Energy in a Magnetic Field (SMES Core Question)

[Rework]

Say one has a solenoid with a given number of amp-turns; if the core is air, we know that the energy to charge a magnetic field to "B" Tesla is recovered when the magnetic field collapses. Now if that core is iron instead of air, we know the magnetic field is greater than the original "B" value, due to the permeability of iron. My question is: when that field collapses, does one recover the same amount of energy as before (with the air core), or a larger amount of energy equal to the larger field? It would seem against all rules of conservation of energy that one would get more energy back, but where does that extra field energy go? -- or is it energy?

So in a SMES, it would seem an air core would be best. Does air have a magnetic field saturation, as does iron?

I would be most appreciative for an answer to this.

 

[berkeman]

Say one has a solenoid with a given number of amp-turns; if the core is air, we know that the energy to charge a magnetic field to "B" Tesla is recovered when the magnetic field collapses. Now if that core is iron instead of air, we know the magnetic field is greater than the original "B" value, due to the permeability of iron. My question is: when that field collapses, does one recover the same amount of energy as before (with the air core), or a larger amount of energy equal to the larger field? It would seem against all rules of conservation of energy that one would get more energy back, but where does that extra field energy go? -- or is it energy?

So in a SMES, it would seem an air core would be best. Does air have a magnetic field saturation, as does iron?

I would be most appreciative for an answer to this.

Welcome to the PF.

It takes more energy to "charge it up" to the same current, so you would get back that higher amount of energy when "discharging" the inductance.

 

[Rework]

But if u(mu) for air is 1.25 x 10^-5, and mu for steel is 5.0 x 10^-3; and B=mu X H, it appears B will be much greater for steel for the same H (amp turns). So for a given amount of H, don't you get more B?

 

[berkeman]

But if u(mu) for air is 1.25 x 10^-5, and mu for steel is 5.0 x 10^-3; and B=mu X H, it appears B will be much greater for steel for the same H (amp turns). So for a given amount of H, don't you get more B?

Yes. And what is the formula for the energy stored in an inductance? How does the higher μ of the iron-core coil affect its inductance?

 

[Rework]

OK -- I'm going in circles here! The stored energy increases with the inductance, and the inductance increases with mu. I understand that. What I don't understand is what happens, say, if you have an air-core solenoid that is fully charged (steady current, no inductance at that point) and you insert a steel core.

 

[berkeman]

OK -- I'm going in circles here! The stored energy increases with the inductance, and the inductance increases with mu. I understand that. What I don't understand is what happens, say, if you have an air-core solenoid that is fully charged (steady current, no inductance at that point) and you insert a steel core.

How are you driving this current in the coil in your through experiments?

And the steel core will get pulled inside the coil (think solenoid actuator):

http://www.solenoidsupplier.com/wp-content/uploads/2017/11/how-to-make-solenoid.jpg

 

[Rework]

OK -- I'm going in circles here! The stored energy increases with the inductance, and the inductance increases with mu. I understand that. What I don't understand is what happens, say, if you have an air-core solenoid that is fully charged (steady current, no inductance at that point) and you insert a steel core.

How are you driving this current in the coil in your through experiments?

And the steel core will get pulled inside the coil (think solenoid actuator):

http://www.solenoidsupplier.com/wp-content/uploads/2017/11/how-to-make-solenoid.jpg
View attachment 222379

So going back to my original post, I say the amp turns are the same; the turns are always the same obviously, but in the case of the steel core, the amps will increase. Is that a true statement?

 

[Rework]

So going back to my original post, I say the amp turns are the same; the turns are always the same obviously, but in the case of the steel core, the amps will increase. Is that a true statement?

My reason for the question is involving a SMES. As far as I know, the current SMES construction is with an air core. not steel or mumetal. Is that for a reason?

 

[berkeman]

SMES

I tried searching on that acronym, and so far no luck. I got this at Wikipedia -- what is SMES?

Science and technology

• Situational method engineering, the construction of methods which are tuned to specific situations of development projects
• Solar Mesosphere Explorer, a NASA artificial satellite in Earth orbit from 1981–1989
• Standard-Model Extension, in quantum field theory
• Subsequent memory effect, in cognitive psychology
• Surface-mount equipment, machines for the assembly of surface-mount technology electronic components
• Soy methyl ester, biodiesel created from soybean oil
• Sevomethyl ether, USP Sevoflurane Related Compound B

Computing

• Secure Memory Encryption, an AMD real time memory encryption technology
• Structure mapping engine, in artificial intelligence and cognitive science
• SME Server, a Linux distribution
• SIGNAL Meta under Eclipse, an EMF-based metamodel of the SIGNAL programming language

 

[Rework]

I tried searching on that acronym, and so far no luck. I got this at Wikipedia -- what is SMES?

Superconductor Magnetic Energy Storage

 

[berkeman]

As far as I know, the current SMES construction is with an air core. not steel or mumetal. Is that for a reason?

Can you provide a link?

 

[Rework]

Can you provide a link?

https://en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage

 

[Rework]

https://en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage

http://www.superpower-inc.com/content/superconducting-magnetic-energy-storage-smes
So as in this web page, they take short makeable magnets and arrange them in a circle to form a toroid, which keeps most of the field within the toroid.

 

[berkeman]

As far as I know, the current SMES construction is with an air core. not steel or mumetal. Is that for a reason?

Interesting question. I'm guessing it's because ferrous metals don't support the high magnetic fields in the SMES interiors. They would saturate at much lower fields, so would probably not help to increase the inductance and energy density. But that is just a guess so far...

The critical current of HTSC wire is lower than LTSC wire generally in the operating magnetic field, about 5 to 10 teslas (T).

10T is way more than an iron core can deal with, I believe. Look at the typical hysteresis curve below -- the vertical axis is all less than 1T:

http://www.magnets.com.cn/mats_pic/SA/Dynamic_Hysteresis_Loop_Cluster_of_Mn-Zn.jpg

 

[Rework]

Interesting question. I'm guessing it's because ferrous metals don't support the high magnetic fields in the SMES interiors. They would saturate at much lower fields, so would probably not help to increase the inductance and energy density. But that is just a guess so far...10T is way more than an iron core can deal with, I believe. Look at the typical hysteresis curve below -- the vertical axis is all less than 1T:

http://www.magnets.com.cn/mats_pic/SA/Dynamic_Hysteresis_Loop_Cluster_of_Mn-Zn.jpg
View attachment 222403

Being an ME, we had to learn EE in one year, so am a bit short on my theory. But am still having a problem. It appears you are getting something for nothing; with the same H, you are getting more B when you have a steel core vs air.

 

[berkeman]

It appears you are getting something for nothing; with the same H, you are getting more B when you have a steel core vs air.

Not really something for nothing. The inductance is increased when you use a ferrous core (of appropriate material for the field strength and frequency of operation), so you can store more energy in that increased inductance. That energy comes from the power source or signal source.

Are you familiar with the use of dielectrics in capacitors? It's a similar situation, although the increase in capacitance from the dielectric over air/vacuum is much more modest than you can achieve with ferrous core inductors.

 

[Rework]

Not really something for nothing. The inductance is increased when you use a ferrous core (of appropriate material for the field strength and frequency of operation), so you can store more energy in that increased inductance. That energy comes from the power source or signal source.

Are you familiar with the use of dielectrics in capacitors? It's a similar situation, although the increase in capacitance from the dielectric over air/vacuum is much more modest than you can achieve with ferrous core inductors.

There is a post on your "Classical Physics" forum, dated May 20, 2013, discussing stored energy. If that is correct thinking, and I understood it,that would also answer my question. I need to learn all I can about SMES. They will get more popular. Thanks for all your help.

 

[Rework]

There is a post on your "Classical Physics" forum, dated May 20, 2013, discussing stored energy. If that is correct thinking, and I understood it,that would also answer my question. I need to learn all I can about SMES. They will get more popular. Thanks for all your help.

BTY, what is the best place on this site to discuss superconductors, etc?

 

[mfb]

The stored energy is proportional to $H B$. Once we fix the coil geometry, H is proportional to the current. The maximal B is given by the superconducting material (and its temperature and current). To maximize the stored energy, you want the largest possible H for the maximal B. An iron yoke doesn't help you here, and if the coil is not perfect you might even get a lower energy density before you risk a quench. It also comes with ugly side effects like eddy currents, which make quick charging/discharging problematic.

I tried searching on that acronym, and so far no luck. I got this at Wikipedia -- what is SMES?

You searched for SME. SMES is a redirect to the correct page: https://en.wikipedia.org/wiki/SMES