WTB: Electromagnets

[apelly]

Yea; simple, right? Bear with me...

A while ago I started a thread about a home made stirring hotplate. As is usual, most people tried to talk me out of it. They did a good job at the time, but I've been sort of dwelling on it. I think I'll make one after all, even though it's a Bad-Idea^tm.

So I've spent quite some time looking for electromagnets.

I find two things:

• Holding magnets. These appear to sit in an iron cup to bring both poles to the contact area. No good for my stir-bar.
• Solenoids.

Thus far, it looks like the best options are either to glue the shaft in a solenoid, or wind my own.
With regard to modifying a solenoid, a bonus there is that they come in a readily mountable format.
Regarding winding my own, well, I could have done it already if I hadn't wasted so much time googling.

What I'm wondering is if my google fu is simply low on this topic (it wouldn't be the only one!)

Do pre-made electro magnets exist for anything other that high school labs?
Are there such things as pre-wound bobbins that will just slip over some mild steel rod?
If so, where the hell are they?

Should I just make some?

[tautech]

When I have needed a simple electromagnet i use the solenoid from an old auto starter motor.
Generally plenty of pull for most requirements, the downside is a reasonable current required and not suitable for continuous duty.
Had a quick look at Electus as they supply the Edu sector, but nothing apparent.
But there will be a an educational supplier for labs and the like.

Do you want to use AC or DC?

[tautech]

What about the coil from an industrial contactor? Easily dismantled, although 230V less common than 440V

[apelly]

Do you want to use AC or DC?

AC

I haven't done any math yet. I thought I'd find out what's available and see how I could work with it.

Tentative plan is 6 magnets in a circle. Opposite pairs are in series, with opposing current. The three spokes thus created are controlled via h-bridges & MCU.

[tjw]

Hi, have not seen your previous discussion so don't know whether the Silicon Chip magnetic stirrer project was mentioned. I have the magazine at home and can take a look if that will help.

regards, tjw.

[apelly]

Stirring hotplate thread is here https://www.eevblog.com/forum/projects/stirring-hotplate/

tjw, looking back at that thread I see that notsob suggested the same thing. I said at the time that I would buy the article. I did not. The brief preview available online obviously failed to inspire me. It looks more interesting now that I read it again.

I feel like I've outlined enough features in my head now that someone else's design would be cheating. I would like to know if you found the article interesting. And I still might buy it anyway, since I've been reminded.

I guess I also feel like winding my own coils is cheating. On the one hand it's industrious, on the other, if I want to make a few, it's a pain in the arse.

[CM800]

What about the coil from an industrial contactor? Easily dismantled, although 230V less common than 440V

Most coils for contactors run on these voltages:
24V
110V
230V

I have never seen one running on 440V ever.

Ebay sell some electromagnets quite cheap i think.

[Rigby]

I'm missing something; why do you need electromagnets?  All the hotplate stirrers I've used were (I think) just bar magnets fixed to a motor.  Though, now that I type it, they probably were electromagnets, given that heat more or less nukes permanent magnets after a while...  They could be insulated from the heat, though.  I don't know.

Ok, electromagnets.

Depending on the stir speed, I would think that winding one's own would be a fine idea.  Magnet wire can be found in lots of places.  You'll need to size the wire based on stuff, and I don't know what that stuff is, but I'm sure it's a lot like inductor math; core material, wire gauge, winding count, etc.

Hotplate stirrers are something that I'm surprised hasn't made it into the form of a kitchen appliance, yet.  Self-stirring pots?  Are you kidding me?  Sign me up!

[SeanB]

Most coils for contactors run on these voltages:
24V
110V
230V

I have never seen one running on 440V ever.

Ebay sell some electromagnets quite cheap i think.

Just took one apart today, 380VAC 50Hz or 440VAC 60HZ. Was a 3 phase DOL motor control, for use on a 3 wire plus PE circuit with no neutral. The contactor was not usable for me as I tend to mostly use 24VAC and 220VAC coils in these style contactors.

[apelly]

Ebay sell some electromagnets quite cheap i think.

Yes, you'd think so wouldn't you. I can't find a thing.

I'm missing something; why do you need electromagnets?  All the hotplate stirrers I've used were (I think) just bar magnets fixed to a motor.  Though, now that I type it, they probably were electromagnets, given that heat more or less nukes permanent magnets after a while...  They could be insulated from the heat, though.  I don't know.

You are right. It looks like most of them are simply permanent magnets on a shaft.

Part of my issue is I lack imagination for projects. The inspiration for this project is that these hotplates cost upwards of $700 in New Zealand, and I reckon I can make a cool one for less. And have fun, and learn. Which is pretty much the point of my projects.

Depending on the stir speed, I would think that winding one's own would be a fine idea.  Magnet wire can be found in lots of places.  You'll need to size the wire based on stuff, and I don't know what that stuff is, but I'm sure it's a lot like inductor math; core material, wire gauge, winding count, etc.

Yes, and current.

I'm a little surprised nobody has come along and said "You idiot n00b, let me google that for you. They're called z-magnets, Xy sells them."

With regard to winding my own, thinking about it caused me to research vacuum pumps for varnishing them. That's opened another whole can of worms!

Hotplate stirrers are something that I'm surprised hasn't made it into the form of a kitchen appliance, yet.  Self-stirring pots?  Are you kidding me?  Sign me up!

That would be kind of cool. Makes me think of putting an electric paint stirrer in my next pot of custard. Come to think of it; makes me think seriously about it!

[neslekkim]

Something like this? https://en.wikipedia.org/wiki/Sextupole_magnet
I wonder if one could use the spools in brushless enginge or similar, and invert its use?

On the other hand, how hot did you intend the stirrer to operate at?, it doesn't seem like the magnets are that bad in heat: http://www.kjmagnetics.com/blog.asp?p=temperature-and-neodymium-magnets

[neslekkim]

ah, better idea here: https://en.wikipedia.org/wiki/Magnetic_stirrer
many elecotromagnets..

I guess the problem will be to find small enough magnets that don't throw out the magnet:
http://www.ebay.com/itm/251335051706
http://www.ebay.com/itm/261469269743


some old patent: https://www.google.com/patents/US1242493

[Fred27]

Tentative plan is 6 magnets in a circle. Opposite pairs are in series, with opposing current. The three spokes thus created are controlled via h-bridges & MCU.

That sounds very much like a stepper motor - or at least the simplified 4-step one you see in diagrams explaining how steppers work. You might find that if you stick to 4 poles (or maybe that a standard stepper motor driver might cover the driver side of things - including smoothing using microsteps.

Sorry I can't be any help with the elctromagnets. I'd try a solenoid and if that doesn't work then wind your own.