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Need Help With Large Electromagnet Design

  1. Dec 1, 2011 #1
    Hello Everyone. I'm very new to this website and am turning to anyone for help.

    I'm trying to design and build a large electromagnet to do some scrap metal lifting for me and my grandpa that can be hooked onto a crane/loader of a tractor/payloader. I'm currently studying electrical and mechanical engineering at a university in Kansas, but I'm only in my second year to have the knowledge to design one correctly. I grew up on a farm and ranch and have all the tools (lathe, drills, mill, MIG welder, TIG Welder, ARC welder, plasma cutter) and abilities I need to do the job correctly because I want to build one tough enough to handle the work.

    I know how to build small simple electromagnets, but I'm not sure what I need to do with a larger scale version.

    * My intentions are to build one that can hold around 1000lbs to, well roughly a car would be nice, so around 5000lbs.

    * Would like to run it off a 12V DC car battery or if needed 24v by hooking two in a series. If making one that can run on 120v or 240v AC current is ideal, than ideas would be appreciated.

    * I'm wanting the total diameter to be around the ballpark of 2 feet.

    *Use an iron core of anywhere around 2" to 6" in diameter depending on what design is best to go with

    What I don't know.

    * What is possible

    * The Best electromagnet design

    * Size of resin wire to be using and wraps

    * Do I need an insulator between the iron core and windings

    * If I can hook hook it up directly to the battery using a switch/relay to control the on/off or if i need an amperage controller

    Below is a drawing of what i assumed it would end up being designed to look like\


    Thanks for your help
  2. jcsd
  3. Dec 2, 2011 #2
    With that scale, you would Like to optimize everything.
    Please, don't shout at me to produce the proof, but I think I have read somewhere that in the best design, the overall diameter of the solenoid is equal to its Length.
    Also, consider using an U-shape Core. U shaped Magnets can lift orders of Magnitude more weight.
    There is an interesting give and Take situation In selecting the wire.
    Roughly, the magnetic Field strength is B = uNI.
    You can Increase N (no. of turns) or Increase Current (I), or Both to increase the Lifting power.
    To increase B if you keep on increasing the turns (N), you will increase the wire resistance and hence decrease I. To counteract this, you need to use Bigger diameter Wire. So, large wire, large turns gives a hell big Solenoid. You don wanna make the strongest possible Electromagnet that is big as a mountain, do you?
    So, where do we start?
    Start by fixing, what you can't change. I would start from the Power supply I have.
    Ok, 12V is the power supply.
    Now, I would fix the overall Core Geometry.
    Ok, I am going to use a U-shaped core with 6" thick limbs and a gap of 6" between the Limb.
    Now, what?
    How much Battery Power Are You willing to give?
    Yes, this is an option, but don't wish too little.
    The battery Power will be roughly, P = 12^2 / R
    where R is the resistance of the wire.
    So, find out R. Now, select that size of wire, which if you wind tightly and fully on the available space would give you the required Resistance.

    A little Difference on Power Chosen don't makes any difference on Magnetic Force, because, if you choose Less power then you come up with higher Resistance requirement, then you will choose thinner wires, hence less current, but you can wind more turns in the available space. Hence the apparent balance.

    If you chose More power, then you come up with Lower Resistance requirement, hence you need thicker wires. There will be larger current, but you can wind lesser no. of turns in available space, Hence apparently the Magnetic Force seems to remain the same.

    Sorry for rough writings.
    And, BTW, welcome to PF.
  4. Dec 2, 2011 #3
    Won't there be more power used than just the resistance losses of heat? That power is in addition to the actual mechanical work done by the magnetic field I think, or am I confused? Basically the load on the field will create a back-EMF that requires an increase in the current, and the EMF times current will be the power in addition to the winding heat dissipation.
  5. Dec 2, 2011 #4
    It would be better if you purchased the electromagnet from someone that has experience building them.
    That said, be sure and put a rectifier in parallel with the electromagnet, or there will be problems with trying to turn it off.
    Also is the magnet for continous duty or does it have a duty cycle.
    The magnet wire getting hot will probably be a problem. Select the wire size so the wire doesn't get too hot.
    The electromagnet should be impregnated with varnish or epoxy to keep the wire in place and to keep the wire cool.
    Cold rolled steel should probably be used.
    How are you going to wind all those turns?
    Why don't you check some places that are using this type electromagnet and see how the electromagnets are constructed and what problems they have when using the electromagnetx.
  6. Dec 2, 2011 #5
    You are correct, it must consume more power during the initial Pull of Scraps to the magnet. But Once its done, it takes a constant power to hold them.
    And as Carl Pugh Pointed out, the duration of time you like to hold the things and duration of its shutdown (which we call duty-cycle) is an important consideration on design.
    If you just want around 1 minute then, design can be made for great power, because, the wires won't heat-up that much in 1 minute even if you flow heavy current.
    But If you are willing to energize the electromagnet for long time, Design Should be done for less power consumption.
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