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    Need pointers to knowledge sources for electromagnetic application

    I'm a hobbyist when it comes to playing with circuits. I took a couple of years of electronics courses at the local junior college back in 1990 - 91. I've been a Network Engineer by trade pretty much my entire adult life, so my ability to engineer circuits is very basic in my opinion.

    A couple of years ago, I made a magnetic stirrer using a motor from an old hand drill, a speed control circuit that I got from Amazon, and an iridium magnet bar that I fastened to ... something ... that was attached to the shaft of the motor (and I don't recall all of the specifics of how I managed to do that).

    The result was a fairly decent magnetic stirrer, but I wanted it to stir vape juice that I would make and the juice was simply too viscous to get any kind of decent vortex in the solution with a Teflon coated magnet shaped like a pill.

    So I basically trashed the project and haven't re-visited it until today.

    What occurred to me is that we can create a magnetic field in a coil that could potentially be much stronger than the magnet I was using, and if I used enough coils which would be physically arranged in a circle (I would think that even 4 could work), I could use an Arduino to fire them in sequence at precise intervals and cause the magnetic pill to spin on center. And with the stronger magnetic field being generated in the coils, it would in theory be strong enough to create that vortex that I want even in the more viscous liquid.

    And THAT is about as far as my knowledge on the subject goes as it applies to this application ...

    What I would like to learn, is the base knowledge that I need to learn in order to understand things like ... how much magnetic strength am I shooting for ... how is magnetic strength even termed and referenced and what is the typical strength of an iridium magnet? Also, what kind of circuit would I need to drive the coils as I would have to assume that the voltage and especially the current needed to create a strong field would be more than an Arduino could produce from its 5-volt pins. So, I assume I would be attaching one output pin to one driving circuit per coil, then simply turn those pins on then off in a sequence at certain timing intervals which could, in theory, be adjusted based on a potentiometer connected to a 5th pin which would be read and the timing of the coil firing would be adjusted based on that pot value.

    Also, when a coil is magnetized, would I need to also magnetize the coil directly opposite of it with an opposite magnetic field so that both ends of the stir pill are "grabbed" tightly, or would magnetizing only one coil at a time work? I tend to think that I would need to fire two coils to make sure that pill stays tightly pulled to the bottom of the beaker.

    I can't imagine that the coils would be very expensive and I certainly have no interest in winding my own unless they happen to be really easy to make and there is a cost advantage to making them myself. Also, I don't know if the coils should have any kind of a ferrous core or if an air core would be preferable since I want the magnetic energy to be sent outward as much as possible...

    The largest amount of liquid that I would be stirring would be about 1,000 ml tops ... most likely it would be around 750 ml. So we aren't talking industrial quantities of liquid here ... just a simple 1,000 ml beaker filled with e-juice.

    In terms of viscosity, the e-juice is about the same as standard olive oil or corn oil ... maybe a tad thinner than that but it's a good reference if you don't know how thick the stuff is. so if you can picture 1,000 ml of olive oil with a teflon coated 1.5 to 2 inch magnet pill down at the center of the beaker spinning really fast ... we ideally want a vortex in the liquid that reaches from the top of the liquid and just about touches the Teflon coated magnet at the bottom of the beaker... various e-juice recipes turn out noticeably better when you can mix them for a long time, then let them sit in a dark place for sometimes a month while the various flavorings etc. oxidize and find their place in the solution ... as it were ... I don't know all the science of steeping, I just know what makes the juice taste better than a typical "shake and vape" approach and this device would really help that process along nicely.

    If this is making sense to anyone and you know what I would need to learn in order to make a serious dent in my magnetic stirrer 2.0 project, I would really appreciate your input.

    I did see that several companies make these things, but the ones that are rated for viscous fluids cost in upwards of three to four hundred dollars and I don't think I could forgive myself for spending that kind of money on this project not to mention the satisfaction of building my own, which has no dollar value yet can be immensely rewarding ... or deeply depressing ... and I would like to avoid the depressing option which is why I'm starting here to hopefully begin with an intelligent vector ...

    I look forward to any feedback and thank you for taking the time to read this.
    Last edited by EasyGoing1; 08-04-2020, 02:52 PM.

    #2
    Re: Need pointers to knowledge sources for electromagnetic application

    Originally posted by EasyGoing1 View Post
    What I would like to learn, is the base knowledge that I need to learn in order to understand things like ... how much magnetic strength am I shooting for ... how is magnetic strength even termed and referenced and what is the typical strength of an iridium magnet?
    You're building a motor with a "free-floating" armature.

    Magnetic force falls off as the inverse square of distance. I.e., doubling the distance results in 1/4 of the available force.

    In your original design, gravity kept the "armature" close to the rotating field (located immediately below it). Any new design would have to conspire to keep the armature close to the stator -- the electrically commutating "windings" that you're describing.

    Why not, instead, fasten a blade that enters the container from above and rests on a rotating surface that surrounds the vessel (sort of like a dasher). This would allow you to directly drive the blade with a friction mate (which you could enhance by providing "teeth" in the mating surfaces *or* a magnetic coupling to hold the blade assembly tight to the rotating surface.

    Or, stir it from above -- like a stand mixer.

    Comment


      #3
      Re: Need pointers to knowledge sources for electromagnetic application

      Thanks for the idea... I was wondering about how to build a magnetic stirrer for a hot plate and never thought about doing it this way...should work. I'm not sure exactly how to best do this yet but suspect I should have multiple electromagnets radiating out from a center hole where the stir rod will end up, not sure what it will do if you don't center the magnet.

      I suspect that you will need to use MOSFET drivers and shouldn't have a problem using them as current buffers, and with really powerful electromagnets you probably could yank the magnet pill through viscous liquids quite easily, much more than the magnet on a stick stirrers.

      May need to be careful with the drive else you will get some weird stepping behavior much like stepper motors. Or you will need more phases to smooth out the rotation which will result in better torque.

      I do wonder how well the magnetic stir rod plastic/teflon will hold up with the stronger field. But it will still eventually depend on how far away the electromagnet poles are from the stir rod through the container and the stirrer case.

      Comment


        #4
        Re: Need pointers to knowledge sources for electromagnetic application

        magnets lose their magnetism when they heat up - it's called currie effect.

        Comment


          #5
          Re: Need pointers to knowledge sources for electromagnetic application

          1. There are hot plate magnetic stirrers out there already, this is a fairly common lab instrument to use magnetic stirrers so you don't have contraptions over the reaction chamber blocking top access. Just that these things aren't in a poor man's laboratory, and I do suspect such devices should already be available, though I wonder if a magnet on a motor is more or less expensive to build versus a digitally controlled electromagnet array. Though you could make *really* flat stirrers without having to deal with gearing or bearings, albeit you still need some separation for heating elements.

          2. As long as you don't heat close or above the curie point you're fine. We're not using magnetic stir bars for making metal alloys where the working temperature is above the curie point of most magnets. Instead these are water or alcohol based solutions which have much lower boiling temperatures that will protect alnico magnets far from losing their magnetism. Indeed this is the reason why magnetic stir bars do not typically use neodymium magnets where even water might not be enough to protect them.

          Comment


            #6
            Re: Need pointers to knowledge sources for electromagnetic application

            Originally posted by Curious.George View Post
            Or, stir it from above -- like a stand mixer.
            I actually do this now ... funny that you brought it up ... but I need to seal the top to keep contaminants out ... its kind of a big deal ... the top down stirring didn't work out well last time ... I like to leave it stirring for a full 24 hours if I can...

            Comment


              #7
              Re: Need pointers to knowledge sources for electromagnetic application

              Originally posted by eccerr0r View Post
              I do wonder how well the magnetic stir rod plastic/teflon will hold up with the stronger field. But it will still eventually depend on how far away the electromagnet poles are from the stir rod through the container and the stirrer case.
              Oh this is wonderful ... you want to build one too? I mentioned that I saw some commercially available but very expensive ... they did show the inside and I saw only 4 coils and they looked kind of like sewing thread spools that were standing up on end ... and they had a short video showing it stir a viscous liquid similar in texture to vape juice ... so really all I need to figure out is the coil config ... the driver circuits ... the amount of voltage / current to energize the coils and then ... the rest should basically build itself ... lol

              Comment


                #8
                Re: Need pointers to knowledge sources for electromagnetic application

                Originally posted by stj View Post
                magnets lose their magnetism when they heat up - it's called currie effect.
                I can attest to this from experience having used heat before to remove magnets from a hard drive and ending up with nothing less than an impotent piece of metal that once was the king of the jungle of magnets ... they certainly don't like heat at all.

                Originally posted by eccerr0r View Post
                2. As long as you don't heat close or above the curie point you're fine. We're not using magnetic stir bars for making metal alloys where the working temperature is above the curie point of most magnets. Instead these are water or alcohol based solutions which have much lower boiling temperatures that will protect alnico magnets far from losing their magnetism. Indeed this is the reason why magnetic stir bars do not typically use neodymium magnets where even water might not be enough to protect them.


                I had considered also building a hotplate into the stirrer but that seemed to me to be an extra level of complexity that I just probably would be better off not dealing with. They do use aluminum for the hot plate element since its not magnetic it doesn't interfere with the field that spins the pill...

                I have no doubt that using an electro magnet would be infinitely more stable than when I did the irididum bar on a motor design. I had the bar as close as possible to the magnet without rubbing on the plastic of the box I built it in and once I got to a certain RPM the pill just bounced around like a Mexican Jumping Bean.... completely useless ... but a stronger magnetic field would most certainly keep it humming along wonderfully and making a very nice vortex ... which is ... I suppose ... my proverbial cloud that I am now chasing ... lol

                Comment


                  #9
                  Re: Need pointers to knowledge sources for electromagnetic application

                  Originally posted by EasyGoing1 View Post
                  I actually do this now ... funny that you brought it up ... but I need to seal the top to keep contaminants out ... its kind of a big deal ... the top down stirring didn't work out well last time ... I like to leave it stirring for a full 24 hours if I can...
                  You didn't visualize what I was saying.

                  Imagine your "mixture" is in a 4" diameter container (glass, metal, ...).
                  Get a 4.5" diameter container that is slightly taller than yours.
                  Invert it.
                  Slip it over the top of yours so that yours is "hidden", entirely, within.

                  Now, affix a "blade" (like a dasher) to the bottom (which is now the "top") of that covering container. So, the blade sits IN your container -- the tightness of fit determined by how wide you make the blade and how tall.

                  The rim of the covering container rests on a surface that is either FIXED (the cover FASTENED to it) while your container sits on a turntable...

                  *or*, the rim rests on a turntable while your container sits on a fixed platform above it.

                  See below:
                  I.e., the cover and your container exhibit relative motion.

                  The base unit has a motorized drive that spins the "white bucket" sitting atop it.
                  The clear plastic cover slides over the bucket -- note that it is taller and has a larger diameter than the bucket.
                  The white "paddle" (blade) is installed in the bucket before the cover is placed on. It engages "catches" in the underside of the cover so it remains stationary -- along with the cover -- as the bucket spins.

                  (the blade assembly "floats", in this application -- for cleaning. In yours, it could be permanently adhered to the underside of the cover)

                  Note that THIS bucket is actually a jacket of eutectic solution that a user chills (freezes) hence the reason it appears so thick.
                  Also, this cover has a hole in it so you can add ingredients while it is "stirring".
                  Last edited by Curious.George; 08-05-2020, 05:03 PM.

                  Comment


                    #10
                    Re: Need pointers to knowledge sources for electromagnetic application

                    Originally posted by EasyGoing1 View Post
                    I have no doubt that using an electro magnet would be infinitely more stable than when I did the irididum bar on a motor design. I had the bar as close as possible to the magnet without rubbing on the plastic of the box I built it in and once I got to a certain RPM the pill just bounced around like a Mexican Jumping Bean.... completely useless ... but a stronger magnetic field would most certainly keep it humming along wonderfully and making a very nice vortex ... which is ... I suppose ... my proverbial cloud that I am now chasing ... lol
                    As you mentioned iridium twice, this is confusing me, I thought iridium was merely paramagnetic and I can't recall any iridium alloys that exhibit strong ferromagnetism. Do you mean neodymium and not iridium?

                    Comment


                      #11
                      Re: Need pointers to knowledge sources for electromagnetic application

                      Originally posted by Curious.George View Post
                      You didn't visualize what I was saying.
                      You're absolutely right ... I didn't visualize what you were saying and now that I see it clearly ... I will say that I love the setup ... and it would be fine for things that you make ... like food items ... but vape juice has to be as clinically clean as possible ... the magnetic stirrer is ideal ... actually, it would be ideal if there was some way to stir it without anything inside the liquid, but to my knowledge, we haven't invented a way to stir solutions without physically touching them, and the coated magnetic pills are perfectly safe.

                      I've been doing some more looking around on how to do this using the electromagnetic idea ... I found one circuit on EasyEDA that I'm trying to comprehend ...

                      I think where I need to start ... in order to be effective anyways ... would be to figure out how much magnetic force I need... I found this Wikipedia on magnetic strength...

                      As I was going down the list, I saw this...



                      I always love finding random comedy in serious writings... lol

                      Anyways, based on that chart, I'm thinking that I need to shoot for a magnetic field strength somewhere around 13 to 15 Kg ... whatever that means ...

                      Comment


                        #12
                        Re: Need pointers to knowledge sources for electromagnetic application

                        Originally posted by eccerr0r View Post
                        As you mentioned iridium twice, this is confusing me, I thought iridium was merely paramagnetic and I can't recall any iridium alloys that exhibit strong ferromagnetism. Do you mean neodymium and not iridium?
                        That was my bad ... the word I should have used is neodymium

                        Comment


                          #13
                          Re: Need pointers to knowledge sources for electromagnetic application

                          Originally posted by EasyGoing1 View Post
                          Anyways, based on that chart, I'm thinking that I need to shoot for a magnetic field strength somewhere around 13 to 15 Kg ... whatever that means ...
                          Okay sorry about more pet peeve about proper units, I think you mean 13 to 15 kG or kilogauss. This is a flux density as written in that web page and not a force which is needed to generate the torque to spin the magnet.

                          Comment


                            #14
                            Re: Need pointers to knowledge sources for electromagnetic application

                            Originally posted by eccerr0r View Post
                            Okay sorry about more pet peeve about proper units, I think you mean 13 to 15 kG or kilogauss. This is a flux density as written in that web page and not a force which is needed to generate the torque to spin the magnet.
                            Well, I'm trying to determine how to choose the coils ... since I'm going to go with a 4 coil design, flux density matters, does it not? I'm thinking that I'm going to be magnetizing two coils at a time, one for each end of the magnetic pill inside the liquid. Current in one coil will flow one direction while the opposite coil will flow the opposite direction (one for the North end of the pill and one for the South end of the pill). Then those coils will be turned off and the second set will be turned on with the same current flow. Then, simply repeat that pattern only with reverse currents in each coil. This would create four steps for the Arduino that would simply be repeated by some kind of timing.

                            If the flux density is strong enough, then it should spin the pill.

                            Here is a screen shot from one companies video explaining how their unit works.



                            I spent some time on Mouser, looking for coils that are arranged in a spool like that (similar in look to a sewing thread spool) but I can't seem to find them, nor do I know what they are called ... any idea on that?

                            Comment


                              #15
                              Re: Need pointers to knowledge sources for electromagnetic application

                              Originally posted by EasyGoing1 View Post
                              Well, I'm trying to determine how to choose the coils ... since I'm going to go with a 4 coil design, flux density matters, does it not? I'm thinking that I'm going to be magnetizing two coils at a time, one for each end of the magnetic pill inside the liquid.
                              You may find that you turn 2 on, then 4 on, then 2 on, etc.


                              Code:
                              1 ++.---.++
                              2 .+++.---.
                              3 --.+++.--
                              4 .---.+++.
                              I.e., the equivalent of half-stepping a stepper motor. This allows you to keep "control" over the armature, better (not requiring it to "jump" as far between steps).

                              Comment


                                #16
                                Re: Need pointers to knowledge sources for electromagnetic application

                                Originally posted by Curious.George View Post
                                You may find that you turn 2 on, then 4 on, then 2 on, etc.


                                Code:
                                1 ++.---.++
                                2 .+++.---.
                                3 --.+++.--
                                4 .---.+++.
                                I.e., the equivalent of half-stepping a stepper motor. This allows you to keep "control" over the armature, better (not requiring it to "jump" as far between steps).
                                Now I'm wondering ... if all four coils shared a common ground, then I connected the remaining leads to a stepper motor driver like a DRV8825 ... I wonder if I could simply use it to control the spin. The DRV8825 is a wonderful stepper motor driver, Ive used it in a few different projects. It allows you to connect anywhere from 12 to 36 volts as the source voltage for the stepper control pins ... and it has a trim pot for current control.

                                Do you know what terminology I could use to locate coils on Mouser? I found all kinds of coils on that site, but I just want the kind that look like a spool of common sewing thread and I don't know how to find them.

                                **Edit - Nevermind, I just discovered their visual search feature and I located the coils.
                                Last edited by EasyGoing1; 08-10-2020, 03:58 PM.

                                Comment


                                  #17
                                  Re: Need pointers to knowledge sources for electromagnetic application

                                  Originally posted by EasyGoing1 View Post
                                  Now I'm wondering ... if all four coils shared a common ground, then I connected the remaining leads to a stepper motor driver like a DRV8825 ... I wonder if I could simply use it to control the spin. The DRV8825 is a wonderful stepper motor driver, Ive used it in a few different projects. It allows you to connect anywhere from 12 to 36 volts as the source voltage for the stepper control pins ... and it has a trim pot for current control.
                                  That would depend on the design of the driver. (you can drive a stepper motor many different ways).

                                  I *think* you want to be able to drive each coil in either polarity (some stepper drivers only offer a choice of "on" and "off" for each coil).

                                  And, you also want to be able to "half-step" (the example I've given).

                                  I prefer to drive motor coils individually with an MCU. This gives you greater control (think: acceleration, braking, back-emf sensing) over the motor -- instead of a "textbook" approach (that ignores the Real World).

                                  But, if you have one handy, just TRY it! You don't need to introduce a liquid (and container); just suspend your "stirrer" over the magnets with a fine thread, for starters.

                                  Comment


                                    #18
                                    Re: Need pointers to knowledge sources for electromagnetic application

                                    Originally posted by Curious.George View Post
                                    That would depend on the design of the driver. (you can drive a stepper motor many different ways).

                                    I *think* you want to be able to drive each coil in either polarity (some stepper drivers only offer a choice of "on" and "off" for each coil).
                                    That's a good point. I'm thinking that I need to light up the coils in pairs and I'm not sure if you can even spin a stepper that way ... definitely something I will need to look into when considering this option.

                                    Originally posted by Curious.George View Post
                                    And, you also want to be able to "half-step" (the example I've given).
                                    Every stepper motor project I've done, I always use half stepping as it typically gives me more granular control and they seem to spin smoother when half stepped.

                                    Originally posted by Curious.George View Post
                                    I prefer to drive motor coils individually with an MCU. This gives you greater control (think: acceleration, braking, back-emf sensing) over the motor -- instead of a "textbook" approach (that ignores the Real World).
                                    BUT, you're talking about applying magnetic force on an object that is mounted on a shaft. The magnetic pill inside a solution will have no shaft. It will spin freely ... so I'm thinking that just lighting up one coil at a time might cause the center of the pill to drift during the spin cycle ... which might not necessarily be a problem as long as the drift does not cause it to get away from the coils ... I still think that lighting up opposing coils - one being North and the other South would be a more stable way to spin a pill that isn't mounted on a shaft.

                                    Originally posted by Curious.George View Post
                                    But, if you have one handy, just TRY it! You don't need to introduce a liquid (and container); just suspend your "stirrer" over the magnets with a fine thread, for starters.
                                    If I had the coils I would try it. I thought maybe I could harvest some from some small motors I have laying around, but when I took one apart (a vibrating motor from a gaming controller) the coils were very small and weren't quite what I was expecting to find.

                                    I did find some on Mauser, but I still don't know how to spec them, so I'm just going to look at maximizing their magnetic flux ratings and their physical size within a certain price range and hope that's enough. The way I figure it, coils to do this job shouldn't cost more than $5 each and I have the photo of that one commercial magnetic stirrer to rough estimate the size.

                                    Comment


                                      #19
                                      Re: Need pointers to knowledge sources for electromagnetic application

                                      Originally posted by EasyGoing1 View Post
                                      That's a good point. I'm thinking that I need to light up the coils in pairs and I'm not sure if you can even spin a stepper that way ... definitely something I will need to look into when considering this option.
                                      There are lots of ways to drive a stepper motor -- depending on how the windings are "brought out".

                                      If you have access to both sides of each winding (for exemple):
                                      - tie one side to +V and drive one of the four "free ends" at a time, in sequence.
                                      - same as above but drive 1, 12, 2, 23, 3, 34, 4, 41
                                      - drive each end of each coil with an H-bridge (so you can reverse the polarity through the coil)

                                      Every stepper motor project I've done, I always use half stepping as it typically gives me more granular control and they seem to spin smoother when half stepped.
                                      Yes, but the torque available changes from one half-step to the next. So, you have to size the motor with how you're going to drive it in mind.

                                      BUT, you're talking about applying magnetic force on an object that is mounted on a shaft. The magnetic pill inside a solution will have no shaft. It will spin freely ... so I'm thinking that just lighting up one coil at a time might cause the center of the pill to drift during the spin cycle ... which might not necessarily be a problem as long as the drift does not cause it to get away from the coils ... I still think that lighting up opposing coils - one being North and the other South would be a more stable way to spin a pill that isn't mounted on a shaft.
                                      This depends on the physical characteristics of your container, etc.

                                      If I had the coils I would try it. I thought maybe I could harvest some from some small motors I have laying around, but when I took one apart (a vibrating motor from a gaming controller) the coils were very small and weren't quite what I was expecting to find.
                                      Find some open-frame relays and disassemble them to salvage the coils. Find a guy who owns/operates an arcade (think: pinball machine, flippers, pop bumpers -- even if it is an electronic model!)

                                      Comment


                                        #20
                                        Re: Need pointers to knowledge sources for electromagnetic application

                                        So I decided, since I couldn't figure out with any kind of certainty - what kind of coil to purchase, I figured I would just make one and experiment with it and if I can generate the kind of magnetic field that I believe I'll need, then I'll make three more and keep going on this project.

                                        I bought a pound of inductor wire on Amazon, then I bought some Ferrite cores on eBay ... then I printed what I needed to construct a spool and I modified a device I built that that I use to make vape coils so I could wind the wire onto the spool. I figured since I need four coils, I'd make each one ΒΌ pound... Anyway, this is what I ended up with as my first prototype:



                                        Now, I figured that I would probably need at least an amp or more to go through this coil to generate any kind of useful magnetic field for this application. But as you can see, the coil ohms out at 72.5 ohms which means that to get a full amp, I'll need 72.5 volts. BUT, the strongest DC power supply that I have is only 38 volts and when I put 38 volts on the coil, the magnetic field doesn't seem very strong to me.

                                        I've seen voltage step up converters on Amazon ... kinda like ... reverse Buck converters I guess ... and they have some that will go up to 90 volts from like 30 ...

                                        But something doesn't quite "feel" right ... I can't put my finger on it... so I'm back in here seeking input.

                                        Should I be looking to feed as much voltage as I can into this coil, or is there some other way to drive it so that I can get a good field out of it?

                                        The ferrite core is 32mm long by 8mm in diameter. The wire that I bought is 30 AWG Magnet Wire - Enameled Copper Wire - Enameled Magnet Winding Wire.
                                        Attached Files

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