Building a homemade crank generator, but no voltage on voltmeter?

  1. Hey guys,

    I built up this design for a hand-powered crank generator with four neodymium magnets, but am having trouble getting a reading through a voltmeter. I want to try and amplify the voltage as well but that will come later.

    Here is our setup

    I am not completely sure if my magnet setup is working correctly. Any help (including ideas to amplify this meager voltage) is appreciated!

    (the hand held shaking generator shows up as a .5 AC voltage on the voltmeter, btw)
  2. jcsd
  3. NascentOxygen

    Staff: Mentor

    Hi PHam! [​IMG]

    I think success or failure to generate an EMF here hinges on what you describe as a "shake". :smile:

    It seems that your coil is a spool of enamelled wire? Please describe in precise detail the motion of that screwdriver when you exert the "shake".

    Also, first you say you are having trouble getting a reading, then you say you have a reading of 0.5v. Please elaborate on these two statements, in combination.
    Last edited: Jan 20, 2014
  4. Thanks for the reply.

    Yes, it is enameled wire bought from RadioShack. We tried both .22 and .30 gauge wires as the spool. The drill bit simply rotates based on how we spin the large wheel on top of the wooden rod, but it is pretty fast (I'd estimate ~10 rotations per second).

    The photo album is actually from 2 different setups, sorry. Should've elaborated. The blue taped PVC pipe is the 2nd generator that, when we shake it up and down with magnets inside, will create a .5 voltage that can be read by the voltmeter. The pulley spinning mechanism does not create a readable voltage.
  5. davenn

    davenn 4,364
    Science Advisor
    Gold Member
    2014 Award

    hi PHam

    welcome to PF

    with the coil size and small magnets you have, yes the generated voltage will be VERY small
    That thing you have to "Amplify voltage" isnt for that purpose !!

    its a switch mode power supply designed to take mains voltage in (120,220,240 depending on your country) and it will output a specific DC voltage say 12VDC ( it will be marked on the case) there's a small white trimpot on the left side of that black terminal strip that can be used to adjust that DC voltage a small amount.
    for a 12V nominal output it will usually adjust from ~ 10.5 to 14.5 V

    Back to the generator .... you need MUCH more wire and MUCH BIGGER MAGNETS !! seriously

  6. davenn

    davenn 4,364
    Science Advisor
    Gold Member
    2014 Award


    and the magnet discs on the far side ( away from the coil) of the drill bit are not doing anything
    they serve no purpose

  7. NascentOxygen

    Staff: Mentor

    To generate a voltage, the magnetic field must cut across the strands of wire. Your first photo shows the path of the magnetic field will be following around (not cutting across) the strands of wire, hence no induced EMF.
  8. FlexGunship

    FlexGunship 727
    Gold Member

    The rotating magnetic field must be perpendicular to the direction of your conductors. Unwind the spool and rewind it in two packs perpendicularly. Higher number of coils increases induced EMF.

    For the love of Pete, don't play with that rectifier. You don't want 110VAC exposed in this setup.
  9. uart

    uart 2,776
    Science Advisor

    As Nacent has pointed out, you have *no* flux linkage threading the coils. Two things that are critical for a generator to work are:

    1. Flux linkage through the coils
    2. That this flux linkage changes with angular position of the magnets.

    In your setup the magnetic flux is directed radially outward and therefore does not thread through the coils as required. In "real world" generators, it's common to use laminated "iron circuits" to make it easier to direct the magnetic flux in such away as it threads the coils efficiently.

    It can be quite difficult to built a functional generator like that without an iron circuit. You could try winding the coils so that there is a gap in the windings on one side and try to get an axle to rotate the magnets in from the side (that is, perpendicular to your current rotational axis).

    Amplification is not the correct term to use here. If you have to use an amplifier then you no longer have a generator, you have a transducer. You could use a transformer to increase the voltage (this is not an the same thing as an amplifier), but in any case that too is kind of pointless. The real solution is to use thinner wire and more turns.
  10. Thanks for all the answers. I'll try arranging them perpendicular, but if that does not work out very well, we'll stick to the simple shake method. Learned a lot (especially about the power supply)!
  11. sophiecentaur

    sophiecentaur 14,725
    Science Advisor
    Gold Member

    Google simple electrical generator images. It will give you an idea about typical setups and some hints on good construction. The movement and the axis of rotation makes a big difference to the output.
    PS a transformer will not actually help with the power output. Your DMM is quite sensitive enough for the job - you should be able to light an LED or small pea bulb this way.

    Many more turns of much thinner wire may improve matters.
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