Directed electromagnet - Page 1
Forum section: electronics / projects

author: Rachie5272
author_rank: Regular Contributor
timestamp: October 13, 2018, 05:51:52 pm
content: I'm trying to design an electromagnet which concentrates most of its energy on a point source some ~5cm away. The end goal is to cause a permanent magnet to oscillate as strongly as possible without the coil overheating.I'm familiar with basic solenoid equations, but I can't find any practical information on how to shape or focus magnetic fields. At the moment I've been playing around with a degaussing coil (coreless copper toroid), but the flux density is very low.What is the best way to proceed with designing such a magnet?

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: October 13, 2018, 06:46:10 pm
content: Install this free software and learn how to use ithttp://www.femm.info/wiki/HomePagePretty musch anything you can do in reality can be modeled using FEMM.You will find that reality and your desires are far apart. But, hey, you will learn a lot.Randall

author: ahbushnell
author_rank: Frequent Contributor
timestamp: October 13, 2018, 07:03:57 pm
content: FEMM is good 2D software. I use it for engineering work.How much field and what frequency.Sent from my SM-G930V using Tapatalk

author: Rachie5272
author_rank: Regular Contributor
timestamp: October 13, 2018, 08:44:21 pm
content: The idea is really just to oscillate a neodymium magnet through a thick tabletop as a demonstration. I don't have a good feel for magnetic field intensity. Frequency is adjustable, 10-1000 Hz. Voltage is ~180V (rectified 120V mains AC), with MOSFET choppers.

author: JS
author_rank: Frequent Contributor
timestamp: October 14, 2018, 03:54:28 am
content: You should use a U shaped core, as those classic red and blue magnets, find a core made of silicon steel, like the one mains transformers are made of. A lamination for toroidal cores could be ideal, as a tape you could cut strips from. EI cores are easier to find, you could cut one E in half and get two squarish U to build this, the size will depend on what you try to archive, and the size of the magnet you want to move, so the field is wide enough for the magnet to fit in. Try at least twice the lenght of the magnet for your gap, maybe three times.Then you should use a full bridge to make current flow in both senses. If you just make the current flow during the entire cycle the number of turns of the coil will be huge for 10Hz and the current at 1kHz will be very low, so I suggest using constant pulse width and change the timing between pulses, I hope that make sense...If using 500μs pulses (full width at 1kHz as max freq) you should do the math for your core and the voltage to get the numbers of turns, so the core doesn't saturate. That's just as an air gapped transformer primary, you could fine the equations around. If the core saturates it will just start heating and the power delivered to the field wont rise any further at that point.If you are more the experimental kind of guy, you could try that experimentally to find the apropiate number of turns, making a random coil on the core, taking it slowly to saturation, where the current starts to increase much higher than the voltage, measure the current and count turns, and then keep that value constant. Add as many turns as you need for the core to be just about to saturate at 500μs, with 180V applied and then you go.JS

author: MrW0lf
author_rank: Frequent Contributor
timestamp: October 14, 2018, 04:24:37 pm
content: Quote from: Rachie5272 on October 13, 2018, 05:51:52 pm

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 15, 2018, 01:00:09 am
content: The FEMM advice is good. But to cut to the chase, the best practical focus of a coils magnetic energy is incompatible with your objective. You make a magnetic core in the general shape of a C, with the ends of the C as close together as practical. The focus point is between the ends of the C. The best variant I can think of for your application is to arrange the C so that the table top lays across the ends of the C. If the gap in the C is somewhat larger than the thickness of the table you should get coupling to the permanent magnet that is as good as is possible.You could cheat a little and incorporate the magnetic core in the table top to limit the distance between the gap and the permanent magnet. In either case the system is unstable, to demonstrate the vibration you are looking for you will need to pin the bar magnet, somewhat like the way they are pinned in a compass. Otherwise your bar magnet will be pull to one or the other of the poles.FEMM will let you optimize the gaps and other geometry for this application.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: October 15, 2018, 07:51:24 am
content: Overall you forget cores if you want long haul action. Use compound permanent magnets that create potential valley at needed height. Superimpose relatively weak alternating field from coreless electromagnet(s) to gain needed special effects. However, if take metals into the play on levitating part and apply lots of power...https://youtu.be/OI_HFnNTfyU?t=783...or if willing just to use stable geometry:

author: Marco
author_rank: Super Contributor
timestamp: October 15, 2018, 08:56:33 am
content: Quote from: Rachie5272 on October 13, 2018, 08:44:21 pmWhat do mean oscillate? If it's levitating then the oscillation is orthogonal, you'd first have to levitate it. You'll need multiple electromagnets and probably some kind of closed loop control (there's tons of modules from China which do this with optical feedback). You might be able to get away with just switching off one of the electromagnets in sequence instead of closed loop control, but I'm not sure.