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Help with building an ElectroMagnet

  1. Aug 1, 2009 #1
    I have been looking through the forum for help on working out what i would need to build an electromagnet to certain specifications, but as i have not done anymore than a grade nine level physics frankly I don't understand most of whats in the posts. So if you reply can you break the physics speak down into plain english please.

    I want to build two electromagnets for use in a harness, they will need to hold about 10-15kg at a guess (I want to hold wings about two-three times the wieght of these http://etaru.deviantart.com/art/Cosplay-wing-tutorial-56518340?offset=50#comments) the lenght of the electromagnets can be 30-45cm long and 2cm wide with up to 2cm thick. What type wire would i need, how many coils and i was planning to run this off a number of 1.5v size D batteries (the thick round ones). Can somebody please tell me the specs of what I am trying to build?
     
  2. jcsd
  3. Aug 1, 2009 #2

    Nabeshin

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    Why electromagnets? Why not just get permanent magnets? Inevitably whatever system you could come up with involving electromagnets will likely be costly in terms of batteries, and perhaps even difficult to make.

    Permanent magnets might be heavier (locally, at least), but they'll get the job done a lot more efficiently, cheaply, and worry free.
     
  4. Aug 1, 2009 #3
    The point with using Electromagnets is that it is easy to remove the harness by myself and not need somebody else who can actually pull them off and not have to worry about where I store them, the cost isn't a problem as I was more building them as a thing to say 'hah I have black wings' and not use them all that often.
     
  5. Aug 4, 2009 #4
    Here are some numbers that will help building a solenoid magnet that will run on a single D cell.
    1) Obtain two 5/8" diameter round steel bars, each about 20 cm long. The ends should be flat, smooth, and perpendicular to the length. Low-carbon or iron bars are best because they will retain the least magnetization when the current is turned off. This larger diameter bar is best because the cross-sectional area and the magnetic flux increase roughly as the square of the length of the wire.
    2)A D-cell battery can supply 1 amp for about an hour. So we will choose 20 Ga wire, which is rated for 1.5 amps. We will use 20 Ga. Formvar (or enamel) coated solid copper wire. Don’t use a larger Ga. wire because its current rating is too low. Don’t use a lower Ga. wire because the resistance is too low. Don’t use bare wire. This wire is about 0.83 mm diameter, so we can get about 11 turns per cm. If we leave about 0.5 cm bare at each end, the coil will be 19 cm long. The coil will be about 210 turns long, or about 1180 cm long. This wire is about 11 ohms per 1000 ft, or about 0.43 ohms for 1180 cm. This resistance is a little low, but we will correct this later. 3) The magnetic field in the bar, at 1 amp, near each end is roughly u u0 NI/2L where u is the relative permeability (about 5000), u0 is 4 pi x 10-7 henrys per meter, NI is 210 amp-turns, and L = 0.18 meters. So we get B=3.5 Tesla, which is too high by a factor of two (for 1 amp). So for each bar magnet, the current should be about 1/2 amp, and the total resistance about 3 ohms. A 200 ft spool of 20Ga magnet is available on eBay for about $15.00.
    4) Obtain eight 10-ohm ½ watt carbon film resistors from a nearby electronics store (they usually come in 5-packs for less tha1 $1.00)
    5) Wrap the iron bars with a single layer of plastic electrical tape. Do not overlap the edges, so there are no ridges. This insulation is to prevent electrical shorts to the bar. With about 30 cm of wire hanging loose, tape down the wire at one end of the bar, and start winding the wire smoothly about 0.5 cm from the end. Wind until you are about 0.5 cm from the other end, tape the wire down, and cut it off leaving about a 50 cm loose end..Now fold this loose end back parallel to the other, and tape this wire against the wound coil. Carefully strip about 1 cm of insulation off each end, using a sharp blade or sandpaper.
    6) Solder 4 resistors together in parallel, and solder one end to one free end of each coil.
    7) Attach each coil to one D-cell battery and test.

    [Added info] Here is a plot of B (Tesla) vs. H (amp turns)
    http://farside.ph.utexas.edu/teaching/jk1/lectures/node47.html.
    We want to be about 400 amp turns per meter excitation, and at this point, the plot of u (permeability) is only 2500, so my original estimate of B is high by a factor of 2. Half an amp in 210 turns will give us about 580 amp-turns per meter, so the iron will be driven into saturation (about 1,5 Tesla). This is OK.
     
    Last edited: Aug 4, 2009
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