Optimized Very Strong Solenoid Design and Construction Questions - Page 1
Forum section: electronics / projects

author: www.MKRD.info
author_rank: Contributor
timestamp: December 05, 2019, 06:55:06 pm
content: Folks,I am in need of an VERY (by standards of what is available for sale in 98% of cases) strong solenoid for coil-gun type of experimentation that will propel the plunger from a pulse of current from control circuitry (low buy quantity / make myself).The specs, crazy as usual:* Dual pull direction, plus minus around the center (thus, core can't "stick" "stop" or "rest" against anything in center like a magnetic stop).* Pull force on the order of 1000++N at 50% duty (as force drops off exponentially with distance from center, this is then the "at center" pull force). Plunger mass is up to 1kg (yes, that is huge. NO, this is not really a coilgun application to shoot a projectile, I am just trying to shove the plunger as energetically as possible for a short distance).* FAST pulsed-DC operation (ON from extended position to the time the plunger moves to the center), up to 20kHz, although this can be restricted if it calls for exotic material like Litz wire, ferromagnetic return, etc.* It would be optimal if I had some distance for the acceleration part, at least 1cm of somewhat linear high-force region, so that I can properly start and stop the control pulse.* Given how much this beast will waste in ohmic losses, I would like to learn how to optimize design parameters to have a high pull force, "long" (of somewhat linear force) stroke, and high plunger acceleration per optimized waste power dissipation and total unit weight (although weight reduction for solenoid assembly as whole can be a 2nd-generation design target).* Available power supply is 40VDC, 15A x 2 for two banks. This sets my voltage/current limits.The questions:1) Most low-power "open-frame" and "D-frame" DC solenoids have the core pull in to the end (magnetic stop) and rest/stick there. I must have a design which has no such stops, and the core can move freely in either direction, with pull being towards the "center" position from either direction. When I tried to contact an Indian company to design me a large solenoid without such magnetic stop, they laughed at me. Why? Is the solenoid design the optimal one for an "electromagnet" which allows core movement in/out in either direction away from center? Is there a better design for fast speed, high pull force, a more linear force, and long stroke that is still applicable for high forces and high dissipation I will have (voicecoil but it is fragile, E core, etc, etc)?2) How do I optimize a solenoid for maximum pull force, maximum plunger acceleration, and minimal wasted power consumption? The force goes up with number of turns per unit length or amount of current. Increasing number of turns increases resistance linearly. Increased resistance requires higher voltage, and ohmic losses increase linearly with V*R. However, the scary part is the quadratic I^2*R ohmic loss rise if someone tries to design a high-current solenoid. On the other hand, trying to use oversize wire to reduce resistance blows up the number of layers and outer diameter of the solenoid and solenoid becomes inefficient. It is apparent that commercial solenoids are on the order of a thousand turns of fine wire in a few layers, and operate at high voltage. How has this sweet spot been established and where is the golden book that will explain this sweet spot for solenoid design optimization for me??? What is the sweet spot for number of solenoid layers?3) For pulsed DC operation in the range of 20kHz-100kHz (yes, that is a wide range, so you can discuss both 20kHz and 100kHz separately), are "exotic" materials needed, such as ferrite magnetic return on the outside for the solenoid, and ferrite plunger core instead of a soft-iron one? Is Litz wire required at such frequencies? What is the "safe" transition frequency from soft iron to ferrites? Aren't powdered metals in the middle between solid/laminated iron and ferrites? Does anybody use powedered metals for intermediate to high frequency solenoids? What is the "safe" transition frequency from solid wire to Litz wire (at what percentage of skin surface)? If using ferrites/powdered metals instead of soft iron, do I lose anything (force produced, maximum magnetic field, efficiency) other than money (because ferrites are expensive) at "in-between? frequency range (I am aware that permeability appears to be larger for iron compared to ferrites)?4) Why are industrial-strength designs to produce bi-directional force (such as sinusoidal shakers) using a winding instead of a soft iron / ferrite core, to push/pull against the outer "solenoid" winding? I am guessing this is related to why large motors don't use permanent magnets, and use armature/stator windings instead. Please explain this to me. Is that due to core saturation? How would I know I run against this limit?5) Why is there a pittance of books that I could find on solenoid design on Amacrap by using search terms "solenoid" and "electromagnet", and any available books are from early last century ONLY? Are there any modern, practical books on solenoid and electromagnet design & construction (well, by being modern, I suppose all I gain is that they will be in print, and will talk about updated construction materials...)??? I am talking about a "cookbook" type and NOT a dry math book; one that will tell me what bobbin material to use, how to make easy calculations, will give me rules of thumb, will tell me how to wind the coil best, how to pot it, what to use for magnetic return and plunger materials, etc, etc, etc!!! Which nice, recent book can tell me all the juicy details of constructing such a beast, including all the salient info of whether I need to pot my core (to increase cooling and decrease windings displacement, etc), should I stick a layer of insulator between winding layers, what materials to use, etc, etc, etc...6) Should I try to construct first sample, what plunger diameter and length should I allocate to this level of force required, and using either soft iron, ferrite, or PM plunger? I am guesstimating one inch diameter would be on the low end. Where do I get a rule of thumb or a nice chart from a book that would size my plunger? The minimum volume would be dictated by magnetic saturation, right (is it volume or cross section)? Fromhttps://en.wikipedia.org/wiki/Electromagnet#Force_exerted_by_magnetic_field,what can come up with as a guesstimate, 1kN, requires a plunger 2cm in radius (how does 2 inches sound for diameter?).7) The bobbin must be non-magnetic and non-metallic due to eddy currents, right? What material should I use for smoking-hot temperatures and very high tensile force that will be produced by the windings? Are there ready-to-buy bobbins available?8) What potting material should I use for maximum heat transfer, high temperature rating, and its ability to permeate all the internal windings to keep them in place? Is vacuum impregnation the way to go?9) For the predicted 1-kW thermal dissipation (I am using four solenoids), since I require 50% duty from banks of two solenoids alternating, continuous operation between the two banks, I may need water cooling if blowing air past from incredibly powerful AC fans won't cut it. I am thinking about placing a cooling pipe touching the outer layer of the solenoid, or to be placed under the top layer of winding (potting will also touch the pipe). However, is any sort of metallic pipe out of the question due to eddy currents, even at the outside layer of a solenoid? What non-conductive pipe material will be decent enough for liquid cooling?10) How would I calculate realistic, practical induced emf magnitudes from the "coasting" plunger exiting the other end of the solenoid, that my driver electronics must withstand?11) Are there any companies that make super-solenoids, particularly one that is open-ended at both ends? Am I going to murder myself looking for any companies that make non-iron, higher-frequency power solenoids? So far I looked on Alibaba for CN companies, but I was very dissapointed by their lies about their ability to R&D and their experience/expertise with anything out of the ordinary. Qty 4 is required. I am willing to spend up to $1k for four pieces, and pushing the cost to $2k is probably out of financial possibility for my project. This is why I have not looked at any US / EU company, fearing the cost.Now come the dumb questions:12) Why would I NOT want to stick a permanent magnet inside the solenoid instead of the soft iron/ferrite plunger core to increase the produced force? I see conflicting reports online regarding permeability is the reason why, and the opposite claim that it actually increases the produced force. If it is a bad idea, then why do voicecoils use exactly that, a solenoid pushing against a permanent magnet?13) I am picking up some information that solenoids are really slow (https://www.eevblog.com/forum/beginners/solenoid-force-calculations/). With parameters force=1kN, coil length=20cm, plunger mass=1kg, current=15A, voltage=24/40V, displacement=1cm, inductance=5mH I get very strange results... The pure application of constant force to the plunger would result in an acceleration of 1000 m/s^2, final velocity of the plunger would be 4.5 m/s, and displacement time would be 4.5 ms. The time to "charge up" this inductor would be 3.1ms for 24V or 1.9ms for 40V. However, per provided source, the time for solenoid to move the plunger will take 42.2ms and final velocity would be 0.24 m/s. What is going on??? Why is the solenoid so slow??? Is there a design which can push plungers on the order of milliseconds, within my design parameters?I am an BSEE by study, embedded by concentration, no grad-level physics background, and not a minute of discussion required sadly on inductors other than whatever little bits I picked up along the way. I am willing to learn, but hands-on and experience-based only; please don't throw dry calculus equations at me: it has been too long since that and dry high-level math does not impress me. I would like to instead see a whole bunch of undergrad-level explanations and pointers for me to where to research next, so that I can study it all, try a design, try to construct it, and make some physical measurements of the results... I tried contracting Alibaba companies, but most lied about their ability to R&D and eventually give up on a design outside of their toy-sized solenoid... One company wants $1k, mostly for NRE, but I MUST KNOW I AM SPECIFYING THE RIGHT TYPE (solenoid, conventional type/shape, soft iron or ferrite magnetic return and plunger, for example) AND MATERIAL (soft iron or ferrite) AT THE LEAST to avoid having them manufacture a part that is useless for high acceleration or fast pulses, for example...