Electromagnet Coil Performing Poorly

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Main Question or Discussion Point

I recently posted about an air cored electromagnet I was designing. It currently uses 300' of 17 AWG magnet wire, coiled around a .5'' diameter tube. I purchased a DC to DC step up to test different voltages and currents. I noticed while using it that regardless of what current I supplied, the magnet got no stronger. I also tried to increase the voltage and there was little result. I got up to 40V 7A. Preformed similarly to 12V 2A.

Honestly hit a wall here. At this point I'm about to take apart a microwave to try to make a magnet out of the transformer's wires. I have no clue why my coil will not work. Some have suggested thinner wire with more turns but that still doesn't explain why increasing the current on my current build isn't working. Im also not sure what gauge wire to select if I were to switch.



Edit- I will be uploading photo of the set up shortly.
 
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Answers and Replies

  • #2
gleem
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Preformed similarly to 12V 2A.
How are you measuring the magnetic field? How similarly? FYI the B field outside a solenoid along its length is negligible. It is strongest at the end opening. The strength of a magnetic field is determined by the concentration of the magnetic field lines. Since the field lines are closed, the concentrated field line inside the solenoid are spread out over all of the exterior space thus reducing the field strength outside significantly.
 
  • #3
berkeman
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I recently posted about an air cored electromagnet I was designing. It currently uses 300' of 17 AWG magnet wire, coiled around a .5'' diameter tube. I purchased a DC to DC step up to test different voltages and currents. I noticed while using it that regardless of what current I supplied, the magnet got no stronger.
I don't remember if I already posted the following comments in your first thread or not:

https://www.physicsforums.com/threads/i-have-been-trying-to-create-an-air-core-electromagnet-with-no-luck.981479/

but you are not going to make a strong electromagnet with an air-core solenoid. Why are you using an air core if you want a strong electromagnet?

And to get a strong electromagnet, you need the two poles of your electromagnet close and co-planar. That minimizes the air gap from your ferrous core to the ferrous metal piece you are trying to attract/hold.

https://imgaz.staticbg.com/thumb/large/upload/2012/lidanpo/SKU092627/SKU092627 (1).JPG

1578951648766.png
 
  • #4
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How are you measuring the magnetic field? How similarly? FYI the B field outside a solenoid along its length is negligible. It is strongest at the end opening. The strength of a magnetic field is determined by the concentration of the magnetic field lines. Since the field lines are closed, the concentrated field line inside the solenoid are spread out over all of the exterior space thus reducing the field strength outside significantly.
I wasn't measuring anything in an intellegent way. I had a thin steel rod in the tube that I was holding with my hand, while I increased the amperage
 
  • #5
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I wasn't measuring anything in an intellegent way. I had a thin steel rod in the tube that I was holding with my hand, while I increased the amperage
Could you please provide the following data?

How much force do you actually expect from the electromagnetic coil? 10 or 20 or more grams force?

How thin is the steel rod you use? What is its diameter?

Do you know how many turns have actually been wound on the tube?

What are the lengths of the coils and the steel rod?

...etc
 
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  • #6
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Could you please provide the following data?

How much force do you actually expect from the electromagnetic coil? 10 or 20 or more grams force?

How thin is the steel rod you use? What is its diameter?

Do you know how many turns have actually been wound on the tube?

What are the lengths of the coils and the steel rod?

...etc
The rod was more than half the diameter of the tube, filled most of it up. 1200 turns. the coil is six inches long, the rod was maybe a foot. Im not sure how powerful I am expecting it to be but I would hope 1200 of feet of 17AWG being powered by well over 30V would be at least more powerful than a fridge magnet, if not much more.
 
  • #7
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My humble opinion is the same as it was way back: an external 'core' would help you a lot.
Try to put the coil inside a steel tube (of similar length as the coil).
 
  • #8
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Because the electromagnetic force of the electromagnetic coil is proportional to the square of the current, it is inexplicable that the force remained almost the same when the current increased from 2A to 7A, that is, (7/2) ^ 2 = 12 times greater than before. 🤔

If you applied 40V 7A which is equal to 280W into the coil, it is very unreasonable if the device doesn't even produce a force greater than a mini fridge magnet, especially the size of your solenoid actuator seems to be already comparatively large.🤔

Since the force should change with the depth of the steel rod pushed into the hollow center of the aluminum tube, have you found out the depth that produces the greatest force?

Have you verified that the magnetic properties of the steel rod are good enough?
You might just use a fridge magnet to test and compare the forces produced by the steel rod and iron, which may help ensure that the steel rod can conduct magnetic flux no less than iron.

Besides, using a magnetically permeable frame to reduce external reluctance or increase the cross-sectional area of the steel rod should also help.:smile:
 
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  • #9
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Because the electromagnetic force of the electromagnetic coil is proportional to the square of the current, it is inexplicable that the force remained almost the same when the current increased from 2A to 7A, that is, (7/2) ^ 2 = 12 times greater than before. 🤔

If you applied 40V 7A which is equal to 280W into the coil, it is very unreasonable if the device doesn't even produce a force greater than a mini fridge magnet, especially the size of your solenoid actuator seems to be already comparatively large.🤔

Since the force should change with the depth of the steel rod pushed into the hollow center of the aluminum tube, have you found out the depth that produces the greatest force?

Have you verified that the magnetic properties of the steel rod are good enough?
You might just use a fridge magnet to test and compare the forces produced by the steel rod and iron, which may help ensure that the steel rod can conduct magnetic flux no less than iron.

Besides, using a magnetically permeable frame to reduce external reluctance or increase the cross-sectional area of the steel rod should also help.:smile:
I will try to test with different materials. I really am unsure why the magnet isn't getting strong. I don't think I wound it wrong, all the layers are wound clockwise. One thing that may affect it is that I am coiling around a 6 inch section of the tube, while the aluminum tube is 2 feet long.
 
  • #10
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I will try to test with different materials. I really am unsure why the magnet isn't getting strong..
Let's roughly estimate how much force the device will generate.

Coil.png

Equation : F = ((NI)^2 u0 A) / (2g)

Assumptions : -
N = Nos. of turns = 1200
I = Current = 2A
uo = permeability of free space = 1.257 *10^-6
A = cross-sectional area of the steel rod = 50mm^2 = 5*10^-5 m^2
g = air gap as show on the image = 20mm = 0.02m
The magnetic reluctance in the space outside the coil is assumed to be small, so its effect can be ignored

Result : -
F = 0.455N = 46 gram-force

By the way, I hope that the structure shown on the image correctly describes the device you are using.
 
  • #11
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Now we can see the shortcoming of not installing an external magnetically permeable return path, normally the force should be much and much stronger when the gap (g) length is shortening. However, the effective magnetic reluctance in the outside space is actually not so small when there is no dedicated return path, therefore, the outside space magnetic reluctance will become comparable to the gap magnetic reluctance as the gap length reduced to a certain extent, and as a result, the expected force at this time will be reduced.
 
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