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Terry Fritz

Thu Jul 06 2006, 03:42AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

Oh!! I am a "double" bad boy now

Not only do I drill through and terminate my primaries inside, but now I am double posting!! But I think I do have significant "new information to add" since this data has never shown up in Tesla coil stuff before

As to:

Also, are there any effects of having ferrous objects in close proximity to the coil, ie, for the screw connections at the top and bottom? I imagine it might lower the resonant freq. somewhat and can cause some losses by induction

See tonight's testing!!! I never would have thought before that the "center of the primary" was where to put the MOT!!

The SISG is the "perfect gap" for this stuff since is it totally repeatable... Bwahahahha!!

Wild stuff!!! Someday, we must make a field mapper program for the primary coil.... I guess "below the center" of the primary is like a funnel with lower fieds...

Cheers,

Terry

Wilson

Thu Jul 06 2006, 09:15AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

Thanks for the pics Terry. The stress plots were very interesting indeed.
Seems like the best method for high power coils is to spiral the wire up the former, and into a notch perhaps, to smoothly bend the wire, which would then be attached to a corona ring.

Could someone post a pic of a corona ring please? I need a gauge of how large it should be and how high it should be raised.

Is there any difference between attaching the topload very close to the last turn, or further away? Does mounting it too close promote arcs to the secondary?

Thanks,
Wilson

Steve Conner

Thu Jul 06 2006, 03:40PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I had a look through my tesla photos and found these pics. TBH, I don't know if the corona ring is any use, but I thought it looked neat.

Terry Fritz

Thu Jul 06 2006, 11:01PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

I worked out the numbers for the "big iron" next to the primary thing.

MOT position Rac(Ohm) Lpri (uH) Power Loss (W (14Arms))

No MOT 0.14 3.19 0
MOT in worst position 0.20 3.25 11.76
MOT in expected build position 0.148 3.19 1.57

On a conventional lossy spark gap coil, even a very close MOT in a bad position would not be noticed. We often see coils with transformers and such under the primary. It might be the case that their added losses are not too great. But the open core MOT will reject eddy and shorted turn currents. That is not the case for say a NST cover.

But I just tried a sheet of copper under the postion I will build the coil to (about 2.5 inches below the secondary) The loss was insignificant. So I thihnk if you space the iron and metal stuff about say 1/2 the primary diameter away you will be fine. Only when you get really close (inches) does it mater. The fields must stay pretty close to the primary coil. MandK could probably calculate such distances too with some creativity.

So bottom line is you can put stuff pretty close, espicially with a spark gap coil.

More...
I put a piece of copper under the primary like this:

It hardly made any difference!!! The magnetic fields around the primary must stay very close the the coil!! I don't know if there are any magnetic field mapping programs out there that could show this?

But it appears you can put metal objects really near the primary!!

Cheers,

Terry

Steve Conner

Thu Jul 06 2006, 11:30PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi Terry

I always assumed that you could put metal objects as near the primary as you liked, as long as they were good RF conductors like copper. That was what gave me the courage to park all the electronics for my DRSSTC (shielded in aluminium boxes) directly under the primary. I put the tank capacitor under there too because I figured it was mostly just a big brick of copper foil with some plastic mixed in.

I'd expect that good conducting objects would increase the resonant frequency but not lower the Q significantly, whereas lossy objects like the MOT would do pretty much what you measured.

My conception of things leads me to believe that if you put the MOT in the least favourable place, putting the copper plate on top of it should restore most of the Q that you lost. Did you try that?

On a related note, I once tried shorting the strike rail on my DRSSTC with a big alligator clip so that it formed a shorted turn. The spark output wasn't affected as far as I could see, but after a few seconds the alligator clip melted in a shower of sparks.

Terry Fritz

Fri Jul 07 2006, 12:32AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi Steve,

I tried a copper plate over the MOT in the bad postion (yellow) as compared to just the MOT (white):

It helped a tiny bit but the loss was still pretty high. But the copper only covered about 1/2 the coil so it was probably not a good low loss shield. So I tried a full sheet about 1.2 inches abouve the coil like this:

And got this:

Still fairly lossy. but for a conventional coil that 0.1 ohm of added primary resistance will not matter!! If a coil has say 15 amps of primary RMS current the heat in the copper would be 22.5 watts. If that were all in a lossy aligator clip, it could very easily burn it up.

My little coil's performance goes way down if it is on concrete (~1.5 inch spacing). But it is much wider too. I will try to find a field mapper which will probably explain a lot about the dimensions and sizes.

Cool stuff!!

Cheers,

Terry

HV Enthusiast

Fri Jul 07 2006, 12:40AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

wrote ...

If a coil has say 15 amps of primary RMS current the heat in the copper would be 22.5 watts. If that were all in a lossy aligator clip, it could very easily burn it up.

22 watts seems like quite a bit. I have a full sheet of copper underneath my DRSSTC III (only 1 inch below primary coil), and even after very long run times (20 minutes or more), the copper is not even noticeably warm.

Relatively conductive material, such as copper and aluminum, will not pose much of a problem as far as losses go and actually are the best materials for magnetic shielding for the frequency we usually see in our coils (few hundred kHz) Copper actually becomes superior for shielding above 200kHz for magnetic shielding even over steel.

On the otherhand, magnetically soft metals, such as Mu metal, are very poor at magnetic shielding for Tesla coil frequencies are pose MAJOR losses in the system if placed in the vicinity of the primary coil. (Mu metal should only be used for 60Hz or lower magetic fields)

I did some experiments awhile ago on shielding my control electronics on my DRSSTC III system since the control electronics were literally less than 1 inch from the base of my primary coil. For the hell of it, i put a sheet of Mu metal in there just for fun, and it got so hot it was glowing red and melted my PVC enclosure top! On the other hand, i put a copper sheet, again only about 1 inch lower than the primary, and it solved my noise problem, and at the same time didn't even get warm, even after long runs. The Mu metal glowed Red after about 30 seconds.

Dan

Marko

Fri Jul 07 2006, 12:40AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Omg d00d. And I had an iron plate just about 3cm, under entire primary.

I knew it was bad but now i see how bad really...

Im' reconfiguring that coil for a while now, since this is most probable reason of my low primary current.

HV Enthusiast

Fri Jul 07 2006, 12:45AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Yeah, iron is definitely not the way to go for Tesla coil frequencies.

Soft magnetic materials have a very high initial permeability, while conductive metals such as copper, aluminum, steel, have relatively low initial permeability. But, as you increase in frequency, the permeability of the soft magnetic materials drops off way way faster than the conductive metals and these curves eventually cross one another if you were to plot the permeabilities.

Terry Fritz

Fri Jul 07 2006, 04:42AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

For my little SISG coil, I got the Ansoft Maxwell 2D thing going:

Here are the flux lines of the coil set up with the MOT under the coil and a fan to the right side:

Very few flux lines make it up to the top terminal and the aluminum fan seems to be transparent to magnetic flux. The big iron transformer bends the lines around.

We then can look at the B field (in Teslas

)

The transformer attracts some of this field and is energized by it slightly.

The plot of field energy is where the "power" is at:

The power stays "very close" to the primary coil turns. That is why those big metal things in the area of the primary do not affect things much. If we increase the sensitivity, we can see more detail:

The energy still clings very close to the primary but you can see it touching the secondary now. If we really turn up the magnification, you can see the transformer having a tiny effect:

Sort of cool!!

22 watts seems like quite a bit. I have a full sheet of copper underneath my DRSSTC III (only 1 inch below primary coil), and even after very long run times (20 minutes or more), the copper is not even noticeably warm.

Two feet of copper surface (front and back) with fan cooling is not going to get warm :o)) But 1 inch seems "too" close to me... But the primary energy loss is probably not a big deal. If it fixes the noise problem that is the main thing there.

Omg d00d. And I had an iron plate just about 3cm, under entire primary.

That might get warm :o)) but I see you are fixing it

Cheers,

Terry

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