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Simple Dc Motor Clarrifocation

  1. Dec 23, 2007 #1
    So I have a project over the holidays, recieved it on Friday, I have to build a simple Dc Motor. Using the textbook as a guide has proven to be very difficult becuase it does not explain itself very clearly, I was hoping some one could provide me with some clarrification. Here are the instructions from the textbook, I apalogize for the poorn quality they are hard to read, my printer never seems to work with this wireless network I have.





    Okay So when tis whole thing is done, Does it rotate in complete cirlces, because it doesn't look like it is possible with the wire attached to the solenoid around the pencil.

    5, - It says to wrap a double layer of coil around the nail, does this mean wind up to the top then wind back down, (possibly creating 2 opposite polarites at each end of the nail? A bad thing?)

    Does it matter that I use a pencil, Can I use a plastic circular coat hanger piece instead?

    Do the wires go going form the nail to the solenoid attatch to the wires on the pencil, or where the tape around the coil is?

    I have started this, almost done, I'm just not sure if I did it correctly So i need some clarrifcation.

    Thank-you for any help
    Last edited: Dec 23, 2007
  2. jcsd
  3. Dec 27, 2007 #2


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    Staff: Mentor

    Since this is more of a project, rather than strictly homework/coursework, I've moved it to the EE forum.

    Yeesh, that's a pretty cobbled plan for a DC motor. First, to answer your question about the pencil and coil being able to rotate, that's what the "communtator" is for at the end of the windings -- the two ends of the windings on the pencil are stripped bare and attached to the pencil, so that "wipers" (which I don't see yet in the plans....) can make rubbing contact with them as the pencil turns. The two wipers would alternate the polarity of the current in the pencil coil as it turns, which helps it to keep getting rotational force in the same direction to keep it turning.

    The stator windings formed by the two nails also confuses me -- it's not very proximal to the pencil coil, so it won't make much of a field to interact with the pencil coil's wires. The intent is for the current flowing in a nail to generate a B field close to one side of the pencil coil as it is close, which generates a force on those pencil coil wires on that side, to help turn the pencil. I doubt whether the friction can be made low enough at the pencil supports to allow full turning of the pencil coil -- it might turn 45 degrees, but it doesn't look like it will have enough inertia to make it through to the opposite wiping angle.

    And for your question about winding the coils on the nails, no, winding up and back down in an overlaid fashion does not cancel out the B field in the nail. As long as all the windings are in the same direction around the nail (like all clockwise as seen from the pointed end of the nail), they will generate field in the same direction in the nail.

    There just have to be better plans for a simple DC motor somewhere on the Internet. Maybe do a search to see if you can find something, and then ask the professor if it's okay to substitute the better design....

    Here's a wikipedia.org article on DC motors in case that helps:

  4. Dec 28, 2007 #3
    Thank-you for moving this thread to somewhere more relevant, and for your input. If I wrap wire around the nail 3 or 4 times instead, will it create a stronger field? I'm using the wie you find inside ethernet cable, Is that sufficient or should I use something else?
  5. Dec 28, 2007 #4


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    Staff: Mentor

    The field you create in the nail is related to the product of the current in the wire multiplied by the number of turns ("amp turns"), so yes, more turns will generally give more field. As for the wire, any single-conductor wire will do. Specialized "magnet wire" which is usually used for real inductors and transformers and motors, has a thinner insulation layer, to allow the turns to be wound more densly. But regular wire (probably around 24AWG like your un-wound Ethernet cable) would be fine.
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