Building a capacitor bank capable of pulsing 16000 A DC

[trini]

alright guys a few weeks of gathering supplies and working out kinks and I'm nearly ready to try this, the last thing left to do is wrap my wire around my frame. Bob I looked into the litz wire, and it seems to me to be extremely similar to amp wire, although less twisted. do you think that i could use amp wire, which is more available to me, even though the strands themselves are twisted around more inside? more specifically, do you think that the twists in the wire will significantly affect the final output field?

 

[Bob S]

trini-
In Litz wire, all the indvidual strands are insulated from one-another. Amp-wire is certainly better than solid wire, and bends better, but not as good as Litz wire in terms of eddy current limitations. A problem I have had with large bundles of Litz wire in high-frequency ferrite magnet coils is making good solder contact will all zillions of strands. Use Amp-wire. Keep us posted on your progress.
Bob S

 

[trini]

Alright I'll go ahead and use the amp wire. I've been looking for good diodes to use with this system, but can't seem to find any. Does anyone have a suggestion (note: operating voltage = 400V, Imax= 16,000 A). Also, would it be practical to create some sort of wheatstone bridge to rectify the system, perhaps using lower rated diodes?

 

[Bob S]

trini-
Please see my thumbnail in post #7 under the thread "Pulse Width of Magnetic Field":

https://www.physicsforums.com/showthread.php?t=329842

When you use a series diode to get a single half cycle current pulse, the capacitor gets charged to a high voltage with reversed polarity. unless the Q of your circuit is very low. So unipolar caps should be used with caution in a resonant circuit.
Bob S

 

[trini]

Bob,

is there any sort of fuse I can use which will allow the first half wave to pass, then blow on the reverse and physically discharge the coil, thus meaning the only pulse i see is my first pulse, because I want the direction of the resultant field in the magnet to be as strong as possible in one direction. Once the field changes direction, it's going to flip over some of the domains in my magnet, which is counter productive. Basically, what simple, reliable solution is there if I cannot source the right diodes.

 

[Bob S]

trini-
I have looked through high current diodes, and have found ones with adequate peak 1-cycle surge current and adequate peak reverse voltage, but they are very expensive (~$600.00). I do not advise paralleling diodes in this situation, because one will invariably conduct most of the current. I Don't have any other suggestions right now.
Bob S

 

[trini]

Hmm well the diode is necessary in any event. What is the difference between a thyristor and a diode in this application? would a suitable thyristor be more economical than a generic diode here?

 

[Bob S]

Your suggestion of a triac (an ac SCR) led me to a search of SCRs. An SCR is actually better than a triac in your case, because they conduct in only one direction, and can hold off lots of volts in the other. They will not conduct after the capacitor voltage changes sign. Here is one:
http://www.pwrx.com/pwrx/docs/t9g0--10.pdf
It is about the size of an ice hockey puck, and costs ~ $160. It can handle a single 16,000 A pulse, and hold off up to 2000 V. You can use it for your switch. You will need to ac couple to gate.

 

[mheslep]

trini-
I have looked through high current diodes, and have found ones with adequate peak 1-cycle surge current and adequate peak reverse voltage, but they are very expensive (~$600.00). I do not advise paralleling diodes in this situation, because one will invariably conduct most of the current. I Don't have any other suggestions right now.
Bob S

Power MOSFETs could be used and controlled w/ current sensing feedback so that they're forced to share the load equally. This is essentially what's done in high power switched mode power supplies.

 

[vk6kro]

The ringing effects may not occur in this case.

They are caused by the coil returning power to the capacitor, but in this case, the power is being removed from the coil to magnetize some rare Earth material and to supply eddy current losses.
Also, the resonant frequency will be pulled down by the magnetic properties of the rare Earth mixture being inside the coil.

So, the damping effect may be much more severe than predicted by BobS's wonderful graphs.

 

[trini]

Guys, I was looking over my calculations this morning, and think i may have misinterpreted the relative permeability factor in my original equations. My original eq'ns considered the permeability and not the relative permeability. The following shows my revised calculation of the required current:

∫H.dl = κ [(N/ι)(I) + dφ/dt] {Re: dφ/dt = -ε0 LI}{κ = μ/μ0}
= κ [(N/ι)(I) - ε0 LI] {Re: L = (N2/ ι) A }
= κ I [(N/ι) - ε0 (N^2/ ι) A ]
Where,
∫H.dl = magnetizing field
K = average relative permeability
N = number of turns in solenoid
ι = length of solenoid
I = current
A = average cross sectional area of surface being penetrated by solenoid

Given:
Bsat = 0.925 T
M = Md(6000)/4∏ = 0.05307 T
=> Hreq = (Bsat / μ0) - M
= (0.925 / 1.25663706 x 10-6 ) – 0.05307
= 736,092 Am^-1
Also;
N = 6
ι = 0.09 m
A = 0.036483 m^2


Now my magnet is to be aligned diametrically, so the cross section of my solenoid will appear as in the attached file:

The pink area denotes the cross section of a bag which will be filled with iron powder, the relative permittivity of which is 700.

The stainless steel tube is non magnetizable, so I have ignored it for this calculation.

The blue area denotes the B powder, the relative permittivity of which is 939,014.

The ratio of the areas of pink : blue = 4.29 : 1
=> average permeability of the magnetic path, K = [(4.29)(700) + 939,014] / 5.29 = 178,075


So,
∫H.dl = κ I [(N/ι) - ε0 (N^2/ ι) A ]
736,092 = I(178,075){(6/0.09) – [ε0 (6^2/0.09)(0.036483)]}
= 11,871,666 I
I = 0.062004 A = 62 mA

this seems very small to me, though the physics does check out. the only question i have regarding this is my interpretation of the relative permeability, as i am not sure if i should just use 700 which is the permeability of just the steel(in which case my required current is about 12 A, which is more understandable). Also with these new current values, i can probably just use a car battery to apply a steady DC current while i heat the powder to activate its thermosetting resin.

 

[Bob S]

Trini-
Here is the central B field for an air-core solenoid of length L and radius r from Smythe "Static and Dynamic Electricity" 3rd Ed., page 297 eq(4). Note that the radial size reduces the central field for a fixed length L

B_z = u₀NI/sqrt(4r² + L²)

[Edit: To include the iron powder, multiply Smythe's formula by the effective relative permeability of the iron powder. It is something like 700, NOT 178, 075. Very few things have permeabilities over 10,000.]

The solenoid inductance calculator used by NASA is available for download at:
http://www.openchannelsoftware.com/projects/Solenoid_Inductance_Calculator/

The analytic equation for a single layer solenoid (thin solenoidal current sheet) is derived by Smythe "Static and Dynamic Electricity" 3rd Ed., page 340

L = pi u₀ a²n²[(z²+ a²)^1/2 - a]

where z= length, a = radius, and n = # of turns.

There are several on-line calculators. One used by ham radio operators is:

http://hamwaves.com/antennas/inductance.html

 

[trini]

Unfortunately I can't use these calculators as my coil is cuboidal in shape rather than cylindrical(to allow for the shape and orientation of my magnet), however my calculations are based off of first principle, so I do not doubt them.

The question here lies in my interpretation of the relative permeability to be employed. I suppose i could always allow for a lower permeability and just use 700, because using a higher current can only serve to create a stronger more uniform field in my rare Earth powder.

 

[Mike_In_Plano]

Few thoughts

1. You'll have to drive the Neo into saturation, so it's perm = 1
2. When considering rectifiers for pulse applications, try evaluating them by I^2t. They'll have a rating for peak current, usually for a 8.3 ms half sine wave. Integrate over this to get the I^2t (fusing rating). Then calculate the integral of your pulse's I^2t to see if the parts will survive.
i.e. Average I=100amp, peak I =2500amp. i(t)=2500 sin(120 pi t) (0 - 8.3ms)
. Integration of i(t)^2 x t (0 - 8.3ms) gives 108.5 a^2 s

3. Multiple rectifiers are fine - use a length of lead in series with each to form a ballast resistor. If the lead drops 2v at peak, the rectifiers will track fairly close.
4. In place of Litze wire, try parallel windings side by side (filer). Or you can look into purchasing some flattened wire.
5. Be very careful - In the kiloamps, Lorentz forces can cause wires to explode outwards.

Mike

 

[trini]

mike, the MSDS listed the permeability at my density(7.6 g cm^-3) as 1.18. if i am not mistaken, as the iron powder is essentially a magnetic path, i am concerned with the relative permeability, correct?

all my previous calculations assumed an air core, but this is now a steel core. because the powder will be within a field 700 times stronger than an equivalent air field. the magnet assumes saturation when it is placed in a field which relative to itself is above a certain value, Bsat(in this case= 0.925 T= 736,090 A/m). what are your thoughts on my deduction?

 

[Bob S]

Hi trini-
The iron powder dominates the field properties inside the coil, and B_longitudinal is continuous, so the iron powder will determine the B field in the sample. Do you have a curve like my annealed-iron thumbnail for the powder?
Bob S

 

[trini]

i have a similar graph but not one including the relative permeability, though i can say it is effectively 939,014 at my density. I think it is safe to assume that a 15A current will more than drive the powder material to this permeability in this core. My scanner is down so i can't upload the graph, but is it agreed that this is the best way to go for my project?

EDIT:

totally missed the point of your question, no i don't have one of those graphs for the iron powder i'll be using, i still have to locate it.

 

[Bob S]

i have a similar graph but not one including the relative permeability, though i can say it is effectively 939,014 at my density. I think it is safe to assume that a 15A current will more than drive the powder material to this permeability in this core. My scanner is down so i can't upload the graph, but is it agreed that this is the best way to go for my project?

trini-
Nothing has a permeability that high. Mu-metal is like 50,000. I suspect that you should use a number more like 700. Your formula for H should look something like my formula from Smythe, and you should specifically be correcting for the width of the coil if it is more than say 25% of the length of the coil.
Bob S

[Edit] For rectangular coil of width w, this should be pretty good:

H_z = NI/sqrt(w² + L²)

B_z = u u₀NI/sqrt(w² + L²)

where w = width of coil, and L is length.

 

[trini]

bob,
I was referring to my rare Earth powder, I may be wrong but since my neo powder has a u = 1.18, then my relative permeability = 1.18/u0 = 939,014. Regardless, even using 700 as my relative permeability, I only need 12 A, so that's why I think a 15 A DC source running for my heating time(1 minute to heat powder to 195 C using IR heating) will overcome the eddy current losses generated. Also, the width of my coil is negligible compared to its length(its a long magnet, 50 cm x 1 cm thick).

http://www.lessemf.com/278.html

that site shows a similarly composed composition with a relative perm of 1,000,000.

 

[Mike_In_Plano]

I worked on a project where GE was building brushless motors for us, and they had a terrible time with an experimental Neo design because the magnetizer was so hard to build. As I recall, they were playing around with numbers like 20kG or so. At that point, a lot of magnetic materials start saturating.

So, you get a goodly many webers through the core, until it saturates. Beyond that, your perm=1, and and additional magnetization is due to pushing a lot of current through the coils.

In the end, we settled for large ceramic magnets - much cheaper.

 

[Mike_In_Plano]

Hmmm, I don't think it's fair to expect any magnetic powder to approach the permeability of a strip of metglas. It (metglass) is incredibly homogeneous. Most varieties don't even have anything you can define as magnetic domains (though some have oriented "micro crystals" which are introduced to optimize the B-H loop).

Powder, on the other hand, is the accumulation of a multitude of diverse particles. Within any given particle,you can make a valid claim that it has a tremendous perm, but the irregular packing between the particles will make the whole appear far less permeable.

 

[vk6kro]

I'd like to see the powder above and below the coil as well as down the middle as in your diagram.
The complete path includes the area outside the coil and the less air gap there the better.

Also, it should taper towards the magnet rather than tend to bypass it. You really want to concentrate the field in the area of the magnet.

 

[trini]

I updated my design module, making this a more typical magnetic circuit:

 

[trini]

Ok so I've been meddling around with my magnetic circuit design, and right now my biggest problem is having enough ampere turns in the space. in my current design(see above attachment), i can fit 6 turns of gauge 0 amp wires per side.this still requires a huge current to fully utilise the size of wire. instead, i have devised a plan to maximise space and minimise required current.

I will construct a series of magnetic flux 'cells'(shown below) which can then be any required length to allow for any desirable amount of turns. the cells will be made out of annealed iron rods, and will 'impale' themselves into the powder core, which I can make into any shape I desire. Each cell will consist of 4 parallel magnetic flux sources, so the total flux into the powder core would be 4 times each of the sources' flux. I will distribute these cells evenly throughout the length of the powder core to ensure uniform field distribution.

I would be foolish to assume there would be uniform permeability throughout the powder core, so my main goal should just be to drive the core to saturation( about 1.2 T). My magnetic material has an Msat of 0.925 T, so I think all I have to do is create a field of that or higher in my air gap to saturate my material, unless Msat means something else entirely.

 

[Bob S]

Hi trini-
I like your coil configuration in post #58 better than #59, because there will be less flux leakage, which will literally defeat some good dipole magnet designs, which are made with higher relative permeabilities.
Your #58 design is called an H-magnet design, and the powdered iron funnels the flux around a bend into the sample, where the H-field (but not necessarily the B field) is concentrated. Do you plan to slide the sample in/out or do you plan on removing ferrite to get the sample in/out?
In any case, the powdered iron/ferrite will saturate before you get to 1.4 Tesla, and there are few alternatives to adding brute-force amp turns.

The best possible design might be a cylindrical version of an H magnet (similar to a gapped pot core), with a gap in the center for the sample, and the sample surrounded by the coil. This would get the coil close to the sample, but it would have to be disassembled (like pot cores) to get the sample out.
You can minimize the size (max the Ga. #) of the wire in your coil by picking a wire size, and then integrating the current-squared times wire resistance over time:
E = ∫I²R dt (energy dissipated in wire during single pulse)
and choose a wire gauge such that the temperature rise in one pulse raises the copper temperature say 25 to 50 degrees.
Remember that the powdered iron raises both the magnetic field intensity and the inductance of the coil.
Bob S.

 

[Mike_In_Plano]

Bob's got a good proposal - The gap through the Neo is HUGE. When your magnet material goes into saturation, you'll have terrible problems with leakage flux. The best way to attack that is to have the coils in closest proximity to the Neo.

 

[trini]

Ok so more tinkering has led me to this. I can't use a pot core because of my magnet shape, but this is essentially the same thing. I have employed rounded edges wherever possible to avoid flux leakage, while the M shape of each half of the core serves to direct the lines to the gap. The tube is relatively impermeable compared to the neo so it itself provides a natural 2mm gap i have to deal with on each side of the neo. I call it a MoM core, see if you can guess why ;)

 

[mheslep]

Ok so more tinkering has led me to this. I can't use a pot core because of my magnet shape, but this is essentially the same thing. I have employed rounded edges wherever possible to avoid flux leakage, while the M shape of each half of the core serves to direct the lines to the gap. The tube is relatively impermeable compared to the neo so it itself provides a natural 2mm gap i have to deal with on each side of the neo. I call it a MoM core, see if you can guess why ;)

You don't really need or intend 0.0001 tolerances do you?

 

[trini]

no no autocad automatically does that, i'll round off to suit.

 

[Darryl]

Ive used one of these devices, only once and I did not like it one bit. I am a bit rusty on the details, but it was at ADFA (Uni of NSW, ACT Aust). It was used for the purpose you require.

It looked Like the TARDIS, a simple (lange box) with 4 red LED's and 1 (red) button on it.

The use of this thing was strickly forbidden near any sensitive electronics or computers, and the red button operated with a broom handle. (long as possible).

As for the design, it was no more than a wacking great capacitor, and large (hockey puck type) diode, (called a recirculating current diode from memory).

The red button, fired a thyrister which in tern fired a switch system I am sure was called an "IGNITRON".

I hope that helps you some.

Edit: Definetly IGNITRON
http://en.wikipedia.org/wiki/Ignitron

 

[Darryl]

Design is not too complex, I don't think. But first things first.

Safety, this is one deadly device, with such a device you DO NOT get a second chance, if you come in contact with a fully charged low value, High Tension (HT) capacitor you never get away scot free.

PLEASE: use all safety you can muster, and then some.

The system would contain 1 small HT electrolytic capacitor to generate the trigger pulse to fire the Iginitron.

And one or a bank of high capacity electrolytics, (possibly supported by various size ceramic or poly type high voltage caps for their high speed.

you will also need to design a high Ohm bleader resistors across all the capactior banks, and a shorting stick in stalled in the cabinet, these electro's can hold a charge for a very long time, and in the presence of RF fields can self charge, always store them with wire shorting out the terminals, and design bleeder resistors mabey 1meg or 10 meg for each cap.

The smaller electro connected to the THYRISTER that with triggered with dump the contents of the smaller electro into the TRIGER terminal of the Ignitron.

That will fire the Ignitron and dump the contents of the big capacitors into the recirculating diode, and onto the load.

I would mount the whole thing on a thick steel place, probably with a copper flashing over and bolted to the steal place, all sitting on a think insulating rubber mat.

I would use a solid state DC-DC converter to generate the High voltage required to charge both big and little caps.

I would have NO connection to your business electrical system when you fire the device and I would use an RF Garage door opener remote control to fire it, I would have a loud buzzer and flashing light when the device is charged, and carefull fireing procedure.

My experience is mainly from high power RF transmitters, mabey 10Kv to 15KV and 10 to 15 Amps, but that is continuous.

The normal method used to pass high current and high voltage is multiple stacked copper strap mabey 3 to 5cm wide and 25 to 50mm thick and possibly 4 or more conductors and usually only joining at the ends, not over the entire length. The conductors would also have anti parsidic radiation resistors mounted along their length, seemingly directly shorted out by the thick copper conductors.

These resisters are doing nothing until the potential difference between the ends of it (shorted by the conductor) changes from zero, this occures from the IR loss at the peak of the current (right when you want it elsewhere) so the resisters then start to pass current passing more current into the Diode and finally the load.

Be carefull :)

 

[trini]

OK so after some deliberation i have realized that a typical magnetic circuit is not the way to go with this, as basically any core i use is just going to saturate then poof there goes most of my flux out the window. Following up on the idea of the magnetic circuit wasn't a complete waste of time though, as it pointed out a very important fact to me. In the calculations thusfar, i used H = 1,600,000 Am, but my magnet is only 1 cm in length, so my required H is really just 16000. Now considering i would be using the region inside of my solenoid to determine my H, the required formula becomes:

H = u(N/l)I (im ignoring self inductance for this coil atm, it will be very minimal anyway)

where u is the AVERAGE permeability of the inside of the solenoid. By making the space inside the solenoid as minimal as possible(as shown in the diagram, note i do cover the tube area, so that the field experienced by the powder is relatively horizontal and not bending at the edges.

so the ratio of NEO to free space is about 60:40, which i would think makes my average u = [(0.6)(1.18) + (0.4)(u0)] / 2 = 0.321792

So then,

16000 = (0.321792)(4/0.012)I
I = 149.16 A

so there you have it, i only really need about 150 A(preferably DC) Surely there is some commercially available product which i can use to run this current through my wire(welding supply maybe?) So please, put forth suggestions, and also, could someone verify that 3 mm(AWG #9) wire can handle about 2-3 seconds of this current?

Note also that if i want to extend the solenoid so as to make the region the magnet experiences more uniform i can, i just have to fill the extra space with steel powder but it will drop my average permeability and thus increase my req'd current.

 

[gary350]

OK. I see what your doing now. Your trying to magnatize a magnet or re-magnatize a magnet or change the direction of the magnetic field of a magnet.

This is what you need. I built this 15 years ago here are some pictures.

http://i67.photobucket.com/albums/h292/mikeweaver/000_0022.jpg

This is a coil of wire wound with #24 enamel coated copper wire. It measures 1 1/4" inside diameter, 2 1/4" outside diameter and the coil width is 1". It has been varnished with polyurethane several times to glue all the wire together. The wire must be glued together other wise this thing will self distruct.

http://i67.photobucket.com/albums/h292/mikeweaver/000_0024.jpg

The capacitor bank is 2 capacitors in parallel. Each cap is 7400 MFD 200 VDV with max surge of 250 VDC.

The power supply that charges the capacitors is. 11 diodes in parallel each diode is rated 1 amp. 1N4007 will work fine. There is a current limiting resistor in series with the diodes it is 18 ohms 5 watts. It plugs into the wall outlet 120 volt AC.

Here is how the power supply works. If I plug this into the wall the capacitor bank will pull to many amps and burn out all the diodes so the current limiting resistor limits cap charging current to 10 amps. With 11 diodes in parallel it is rated 11 amps. It takes a few seconds for the cap bank to completely charge. Once it is charged I un-plug it from the wall.

http://i67.photobucket.com/albums/h292/mikeweaver/000_0023.jpg

Next I already have one of the alligator clips connected to one of the wires on the coil. I tough the other alligator clip to the other wire on the coil and all the current that is stored inside the caps is discharged into the coil. This produces a giant magnet field that can change the direction of the magnet field of the existing magnet or it can also super charge the magnet.

The tiny magnet inside the coil in the picture measures 1/2" thick 3/4" diameter. It has a lifting power of only about 1 lb. After I discharge the cap bank through the coil it super charges the magnet and it will lift over 250 lbs. The super magnet field has a half live of about 2 seconds so it drops from 250 to 125 lbs. in about 2 seconds, then it drops to 62, then 31, then 15, then 7.8, then 3.9, then 1.9, and so on. After about 20 seconds the magnet is almost back to normal it will lift about 2 lbs for several more hours but by tomorrow the magnet field is pretty much back to normal.

If I place the magnet in the coil in the wrong direction I can reverse the direction of the magnet field on the magnet.

If I place the magnet in the coil at a 45 deg angle I can relocate the magnet field at a 45 deg angle on the magnet.

If I place the magnet in the coil on its side 90 degs then the magnet field will be change so it comes out the side instead of the ends.

If I wind 2 coils and place both coils 90 deg from each other on the magnet I can force the magnet to have 2 north poles and 2 south poles 90 degrees apart.

If I wind 6 coils and place them around the magnet I can make the magnet have 6 north poles and 6 south poles.

Is this what your looking for?



Notice the contraption I built is not very fancy but it works. I was never able to find a switch that would not weld itself shut after discharging the cap bank. A push buttom to charge would be nice and a push button to discharge would be nice too.

I can not tell you for sure how many turns of wire is on this coil. #24 enamel coated copper wire is 46.3 turns per inches. Doing the math 46.3 x 23.15 = 1017 turns. There is probably about 1000 turns on the coil.

It has been too long since I did this I have forgotten just about every thing. As I recall if you reduce the coil size by half it doubles the magnet field and if you double the number of turns it doubles the magnet field. Maybe I should say it concentrated the same amount of power into a smaller space so it effectively doubles the power for a given space. Something like that.

All that is required to change a magnet or magnatize a magnet is to over come a certain power rating for each magnet. It is like trying to charge a car battery you have to excede the voltage rating of each cell by 1/2 volt to make the battery take a charge.

One more thing. The voltage at the wall outlet is 120 VAC after going through the diodes it should be about 170 VDC in the cap bank. You don't need high voltage to super charge or remagnitize a magnet you only need a strong magnet field strong enough to do the job. I not sure how much power is discharging out of the cap bank but it all comes out in one big PULSE that is what makes this device work. The caps take a few seconds to charge and a micro second to discharge.

 

[Bob S]

Gary's setup may work, but it has serious safety hazards. Any circuit that works off the 120 Vac should use an isolation transformer if possible. Also, a 3-conductor plug (hot, neutral, ground) should be used, and all equipment grounded. Lastly, if using diodes in parallel, the individual diodes should have series resistors (in this case about 120 ohms). This is because as a diode gets warmer, its forward voltage drop decreases, and it will steal current from the cooler diodes.

 

[mheslep]

I would have thought a complete air gap between the grid and the apparatus was warranted. That is charge the capacitors, disconnect from the grid, and then fire the apparatus from the capacitor.