# I have been trying to create an air-core electromagnet with no luck

• NathanSM
In summary: I don't know. The battery may have an internal resistance of just over one ohm, and you may have a current of about 7 amps. That is my best guess=in any case, the wires probably weren't designed to carry 7 amps, and the battery won't last very long if the circuit is drawing 7 amps. If the iron core is iron that readily becomes magnetized, it shouldn't... I don't know.
NathanSM
TL;DR Summary
I am not very experienced and this is my first post on this forum. My electromagnet so far has not been preforming as I expect. All of the maths I've done so far projects it as being much stronger than it is.
The magnet I have been created has been coiled around an aluminum tube. This tube has an OD of .5'' and an ID of .375''. This leaves 0.125'' as the thickness of the aluminum. The electromagnet was created with 17 AWG magnet wire. This wire is 157'long and has a diameter of 0.0469''. It is coiled around a 3'' length of the tube. The battery powering the circuit is two 7.1 Volt, 3000 mah batteries that were put into series to create a 14.2 volt battery.

To summarize:
507 feet of 0.0496'' magnet wire
3'' long, .5'' outer diameter aluminum tube
14.2 Volts Applied for a short duration
...
I am rather inexperienced and I have been struggling to find some of the information I feel is required to build a proper electromagnet. I intended to build a magnet based off of the batteries that I already had. I used this calculator to determine much of what I needed.

http://production-solution.com/coil-calculator.htm

After realizing just how weak the magnet was I looked back at that calculator to find my error. I noticed that the calculator was for iron cored magnets while I was building an air cored magnet. I tried to wrap the wire around an iron core and saw little improvement. I also tried to do all of the math that the calculator preforms automatically but I could not find all of the formulas necessary. The magnet itself heats up quit a bit after staying plugged in for a while, however for its intended use it only needs very temporary activation. Any ideas as to why my magnet is such a failure?

I would recommend a small resistor in series in the circuit= about ten or twenty ohms. Without it, you could drain the batteries very quickly. If your electromagnet doesn't do well with an iron core, I think the batteries might have been drained. The wire you use needs to be insulated=presumably it is.

tech99
I would recommend a small resistor in series in the circuit= about ten or twenty ohms. Without it, you could drain the batteries very quickly. If your electromagnet doesn't do well with an iron core, I think the batteries might have been drained. The wire you use needs to be insulated=presumably it is.
To clairify, the batteries never heated but the coils got VERY hot. I could try the resistors though. I don't know how fast the batteries are discharging though. I can leave it plugged in for a while and it will continue to hold its force

NathanSM said:
To clairify, the batteries never heated but the coils got VERY hot. I could try the resistors though. I don't know how fast the batteries are discharging though. I can leave it plugged in for a while and it will continue to hold its force
It would help to have a voltmeter to test the batteries, but a light bulb that works at about 12 volts would also do the job. When the light bulb gets a lot dimmer, you know the batteries are losing their charge. In any case, it is important to have a resistor in series in the circuit. Otherwise, basically the only resistance is the internal resistance of the batteries. You could even use the light bulb as your series resistor.

It would help to have a voltmeter to test the batteries, but a light bulb that works at about 12 volts would also do the job. When the light bulb gets a lot dimmer, you know the batteries are losing their charge. In any case, it is important to have a resistor in series in the circuit. Otherwise, basically the only resistance is the internal resistance of the batteries.
Ive tested the batteries and they are alright. So a small resistor? 20 ohms? The length of wire actually does have about 0.8 ohms inherently. I am using a 14.2V battery and the voltmeter read only 6V on either side of the coil. The battery is healthy though

NathanSM said:
Ive tested the batteries and they are alright. So a small resistor? 20 ohms? The length of wire actually does have about 0.8 ohms inherently. I am using a 14.2V battery and the voltmeter read only 6V on either side of the coil. The battery is healthy though
The battery may have an internal resistance of just over one ohm, and you may have a current of about 7 amps. That is my best guess=in any case, the wires probably weren't designed to carry 7 amps, and the battery won't last very long if the circuit is drawing 7 amps. If the iron core is iron that readily becomes magnetized, it shouldn't need 7 amps of currents in the coils if you have enough turns per unit length, to make a strong electromagnet. How many turns do you have in the coil? (=approximately). A good number would be at least ten turns per centimeter, and twenty turns per centimeter would be better.

NathanSM said:
Summary:: I am not very experienced and this is my first post on this forum. My electromagnet so far has not been preforming as I expect. All of the maths I've done so far projects it as being much stronger than it is.

After realizing just how weak the magnet was I looked back at that calculator to find my error. I noticed that the calculator was for iron cored magnets while I was building an air cored magnet. I tried to wrap the wire around an iron core and saw little improvement. I also tried to do all of the math that the calculator preforms automatically but I could not find all of the formulas necessary. The magnet itself heats up quit a bit after staying plugged in for a while, however for its intended use it only needs very temporary activation. Any ideas as to why my magnet is such a failure?

I have the following suggestions.

First, try to measure the current flowing through the electromagnet to ensure that the battery can actually provide the required amount of current in the short period of time.

Second, try to perform a complete calculation to determine the magnetic flux flowing through the closed loop of the magnetic circuit, and the magnetic flux density of each segment in the loop. This needs to consider all the magnetic properties of different material segments in the entire magnetic circuit, then estimate the actual magnetic flux density and electromagnetic force in the gap between the iron core and the arm contact.

There may be other factors to consider, such as hysteresis loop and saturation characteristics..etc.

Hope your electromagnet will eventually produce the required force and work as expected

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alan123hk said:
I have the following suggestions.

First, try to measure the current flowing through the electromagnet to ensure that the battery can actually provide the required amount of current in the short period of time.

Second, try to perform a complete calculation to determine the magnetic flux flowing through the closed loop of the magnetic circuit, and the magnetic flux density of each segment in the loop. This needs to consider all the magnetic properties of different material segments in the entire magnetic circuit, then estimate the actual magnetic flux density and electromagnetic force in the gap between the iron core and the arm contact.

There may be other factors to consider, such as hysteresis loop and saturation characteristics..etc.

Hope your electromagnet will eventually produce the required force and work as expected
I have tried the first suggestion, and the batteries should be able to exceed the draw of the coil. For the second suggestion I've got some studying to do. Ill have to find all of the formulas online and figure out what they all mean. Definetly a hastle but I'll learn some and it will certainly be impressive to record. However the coil does not have an iron core.

The battery may have an internal resistance of just over one ohm, and you may have a current of about 7 amps. That is my best guess=in any case, the wires probably weren't designed to carry 7 amps, and the battery won't last very long if the circuit is drawing 7 amps. If the iron core is iron that readily becomes magnetized, it shouldn't need 7 amps of currents in the coils if you have enough turns per unit length, to make a strong electromagnet. How many turns do you have in the coil? (=approximately). A good number would be at least ten turns per centimeter, and twenty turns per centimeter would be better.
I will have to look into the internal resistance, but the wire is 17 AWG, and it only has to support high amperages for very short amounts of time. It is somewhere are 640 turns with nearly 10 layers of wire.

A useful link is as follows:

https://en.wikipedia.org/wiki/Force_between_magnets

Force between two nearby magnetized surfaces of area A = [(B^2)*A]/(2*μ0)

A is the area of each surface, in m2
μ0 is the permeability of space, which equals 4π×10−7 T·m/A
B is the flux density, in T

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You could use just one 7.2 volt battery perhaps, to reduce the current. You need a "soft iron" core that fills the centre of the coil if possible. An ordinary black steel or iron bolt is not so good but should also work.

NathanSM said:
17 AWG... somewhere are 640 turns with nearly 10 layers of wire.
Could you please upload a photo about the coil and the core together? Sounds like a very special design: easy to miss the point with those.

On the coil the actual DC current will be limited by the resistance only, and you need very specific wire diameter to be able to bear that current (heat production) once it is made into a coil. What you wrote so far sounds like a pretty unbalanced setup: too low resistance for not enough turns => high current (likely with a decent voltage drop on the battery), but not enough strength at the end.

Air core electromagnets are wimpy performers in comparison with ferromagnetic cores magnets. Takes a lot of wire and many turns. With low voltage power source and thin magnet wire don't expect good results.

zoki85 said:
Air core electromagnets are wimpy performers in comparison with ferromagnetic cores magnets. Takes a lot of wire and many turns. With low voltage power source and thin magnet wire don't expect good results.
Air cored is essential to my design unfortunately

tech99 said:
You could use just one 7.2 volt battery perhaps, to reduce the current. You need a "soft iron" core that fills the centre of the coil if possible. An ordinary black steel or iron bolt is not so good but should also work.
Using only one 7.2 battery was extremely weak, which is why I tried putting two in series. Unfortunately I can not use an iron core.

Rive said:
Could you please upload a photo about the coil and the core together? Sounds like a very special design: easy to miss the point with those.

On the coil the actual DC current will be limited by the resistance only, and you need very specific wire diameter to be able to bear that current (heat production) once it is made into a coil. What you wrote so far sounds like a pretty unbalanced setup: too low resistance for not enough turns => high current (likely with a decent voltage drop on the battery), but not enough strength at the end.
I will try to get a picture as soon as I can, may be on Sunday. So are you saying I need either many more turns or wire or a smaller gauge of wire that supplies more resistance? I used that calculator to determine I needed 17 gauge, but I'm assuming there must be another way to get a more fitting gauge wire.

I think you need many turns of thicker wire, higher voltage and more power

zoki85 said:
I think you need many turns of thicker wire, higher voltage and more power
Possibly. The wire is heating up a lot, but as you get thicker wire, the price for enameled wire goes up quickly. One other poster said the wire may have had too little resistance, and larger wire has less resistance.

NathanSM said:
So are you saying I need either many more turns or wire or a smaller gauge of wire that supplies more resistance?
You have a given cross section available for an usual coil. You can fill up that cross section with many turns of thin wire, or less turns of thick wire: as long as you are within the same cross section, you will have a ~ constant current/surface there. Thin/thick in this case determines the resistance, and through that the so called 'voltage' of the coil.

AWG 17 feels like an absolute overkill for 14V, in usual (electronic) application. What were you aiming for with your calculations? Some kind of actuator, so you need this to pull something?

Rive said:
You have a given cross section available for an usual coil. You can fill up that cross section with many turns of thin wire, or less turns of thick wire: as long as you are within the same cross section, you will have a ~ constant current/surface there. Thin/thick in this case determines the resistance, and through that the so called 'voltage' of the coil.

AWG 17 feels like an absolute overkill for 14V, in usual (electronic) application. What were you aiming for with your calculations? Some kind of actuator, so you need this to pull something?
So are you saying regardless of the gauge the resistance will be determined by the cross section of the entire coil of wire, and not the actual AWG of the wire? And by overkill do you mean too low (large) gauge of wire? And yes, I do need this to pull a good amount of weight

NathanSM said:
So are you saying regardless of the gauge the resistance will be determined by the cross section of the entire coil of wire, and not the actual AWG of the wire?
No: the available 'force' (flux) will be determined by the cross section (in a given setup), and you match the coil to the voltage source by selecting the right thickness for the wire, according to the acceptable current density for the setup. Maybe you should take a look on transformers, how this is done there.

You can boost the pulling force of an air-core coil by making a core on the outside of the coil: like here.

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Rive said:
No: the available 'force' (flux) will be determined by the cross section (in a given setup), and you match the coil to the voltage source by selecting the right thickness for the wire, according to the acceptable current density for the setup. Maybe you should take a look on transformers, how this is done there.

You can boost the pulling force of an air-core coil by making a core on the outside of the coil: like here.
I have looked at one air core coil design in which the coil is surrounded by iron but I saw no benefit in that. The plunger idea makes sense but I need the center of the coil to remain open so that objects can't enter and pass though the middle of the spool. And as you mentioned I have been struggling to select the correct gauge of wire. I am not sure how I am supposed to find what current density I need for the strength I want, and consequentially, I do not know what gauge of wire I require.

This is the wire not coiled around the aluminum tube.

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NathanSM said:
I have looked at one air core coil design in which the coil is surrounded by iron but I saw no benefit in that. ..
Why not?
Seems you are avoiding presence of the iron at any cost...
BTW, have you considered multi-layer wound coil design on your coil form?

zoki85 said:
Why not?
Seems you are avoiding presence of the iron at any cost...
BTW, have you considered multi-layer wound coil design on your coil form?
The coil is multi layer, that is my mistake for not specifying

NathanSM said:
This wire is 157'long and has a diameter of 0.0469''. It is coiled around a 3'' length of the tube. The battery powering the circuit is two 7.1 Volt, 3000 mah batteries that were put into series to create a 14.2 volt battery.

To summarize:
507 feet of 0.0496'' magnet wire
3'' long, .5'' outer diameter aluminum tube
14.2 Volts Applied for a short duration

But my point is why not to make a multi-layer coil using all 507' of AWG 17 wire and more batteries in PSU

zoki85 said:
But my point is why not to make a multi-layer coil using all 507' of AWG 17 wire and more batteries in PSU
Oh wow, I did not mean to type 507. 157 is all I've got and it is wraped over a three inch length of tube. I could add more batteries but it already seems like the coils are getting very hot without really making that strong of a field.

NathanSM said:
To clairify, the batteries never heated but the coils got VERY hot.

of course they do, high current through large diameter / low resistance wire

zoki85 said:
I think you need many turns of thicker wire, higher voltage and more power

No, lots more turns of thinner wire

NathanSM said:
I will have to look into the internal resistance, but the wire is 17 AWG, and it only has to support high amperages for very short amounts of time. It is somewhere are 640 turns with nearly 10 layers of wire.

OK you need much finer wire

24 - 28 SWG and upwards of 1000 turns

Most small electromagnets, say ones in relays, are 26 - 30 SWG/AWG and several 1000 turns

you are way off the mark with the wire size you are using

Merlin3189
davenn said:
of course they do, high current through large diameter / low resistance wire
No, lots more turns of thinner wire
OK you need much finer wire

24 - 28 SWG and upwards of 1000 turns

Most small electromagnets, say ones in relays, are 26 - 30 SWG/AWG and several 1000 turns

you are way off the mark with the wire size you are using
I understand what you are saying, The electromagnet does need to apply a good bit of force, and I want to learn what gauge of wire I need give the voltage/battery

davenn said:
OK you need much finer wire

24 - 28 SWG and upwards of 1000 turns

Most small electromagnets, say ones in relays, are 26 - 30 SWG/AWG and several 1000 turns

you are way off the mark with the wire size you are using
and with battery bank voltage too

NathanSM said:
, and I want to learn what gauge of wire I need

you should be aiming for a coil resistance of around 100 Ohms
A 12V relay coil is typically around 250 - 300 Ohms and more than 1000 turns

davenn said:

you should be aiming for a coil resistance of around 100 Ohms
A 12V relay coil is typically around 250 - 300 Ohms and more than 1000 turns
I meant "I want to learn how to determine what gauge I need" Does this mean I could use any combination of length/gauge that would reach 100 ohms? How do I figure out how many ohms are required?

The field strength of a solenoid is proportional to (N×I)2, where
N is the number of turns
I is the current in Amps.

So you want to get the product of N and I as high as your physical design will allow. For instance that may be 10 turns at 100A or 2000 turns at 0.5A.

Of course the wire size needed depends on your choice of current.

And the voltage needed depends on the chosen current and on the resistance (length) of wire in the winding.

Also keep in mind that the magnetic field strength decreases with the square of the distance from the coil.

A quick search found some calculators and a discussion related to the above. They have 'Area' as a variable, which seems to be the cross section area of what you are trying to pick up/move (as long as it fits in the core of the solenoid), and a variable 'g' which is the distance from the solenoid to what you are trying to attract.

Note: My knowledge of this subject is limited, so these calculators could be completely wrong too! Hopefully others here can chime in on their relevance.

https://daycounter.com/Calculators/Magnets/Solenoid-Force-Calculator.phtml
http://onlinecalculators.brainmeasures.com/Electric/SolenoidCoil.aspx
https://www.eevblog.com/forum/beginners/solenoid-force-calculations/

For further information, above found with:

Hopes this helps!

Cheers,
Tom

p.s. I can see how that external core that @Rive suggested would help, alot, when the target object is already inside the coil by concentrating the field there. I believe however that this would reduce/eliminate the external field of the solenoid. From your brief description, it sounds like this is not what you are after.

Tom.G said:
The field strength of a solenoid is proportional to (N×I)2, where
N is the number of turns
I is the current in Amps.

So you want to get the product of N and I as high as your physical design will allow. For instance that may be 10 turns at 100A or 2000 turns at 0.5A.

Of course the wire size needed depends on your choice of current.

And the voltage needed depends on the chosen current and on the resistance (length) of wire in the winding.

Also keep in mind that the magnetic field strength decreases with the square of the distance from the coil.

A quick search found some calculators and a discussion related to the above. They have 'Area' as a variable, which seems to be the cross section area of what you are trying to pick up/move (as long as it fits in the core of the solenoid), and a variable 'g' which is the distance from the solenoid to what you are trying to attract.

Note: My knowledge of this subject is limited, so these calculators could be completely wrong too! Hopefully others here can chime in on their relevance.

https://daycounter.com/Calculators/Magnets/Solenoid-Force-Calculator.phtml
http://onlinecalculators.brainmeasures.com/Electric/SolenoidCoil.aspx
https://www.eevblog.com/forum/beginners/solenoid-force-calculations/

For further information, above found with:

Hopes this helps!

Cheers,
Tom

p.s. I can see how that external core that @Rive suggested would help, alot, when the target object is already inside the coil by concentrating the field there. I believe however that this would reduce/eliminate the external field of the solenoid. From your brief description, it sounds like this is not what you are after.
Wow this really helps a lot. I should clarify that the object attracted to the magnet is passing through the center of the coils, so yes, I could surround the coil in iron to contain the field. What is the field strenght of the coil measured in? Tesla? Also I'm am struggling with which gauge of wire to select as I all really know is the voltage of the battery I have.

NathanSM said:
I meant "I want to learn how to determine what gauge I need" Does this mean I could use any combination of length/gauge that would reach 100 ohms?

That would be a minimum and it is more to stop the coil getting too hot because of high current flow
and draining the battery(s)

NathanSM said:
How do I figure out how many ohms are required?

The Ohms required isn't your main requirement

In post #33, @Tom.G gave you the main requirements

The field strength of a solenoid is proportional to (N×I)2, where
N is the number of turns
I is the current in Amps.
NathanSM said:
I could surround the coil in iron to contain the field.

the physics of the coil will already contain the magnetic field reasonably well ( sort of)
putting the coil in a metal box isn't going to help concentrate the magnetic field.Only an iron core will do that

NathanSM said:
What is the field strenght of the coil measured in? Tesla?

absolutely tiny ... microTesla or less

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