Anyone have experience working with electromagnets?

Thread Starter

EasyGoing1

Joined Dec 10, 2016
14
Hello,

I've been experimenting off and on over the last few months with electromagnets for a specific application. But I seem to have found myself with a big question mark over my head after all of the experimentation I've done.

What I need to create, is a set of four coils, each with more magnetic force than can be found in an N45 Neodymium magnet. Preferably twice as much magnetic force. And each coil should be no larger than a spool of thread and of course, it should be able to be powered from standard home electricity and ideally emit minimal heat from the coils.

What I have found in my pursuits, is that I face walls in that when I obtain what seem to be decent coils, in order to get any kind of decent magnetic strength out of them, I really have to step up the voltage and the current which causes them to get really hot fairly quickly and the magnetic force that I'm getting out of them certainly does not rival or come close to N45 strength.

Common sense tells me that electricity should have the ability to create a magnetic force as strong or stronger than N45 but finding that solution has been a royal pain.

Is there anyone out there with experience in this area who can offer some useful discussion on this topic?

Thank you,

Mike Sims
 

AnalogKid

Joined Aug 1, 2013
8,819
Since you don't say how much force you need, there is no clear answer. Generally speaking, big electromagnets get hot. For any particular wire gauge and length you can look up its resistance. From there, Joule's Law will give you the power dissipation. Simple rule of thumb - if it is the size of a light bulb and dissipates 60 W, it's probably gonna get as hot as a 60 W light bulb.

You can calculate an electromagnet's force if you know the wire gauge and length, number of turns around the core, and the core's electromagnetic properties. There probably are online calculators for this.

ak
 

MrChips

Joined Oct 2, 2009
22,536
The magnetic force is directly proportional to the current and the number of turns. Hence you need a very large number of turns.

The reason the coil gets hot is because of the resistance of the wire.
Hence you need very fat wires to keep the resistance low.

You will end up with a very big coil.
 

Papabravo

Joined Feb 24, 2006
14,868
The magnetic force is directly proportional to the current and the number of turns. Hence you need a very large number of turns.

The reason the coil gets hot is because of the resistance of the wire.
Hence you need very fat wires to keep the resistance low.

You will end up with a very big coil.
OR....room temperature superconductors. Those might just be outside normal budgetary limitations.
 

MisterBill2

Joined Jan 23, 2018
7,732
Physics does indeed limit the amount of magnetic attractive force, and one commonunit of magnetic flux is "ampre-turns". The directing of flux is where gains cam be made, which suggests that continuous paths of the most permeable materials will provide the most attraction. In addition, the attractive force decreases with the square of the distance , so having a minimum air gap will provide the maximum force.
And, exactly as has been stated, all magnet coild will heat because of the current passing through the resistance. And superconductors are both expensive and inconvenient, and thus may not be applicable to this unknown application.
 

MaxHeadRoom

Joined Jul 18, 2013
22,036
My experience with modern Neodymium magnet material, a electro-magnet set up will be quite a bit larger than a equivalent strength Neodymium version.
What equivalent size to a spool of thread are you aiming for?
Max.
 

MisterBill2

Joined Jan 23, 2018
7,732
One interesting concept would be to use that neodymium magnet as the core and then the coil current to add to the field or counteract it by reversing the polarity. Then there would be switchable attractive force.
Of course heating would be even more of a consideration, so the whole concept may not be useful.
But since this is all guesses it does not matter.
 

MaxHeadRoom

Joined Jul 18, 2013
22,036
it should be able to be powered from standard home electricity and ideally emit minimal heat from the coils.
The Heat produced is going to be directly related to the coil resistance, so the wattage can easily be calculated.
When you say "standard Home Electricity'" what are you referring to?
How are you powering them?
Not mains AC surely? o_O
Max.
 

MisterBill2

Joined Jan 23, 2018
7,732
Here is a radical idea, which is that instead of magnets, the TS consider commercially available solenoids. That would both remove much of the needed experimentation and also provide a ready-made and safer device. Ofcourse, they will still get hot.
 

MrChips

Joined Oct 2, 2009
22,536
Let's back up for a moment.
What is the application?
If it is for picking up objects, as MisterBill already said, the attractive force decreases with the square of the distance.
If it is for motion as in a plunger solenoid then that is a different story.
 

Thread Starter

EasyGoing1

Joined Dec 10, 2016
14
The magnetic force is directly proportional to the current and the number of turns. Hence you need a very large number of turns.

The reason the coil gets hot is because of the resistance of the wire.
Hence you need very fat wires to keep the resistance low.

You will end up with a very big coil.
And a lot of juice to power it. I've built coils with 18 gauge wire and I didn't have enough power to get any decent strength from it.


You can calculate an electromagnet's force if you know the wire gauge and length, number of turns around the core, and the core's electromagnetic properties. There probably are online calculators for this.

ak
It would be nice to be able to calculate the wire gauge, the number of turns, and total height, based on desired strength, core material and available voltage, but I can imagine that coming up with such an equation would be damn difficult.

Here is a radical idea, which is that instead of magnets, the TS consider commercially available solenoids. That would both remove much of the needed experimentation and also provide a ready-made and safer device. Ofcourse, they will still get hot.
I've contacted over 10 manufacturers of coils and provided them all with the parameters of my project and only two replied and both had nothing helpful to tell me. I was only asking them which coil they manufacture could accommodate my needs so I can purchase the coils. Either my letter was too long, or I wasn't a large enough of a purchaser for them to even pay attention to me.

The resultant magnetic field of an electromagnet with a ferromagnetic core is not linear with current. Once the core reaches manetic saturation the field will increase much more slowly with increasing current.
https://en.wikipedia.org/wiki/Electromagnet#High-field_electromagnets
That's interesting, I did not know this ... what this means, at least the way I understand it, is that once you reach some optimal level of current, increasing it does not provide enough additional magnetic strength to justify the added current. This means there is most likely a "sweet spot" for any given coil in terms of the current used to power it - at least at room temperature... If you add cooling to the equation, then obviously that sweet spot changes.

The Heat produced is going to be directly related to the coil resistance, so the wattage can easily be calculated.
When you say "standard Home Electricity'" what are you referring to?
How are you powering them?
Not mains AC surely? o_O
Max.
What I mean by "standard home electricity" is that I should be able to plug it into a 110 outlet and get what I need since I'm not a corporation who might be able to afford non-standard electric service such as a company that might play with very large electromagnets and hence purchases unusually high voltage from the electric company.

One interesting concept would be to use that neodymium magnet as the core and then the coil current to add to the field or counteract it by reversing the polarity. Then there would be switchable attractive force.
Of course heating would be even more of a consideration, so the whole concept may not be useful.
But since this is all guesses it does not matter.
I had actually considered this but quickly turned away from it since I need to also invert the magnetic field several times per second. The core would just oppose the reversal of poles half the time.

Let's back up for a moment.
What is the application?
If it is for picking up objects, as MisterBill already said, the attractive force decreases with the square of the distance.
If it is for motion as in a plunger solenoid then that is a different story.
Well, I was hoping to avoid this discussion ... trying to keep it simple which is why I kept my question focused on the strength of a standard neodymium magnet.

BUT, since you asked, what I am trying to do is build a magnetic stirrer that can handle viscous liquids without difficulty. I've built one before, using a motor and a neodymium bar magnet as the tracker. I centered the bar magnet on a motor post and mounted it under a plastic project box. Then on top of the box, I had a glass beaker with the solution in it along with a Teflon coated magnet that would spin as the bar magnet spun. The problem was, that with the viscosity of the liquid that I'm mixing, I could not attain decent RPMs without the Teflon coated pill losing attraction to the bar magnet and skipping around inside the solution.

I have already successfully done what I am trying to do here, but the attraction of the pill to the electromagnets is no where near strong enough yet.

Here is a video that I made last week of my current progress.

See this is where my ignorance comes into play. How many 'N' would a magnet like this be rated at? Is N52 the same as 52N in this context?
 

Thread Starter

EasyGoing1

Joined Dec 10, 2016
14
As I started looking at these coils more, I noticed that on some of the Amazon items, there is this picture

Screen Shot 2020-12-27 at 6.18.33 PM.png

And If I'm reading this correctly, it seems like once a current is applied to the coil, the magnetic field has a "lingering" effect in that it takes time for it to dissipate once you remove the current from the coil. This tells me that a current applied to the coil magnetizes the core and that magnetism doesn't go away by simply removing the current.

This could be a problem for me since I need to reverse the polarity of the coil up to 20 times per second and it seems to me that the core of these coils would resist a change in polarity such that in my application, I would end up getting no magnetic strength from it at all.
 

AlbertHall

Joined Jun 4, 2014
10,780
And If I'm reading this correctly, it seems like once a current is applied to the coil, the magnetic field has a "lingering" effect in that it takes time for it to dissipate once you remove the current from the coil. This tells me that a current applied to the coil magnetizes the core and that magnetism doesn't go away by simply removing the current.
This is about heat, not magnetism. The off time gives the coil time to cool down the magnetism will disappear quickly.
 
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