a ring rather than a point.""
Well, this prompted me to tackle a rather large programming task--I used, for the first time, a free programming language called "PROCESSING" (I got the manual from ebay). I decided to plot the B-field inside the coil using the Biot-Savart law. This allows me to accurately plot B at any point in space. Using various, and sometimes huge, parameters for winding configurations and current, I was easily able to get beautiful full color plots of the -entire- cross sectional B-field (not just axial). I color-coded the B strength in 24-bit color following the continuous spectrum with red representing the most intense B and blue the weakest. Somewhat, to my surprise, I could not find any maxima forming in the central region of even the thickest coil with impractically huge currents flowing. The central region of a cross-sectional area showed mostly blue and cyan (about 80%) while the green, yellow, reddish, hugged the inner wall.
In quarter-shrinking, I always found that the outer perimeter of the coin was the most distorted--usually with a ripple'ed edges. Does anyone know how, or if, the 'frequency' of those ripples have anything to do with the LC resonant frequency? NOW, would the quality of the shrunken coin be improved if, during the shrinking process, the coin is confined to that uniform central region? Of course this would require the coin to be centered in a larger diameter coil, and probably also need a significantly more energetic capacitor charge since a fraction of the field (the extremely non-uniform outer portion) is not utilized.
Briefly, here is how I see the general coin-shrinking process (please correct me or add/subtract information as needed): The coil's counter mmf results in a huge coin current of about 10 times that in a 10-turn coil due to 'transformer action' + the coin acting as a single shorted turn. The direction of this induced coin current obeys Lenz's law and flows as circular eddy currents in such a direction as to interact with the induced B-field producing a Lorentz force directed radically to the coin's center thus shrinking it.
Registered Member #4266
Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874
I would say that the two fields are oppsite, but that the coin can produce larger current to equal the primary, amp.turns, but the strength ist related to turns more j,E,B, so it can compress more than the primary.
It should show closest to the wire as at that point is the closest field reverasble., oppsite equal,oppsite.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
If you see ripples in the metal imho the current is to low and pulse is too long.
Here is why i think this: While the coil inside shrinks the coil outside expands. I am just guessing here, but since those guys here have no ripples (correct me, but i dont know what to look for - i just find them good enough).. i think we need to think for the difference to other guys. The link presents a vaporizing coil. It has a very high expansion force.
What i think can go wrong - specially then the pulse is too long - that the outside coil doesnt perfecly expand but due to springaction it kind of oscillates during expansion with one of its mechanical eigenfrequencies. The faster the outside coil expands (explodes) the needed eigenfrequency gets higher and higher until its just smoth. Maybe if you could give an example what effect you want to avoid (pics!) the would be a more solid discussion.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
DerAlbi: In the video, I think these guys are using WAY to much capacitance and energy! The ripples I spoke of are along the thickness of the edge of the coin, not over the surface. If you inked the edge of the quarter and rolled it down the floor, it would draw a sine wave! At the outer perimeter, the coin is not flat for only about 10% along a diameter. -------------- Andy: Yes, I think your last statement may explain the strange perimeter pattern, though I am still wondering if this sinesoidal deformation is related to the LC constant. --------------- The following is from several years ago...At first I only had ten 32uF 4.5kJ, I don't even remember if I ever used all ten. The result was a quarter that you had to compare with another one to see it was actually smaller (in dia.). Then I got my first giant energy capacitors! Two orange Maxwell energy caps each rated at 12kV @ 70uF. I used them in parallel and charged them with a rectified NST. each coin was centered in ten turns of 10AWG magnet wire. First shot: @ 5kV (1750J) gave -noticeable- results. Then after a 10kV shot, it took a while to find the quarter (in a giant holography sandbox) and there was a LOT of 'thinned' razer sharp exploded shrapnel (NEVER do this unless the coil is covered and in something virtually bullet-proof!). When I finally found it--I was shocked--looking at that TINY little quarter (it was heads). It was too hot to pick up.
So I found it took a variable (use a variac to the NST) energy cap bank (140uF in my case) of a minimum of, say, 2kJ to a maximum of 6kJ to 7kJ to control the quarter diameter from just un-noticable to -much- smaller than a dime (7kJ). I could control the voltage to get diameters precisely equal to a nickel, a penny, and a dime. And much smaller, although past this they start getting very thick--to the point they look fake. Note that this is all with the same ten-turn 10AWG wire and 140uF capacitance. I intend to broaden these two parameters. A good project would be to find the least-energy capacitor bank to shrink to the smallest desired diameter.
A RE-USABLE COIL: I found (to my surprise) that if I coated the coil in fiber (cloth) resin it became much more resilient to the current pulse! I got a few shots at medium energy until it was un-usable. The coil did not explode, but the coin AND wire shrunk! I remember then cranking the energy up and the coil exploded while the coin shrunk less, of course, than normal for this energy level. Then I made big (really big) blocks of cloth fiber, but have not tried them YET! I encased wire of 10AWG, 3-layer 10AWG, ribbon magnet ~1/4" wide x 1/16" thick, and a monster coil of ten turns of huge square (1/4" x 1/4") magnet wire which took a special tool to wind. I will post some photos of this stuff soon (people got all my quarters though, I did manage to hold onto my last one until I traded it for a laser ruby!--I will make more). I will include a photo showing the rig with which I plan to shrink coins in the uniform field, but I am afraid that these big potted 'shrinkers' may not protect the -inside- of the coil in the blast BUT my new method very well could, as the coil will be protected inside also.
A REALLY strange effect: After looking at the dime-sized quarter a while, a regular quarter appears "freakishly" expanded.
The central region of a cross-sectional area showed mostly blue and cyan (about 80%) while the green, yellow, reddish, hugged the inner wall.
For long and thin tightly wound solenoids the field is nearly homogeneous inside the coil. For shorter thicker ones, the filed is somewhat weaker in the center. If the coil is not tightly wound, i.e. you have some room between the windings, the field is a bit stronger near the winding and weaker inbetween. This does not depend on current strength. A larger current will just multiply the field by a constant factor.
Does anyone know how, or if, the 'frequency' of those ripples have anything to do with the LC resonant frequency?
Possibly this has to do with the fact, that the field is not homogeneous near the windings. Or the coin might just buckle in some way.
Note that this is all with the same ten-turn 10AWG wire and 140uF capacitance. I intend to broaden these two parameters. A good project would be to find the least-energy capacitor bank to shrink to the smallest desired diameter.
There is an optimal time constant (i.e. 1/sqrt(L*C)) for shrinking. The current induced in the coin will increase for shorter pulses, since the voltage induced in the coin is proportional to the rate of change of the magnetic field. At a certain point, though, the induced current will produce a strong field opposite to that of the coil. That will limit the coin current. Going beyond that point will only increase losses due to ESR and coil resistance. That has been discussed e.g. here:
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Signification wrote ... Briefly, here is how I see the general coin-shrinking process (please correct me or add/subtract information as needed): The coil's counter mmf results in a huge coin current of about 10 times that in a 10-turn coil due to 'transformer action' + the coin acting as a single shorted turn. The direction of this induced coin current obeys Lenz's law and flows as circular eddy currents in such a direction as to interact with the induced B-field producing a Lorentz force directed radically to the coin's center thus shrinking it.
Let me begin with a respectful bow toward someone who has actually made and used a quarter shrinker. My 52 uF 20 kV (10.4 kJ) Aerovox capacitor was purchased with that in mind, but I stopped after cutting cans in half with 1 kJ. Too many projects, too little time. And I don't want to risk damage to the capacitor, lately serving as a door stop.
Using "transformer action" formula to predict current in the shorted turn is wrong in my opinion. It works in ordinary transformer applications, where the secondary ampere-turns practically cancel the primary ampere-turns. A small remainder of ampere-turns cyclically magnetizes the core, enough that the induced "back EMF" matches the applied volts per turn.
I think a better model starts with the secondary volts per turn (similar to primary volts per turn) divided by the resistance of the shorted turn. In the Al-can case, that's substantially less than N times the primary current. The resulting current does reduce the B-field that would be there if no workpiece were present, and reduces the inductance seen by power source. Has anyone here seen a quantitative model published, even in the simplified case where workpiece and coil don't move?
The system ought to be linear for all energy levels too low for metal to move during the shot. (Except the coil and workpiece resistivity increase progressively during the shot because of Joule heating.) In a quarter-shrinker fired with insufficient energy to move things, before-and-after temperature measurements could show how 100% of original capacitor energy is partitioned between coil heating and workpiece heating.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
klugesmith wrote ...
Signification wrote ... Briefly, here is how I see the general coin-shrinking process (please correct me or add/subtract information as needed): The coil's counter mmf results in a huge coin current of about 10 times that in a 10-turn coil due to 'transformer action' + the coin acting as a single shorted turn. The direction of this induced coin current obeys Lenz's law and flows as circular eddy currents in such a direction as to interact with the induced B-field producing a Lorentz force directed radically to the coin's center thus shrinking it.
Let me begin with a respectful bow toward someone who has actually made and used a quarter shrinker. My 52 uF 20 kV (10.4 kJ) Aerovox capacitor was purchased with that in mind, but I stopped after cutting cans in half with 1 kJ. Too many projects, too little time. And I don't want to risk damage to the capacitor, lately serving as a door stop.
Using "transformer action" formula to predict current in the shorted turn is wrong in my opinion. It works in ordinary transformer applications, where the secondary ampere-turns practically cancel the primary ampere-turns. A small remainder of ampere-turns cyclically magnetizes the core, enough that the induced "back EMF" matches the applied volts per turn.
I think a better model starts with the secondary volts per turn (similar to primary volts per turn) divided by the resistance of the shorted turn. In the Al-can case, that's substantially less than N times the primary current. The resulting current does reduce the B-field that would be there if no workpiece were present, and reduces the inductance seen by power source. Has anyone here seen a quantitative model published, even in the simplified case where workpiece and coil don't move?
The system ought to be linear for all energy levels too low for metal to move during the shot. (Except the coil and workpiece resistivity increase progressively during the shot because of Joule heating.) In a quarter-shrinker fired with insufficient energy to move things, before-and-after temperature measurements could show how 100% of original capacitor energy is partitioned between coil heating and workpiece heating.
Even when it does move things, the energy should still end up as heat
Has anyone here seen a quantitative model published, even in the simplified case where workpiece and coil don't move?
IMHO a transformer model with a coupling lower than one might work. A k<1 will account for not all of the primary field going through the secondary and vice versa.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
DerAlbi wrote ... ... those guys here have no ripples ... The link presents a vaporizing coil. It has a very high expansion force.
It always bugged me that the Hackerbot video caption says their coil vaporized. The available energy is not enough to vaporize (or even melt) more than a tiny fraction of the copper mass. Aside from heating, the workpiece and coil consume energy in plastic deformation, being torn to bits, and being launched at high velocity. The last part soon converts to air motion, damage to the fragment shield, and heat.
Sig's report of re-using shrinker coils was enlightening. My thoughts in that department had drifted toward a flexible shaft coupling geometry.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
I remember once when Washington's right profile was deeply imprinted in a piece of the rosin--I must have had the quarter un-centered in that direction. I just checked the coupler on my homemade ciolwinder--A great match!
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COIN SHRINKING: in a B-field of improved uniformity
