Any coil design experts out there? - Page 1
Forum section: electronics / projects

author: EasyGoing1
author_rank: Regular Contributor
timestamp: April 14, 2021, 03:22:14 pm
content: Without wasting a lot of time going into backstory, I've been tinkering off and on with a project I want to do but at this point, I've been stopped by the coil design...What I want to build, is a magnetic stirrer that does not use a motor. And I need the electromagnetic field that is generated by the stirrer to be stronger than an N50 neodymium magnet at a position of about 5mm above the coils.I've built many coils ... I've bought a few ... I've posted topics in other forums ... and so far, I've been unable to really wrap my head around this problem nor have I been able to find someone who understands it well enough to tell me exactly what I need to do.I've successfully built a working prototype of sorts ... I've gotten the magnetic pill to spin inside glass of wateras you can see here. I believe I was feeding those coils 19 volts and I was using a stepper motor driver to make em go controlled by a microcontroller ... and those coils get pretty hot fairly quickly, which is a problem. Also with those coils at that voltage, they still weren't nearly strong enough for what I need.I know this can be done, because they make these commercially, but they are fairly expensive, and buying one won't do anything for my education either, which is a big part of why I'm doing this in the first place.So if anyone out there has knowledge on coil design and understands what I need to do to spec or build the right coils, I'd be grateful for any comment.Thank you,Mike

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: April 14, 2021, 03:43:29 pm
content: Hi Mike,the url link you posted is a bit malformed but for anyone else the video is at:Did you try to use the stator of a stepper or three phase motor around your flask? That will definitely make magnetic bits spin inside

author: Benta
author_rank: Super Contributor
timestamp: April 14, 2021, 04:04:57 pm
content: What you're actually trying to design is a permanent magnet synchronous motor (PMSM).For smooth operation, a three-phase, multiple pole stator would be my approach due to the low speed.Reliable startup of this kind of motor is a bit involved, but can be handled without problems in the control electronics/firmware.

author: EasyGoing1
author_rank: Regular Contributor
timestamp: April 14, 2021, 04:47:16 pm
content: Quote from: pardo-bsso on April 14, 2021, 03:43:29 pmBoy ... you've definitely gone above my pay grade as I have no idea what a "stator or a stepper" is... but I have Google and I'll see what I can learn.Not sure if this matters, but I'm trying to do this without any motor at all ... no mechanical moving parts ...

author: bob91343
author_rank: Super Contributor
timestamp: April 14, 2021, 05:49:31 pm
content: A stator is the part of a motor that does not move. It's a coil with associated magnetic circuit.A stepper is a stepper motor. That is a motor that does not move smoothly, but goes in jumps, sort of digital instead of analog. That way a digital signal can move the motor a specific distance, rather than the more usual analog motor which moves as long as it is driven.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 14, 2021, 06:29:33 pm
content: It may help to think about magnetic field lines and how to close them. A coil in free air has very little influence over a magnet. If you wind that same coil around an iron form and bring both ends in close proximity to the magnet, you can generate far more force. Yes, you're building a motor directly, rather than spinning a magnet like common stirrers. The stirring element is the rotor of the motor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2021, 10:19:11 pm
content: Yeah... you're going to have to relax your requirements. An air cored winding gets very hot indeed, very quickly indeed, making 1.5T!What size magnet are you using? N50 is the grade, it tells us nothing about the size, shape or energy of the thing.Exactly how big of a magnetic field do youreallyneed? Technically the dipole moment is what you're after: you're trying to make a magnetic stirbar spin, and the flux density required to do this doesn't really matter, just that it gets enough moment to pull it around.Typical planar and flat coils will do fine in the 10mT range, which is enough to spin, like, something in a flat or round bottomed flask I suppose, in a not very viscous fluid, at a modest speed.Do you know how much torque you need for a given stir rate and viscosity?Nevermind granular or lump materials, maybe you can approximate an average or worst case viscosity for that?...It would be nice if there were a way to gear down a stirbar, so that for example a pile of small ones could spin very fast (which won't need much flux density). Alas, fluid dynamics just doesn't work that way...The point of that being, magnetic fields are relatively weak, at least fields that we can create easily. But we can vary them extremely quickly, equivalent of millions of RPM in this sort of arrangement. So while the torque may be feeble, we can compensate by cranking RPM up, and power is proportional to both. That's why small motors are so fast and loud, and why gearboxes are so necessary to make use of them. You lose this advantage in direct-drive applications, which usually results in ugly compromises, like really wide hub-shaped motors -- the large hub means more magnetic field area to get the torque needed.BTW, the limiting factor is literally just copper -- if we had an about 10 times better conductor, we'd be sitting pretty good actually, with applications like this. Well, your hotplate and any surrounding metal might not be so happy (being pulled/pushed/spun by the fields, and heated up by induced currents unless made of the same mythical metal), but the core idea would work. We can actually do even better than that, in fact infinitely so -- but only at low temperatures, where superconduction becomes possible. Go figure, eh?Tim

author: langwadt
author_rank: Super Contributor
timestamp: April 14, 2021, 10:37:23 pm
content: Quote from: T3sl4co1l on April 14, 2021, 10:19:11 pmI'd expect a 1.5T electromagnet to be big and water cooled ...the magnetic stirrers I've taken a part spins a permanent magnet with a motor ...

author: EasyGoing1
author_rank: Regular Contributor
timestamp: April 15, 2021, 02:28:06 am
content: Quote from: T3sl4co1l on April 14, 2021, 10:19:11 pmI have Ferrite in the core of these coils... bought the little rods specifically for this project.Quote from: T3sl4co1l on April 14, 2021, 10:19:11 pmDid you see the video I posted in my original post? The stir bar is visible enough to get an approximation of how large it is. Notice I got the thing to actually spin with some decent RPMs though there is no way it would work in viscous solution which is what I'm after.I built one of these with a motor and an N45 rated bar magnet and I couldn't get it going fast enough without the stir bar getting knocked out of place because of the resistance in the solution. Although if an N55 is significantly stronger than an N45 it might work ... but I really wanna build this motorless stirrer... it's become a challenge that I don't want to surrender to if you understand. :-)Quote from: T3sl4co1l on April 14, 2021, 10:19:11 pmYes, but pull it around in a solution about the same viscosity as vegetable glycerin.Quote from: T3sl4co1l on April 14, 2021, 10:19:11 pmI wouldn't know ... nor would I know how to find out ... when I think about torquing down on a nut with a torque wrench, I'm thinking 20 or 30 foot lbs might be enough? Maybe not... maybe too much ... I really don't know...Quote from: T3sl4co1l on April 14, 2021, 10:19:11 pmThis much I HAVE learned in this process. ... oi!Quote from: T3sl4co1l on April 14, 2021, 10:19:11 pmThat stepper motor driver that used in that video did pretty good in terms of timing for the firing of the coils...Quote from: T3sl4co1l on April 14, 2021, 10:19:11 pmWell ... if I could suspend my coils in liquid Nitrogen and build them with superconductive material ... I wouldn't be here posting this question, I'd be spinning my television at 200 RPMs LOL