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Some General Electromagnet Questions

  1. Oct 12, 2007 #1
    Hi, Im fairly new to magnetism and had a few questions regaring the construction of electromagnets. Im trying to build one for a simple application and was hoping for some advice. My application requires a small, preferably short, large-radius cylindrical electromagnet that will act as a permanent magnet when turned on. My questions are as follows:

    1) What properties of an electromagnet effect its magnetic field strength? Does the size of the core matter? The length of the coil? The thickness of the coil? The gauge of wire used for the coil? The core, is iron way better than steel? The current? What is an easily accessable source of decent current for a strong magnetic field strength? The voltage?

    2) How long does Hysteresis last? If the electromagnet is on, and then turned off will the magnetic field strength decrease significantly or does hysteresis keep the magnetic field just as strong initially?

    3) Will an electromagnets north pole repel a permanent magnets north pole the same as two permanent magnets poles would?

    4) Ive been doing research in magnetics and related topics for an internship of mine, and have been having trouble finding sources about magnetic analysis of a small metal structure. Does anyone know of a way I could get info about magnetic analysis of small nickle structures. The application requires magnetic fields to guide them through a fluid filled maze. Anyways, my job is to design and fabricate the structures, although when designing them I have to do magnetic analysis of the metal skeletons before fabricating them. I have never done this before and am looking for a source to help me learn.

    Thats pretty much it.

    I would greatly appreciate any feedback on any of the questions. Thank you so much for your time and help.
  2. jcsd
  3. Oct 22, 2007 #2


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  4. Oct 22, 2007 #3
    Thanks a lot for your help but im still having trouble finding the information I want. I have read books, and many websites, threads, and probably every electromagnet question on this site and keep reading the exact same things, just repeated. What im wondering, as I think many other people are too, is how to make a fairly simple powerful electromagnet.

    For example, Edmund Scientific has an eletromagnet that is small yet produces something like 200 ibs of lift off of two D batteries. How do they design their simple, small electromagnet to make it this strong?

    I have made various weak electromagnets with different sized cores, radius, wire gauge, voltage, and turns of wire, but have found them all to be very weak and about the same observable strength.

    I understand that more amp-turns, a soft iron core, and a high current are all qualities of a strong electromagnet yet can not seem to make it work.

    I guess what I'm looking for is a recipe or a detailed explanation of how to build a strong electromagnet similar to the one edmund scientific has using easy to come by materials and resources.

    I also wondered how to layer coils. I think that layered coils may be a way to increase the strength quite a bit. When layering is there a way to do it all without cutting the wire, or do you have to make each layer independently around the previous one?

    I would greatly appreciate any advice on this topic, and I think many others would as well. Thank you very much for your time and help!
  5. Oct 22, 2007 #4


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    While I don't know the design for the Edmund Scientific electromagnet, the most likely explanation is that they use a U shaped core, rather than a straight bar.
    For a straight bar core most of the magnetic field "leaks" away and does not contribute to the pull. A piece of metal bridging the two ends of the U will contstrain most, if not all, of the magnetic flux to pass thru it.

    Perhaps this link will help some
  6. Oct 23, 2007 #5
    Thanks for your help. Your source was helpful and I learned a few things I had not known. The fact that the Edmund Scientific electromagnet is probably a U shaped core in order to maximize pull makes perfect sense. Although I am still curious how to construct a strong dipole electromagnet for use with pulling and repelling. Thanks again
  7. Oct 23, 2007 #6


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    I don't believe that it is possible to make a magnet that repels non-magnetized ferrous metal. The attraction between a magnet and non-magnetized ferrous metal comes from the shape of the magnetic field -- it gets stronger and more concentrated closer to the magnet, and the non-magnetized ferrous metal gets attracted into the region of higher field density. This is similar to why an electric dipole gets attracted into regions of higher electric field, no matter what the polarity of the electric field.

    BTW, you can make a cylindrical "U" shaped electromagnet by having an inner cylindrical core with the coil around it, and an outer hollow cylindrical tube that slips over the inner core and coil (both the inner rod and outter hollow cylinder are made of good ferrous material). At one end, you seal off the end with a disk of the same material, and the other end is left open. So when you energize the coil, that generates a magnetic field in the inner rod, and when you clonk the electromagnet on something ferrous, that lets the field return to the rod via the outer hollow cylinder. If the Edmund Scientific electromagnet appears to be cylindrical, it probably uses this type of coaxial construction.
  8. Oct 23, 2007 #7
    Thanks! I meant repelling a permanent magnet not unmagnetized ferrous material. Sorry, I should have specified. I see how a U shaped core could work really well for attraction but im still curious about a strong dipole electromagnet with a north and south end in order to both attract ferrous material and repell a similar pole of a permanent magnet. Is this possible? Can you make an electromagnet with two opposite poles thats as strong as lets say a similar sized permanent magnet? Thank you everyone for your help.

    Also, im still hoping for some advice on how to layer coils and if it will significantly increase the magnetic field strength.

    Does anyone have any advice on somewhere I could purchase an iron rod core, or a U shaped core for this purpose?
  9. Oct 24, 2007 #8


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    Iron or steel won't be that much different.
    A U-bolt clamp could be used for a U core.
    Some iron plumbing pipe, end cap and a bolt could make up the coaxial type Berkeman described.
    Any good hardware store will have these and bar stock (rod).

    Edit: For moving things back and forth try looking into linear motors.
    Last edited: Oct 24, 2007
  10. Jan 12, 2011 #9
    i don't know if you are still even checking this thread because it is so old but i hope i can answer some of your questions.

    yes a electromagnet can repel a permanent magnet, just like a they repel each other if you match the south pole to the south pole or the north to the north they will repel.

    and yes more coils will strengthen an electromagnet i have found that the beginning coils will strengthen the over all strength a little less than 20% of your base coil, but remember that the farther from the core the less each coil will contribute. about 1 inch away from the core is when the coils will help less and less.

    and as for the gauge of the wire i suggest that using 22 or 24 gauge wire will be best because to small will give you a higher resistance while to large will give you less turns of your wire so you have to find a midpoint.

    and no the core will not stay magnetized for long at all, almost as soon as your electromagnet is disconnected from your power source your core will demagnetize.

    hope i helped you at all if you are still even checking this thread.
  11. Mar 31, 2011 #10

    Here is my query..

    Concept: using Electromagnet Fields of varying strength(Torque) is created based on the PWM concept.

    Design:We are using Circular Electromagnet.
    Supply to the coil (5V,500ma) this input to the core was given on varying PWM,In order to generate various holding torque For example 100% duty cycle - Torque Max of 150Milli-Nm is measured..similarly for other duty cycle corresponding Torque shall be generated..

    What happened was, it is generating the magnetic field which is not consistent in the sense if we measure the strength (Say Torque) it keeps on varying and this varies from core to core the results are not consistent to come to a conclusion.
    So my doubt is
    1.Does the External temperature change causing this megnetic field variation?
    2.Heat dissipation due to continuous current flow causes this field change?
    Is there something else i am missing in the design.

    Your ans and suggestions helps me a lot..we are stuck with this unknown behavior..It's like mystery..
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