High permeability solenoid cores and core geometry. - Page 1
Forum section: electronics / projects

author: PythonGuython
author_rank: Contributor
timestamp: January 26, 2022, 06:33:59 pm
content: I'm working on a class project for my feedback controls class. We're making a magnetic levitator. We're wrapping the coil for our own solenoid, and we want to maximize the strength of our solenoid so that we can levitate an object asheavyas possible asfar awayfrom the solenoid as possible. One way to do this would be to make a bigger solenoid that carries more current with more turns. Another way could be to maximize the efficiency of the core.According to Wikipedia (https://en.wikipedia.org/wiki/Permeability_(electromagnetism)), 99.8% iron has a relative permeability of 5000, while if we were to splurge for 99.95%, we would up our permeability to 200,000. Which would up our flux density by 40 times (according to theory of course).I have two concerns with this:1) the theory breaks down at such high permeabilities2) We end up saturating the iron core, thus negating the benefit.Another aspect we're considering is core geometry. We took a look at some solenoid actuators around the lab, and they have sharpened cores like the diagram shown on this website:http://www.solenoidcity.com/solenoid/manual/construction/construction.htmMy guess is that this focuses the field lines to a point, increasing the flux density. Maybe we should try this?Thoughts on any of this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2022, 08:01:58 pm
content: Permeability is largely irrelevant, because the average permeability of the system (including airgap) is close to 1. The pole pieces just need to be high enough that their effective length (actual length divided by mu_r) is negligible. So, over 100 is probably fine. And that's a given for almost any choice of material, so no worries there!Your issue is the geometry of the field itself. Field can't be put just anywhere, arbitrarily. It forms a closed loop, pulled into a minimal surface; something like nested air bubbles, held up by internal pressure. That pressure here, being the energy stored in the field within a given respective surface.So, a "long" field is a wide field. There's no way to "throw" field axially; it can be made shorter (closer to the surface) however, though obviously that isn't useful here. (It does have the advantage of allowing higher field strength, e.g. Halbach array. In fact, this is what 'fridge magnets are: the ferrite-loaded plastic is very weak indeed (~10mT), but turned back on itself as a ridged Halbach array, it can achieve reasonable holding strength.)Basically starting with an open structure, like a horseshoe magnet, is a good plan. Rather than leaving one end of the solenoid sticking out, attach pole pieces and bring them around so both can interact with the armature. The magnetic path length is much shorter: the field from the rear of the magnet doesn't have to return on a long path through air (mostly returning to the coil without exerting any force), instead both poles act symmetrically upon the armature.And you can always increase field strength, up to the point where the pole pieces saturate. For a generous air gap, power dissipation will probably be the limiting factor, but you should still be able to get up to saturation without, say, requiring water cooling for example. Needless to say that'll be a big ball of copper wire though.Saturation is where all the magnetic domains in the material are lined up parallel; each domain has its characteristic intensity, and when all add up together, that's simply how much you get. Field continues to build only slowly with increasing magnetization, whether because of material properties or because the (through air) field from the coil itself starts to dominate. (In which case, this sideways geometry probably isn't as good as just a straight-up solenoid, with diameter equal to the spacing of the pole piece tips -- but by the time you're looking at something that strong, you're making it out of superconductors, and it's an occupational hazard with respect to nearby metal bits.)Since field drops off roughly 1/x^3 or something like that, you don't gain much distance from cranking up the field, even quite dramatically. So having the geometry to start with, is a big win.Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: January 26, 2022, 09:29:09 pm
content: The shapes ( cone, point, flat etc) of the moving armature and the pole piece affect the force versus displacement function.As T3 mentions, the permeability does not change things much and anyway it is varying with H [A/m]. A good general constant approximate permeability for initial calcs and model is ur = 66 , which could be adjusted after a real test.Another analytical approximation I used a lot for low carbon steel is : Ur = 720000/ (H^0.92) where H is in A/mI did some FEM years ago on DC solenoid forces with shaped armatures but don't know if I can find them now.

author: PythonGuython
author_rank: Contributor
timestamp: January 26, 2022, 10:51:57 pm
content: I'm certainly questioning my understanding of magnetism. From what I read, are you suggesting I try a horseshoe solenoid?Let me know if the diagram below approximates your advice.

author: mag_therm
author_rank: Frequent Contributor
timestamp: January 27, 2022, 02:41:20 am
content: Did you read about "skull melting"?In this the interacting magnetic fields form a negative feedback system to hold the moving part firmly in position.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 27, 2022, 03:54:31 am
content: Check "The signal path" on Youtube. The guy did something similar 5-ish years ago. He used to show up here regularly, not sure about now.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2022, 12:35:28 pm
content: Yes, something like that. Note that the thickness of the magnet (depth axis in the image) is a free variable; probably it should be comparable to the width, say 10-25% of it? Cross section can be round or rectangular. Standard transformer cores are rectangular, which may be a useful starting point. Coils can be as shown, or along the 'spine' (which would be rather hard to wind with a solid 'C' core, but if using pole pieces assembled around the winding, that's fine).Quote from: mag_therm on January 27, 2022, 02:41:20 am"Skull melting" turns up some pretty sick image results.Do you mean theskull crucibletechnique? (Or the similar technique with arc furnaces.) But that should be pretty well in-place; perhaps you mean levitation melting?Magnetic levitation -- when it works -- is stable because of how diamagnetic forces interact. It doesn't work, with just permanent magnets -- there's no stable arrangement (without an active control, or gyrostabilization which does works, albeit over a narrow operating range). With diamagnets (hence the demo with diamagnetic materials like bismuth or pyrolytic carbon; or as here, for metals at AC), it's pretty easy to arrange. The price paid is, the metal dissipates a lot of heat. Fine for heating/melting things though.Tim

author: eugene
author_rank: Frequent Contributor
timestamp: January 27, 2022, 03:48:47 pm
content: Quote from: PythonGuython on January 26, 2022, 10:51:57 pmLet me try to explain it in another way. Instead of thinking in terms of permeability, which is loosely analogous to electrical conductivity, think in terms of reluctance, which is loosely analogous to electrical resistance. A material with high permeability has low reluctance to magnetic flux.But consider that the flux must flow in closed loops. That means that part of its path is through the magnetic core, but a significant part of the path is through air. Each part of the path introduces some reluctance and the net reluctance of the entire circuit is the sum of the two. It's not unlike an electrical circuit that includes two resistors in series. The net resistance is the sum of the two resistances. If the two resistances are very different in magnitude, then the sum will be dominated by the larger one. If you have a 1M resistor in series with a 100 ohm resistor, the sum is changed very little if you replace the 100 ohm with 10 ohm or even 10 milliohm.In your case, the reluctance of the path through air dominates. By comparison, the reluctance through the core is so small that it hardly matters. In order to reduce the reluctance of the air path, you need to shorten that path. If your core is just a straight bar, then the flux needs to leave one end of the bar, travel through air until it gets to your levitating object, then travel back through air to the other end of the bar. By bending the bar into the shape of a horseshoe, you shorten the path from one of the two ends to the levitating object.Notes:1. Some physicists would say that I've bent some of the facts and misused some terminology in this explanation. They would be correct. I'm just hoping to improve your intuitive understanding.2. What you are describing is not technically a solenoid, but just a simple electromagnet. I would feel more comfortable if you stopped calling it a solenoid.

author: PythonGuython
author_rank: Contributor
timestamp: January 27, 2022, 06:13:02 pm
content: Thanks to all for the help. I feel like I can parse what T3 said after eugene's help. I just ordered 1250 feet of transformer wire. I'll build both the vertical and horseshoesolenoidelectromagnet and bump this thread when I have results!

author: eugene
author_rank: Frequent Contributor
timestamp: January 27, 2022, 11:15:04 pm
content: If you've digested my last post, then I'd like to go one step farther. Getting maximum lift means maximizing how much of the available flux goes through the thing you're trying to lift. Your electromagnet will generate a certain amount of flux, but as Tim pointed out, it goes everywhere, literally. Most of it will travel from one end of your core to the other without ever going through the object being lifted. By bending the straight bar into a horseshoe shape you send more of it in the direction of the object.That's how refrigerator magnets work. Those flexible ones that often have business cards glued to them do NOT have north on one side and south on the other. In reality, they are composed of narrow strips of alternating polarity. The flux goes from a north facing strip through the steel door and then onto a neighboring south facing strip. A very short and direct path that wastes very little flux.Larger refrigerator magnets are made with a relatively wimpy ceramic magnet sandwiched between two steel plates. The steel plates direct almost all of the flux through the steel in the fridge door. If you remove the plates you'll find that the magnet will barely stick to anything.It's all about maximizing the amount of available flux that goes through the the thing you're trying to lift (or stick to.) That's the really important reason why bending the straight bar into a horseshoe shape. I encourage you to build both and see for yourself.