# Inductance and superconductors

Discussion in 'General Electronics Chat' started by wes, Aug 28, 2010.

1. ### wes Thread Starter Active Member

Aug 24, 2007
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I have a question about inductors and superconductor's.

I have read that superconductor's expel all magnetic fields (or at least most )

so if a you had a inductor made out of a superconductor then how could it produce a inductance since each field generated by each wire could not induce a voltage in the next. if it would then why exactly since the field is expelled by the next wire?

Also what would the inductance be for it if say it's inductance would normally be 1 mh in a normal state but in a superconducting state what would it be?

2. ### marshallf3 Well-Known Member

Jul 26, 2010
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Shoot, I used to know some of this stuff but I can tell some of the reading you've been doing hasn't explained things thoroughly enough.

If I recall the inductance still exists, it just has no resistive component to it.

Jun 1, 2009
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I'm guessing that means it doesn't act as a classical inductor, since the magnetic fields are forced out of the super conducting region there is no interaction electrically but it still produces a magnetic field around the wire? That's my guess at least.

4. ### Wendy Moderator

Mar 24, 2008
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Actually it will be the classic inductor squared, with no resistance there goes a lot of pesky negligible factors that aren't. It will also store energy indefinitely (IE, forever) if you short the leads while conducting current, thus forming a permanent electromagnet. Basically the Q of the circuit will be exceedingly high.

It will be fun to play with.

Remember, they are using superconducting electromagnets for many industrial and scientific uses. Electromagnets is just another word for inductor.

5. ### timrobbins Active Member

Aug 29, 2009
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An ideal wire, or coil of wire, carrying a current will have a magnetic field in space dependant on the geometry of the wire. At the wire surface, the electric field doesn't penetrate the wire, because there is zero resistance throughout the wire corss-section. Hence there is no field in the wire itself - it is all external.

When the wire has resistance then the field extends in to the wire, reducing as you go in further, and the term 'skin depth' describes how far in the field extends. In normal wire the skin depth is frequency dependant. Superconductors have a very small 'skin depth' that is not frequency dependant, for normal superconducting conditions.

Skin depth is a term readily appreciated by the early HAM and rf transmitter community, and lately by switchmode designers - and maybe some other exotic applications (?).

Ciao, Tim

6. ### marshallf3 Well-Known Member

Jul 26, 2010
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I have noticed an increase in availability of Litz wire lately at some of the parts websites.

7. ### retched AAC Fanatic!

Dec 5, 2009
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That's funny you say that. I have had a funny feeling about seeing much more lately. It seems during times of war and major innovations, companies make more odd items. If you figure that they set up a factory tooled to produce litz, they would start marketing to sell inventory.

Dunno. Odd you noticed too.

8. ### wes Thread Starter Active Member

Aug 24, 2007
242
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Ok so if anything then the superconductor inductor, lol would create a larger inductance for the same size inductor as a normal one

same number of turn
same size in all directions
everything is the same except the one is normal wire and the other is superconducting wire

also anyone heard of bifilar coil
they are supposed to create 0 inductance (well almost 0 )
is there still a net magnetic field produced by the coil even though each wire's current is in the opposite direction? I would think there would still be a small field since the very end wire's magnetic field on one side is not being opposed

Mar 24, 2008
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10. ### marshallf3 Well-Known Member

Jul 26, 2010
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Makes sense when you think about it. Switching PS ICs are operating at higher frequencies and producing more amps. Litz wire has another advantage than just minimizing the skin efect - it's easier to wind the equivalent AWG on a toroid form.

I assume you've seen this relatively new IC? LTC3788

11. ### retched AAC Fanatic!

Dec 5, 2009
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No, no I have not.

Interesting. If I had time before work, I would play with it in LTspice...

I guess after work it is.

Very interesting.

12. ### marshallf3 Well-Known Member

Jul 26, 2010
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2 separate outputs of up to 60V @ 10A each and no heat sinks required - even on the MOSFETS.