Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Faraday generator self excited

  1. Mar 20, 2015 #1
    Good day, some time ago i asked a similar question but i didnt get a definitive answer.
    I will try to write as simple as possible, A faraday disc (generator) a constant rpm torque source that spins the generator , now i put some load on the brushes attached to the disc and a capacitor in paralel that is charged, current starts to flow to the cirucit and the disc creating a magnetic field , can this magnetic field from the current in the disc also generate more current as long as there is input power that rotates the disc?
    I guess the question is can this disc generate current without external magnets just it's own current and b field assuming there is input energy provided as torque that spins the disc?
     
  2. jcsd
  3. Mar 21, 2015 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member


    no because you need more energy to spin the disc than what will be created

    you are trying to do a PMM ( Perpetual Motion Machine) these will never work as they break the laws of physics
    and discussion of such is against PF rules

    please refamiliarise yourself with the rules
    https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/

    Expect this thread to be closed

    Regards
    Dave
     
  4. Mar 21, 2015 #3

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    davenn I think you are misunderstanding the question. This does not sound unreasonable to me. Maybe a diagram of some kind would shed some more light? I think the OP is suggesting that the magnetic field needed to make the disk generator work is achieved through output from the generator. Nothing is said about eliminating the mechanical input.
     
  5. Mar 21, 2015 #4

    Nugatory

    User Avatar

    Staff: Mentor

    It is possible to build a generator that uses its own current output to power the electromagnets that generate the magnetic field that generates the current. Modern automotive electrical systems work this way - there are no permanent magnets in the alternator. This design has the nice property that it's easy to regulate the output voltage by controlling the current back through the alternator's windings. These systems also include a storage battery to generate current and hence a magnetic field before the alternator starts generating current to maintain itself.
    (Some systems will start without an initial current source - this is the result of weakly magnetized iron and steel components in the alternator housing, and is somewhere between a distraction and a disfeature).

    In principle a Faraday wheel could be operated this way. In practice, a Faraday wheel is an inefficient electrical generator and I expect that it would be hard to rig something up that came anywhere near the conversion efficiency of a well-designed alternator.
     
    Last edited: Mar 21, 2015
  6. Mar 21, 2015 #5
    Thank you Nugatory and supernova you understood me, I was a bit unclear in my wording i expect thats the reason Dave misunderstood me.

    The idea was simple , could a single faraday wheel be the b field source and the current producer at the same time , starting up order would be as follows, attach mechanical torque spin it to some rpm, then inject a current from a charged capacitor for example , as soon as current starts to flow through the disc it now has also a magnetic field around it and as long as there is rotation this magnetic field would produce a further flow of current in the disc , so the very disc that produces the current is also the " excitation coil" for the magnetic field that makes the current possible. Is my reasoning correct?

    Yes the faraday wheel as a single wheel with brushes would be a inefficient generator , but i used the simple wheel as an example for sake of simplicity.I have another more complicated version in mind that uses the same lorentz force method of current that the faraday disc.

    So could i use a single disc for excitation and current production at the same time with no external magnets and nothing else except a kickstart current source?
     
  7. Mar 21, 2015 #6

    tech99

    User Avatar
    Gold Member

    It is a tricky question, but I think as follows. When a current flows between the axle and the brush, it will create a magnetic field as if there were a wire. The field would surround such a wire as if you were gripping it. This is different to the field from the usual exciting magnet, which is parallel to the axle. When an electron in the metal of the rotating disc approaches the usual magnet, it receives a deflecting force in the radial direction, which is along our "wire", creating a current. But in the case you suggest, the electron might receive a push towards the axle as it approaches, and a push towards the brush as it leaves (or the converse) due to the reversal of the magnetic field each side of the "wire". If you look at the overall picture, the net movement of electrons along the radius is zero, meaning no current in the external circuit, although we are seeing local currents in the disc.
     
  8. Mar 21, 2015 #7

    davenn

    User Avatar
    Science Advisor
    Gold Member


    Im still reading this as that you want the system to be self powering
    as in you will give it an initial kick into motion from the spin up and charged capacitor
    and it will then continue to run on its own .... yes or no ?

    if the answer is yes, then my answer is still no, as per my original post
     
    Last edited: Mar 21, 2015
  9. Mar 22, 2015 #8
    No, the generator is constantly attached to rotational torque source as every normal generator, the discussion here is about whether i have to use a separate exciter coil for the magnetic field that would further create current or can I use the same disc for both current induction and b field excitment from which the current would then be produced.

    In all cases the rotational input remains there ,there’s nothing PMM here davenn please read once again.

    I am asking this because if I could use the same winding or disc shape to make the field which would then produce current in the disc which would further continue the field , it would make my generator with less parts and less material needed.

    tech99 - do you mean the net current would cancel out because of the fact that the b field in the disc would push electrons into opposite directions at the edges where the field lines terminate into poles or what?

    Ignoring all the brush connections and other things, for example what if I had 3 discs parallel to each other close spaced and spinning together the middle disc would then see a magnetic field with an opposite pole coming from each of the side discs just as if i had two permanent magnets surrounding the middle disc? Or just two discs , again each one of them would see the magnetic flux as cutting through it at 90 degrees or close that should work also ?
     
  10. Mar 22, 2015 #9

    davenn

    User Avatar
    Science Advisor
    Gold Member


    that's OK .... it was VERY unclear in your earlier posts
     
  11. Mar 24, 2015 #10
    I understood your concept. But i think the efficiency would be very low. the input power to this machine is torque n some starting current to provide the initial magnetic field; the output is current.

    Now this output is being supplied to:
    a) the magnets that provide the magnetic field across the disc
    b) electrical load

    for the output to be sufficiently large, the input torque has to be large. At high speed, the frequency of voltage generated n current flowing would be very high. At high frequency you'd have high losses.
     
  12. Mar 24, 2015 #11

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    Huh? Since when did a faraday disk generate AC when supplied with DC field?
     
  13. Mar 24, 2015 #12
    My bad....I was thinking of the lines of an alternator.

    I think at high speed the brushes will wear off faster.
     
  14. Mar 25, 2015 #13

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    I would estimate that a car alternator uses slip rings that have a diameter of around 2 inches. I see no reason a faraday disk should wear any different than an alternator assuming the setup is similar and the amount of current passed is similar.
     
  15. Mar 27, 2015 #14
    OK, so could you guys please tell me whether that would work or not , referring to post #8 in this thread and the main question that I asked, Could a drum type disc or more complicated structure still based on faraday disc principle be used both as the exciter magnet and current producer without the need for separate exciting coils?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook