Electromagnet with E shaped iron Core

[eointremont]

Hello all! 

I'm designing an experiment of using a variable voltage power supply (0-30V, max 10A) to power magnetic copper wires wrapped around an E shaped iron core. The wire has a resistance of 0.000533 ohms per cm, the diameter of the wire is 0.065cm, the iron core's dimensions are 2.54x3x6.5 cm3. 

From the calculations, I found that it takes roughly 40 'turns' of the wire around the core; this is in total 760cm of wire length (minus the wire length between the core and the power supply). The total resistance I found in the coil would be 0.405 ohms. From Ohm's Law, I would get around 74A of current from this setup.

Questions:

1. To calculate the magnetic field of a point above the iron core, would I use B=mu*(N/L)*I? Or is this only for the center of a solenoid?

2. The max current running through the wire is 10A. Based on the setup, I'm wondering if this is sufficient to produce a "strong" magnetic field. To find out this strength, would I need to find out the magnetic field, and then its corresponding force at a certain point?

3. The experiment is for maglev purposes actually; I have a superconductor that I'd like to try to see how variable voltage can change the distance between it and the iron core. With this in mind, would my calculations also require a force diagram of the weight of the superconductor and the magnetic force and then I'd be able to find the distance between them at a certain voltage?

4. Finally, I am very new to electricity and would like a lot of feedback on the setup and most importantly safety tips. Any feedback is welcome. 

Thank you!

 

[studiot]

Here are some useful links to the Elihu Thompson Experiment.

https://www.google.co.uk/search?ei=pR7cXrKxMrPYxgOBwoS4Cg&q=elihu+thomson+experiment&oq=elihu+thomson+experiment&gs_lcp=CgZwc3ktYWIQAzIGCAAQFhAeOgQIABBDOgIIADoICAAQCBANEB5Q7CZY9EFgrkRoAHAAeACAAX6IAZYIkgEEMTAuMpgBAKABAaoBB2d3cy13aXo&sclient=psy-ab&ved=0ahUKEwjyl5eVpO7pAhUzrHEKHQEhAacQ4dUDCAs&uact=5

 

[Danijel Gorupec]

This is a real-life case with an iron core, and as you seem to be interested in relatively precise calculations, I would suggest using computer software to simulate your magnetic circuit. There are even few free software packages, but I am not sure if these would be good enough for your (I don't know of any free 3D simulator, but maybe 2D can be good enough for you - like 'femm').

If I understood you correctly, only 40 turns of wire can fit inside your core? At 10A this gives 400Ampere-turns - sounds like it might be OK. However I expect cooling problems and your coil might only be energized for a limited time... I don't understand when you say 'around 74A of current' - how did you get this number?

(If you find out the 10A is limiting you, you can always use a bit thinner wire and fit more turns into your core - then you will be able to obtain more ampere-turns using your limited power-supply. However heating problems will limit you severely.)

Then again, you might be a master of cooling. The superconductor part seems interesting - how do you cool it?

 

[eointremont]

11 hours ago, Danijel Gorupec said:

This is a real-life case with an iron core, and as you seem to be interested in relatively precise calculations, I would suggest using computer software to simulate your magnetic circuit. There are even few free software packages, but I am not sure if these would be good enough for your (I don't know of any free 3D simulator, but maybe 2D can be good enough for you - like 'femm').

If I understood you correctly, only 40 turns of wire can fit inside your core? At 10A this gives 400Ampere-turns - sounds like it might be OK. However I expect cooling problems and your coil might only be energized for a limited time... I don't understand when you say 'around 74A of current' - how did you get this number?

(If you find out the 10A is limiting you, you can always use a bit thinner wire and fit more turns into your core - then you will be able to obtain more ampere-turns using your limited power-supply. However heating problems will limit you severely.)

Then again, you might be a master of cooling. The superconductor part seems interesting - how do you cool it?

 

Thanks for your answer. I'll check out Femm as I know it'll be helpful in the near future. 

Actually, I can fit about 80 turns of wire, but I chose 1" or 2.54cm as the height of the core. I could always use more turns, but I was wary about overheated the wire as you mentioned.

I got 74A of current by using Ohm's law - 30V divided by 0.405ohms. Not sure if I had done that correctly, but I guess it wouldn't really matter since the max the power supply would give is 10A anyways. 

I'm simply using liquid nitrogen for cooling the superconductor. So far I've only tested it on permanent magnets and it works, but I want to experiment with an electromagnet that I can control the magnetic field strength (basically the current). This also had me thinking about the risk of electricity conducting through the condensing water vapor caused by the cooled superconductor. Basically, the concept should be the same; if the superconductor levitates with a permanent magnet, it could surely work with an electromagnet (DC current). It's just I'd like to make sure things are safe and effective before investing in buying more materials (such as the power supply, which I haven't bought yet).

Are you familiar with overheating of wires? Any tips on cooling?

20 hours ago, studiot said:

Here are some useful links to the Elihu Thompson Experiment.

https://www.google.co.uk/search?ei=pR7cXrKxMrPYxgOBwoS4Cg&q=elihu+thomson+experiment&oq=elihu+thomson+experiment&gs_lcp=CgZwc3ktYWIQAzIGCAAQFhAeOgQIABBDOgIIADoICAAQCBANEB5Q7CZY9EFgrkRoAHAAeACAAX6IAZYIkgEEMTAuMpgBAKABAaoBB2d3cy13aXo&sclient=psy-ab&ved=0ahUKEwjyl5eVpO7pAhUzrHEKHQEhAacQ4dUDCAs&uact=5

 

Thanks! I'll check this experiment out. Very helpful!

 

[Danijel Gorupec]

Thanks for explaining the 74A... yes, as you said, it does not have much meaning as your power supply is limited to 10A

About 6 amperes per square millimeter of wire cross-section is usually considered an upper-limit recommendation for coils in commercial products. However my experience is that in experimental setups you can go significantly higher. I guess you should be able to go to 10A per square millimeter without a need for forced cooling (with just some basic common sense regarding passive cooling).

But I guess you might want more than 10A per square millimeter (in your case, the wire is about 0.33 square millimeter, which means that with passive cooling you should be able to use about 3.5A of current). So you might want to arrange some sort of forced cooling or use the coil only intermittently.

Anyway, this is just a rough estimation.

If you decide to use a passive cooling then I would recommend to use as much wire turns as you can. This will allow you to work with a lower amperage for the same effect. If you use a forced cooling, then I am not sure - maybe it does make sense to leave some space between wire turns so that the cooling fluid can pass (but this is also harder to build, so I would go with simple compact coil as neatly and tightly wound as possible)

[eointremont]

Danijel, 

Thanks for the tip; I was actually intending to only use passive cooling which means I'll take your advice and add more turns to the core. Question, is it recommended to wind over the first layer of coils (the coils I have are enameled, so electricity shouldn't travel through the second layer of winding)? Would the net magnetic field just be amplified? 

Have you also worked with an e shaped iron core? I extracted mine from a microwave transformer and removed the base. Winding around the middle section is a little tricky, especially if I want to get a tight and compact wound. This is due to the geometry of the core being rectangular rather than circular. Just wondering if you have any suggestions on winding on an awkward shape like this and also if there are any "adhesives" (for a lack of better word) for the coils so that they don't come undone. Picture is attached for a better visual (note that this winding is not the finished product, I'm still trying to figure out how to keep the wounds tight and compact like you stated). Thanks!