Ways to Increase a Solenoid Magnetic Field

An air-core solenoid does not saturate. At high field strengths this is pretty much your only option. What is your target field strength?

Given the geometry of your coil and the target field strength, work backward to determine the current necessary to produce that field. From the current, conductor composition and permissible temperature rise you can determine the required wire gauge and length.

Hi.

As I said I need to increase the magnetic field to at least 3 tesla because that was the original desire. This problem happened in the semi conductor lab of my professor. He wanted 3 tesla but the solenoid with iron core only gave to him 2 tesla by increases in the current. That was his original problem. In class, I can present any solution even if it's too expensive like the superconducting solutions I have mentioned above and the solution can even far exceed the desired magnetic field of 3 tesla. I found many pappers about 35 and 50 tesla solenoids and that is one of the solutions I'll be presenting.

Is it clear now? Hope it is.

Just to make it even more clear I'll post here the first short text about the problem we would have to discuss in class that my professor gave to us.

It is:

• We need to increase the magnetic field supplied by an electrode. The initial solution was to increase the current in the solenoid but the value obtained was not sufficient. A possible solution is to think about a better core that allows a bigger magnetization. What material or alloy would be indicated?

This class is a class we have after the class "introduction to solid state physics". Even though the change of the core is mentioned in the question that doesn't need to be our only solution to the problem.

Thanks

It's been some time since I've done magnet design but you must change your core design. Once the core saturates, the current to field transfer ratio is based on the permeability of free space instead of the unsaturated core's permeability. This is nearly a definition of core saturation. Super conducting the windings will not help your peak field, this will only reduce the voltage required to run the same current in those windings. This will make a more efficient magnet, but not a stronger magnet. Now how you change the core design can be just as simple as adding more laminations. But with a desired 50% improvement this does not seem a likely solution. To recommend a more precise answer requires a three dimensional understanding of this magnet that will be difficult to post here. One possible approach would be to direct the two poles from the solenoid into a smaller gap than you presently have now.

But as with any homework, your professor is likely giving you a hint about what material he/she considers critical. Study your texts and lecture notes on magnetism along with the mechanical topology of this solenoid to get an answer. For I'm sure your professor will not just want only a new design. You will have to be able to support and explain your design changes IN DETAIL.

Before you present, feel free to pose your suggestions here. We'll gladly comment them to death. But we won't be there when you present or for your later exam.

Good Luck

I'm astonished he got 2 T with an iron core. I would have guessed you might get 1.8 T or 1.9 T with some sort of pulsed magnet, but 3 T? That's asking a lot. None of the designs for high field magnets are something you might toss off in a dozen hours of work. Even a superconductor is not an easy solution since you have to watch exposing the coils to high fields (Have you ever seen 100 L of He boil instantaneously? It's rather beautiful.).

So, I'd look at an air core. I'd consider how to get the high current desnities I need, how to keep the coil from destroying itself, and how to cool the losses. There are some good books around, but I don't think there are any "one weekend" solutions.

Ferromagnetics saturate at around 2T and I suspect he's citing this info, rather than saying he's actually built a solenoid with this strength.

If the OP goes with a superconductor-based design, he needs to be aware that practical superconductors 'quench' at high field strengths; they stop being superconductors. One way to boil 100 of He real fast.

Steady State or Pulsed field?

Start here http://www.magnet.fsu.edu/

An iron core will saturate at ~2 Tesla with "low current". Keep increasing the current (and cooling) until you hit the 3 Tesla target. This will be the same current as if the solenoid had an air core. At that point any iron may only be restricting necessary coolant flow.

Graphic approximation below shows one specific case, but should give you a rough idea of what to expect.

Understanding why ferromagnetic materials such as iron saturate is key to understanding their limitations. Once a magnetic material's domains/atomic spin axes are all aligned with the field, the material is saturated. Beyond that, as MJB said, and the solenoid behaves as if it had an air core.

Sorry,

I should have acknowledged the obvious.

I support your request.

Wikipedia describes plagiarism thus:

"Plagiarism, as defined in the 1995 Random House Compact Unabridged Dictionary, is the "use or close imitation of the language and thoughts of another author and the representation of them as one's own original work."^[1] Within academia, plagiarism by students, professors, or researchers is considered academic dishonesty or academic fraud and offenders are subject to academic censure, up to and including expulsion. In journalism, plagiarism is considered a breach of journalistic ethics, and reporters caught plagiarizing typically face disciplinary measures ranging from suspension to termination of employment. Some individuals caught plagiarizing in academic or journalistic contexts claim that they plagiarized unintentionally, by failing to include quotations or give the appropriate citation. While plagiarism in scholarship and journalism has a centuries-old history, the development of the Internet, where articles appear as electronic text, has made the physical act of copying the work of others much easier.

...

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OK, you've made your point. Get over it.

Just curious .. what do you want to do with this filed ? Generate mechanical force ? something else ? The use of the word "SOLENOID" makes me ask.

i do believe from the quality of the posts so far that this is probably a stupid question, but humour me anyway

Very strange. One of my posts doesn't appear in this topic. So, let's text it all again.

First, thank you all for the repplies. I'll comment post by post.

#3 redfred REPPLY: You mentioned "mechanical topology". There is no such a thing. The only thing we (group of 3 people) have about this problem is the text I posted here after the black dot marker (in my last post as guest). We don't have a scheme of the solenoid that was used and we don't know what use this solenoid had. that doesn't even matter. What matters is that there is a solenoid with iron core and that by just increasing the current it wasn't possible to get more than 2 tesla magnetic field. It's a general problem to allow the students (us) to discuss general solutions. Another thing that I think that needs to be explained is that I don't need to invent any new solution or scheme never before thought. I just need to find real solenoids, real projects and ideas that would most probably solve the problem. It's a job of finding projects and solutions that already exist and that would most problably solve the problem. For instance, I mentioned here a superconducting solenoid that gives arround 50 Tesla. That is one possible solution in general and I'll present it (LINKs: http://nufact06.physics.uci.edu/Workshop/Slides/DSummers.pdf http://pac07.org/proceedings/PAPERS/MOPAN118.PDF). Again, I don't have a scheme of a previous solenoid, I don't know the use it had, it's just about finding possible solutions that IN THEORY would solve the problem. In a specific project you might say 50 Tesla is too much and we don't want it. But the problem that was presented was: Solenoid that saturates with 2 Tesla mag. Field. How can we increase it?

#4 TVP45 Hi. A few people have mentioned air core as yourself, but I don't understand how that can be a solution. Weren't we using an iron core in the first place to take advantage of the ferromagnetic properties it has and get a greater magnetic field that would not be possible with air core? Isn't that the main point of using a core different than air in solenoids, to increase the field? If you have any material to help me understand how and why an air core would allow me to get greater magnetic field than a ferromagnetic core please post it here. I really don't understand it.

#6 mjb1962853 GREAT! You hitted the bull's eye!!! One of the main things that I want to discuss in this presentation is exactly why an iron core will saturate arround ~2 Tesla. Not just that, most ferromagnetic cores saturate arround 2 tesla. So, I want to explain in detail WHY and HOW that happens because that is the main limitation that was faced in lab and created this whole problem for us to solve! About iron cores getting 3 tesla by just increases in the current I disagree. That is not what I found in the pappers and web pages I have read and that is not what happened in the lab as mentioned in the problem. I don't know how much the cooling (super cooling?!) you mention will influence the field, but by just increasing the current it was not possible to get 3 tesla as the project text said to us. Anyway, if you can get me any material or papper that explains why most ferromagnetic cores will saturate arround 2 Tesla that would be great. That's my first main goal! A graph with different materials plotted showing the saturation would also ilustrate the problem in a huge level.

lynlynch I had already seen these images in the other post here about solenoids. They are nice but now of much use to me. Thank you anyway.

Thanks again you all. I'll study the materials I have in hands so far and search for the topic mjb1962853 mentioned that explains why ferromagnetic cores saturate arround 2 Tesla and find some good graphs to ilustrate that for as many materials as I can find. Thanks.

It is useful to understand the basic concepts of magnetic circuits: MMF, reluctance, "mu0", "muR", area, and mean path length of iron, air, and any other material in the circuit. Too much info to discuss in an email or general forum. That's what paid university courses are for ;-)

Take a closer look at the graph provided. It shows that while an iron core solenoid can reach 2 Tesla with minimal current, an air core solenoid (superconducting or not) will need several hundred times that current to reach and then exceed 2 Tesla. If you increase the current in your iron core coil to sufficient levels, you WILL eventually reach/exceed 3 Tesla.

There is no free lunch here. If you need high B-fields, you need high current densities and sufficient cooling to keep everything from melting. If 3 Tesla is required, increase the current by at least 2 orders of magnitude (maybe 3) and remove the iron to make room for a forced cooling system.

I believe your questions have been answered by several posts above, but you are still looking for an easier solution which doesn't currently exist. Good luck in you search!

G'day gals, guys, gurus & doraimom,

• "One of the main things that I want to discuss in this presentation is exactly why an iron core will saturate arround ~2 Tesla. Not just that, most ferromagnetic cores saturate arround 2 tesla."

To understand what causes a ferromagnetic media like iron to saturate you need to first understand what causes such a material to have magnetic properties in the first place.

Now the correct name is the electromagnetic force which is one of the three fundamental forces that make the universe go round, up, down and any other which way your heart desires.

Now the interesting thing is that when you have a moving electric charge (current flowing through a coil) it will produce a magnetic field according to the right hand rule.

So the question now becomes where are the moving electric fields within a ferromagnetic material and why do they reach a point of saturation?

When you expose a ferromagnetic material to an electric field the outer unpaired electrons will all be forced to spin in the same direction and it is this motion that is responsible for the magnetic effect. Also the number of electrons that remain spinning in their new forced direction after the exciting field is removed are responsible for the residual magnetic field.

So, once you have forced all of the electrons to spin in the same direction there is no point increasing the excitation because there are no more electrons left to swithc and the material has become saturated.

An important point to note is that when I speak of electron spin I am talking about the electron spinning on its axis rather than the axis of the electrons orbital spin or path around the nucleus. The orbital spin is responsible for the diamagnetic effect while the electron spin is responsible for the magnetic effect.

Regards, masu

"When you expose a ferromagnetic material to an electric field the outer unpaired electrons will all be forced to spin in the same direction and it is this motion that is responsible for the magnetic effect."

Shouldn't that be magnetic field?

No!

When you have a current flowing or in other words a moving electric charge you have an electric field which then produces a magnetic field.

That's why it is called the electromagnetic force and the two are inseparable.

So no the correct term is electric field and it is this that is forcing the unpaired electrons to all be flipped so they are spinning in the same direction.

Also when you remove the exciting electric field the unpaired electrons do not always revert to their original spin orientation and it is this that is responsible for the residual magnetic field.

Now when you have a residual magnetic field it is because not all of the electrons flipped return to their original state giving an overall imbalance of the unpaired electrons spin direction. Now as the electrons are spinning you have a moving electric charge which creates an electric field which in turn produces a magnetic field.

You can have an applied electric field without charge movement. Without charge movement there is no magnetic field. The space between two charged capacitor plates has an applied electric field. If you place a piece of ferromagnetic material into this space, magnetic moments do not realign.

If you apply a Magnetic field to the ferromagnetic material, the magnetic moments do realign. That Magnetic field can generated from moving charge (solenoid coil current) OR a permanent magnet.

Will you take a look at US Patent # 7608348 ? Lists many materials...maybe interesting for you.

I have no idea of how to view that. Can you explain it to me?

First, go into www.freepatentsonline.com

Then register, it is free.

Log in using your User ID and password.

Then in the search field, type in "7608348". It will show the title etc of the patent, and an Adobe Acrobat pdf icon. Click on it and voila! You can then save it onto your hard disk for later study.

This is a wonderful website and once you get more familiar, you can do advanced searches using keywords...a world of great inventions and innvations is open to you.

Good hunting, ask if you need any more help.

regards,

kvsridhar

mjb is right. usually for reasons of "efficiency" and linearity low values up to about 2 T are used. In your case -academic one- it is possible to accept a VERY low efficiency and use an approach as following sketch shows :

The parallel magnetic field is much more important than the one in the small gap where the field strength could be reached. Such field concentration is often used in different devices although not up to 3 T. It can with 1 or 2 solenoids.

Since you are a student it could be for you of interest to compute the "efficiency" as ratio between the field going through the gap and the total field in the cylindrical part of the core assuming that the last is at a 2T intensity.

As an indication you could consider the field lines as circular and make an integration.

It is not as difficult as it appears. Of course it is only an approximation but good enough for many applications.

Using a gadolinium mixed rare earth core the magnetic field will achieve 60kG or 6 teslas with a regular electromagnet winding scheme.

There are other ways to columnate multiple lesser magnetic fields into one depending on the area of field density and distance of reach you are proposing.

T.Lewis PhD

ted.lewis41@yahoo.com

I'm very interested in more detail regarding a 6 T Bsat material. I'm sure others here would be very interested also. Can you please supply an info link?

doraimom... this site provides a basic review of magnetics which may help answer some of your questions.

http://info.ee.surrey.ac.uk/Workshop/advice/coils/mu/#bhcurve

How about if you allows the core to spin as well then you can use the Air Bearing NASA is working with some business on to let it run cooler.

Hi. I didn't find any solenoid with air bearing NASA as you mentioned in my research.

See: http://br.search.yahoo.com/search?rd=r1&p=Air+Bearing+NASA&toggle=1&cop=mss&ei=UTF-8&fr=yfp-t-707

Hi. My job is almost complete.

One kind of alloy I'll be talking about and need help are: Permendur, 2V Permendur and Hiperco.

If you look at this link: http://www.coilgun.info/theorymath/home.htm. you'll see that these three materials have greater saturation flux density. The problems are:

1) I didn't find enough info about them in the internet. Almost none.

2) I contacted a seller of this alloy and he repplied to me that they are all the same alloy. So, why would they be, at the table above, as different materials with different saturations? To understand that and get to know who is right, I need info about this alloys. What they are, where did their names come from, etc.?

Thanks again

G'day gals, guys & gurus,

Something I don't think anybody has brought up is the topic of laminations.

If you look at the core any transformer, solenoid or electromagnet you will find that the core rather than being constricted from a solid block is rather constructed from multiple layers and there are two extremely good reasons for this.

The first reason is that it makes it much easier to manufacture and assemble. Not only is it much easier to machine a whole stack of identical flat metal sheets but it allows you to wind the coil away from the core then insert it as the core is assembled from the flat components.

However it is the second reason that is most important. When you induce a magnetic field within a core the magnetic field then indices small circulating electric currents called eddy currents within the metal of the core as shown by the red circles in the image on the right. Now while these eddy currents tend to cancel each other out in the core of the material they do not towards the outer surfaces and can induce considerable circulatory currents around the surface as shown by the green lines and arrows. Unfortunately these not only oppose the magnetic field you are trying to create and thus reduce the overall flux density but heat the core which then reduces the materials magnetic permeability and thus reduced the overall field strength even further.

However, by creating the core out of laminations that are coated with a non conductive material (usually a lacquer or clear paint) you dramatically reduce the eddy currents and therefore increase the strength of the induced magnetic field.

Something else that is extremely important when working with magnetic fields that has not been discussed is hysteresis. To explain hysteresis I will be referring to the diagram on the right which shows the current in the exciting coil on the horizontal axis while the magnetic field strength in the core is shown on the vertical axis.

If you were to start with a brand new core that had never been exposed to an exciting electric field then you would be sitting at the origin with no magnetic field being present in the core.

Now if you increase the current in the exciting coil the magnetic field strength will increase as shown by the red line until the core reaches saturation at point "a" on the graph. Any further increase in the current has no further increase in the magnetic field as the core is now saturated.

If you now reduce the current back to zero as shown by the green line the magnetic field also reduced but only to point "b" not back to zero. This is because not all of the unpaired electrons will revert to their previous random state and this results in a residual magnetic field being present when the current in the exciting coil is removed.

Ok, if you now reverse the current in the exciting coil the magnetic field strength starts to reduce along the purple line passing through zero at point "c" and finally saturating in the reverse direction at point "d".

Reversing the current again results in the field strength moving along the blue then red lines until it again saturates in the positive direction at point "a" .

So what does this mean?

Well if you are using an alternating current to excite the coil as is often the case each reverse means that the current first has to get rid of the residual magnetic flux before it can start increasing it in the opposite direction. This makes the core far less efficient and causes it to heat up which again reduces the saturation flux density.

Another problem stems from the residual flux when the current in the coil is removed. If you are using the magnetic field to operate some sort of mechanical device which is what you are doing in any sort of electromechanical device, then the residual magnetic flux may not allow the device to be released. This could be extremely detrimental as it could result in a device that was in effect a one shot or one operation device that once energized would never reset or release.

This is basically an overview of eddy currents and hysteresis which in the world of electromagnetic devices are extremely important topics. I haven't gone into the mathematics behind predicting the effect these will have primarily because I can't remember it from my university days 30 plus years ago. However, I would definitely recommend getting hold of a good text book on electromagnetic devices that contains the mathematic models needed to calculate the effects these phenomena will have on the device you are designing.

Regards, masu

Very correct but the solenoid was as far as it was understood a DC one so that the eddy currents affected only the magnetic field building period. Or am I wrong?

To a certain extent a DC solenoid is immune to the problems of eddy currents, however, the hysteresis is something that you need to allow for in order to avoid stuck or jammed solenoids that fail to reset when the power is removed.

Solenoids will usually have a return spring to bring the plunger back to the un-actuated position when the supply is removed. The key thing is, of course, that the spring force should be higher than the force due to Br the remanent flux, as one can see on the hysteresis curve. And of course a safety margin for that sticking possibility.

A wealth of knowledge in this thread... Doraimom, however, wanted a material with a high saturation ... Hiperco seemed to be the best so far... yes?

If the solenoid core was inside some sort of metal pipe with some light oil to lubricate and a second coil in the 'off' position that shunts the back emf into something useful? The more powerful the coil the more likely you are to get a blown switch so putting it to some work seems like a good way to make use of it.

oh, by the way, thanks for answering my question that led me here - does the problem of saturation limit the energy potential, and can rare earth type magnets be used.

First off let me explain a couple of terms. If a relay is set it means that the coil is energized while when a relay is in the reset position the coil is de‑energized. I've use these terms here because relays can have normally open, normally closed and changeover contacts so you can't say that the relay is closed or open as it would depend on the configuration of the contacts.

"does the problem of saturation limit the energy potential"

In a word yes. Once the core is saturated any extra current you put through the coil ends up getting converted to heat and given enough heat you will burn the coil out. Also designers want to use as little metal as possible in the core so you can bet your bottom dollar that with any decent quality relay the core will be very close to being in saturation so there's absolutely no point in trying to pump in more current.

"can rare earth type magnets be used."

That depends. Rare earth metals are usually used to make permanent magnets which is not what you want in a solenoid core as it would mean that it would remain magnetized after you removed the exciting current and the contacts would remain in the set position unless you demagnetized the core.

However, they do make special self-holding relays that use cores that remain magnetized after exciting current is removed, but these relays also contain a permanent magnet. When you apply an energizing current the core becomes magnetized and attracts the permanent magnet setting the relay. Because the core remains magnetized after the exciting current is removed the relay remains in the set position. To reset the relay you have to apply an energizing current in the opposite direction to that used to set the relay. This causes the core to magnetize in the opposite direction and thus repels the permanent magnet which resets the relay. The relay then stays in this reset state until another setting current is applied.

These relays are used where you have to limit power consumption as they require no power to remain set. They are also used in situations where a relay must remain in a certain position even if the power is lost.

Yes, for the purpose of a relay, the solenoid could not be magnetised and function. The reason for this that I have in mind is using the core as an impact driver to propel a projectile similar to some types of electric nailgun. For this purpose the magnetisation of the the solenoid core is a potentially useful thing, as the device could be used for inducing charge with axial motion to load capacitors (like the rare earth magnet powered capacitor-loading emergency torches you shake axially, or with a sleeved coil like a pump action cocking slide). so long as there is nothing ferromagnetic within the working range of the magnet in the device it would have no negative effect on the amount of linear force when the coil is energised so long as the means of returning the core to the ready position did not interfere with the operation cycle (such as ferromagnetic metal springs and such).

if hysteresis is not a problem in the device then it doesn't matter if the coil is magnetised. something i was thinking about though, was degaussing these magnets, but it would take a lot of power to do it (probably not the same coil as the operating core, would have to be overkill), and once degaussed unless one was maximising the field to this level regularly or applied a counter-current after the operation cycle to ameliorate any hysteresis.

also for such use as a relay a leaf spring type switch with a permanent magnet, one could use a coil to nullify the field so the spring returns to the off position. one would have to invert the current compared to a regular powered-engaged type relay it would be a powered-disengaged.

"one would have to invert the current compared to a regular powered-engaged type relay it would be a powered-disengaged."

It would be a darn side easier to just use a relay with a normally closed set of contacts.

there is some purposes for which an active-low state is useful.

"there is some purposes for which an active-low state is useful."

Could you describe a situation where either a normally closed set of contacts on a relay (in other words they open when the coil is energized and close when it is de‑energized) or a self-latching relay wouldn't fit the bill?

a latch release, for example

I think you would still be better served with a self-latching solenoid as you only need to supply a pulse of power one way to set it and a reverse polarity pulse to reset it. That way for continued operation in either position you would not have a power consumption.

If we're talking about relays or contactors then the simplest solution would be either a unit with normally closed or changeover contacts.