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Does anyone know the shape of the magnetic field surrounding the pancake coil?

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LAN_403

w1vlf

Wed May 21 2008, 11:53PM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

Hello Folks,

In an effort to learn more about the transfer of energy from the pancake coil to the AL disc.
Does anyone know what the shape of the magnetic field looks like aaround the pancake.
For example is it a dome shape on both sides with the coil where the equator would be?

Is a flat disc necessarily the best shape to generate Eddy currents in itself?

This is on area that I cannot even speculate about.

How about it... Maybe there is a simulator out there that I missed?

Spent alot of time looking with very little results.

Any ideas?

Thank you
PauLC
W1VLF

Turkey9

Thu May 22 2008, 02:10AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

the magnetic field lines flow across the surface of the coil, either from the outside towards the middle, or vice versa depending on the polarity. They go through the center of the coil and flow back along the back surface. I attached a pretty bad drawing of what i'm talkin about.

A flat disk adjacent to the coil allows for the field lines to flow through the most material to create the most eddy currents. Any material not near the surface of the coil won't have eddy currents in it so will just be dead weight.

OZZY

Thu May 22 2008, 07:06PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

Here are the results of some finite element models I`ve been working on using FEMM. The first picture shows flux lines around half a pancake coil, the vertical line is the line of symmetry and the rectangle is a cross section of half the coil, the outside diameter is 50mm, inside diameter is 10mm and thickness is 3mm. The coil is powered with A.C. at 2.4KHz.

The next picture shows the same coil with an aluminium disc 1mm above it. The disc has the same dimensions as the coil.

As you can see the disc blocks the flux, at 2.4KHz the magnetic field can`t penetrate 3mm of aluminium. Most of the flux is squashed into the air gap. The last picture shows a plot of current density.

The current density at the bottom of the disc is 3 times higher than at the top due to skin effect.

w1vlf

Thu May 22 2008, 08:29PM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

Turkey9 wrote ...

the magnetic field lines flow across the surface of the coil, either from the outside towards the middle, or vice versa depending on the polarity. They go through the center of the coil and flow back along the back surface. I attached a pretty bad drawing of what i'm talkin about.

A flat disk adjacent to the coil allows for the field lines to flow through the most material to create the most eddy currents. Any material not near the surface of the coil won't have eddy currents in it so will just be dead weight.

Thanks Turkey9,

This is what I was looking for, but could not find on the internet.
Thank you for taking the time to draw out the diagram.

This purpose of this thread is to determine how to extract the maximum energy from the work coil.
Then some how see how that fits with my fixed weight projectile of 46 grams.

Thank you
PayLC
W1VLF

w1vlf

Thu May 22 2008, 09:02PM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

OZZY wrote ...

As you can see the disc blocks the flux, at 2.4KHz the magnetic field can`t penetrate 3mm of aluminium. Most of the flux is squashed into the air gap. The last picture shows a plot of current density.

The current density at the bottom of the disc is 3 times higher than at the top due to skin effect.

OK OZZY,

Thank you for this very much.
I am afraid I need to ask you more questions now.

1) Your model assumes 2.4 Khz, so would it be fair to equate that to a (capacitor discharge) time of 1/2" cycle of that... or .0004 seconds? or 400 micorsec?

2) How thick is the disc? If it is 3 mm and the current is that at the top of the disc, any thicker disc would be of little use in increasing the current??

Forget about the mass of the disc and the transfer of power to the ball for a moment. Would 3 or 4 skin depths optimize the current extracted from the coil field??

Back to you OZZY and thank you

PauLC
W1VLF

Dr. Slack

Fri May 23 2008, 07:04AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If you had another alli disc backing the coil, would that serve to increase the flux in the projectile disc by "deflecting" it upwards, or would ferrite backing acheive that end by providing an easier completion path for the flux? Sorry, total failure of intuition here, FEMM ? The question is whether a coil backing will increase the throw, and if so what type.

Dr. Shark

Fri May 23 2008, 03:54PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

The Al disc does not "deflect" or "squeeze" the flux like iron does, but sets up an opposing field to cancel it. Therefore you would actually end up with less flux on the projectile side of the coil than without any backing.
Ferrite (or iron, since the frequency is rather low) would work at least in principle, but saturation would limit the usefulness to very small currents. Intuitively I think there is very little hope for a multi-kJ launcher to profit from an iron core.

Paul, if you don't mind me stepping in for OZZY, the answer to both of your questions is yes. Actually an "infinitely" thick disc would be required to catch all of the field, but by definition of the skin depth, you would capture 95% or 98% repectively with a 3 or 4 skin depth thick disk.

OZZY

Fri May 23 2008, 05:02PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

Yes, I used a frequency of 2.4KHz to represent a pulse of about 400 microseconds. I am not sure how acurate the model is but so far it agrees very well with the theory put forward by Dr Shark.

You need to use a disc thicker than the skin depth, I am sure Dr Shark is right that more than three times the skin depth is a waste as the extra metal is just extra mass. However consider this, if the disc is too light or the force is very high then the disc will move away from the coil before the current peaks and the efficiency will be low. The force is only effective over a very short distance, I suspect the force distance relationship is exponential. In the example I posted the distance constant for force=1/e is about 5mm. More work is needed, designing an optimal induction launcher is not a simple problem.

Ozzy

w1vlf

Fri May 23 2008, 05:59PM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

Dr Shark, OZZY,

There is a skin depth caculator located here.

According to it, in AL, 1 skin depth at 2.4KHz is approx. 1.7 mm so 3 x that would be 5mm. my disc is currently 1/4" or 6.3 mm thick.

If 4 skin depths can capture 98% of the energy it seems like that would be adequate.

Until you take into account OZZY's point of the needing extra mass to prevent launch well in advance of the maximum current.

I have built a new coil using flat wire. The flat wire makes it very easy to to wind a nice round flat surfaced coil.
See

Scroll to the bottom of the page.

This will be come my reference coil, and all further testing will be done using it. The coil 3" in diameter and has an inductance of 13 uHy. This is a very solid, very tight coil.

The larger diameter will allow me to use a larger AL disc. It would seem that but means of monitoring the current pulse at launch that one could glean information about the projectile timing, and then experiment with certain factors, mass, thickness , shape, voltage, capacitance etc To optimze the launch of the projectile.
I have a storage scope so this is one direction to start research.

Thank you again gentlemen for your help.

PauLC
W1VLF

Quantum Singularity

Sat May 24 2008, 03:10AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

I did a quick google search for "magnetic field flat coil" the first result was this:

take a look at Figure 5. Wish it was more zoomed-out, but basically its just an elongated toroidial shape. Fewer turns would be less elongated, more turns means more elongated.

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