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

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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.

Main Question or Discussion Point

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?
 

Answers and Replies

  • #2
Charles Link
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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.
 
  • #3
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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 dont 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
 
  • #4
Charles Link
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To clairify, the batteries never heated but the coils got VERY hot. I could try the resistors though. I dont 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.
 
  • #5
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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 lenght of wire actually does have about 0.8 ohms inherently. Im using a 14.2V battery and the voltmeter read only 6V on either side of the coil. The battery is healthy though
 
  • #6
Charles Link
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Ive tested the batteries and they are alright. So a small resistor? 20 ohms? The lenght of wire actually does have about 0.8 ohms inherently. Im 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.
 
  • #7
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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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  • #8
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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.
 
  • #9
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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.
 
  • #10
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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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  • #11
tech99
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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.
 
  • #12
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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.
 
  • #13
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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.
 
  • #14
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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
 
  • #15
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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.
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.
 
  • #16
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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.
 
  • #17
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I think you need many turns of thicker wire, higher voltage and more power
 
  • #18
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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.
 
  • #19
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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?
 
  • #20
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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
 
  • #21
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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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  • #22
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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 cant enter and pass though the middle of the spool. And as you mentioned I have been struggling to select the correct gauge of wire. Im 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.
 
  • #23
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This is the wire not coiled around the aluminum tube.
 

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  • #24
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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?
 
  • #25
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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
 

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