Superconducting Triax HTS cable @ 3000A

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I believe that it is both.

Certain materials start to 'super-conduct' when the temperature is low enough, generally in the range of liquid Helium temperatures (4K, -269C). High temperature superconductors are interesting because they actually super-conduct at liquid Nitrogen temperatures (77K). Liquid nitrogen is much easier to work with and much less expensive than liquid Helium.

When a material is in the super-conducting state, its electrical resistance drops to zero. Not close to zero, but really zero. You can take a ring of material, place it in a magnetic field, and then cool it down to the super-conducting state. When you remove the external magnetic field, you will induce current flow in the ring, essentially creating a permanent electro-magnet. Superconducting solenoid coils are used as high field magnets for things like NMR machines.

All this depends upon keeping the material cool enough. If anything heats the material up, then it will cease to super-conduct, leading to some interesting (and scary) failure modes. Even though you don't have standard resistance heating of the conductors (since they have _zero_ resistance), you still get AC heating of insulation, eddy currents induced in any nearby conductors, heat leaking in through your thermal insulation, etc. Additionally, intense _magnetic_ fields can cause the material to stop super-conducting. If anything causes a small portion of the material to stop super-conducting, then you will get resistive heating at that point, which causes heat production at that point, which causes more and more material to stop super-conducting.

This can happen in the high field superconducting magnets, where all of the energy stored in the magnetic field goes into the no longer super-conducting portion of the coil. According to someone who ran a spectroscopy lab at school, if you are lucky this just means boiling off a bunch of Helium. And if you are in the same room with a high field magnet when it 'quenches', then you will probably 'see stars' from the changing flux field passing through your head.

-Jon
 
winnie said:
When a material is in the super-conducting state, its electrical resistance drops to zero. Not close to zero, but really zero.
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That is just so hard to comprehend, it seems imposable.

Is there a layman's explanation for this change to 0 resistance.

Does this mean a 14 AWG could carry infinite current?
 
iwire said:
That is just so hard to comprehend, it seems imposable.

Is there a layman's explanation for this change to 0 resistance.
Click to expand...

I've seen explanations, but I've never been able to grasp them. I know that it works, I don't know why.

iwire said:
Does this mean a 14 AWG could carry infinite current?
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No. The current would not be limited by resistive heating, but there are other limits. As mentioned above, certain conditions need to be met for the material to superconduct. If the temperature is too high, or the magnetic field is too high, or if the current is too high, then it will stop superconducting.

See http://www.amsuper.com/products/htsWire/103419095991.cfm

This wire has the same cross section as 17ga wire, and can be purchased capable of carrying 155A. Exceed this, and it stops superconducting.

For some fun reading, do a search on "training superconducting magnets". This is the process of pushing magnets right up to the point where the coils stop superconducting.

-Jon
 
winnie said:
I've seen explanations, but I've never been able to grasp them. I know that it works, I don't know why.
Click to expand...

I thank you for the entirely candid answer. :)

If your not getting it I don't have a chance.

155 Amps on 17 AWG.....I will have to try that. ;)
 
winnie said:
I've seen explanations, but I've never been able to grasp them. I know that it works, I don't know why.
Click to expand...

Physicists don't really know why superconductivity works either, they just know they can measure it in the lab. That's what makes superconductive physics so frustrating, there really isn't a good mathematical explanation for the phenomenon at a basic level.

You can't really put infinite current through a superconductor, but you can put very large currents through them. And it is possible to store current in a superconducting loop, it just keeps going round and round, chasing it's own tail. I suppose eventually there would be some loss but not due to IR heating. People have been working for years to put superconductors into distribution wires. If you were a utility, you'd pay a lot to have lossless distribution!

--Lawrence Lile, P.E.
Project Solutions Engineering
 
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