??? Are ELECTROMAGNETS really that weak ? - Page 1
Forum section: electronics / beginners

author: RJSV
author_rank: Super Contributor
timestamp: October 21, 2020, 06:30:48 pm
content: Hello, I've been trying to experiment with this magnetic drawing board (EtchaSchetch) and my electromagnet is virtually useless, for making screen marks. I'm playing around with having a 'magnetic non-volitile display'.Problem seems to be apparent strength of the field. Meanwhile, the little permanent magnet (writing stylus) makes marks from as far away as 1 mm, from the pad screen.Only when I tried putting 9 volts across the coil, briefly, did the electromagnet cause a barely visible spot to appear. (Meanwhile causing auditable glitches in nearby radio receiver).I had planned to experiment with the magnetic display, that seems capable of approx 30 by 40 dots across the screen.Does this mean that in any motor, etc, the electromagnetic part is very weak, compared with permanent magnets used?

author: RJSV
author_rank: Super Contributor
timestamp: October 21, 2020, 06:31:39 pm
content: Here's a second view.

author: ucanel
author_rank: Regular Contributor
timestamp: October 21, 2020, 06:48:54 pm
content: The path that magnetic field lines follows is important.It depends on coil construction and if it has something like ferrite core or similar.Try placing an iron maybe a nail into the coil and use it as a pen.I did not think about does it work or not butmaybe you can place a metal sheet under the drawing board.For actual question you may compare Tesla values for generic magnets and electromagnets.You may further search "magnetic field strenght" .Reminded me that:https://hackaday.com/2019/12/18/escher-etch-a-sketch-as-a-service/

author: RJSV
author_rank: Super Contributor
timestamp: October 21, 2020, 07:26:30 pm
content: Thanks, ucanal I posting another pict, shows the coil wrapped on a spool, this was from a toy 'dancing bird' where it pushed a permanent magnet disk, and got pretty good movement, didn't check voltage but probably 3 volts driving the coil, maybe even an audio channel driving that (wing movement).I'm thinking why don't (motors) just do coil opposed to another coil, but those exist, seemingly when there is AC mains power available.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 21, 2020, 08:23:29 pm
content: For electromagnets it depends a lot on the presence of an iron or similar core and it's form.Another point is that electromagnets get weaker when you scale them down. So things that work at the meter scale may be rather weak at the mm scale.The modern rare earth magnets are surprisingly strong. The older style ferrite or Alnico magnets where quite a bit weaker.

author: TimFox
author_rank: Super Contributor
timestamp: October 21, 2020, 08:57:34 pm
content: At the large-magnet end of the spectrum, there is an inherent maximum field for permanent magnets < 0.4 T or so (since the hysteresis curves saturate at physical values), and iron-core electromagnets are similarly limited by saturation of the “soft” ferromagnetic alloys at around 20 T. but air-core electromagnets are limited by cooling (removing the large heat generated in the copper windings) and mechanical strength (holding the wires together against magnetic repulsion). Superconducting electromagnets also have mechanical limitations, along with the “critical” field that the conductor can tolerate and stay “super”. Niobium alloys can go up to about 44 T at the conductor. Higher fields have been obtained with normal conductors, but sometimes this involves a device designed to explode after a brief period of magnetic glory.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 21, 2020, 09:52:17 pm
content: The main trouble you have is because you are using your "electromagnets" / inductors in the wrong way.In your inductors, there is very little of the magnetic field out of the top and bottom of the inductor, but most of it is going from the side of the "ring" on the top to the "ring" on the bottom.If you want it to work better, you have to motivate the magnetic field to go more through your etch-a-scetch.Just rotating the magnet 90 degree and holding the side against the etch-a-scetch probably already is an improvement, but even then most of the magnetic field will be unused.

author: james_s
author_rank: Super Contributor
timestamp: October 21, 2020, 10:53:51 pm
content: The magnetic field strength is proportional to the current through the coil. A little coil from one of those solar toys is designed to run at very low current and produce a weak field, just enough to get a light weight balanced object to oscillate. If you want a stronger magnet you have to wind it using fewer turns of thicker wire, and use a suitable core to concentrate the field. Generally speaking, electromagnets can be made much stronger than permanent magnets. There is a point at which a given core will saturate but even ordinary steel ought to allow a reasonably strong magnet.

author: RJSV
author_rank: Super Contributor
timestamp: October 22, 2020, 01:38:37 am
content: Thanks folks, info gives me enough confidence to try using an armature with windings. I have a spare motor armature, for salvage some wire.The plan is to (1) make an oscilloscope style 'sweep' using a electro-magnet to write patterns as scan progresses. Other mode (2) will be attempts to do crude dot matrix. Screen resolution is approx 40 by 30 max, before lines start bleeding into each other.It's non-volitile !

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2020, 02:06:01 am
content: You can buy magnet wire in a whole range of sizes, it's not very expensive. A nail is the classic electromagnet core.

author: Brumby
author_rank: Unknown
timestamp: October 22, 2020, 05:00:51 am
content: As well as the information given above - distance is another very important factor as the strength of a magnetic field diminishes by a power of 5 the further away it is...That is to say if you move the fieldtwice the distancefrom a given point, the magnetic field at that point is 25=32 times weaker.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 23, 2020, 08:03:03 am
content: The magnetic field does not follow a simple power law and it depends on the magnet configuration how fast the field gets weaker. With distance is gets weaker quite a bit.The saturation in iron alloys is at around 2 T, so the core does not help much beyond that point. This is already a quite strong field. The 0.4 T limit is valid for the cheap ferrite magnets, with suitable form the rare earth magnets can reach about 1.2 T. However this limit is the field in the material, the usual magnet shapes have more like 1/10 to 1/3 this value at the ends. Only a nearly closed core like an extreme horseshoe form can come close. Producing the flied in air is what needs much of the magnetic energy. So a nearly closed core can have the field much more concentrated and stronger.Inside electro-motors there is very little air gap and this way the field can be quite strong.

author: RJSV
author_rank: Super Contributor
timestamp: October 23, 2020, 07:04:37 pm
content: Here is some detail, I'm often finicky, when using tiny wire!In the picture, you can see a sort-of square bobbin, for coil with screw armature. This provides the two contacts for a coil, and isolate any cable stresses from the solder joint (to each fine wire).I like to use a half-inch piece of stiff wire, each goes into the screw socket of the little terminal block.The magnet wire, a bit thicker than radio coil stuff, is 12 ohms resistance, over salvaged 6 ft. (2 m).That's about 250 ma at 3v.Pict also shows PM motor rotor, had typical 3 coils in delta

author: RJSV
author_rank: Super Contributor
timestamp: October 23, 2020, 08:06:13 pm
content: In the enclosed picture, it was interesting to see the 'Magnetic Pad' has a neat enclosed 'dust pouch'; that has the magnetic stuff in 6-sided sections, about 3mm each.Also interesting, this flat plastic pouch can be reversed, for a sort-of reverse video. However, this image is more blurred, for some reason.It was interesting, to see how simple that erasing bar moves, seems like it would bind. In my salvage box(es), there is a perfect linear actuator, (off of a haunted house Halloween thingy).