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Electric motor contest - suggestions?

  1. Dec 1, 2005 #1
    there is a contest to build an electric motor for which we can get some bonus points on the final in my E&M class. they give us a very limited kit and we can only use what comes in the kit which is a couple pieces of thick poster board, 6 feet of insulated wire, a couple paper clips, a couple tacks, and 2 magnets. the only object is to make the motor spin as fast as possible (no extra external load).

    i pretty much have an idea of how i want to build it but im unsure about a few things. what is the optimum coil size? the magnets are circular - should i wind the coil to have the same diameter as the magnets? i figure that if you make it smaller, it will have less area, but you can put the magnets closer to create a stronger field. conversly, if you make it bigger the magnets will have to be farther apart giving a weaker field but there will be more area - more flux. what do u guys think? also, does it matter how neat you wind the coil - any certain technique or pattern for winding it that will make it run faster? also any other suggestions are appreciated. thanks.
     
  2. jcsd
  3. Dec 2, 2005 #2
    Felix83, for maximum “no-load” speed I think you need a low PM field flux density. That is because the back-emf is proportional to speed and flux, so you will match the supply voltage at lower rpm with a high field.
    However, whilst there might be no external load, the motor will have to overcome its own friction, windage, etc, so it has to develop some power just to turn. Therefore, I think you should make the magnets moveable. Set them close to the coil(s) to get the thing going and to develop it, then gradually withdraw the magnets and “tweak” the set-up to optimize for maximum no-load speed.
    I imagine you power source is DC from a battery so that you’ll need brushes. They are likely to cause the largest friction loss. (I used candle wax to lubricate mine. I’d rub it all off, but enough seemed to remain, and it didn’t accumulate dust – which is an insulator – like oil or “short” the segments like graphite.) Their contact pressure can be minimized by reducing bounce (so fix up a circular commutator if you can) and by needing the minimum current. The latter would probably favour more turns, but they would increase the windage and out-of-balance drag. The less the turns, the less the back-emf, so I’d arrange that you could remove some turns during development.
    Since you are not seeking power, don’t use any iron in the coil(s) since that would increase their electrical time constant. If the coil is rotating, consider winding a non-circular one and then working it (between your fingers or wood, but not enough to damage the insulation and cause a disastrous “shorted turn”) into a narrow cylindrical armature. Could you possibly glue stub shafts to the ends of the coil to save having iron passing through the centre?
    Don’t bite off more than you can chew, though. Good luck!
     
  4. Dec 2, 2005 #3

    NateTG

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    To make the motor turn as quickly as possible, you'll obviously want the part that spins to be as light as possible. So, you're going to want to make the motor as small as possible.

    There is a trade-off because you'll want to have enough angular momentum to keep the motor spinning once it starts.

    I'd consider starting with something like:
    http://fly.hiwaay.net/~palmer/motor.html

    but you'll almost certainly want to improve on that a bit.
     
  5. Dec 2, 2005 #4
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