#### trainwreck

Joined Apr 3, 2023
9
Hello, this is my first post here so if I am in the wrong place please let me know!

I am trying to learn about electromagnets, and so decided to just make one and try things out as it seems simple enough. I am using 18awg magnet wire wrapped around a relatively large 3" diameter by 5" tall mild steel core. I wasn't thinking when I did the winding so I didn't count the turns but the math would indicate that I have somewhere between 150 and 200 turns based on 207 ft of magnet wire.

So, my questions that I have come across so far, in no particular order:
1. Does the "neatness" of the winding have any effect on the strength of the magnet? For example, some turns overlapping or crossing over other turns, the coil not being symmetrical I.E. bulging in the middle and thinner on the ends, etc?
2. I have noticed that the magnet is significantly stronger at the edges of the core near the windings than in the middle. Does this mean that the core is far from saturation and I need either more windings, more current, or a smaller core?
3. Is there any kind of ideal shape or aspect ratio I should be shooting for if my goal is to maximize the distance at which the magnet can attract from?
1. Secondarily to the above, is the attraction distance just a function of field strength or is it a separate property entirely?
4. Would I be better of using smaller magnet wire to get significantly more turns? The equations I've come across online imply that both current and number of turns are equally impactful on the overall strength of the magnet.

This is all I have so far, but I'm sure as I continue to experiment I will have many more. Thanks for reading and I appreciate any input I receive!

#### crutschow

Joined Mar 14, 2008
32,840
For maximum attraction you want the magnetic circuit to be completed through the item you want to attract, so both poles of the magnet should be at the same end, such as u-shape (horseshoe).
Generally the most efficient configuration is to have the u-shape in a circle (below):

The magnet field strength is proportional to ampere-turns (winding amperes times number of winding turns) so an increase in either (or both) will increase the magnetic strength (until the core saturates).
Typically many turns of fine wire are used to minimize the current required for a given field strength.

#### ZCochran98

Joined Jul 24, 2018
273
Welcome to AAC!

I'll try to answer those questions as best I can.
• It's hard to quantify the effect of the "neatness" of the coil, as the net magnetic field will still be mostly as expected of a "neat" electromagnet. For the net effect, as long as the wire is coiled in the same direction (either clockwise or counter-clockwise) around the core, the axial field will still be strong. The neatness, however, will help improve future windings and assert that more of the field IS aligned axially along the core.
• By "at the edges of the core" I take it to mean you are referring to the end of the core? Like, if the core is a cylindrical, then the "edge of the core" is at one of the ends? If so, then the reason for that is because most of the magnetic field is pointed axially along the length of the core, due to the circular windings. Because the windings form a circle, far away from the core ("far," being a relative term) you have opposing magnetic fields cancelling each other out, but along the axis of the circles the magnetic field is always facing in one way, so it makes it stronger along the axis. Whether or not the core is near or far from saturation depends on a number of other factors (mostly the current and the material of the core), so it's hard to say, but the strength at the ends/edges of the core vs at the side of the core makes sense from a physics point of view.
Now, if you mean by "edges" that the field is stronger at the corner of the top/bottom of the core, then that also makes sense, as field lines tend to "bunch up" around corners (the math behind it is a little more complicated). This is true for electrostatics (if you have a constant potential on the rod the electric field) and with magnetics (like in your case).
• @crutchshow beat me to the comment about making it stronger in one(ish) direction by turning it into a horseshoe
• Also to @crutchshow's point, the more turns you can cram into the same space, the higher the magnetic field will be for a given amount of current (as current density is the easiest-to-control factor for making a magnet). HOWEVER, be warned: smaller wire = smaller amount of current the wire can handle without burning!

EDIT: A little while back I had a post about Helmholtz coils and the mathematics surrounding them (in particular, rectangular ones). While most of that post is irrelevant to your question, the colorplot I had in it of the magnetic field strength may be of interest to your second question. That colorplot is of a top-down view of the coil, and while it's rectangular, the idea about magnetic field strength near the edges being stronger and weaker near the center is demonstrated there, too, purely through some (painful) math. I had actually almost forgotten about that post.

#### trainwreck

Joined Apr 3, 2023
9
• By "at the edges of the core" I take it to mean you are referring to the end of the core? Like, if the core is a cylindrical, then the "edge of the core" is at one of the ends? If so, then the reason for that is because most of the magnetic field is pointed axially along the length of the core, due to the circular windings. Because the windings form a circle, far away from the core ("far," being a relative term) you have opposing magnetic fields cancelling each other out, but along the axis of the circles the magnetic field is always facing in one way, so it makes it stronger along the axis. Whether or not the core is near or far from saturation depends on a number of other factors (mostly the current and the material of the core), so it's hard to say, but the strength at the ends/edges of the core vs at the side of the core makes sense from a physics point of view.
Now, if you mean by "edges" that the field is stronger at the corner of the top/bottom of the core, then that also makes sense, as field lines tend to "bunch up" around corners (the math behind it is a little more complicated). This is true for electrostatics (if you have a constant potential on the rod the electric field) and with magnetics (like in your case).
I did indeed mean that the field is stronger near the circumference of the end of the cylinder that is the core of the magnet. I don't know why, but I was expecting it to be strongest in the center, perhaps a poorly informed intuition based on the thought that the field would be the most "balanced" in the center or some other such thing.

I also didn't consider a U shape magnet. So effectively, it seems like the best thing for me to do would be too take my core to the lathe and bore a groove into the end that the magnet wire coil would sit in. Sounds like I have new things to try and new magnet wire to buy!

Thanks for the answers, expect updates in a few days or so!

#### crutschow

Joined Mar 14, 2008
32,840
bore a groove into the end that the magnet wire coil would sit in
Not sure what you mean.
The wire needs to go around the magnetic material not sitting in it.
Here's an example:
Note the winding directions.

#### trainwreck

Joined Apr 3, 2023
9
Not sure what you mean.
The wire needs to go around the magnetic material not sitting in it.
Here's an example:
Note the winding directions.
View attachment 291385
The original picture you posted showed a groove in the end of a bar of steel with the windings sitting in that groove, like this.

#### WBahn

Joined Mar 31, 2012
29,130
Hello, this is my first post here so if I am in the wrong place please let me know!

I am trying to learn about electromagnets, and so decided to just make one and try things out as it seems simple enough. I am using 18awg magnet wire wrapped around a relatively large 3" diameter by 5" tall mild steel core. I wasn't thinking when I did the winding so I didn't count the turns but the math would indicate that I have somewhere between 150 and 200 turns based on 207 ft of magnet wire.
So #18 AWG magnet wire is about 7 Ω / 1000 ft, so the DC resistance of your coil would only be about 1.4 Ω. The current rating is about 1.2 A, so if you are applying anything over about 1.5 V, you might be asking for trouble with overheating.

You might consider going with smaller wire and a lot more turns.

To first order, the power is going to be proportional to the ampere-turns, so it is largely a wash. But delivering higher current at low voltage is often harder than delivering the same power at a lower current with a higher voltage (within reason).

#### crutschow

Joined Mar 14, 2008
32,840
The original picture you posted showed a groove in the end of a bar of steel with the windings sitting in that groove, like this.
View attachment 291387
So what's the size of this bar of steel?

#### MrChips

Joined Oct 2, 2009
29,187
The number of turns ought to be in the thousands.
Hence 18AWG is going to make a very large coil. I would think that you want about 28-32AWG.
Resistance of 28AWG is 65Ω per 1000ft.
Resistance of 32AWG is 164Ω per 1000ft.

#### trainwreck

Joined Apr 3, 2023
9
So what's the size of this bar of steel?
What I'm using now is 3" diameter, but I have a machine shop so I can make pretty much any size and shape needed. Would it be better for me to use a smaller diameter but longer core? I went with wide and short originally because I was thinking of junkyard magnets and that seems to be the shape they always are.

#### Janis59

Joined Aug 21, 2017
1,717
About how far You are from saturation tells formula Ampers*turns=B*[L(c)/mjumju(0))+L(air)/mju(0) where for mild steel mju is 4400, mju(0) everywhere is 12.6E-7, B(max) for steel is 0.8...1.0 Teslas, L(c) is magnetic path into core (meters) and L(air) ir air gap (meters).

Thus, maximum what iron may bear is the 10 kg per each cm2 of core cross section. However, better take a clever reserve, so 5 kg/cm2.

Last edited:

#### shortbus

Joined Sep 30, 2009
9,938
What I'm using now is 3" diameter, but I have a machine shop so I can make pretty much any size and shape needed. Would it be better for me to use a smaller diameter but longer core? I went with wide and short originally because I was thinking of junkyard magnets and that seems to be the shape they always are.
That lifting magnet is just a larger version of what Crutschow posted. To make what you already have work like a lifting magnet the easiest way would be to take a piece of pipe or tube that will fit over your coil. then weld a round washer like part to it, then the hole in the washer would get bolted into a taped hole in the core you already have,

Here is a cutaway of a lifting or sometimes called a pot magnet. You said you have a machine shop, do you have a surface grinder? the electromagnetic chuck on a grinder is made the same way, although the coils are oval shaped not round.

#### DC_Kid

Joined Feb 25, 2008
963
There's ideal, and then there's not ideal (about 100% of real stuff).
To get best efficiency (less wire, more field), use hex wire to remove air gaps in the windings. Not easy to do, but a better e-mag. Hex windings will yield less wire and smaller physical size vs round wire that makes same strength mag field.

Very large e-magnets, like Tgauss items, have stacked windings where each layer is very very thin with very thin insulator between, thus maximizing # of turns, thus max gauss. But for DIY homemade stuff, even with access to machine shop gear, donuts and logs are probably the best you'll achieve.

Now, for further reading, I recommend seeking info from CMMR research places, like this one, but there are many.
https://cmrr.ucsd.edu/resources/secure-erase.html

#### MisterBill2

Joined Jan 23, 2018
15,579
What IS affected by the "neatness" of the windings is the volume of the coil, the capacitance between turns, the resonant frequency, the heat transfer, and possibly the mechanical durability of the coil. Those factors may, or not, matter in some instances.

#### DC_Kid

Joined Feb 25, 2008
963
What IS affected by the "neatness" of the windings is the volume of the coil, the capacitance between turns, the resonant frequency, the heat transfer, and possibly the mechanical durability of the coil. Those factors may, or not, matter in some instances.
Indeed. Other factors do come into play in real applications. DIY home testing trying to make best "e-magnet" is done using tight-neat windings and a good core.

#### MisterBill2

Joined Jan 23, 2018
15,579
If you ever wind transformers it is quite important. At least for most kinds of transformers. Evidently electric fence transformers are slightly different.

#### Duane P Wetick

Joined Apr 23, 2009
440
Hello, this is my first post here so if I am in the wrong place please let me know!

I am trying to learn about electromagnets, and so decided to just make one and try things out as it seems simple enough. I am using 18awg magnet wire wrapped around a relatively large 3" diameter by 5" tall mild steel core. I wasn't thinking when I did the winding so I didn't count the turns but the math would indicate that I have somewhere between 150 and 200 turns based on 207 ft of magnet wire.

So, my questions that I have come across so far, in no particular order:
1. Does the "neatness" of the winding have any effect on the strength of the magnet? For example, some turns overlapping or crossing over other turns, the coil not being symmetrical I.E. bulging in the middle and thinner on the ends, etc?
2. I have noticed that the magnet is significantly stronger at the edges of the core near the windings than in the middle. Does this mean that the core is far from saturation and I need either more windings, more current, or a smaller core?
3. Is there any kind of ideal shape or aspect ratio I should be shooting for if my goal is to maximize the distance at which the magnet can attract from?
1. Secondarily to the above, is the attraction distance just a function of field strength or is it a separate property entirely?
4. Would I be better of using smaller magnet wire to get significantly more turns? The equations I've come across online imply that both current and number of turns are equally impactful on the overall strength of the magnet.

This is all I have so far, but I'm sure as I continue to experiment I will have many more. Thanks for reading and I appreciate any input I receive!
Amperes x # turns gives a good approximation of magnetic strength. Pole geometry is another;ie Deep field vs shallow field. Watch your heat ! 1/2 watt per sq in.of coil dimension (air cooled). Strength will also be a function of gap between poles and work piece. Field gradient will be maximum at edges.

Cheers, DPW [ Spent years making heaters out of op-amps.]

Joined Jul 18, 2013
27,593
What I'm using now is 3" diameter, but I have a machine shop so I can make pretty much any size and shape needed. Would it be better for me to use a smaller diameter but longer core? I went with wide and short originally because I was thinking of junkyard magnets and that seems to be the shape they always are.
That version you show of scrap yard magnets is the same configuration as Post #2, which is used in practically all lift magnets.

#### MisterBill2

Joined Jan 23, 2018
15,579
The fact is that magnetism and electromagnets have been around a long time and so the books that may be obsolete in some aspects still are good for learning about magnetism and electromagnets. We have better magnetic materials now and better insulation for wires, but the math and fields are the same. So there is a lot to be learned from old textbooks, which often are much cheaper.

#### trainwreck

Joined Apr 3, 2023
9
Amperes x # turns gives a good approximation of magnetic strength. Pole geometry is another;ie Deep field vs shallow field. Watch your heat ! 1/2 watt per sq in.of coil dimension (air cooled). Strength will also be a function of gap between poles and work piece. Field gradient will be maximum at edges.

Cheers, DPW [ Spent years making heaters out of op-amps.]
When you say deep field vs shallow field, do you mean how far the field extends from the magnet? What aspects of the design control the depth of the field?

I'm also not certain what you mean by pole geometry? Is it the same as what others have said regarding having both poles pointed the same direction (I.E. a "U" shape)?

I also have another question. How does the shape and size of the coil of wire affect the strength? For example, imagine these are coils of wire wrapped around a core. They both have exactly the same volume, but given that right cylinder has a smaller diameter, that would mean there would be significantly more turns than on the left. If we assumed that the bottom was the "grabbing" surface of the magnet, does the fact that most of the turns of the wire are farther away affect the strength? Does how far away from the core the wires are radially affect the strength?