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Engine for small generator 3-30V

  1. Dec 28, 2011 #1
    Hello, I am a new user here but have found links to this site in many of my Google quests so I thought this may be the place to start a more focused search for information.

    I am doing a project with my son, we have created some spinning discs using magnets as a power source. I am interested in attaching a small electric engine (or coil) to produce electricity that could power a small LED or compact fluorescent light bulb.
    I am assuming it would be better to buy a small engine and reverse it than to build the coil because it would probably be better produced and more efficient. But I am not completely understanding how much torque and rpm's are needed to generate electricity from said motors and what tools should be used to measure the performance of the set-up. I was considering buying the Greenlee DM-300 multimeter because of its low voltage accuracy and maybe trying this method of measuring rpm's <http://www.fastcharged.org/electronics/handyrpm> unless there is an easier cost effective way.
    Also are their numbers I should look at to find out what rpm and torque is needed to spin the coil? Can anybody recommend a good engine for this?

    Thanks much in advance.
  2. jcsd
  3. Dec 29, 2011 #2


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    Staff: Mentor

    A small electric motor that is easiest to get hold of would be one that you salvage from a toy, such as a cheap slot car or battery-operated toy train. You can feel the torque needed to turn them by turning the shaft between two fingers. You should be able to detect a generated voltage if it is spun at even a low speed of a few revs/sec.

    BTW, if it runs off electricity it is a "motor", if it burns gasoline it's an "engine".

    The cooling fan inside the case of desktop PCs is very easy to spin, and is powered by a low voltage. I've never examined them closely so don't know whether they can be used as a generator. You could salvage one from a dumped PC and try blowing through the fan to see if it will generate DC.
  4. Dec 29, 2011 #3


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    Staff: Mentor

    The way to actually calculate the torque is to equate the electrical output to the mechanical input and add 10% or so. power = torque * rpm.
  5. Dec 29, 2011 #4

    jim hardy

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    Science Advisor
    Gold Member

    English units:

    horsepower = 2PI X torque X rpm / 33,000 ; torque in ft-lbs.

    should be easy swap to SI units.
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