self excited generator

Discussion in 'Physics' started by Salvador1, Apr 3, 2015.

  1. Salvador1

    Thread Starter New Member

    Jan 9, 2015
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    Hi , I've been struggling with this on many other places so decided to come here who knows where help might arive.

    So not to make long poems here goes. I will talk about the simplest possible setup for the sake of simplicity and the argument , keep in mind the geometry and other things might be updated.

    A faraday disc , brushes and a load attached , I now attach a torque source to the disc which spins it , then I inject some current through the disc into the load or I could say the circuit, this produces a current this current going through the disc should create a magnetic field around it , so here is the question can this magnetic field then continue to serve also as the exciteent field for further current generation in the disc , assuming there is constant torque provided to the disc spinning it ?

    In other words normally you would use a permanent magnet which provides the field and then the disc spins and due to the magnetic field flux cutting the disc under Lezn law the electrons will drift from rim to center or vice versa but as we know from the faraday paradox it doest matter either you spin the magnet with the disc ir keep the magnet stationary and spin just the disc current is generated either way , so I was thinking does the wire (disc) care at all whether the field is coming from a separate magnet or coil or the field is the result of the very current flowing through the disc , shouldn't it be the same , so would I get current produced without external magnets just the field of the disc itself?
     
  2. Kermit2

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    Feb 5, 2010
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    It has been so long I am not sure I remember the correct name of the person who first stated this "law", but it states that the magnetic field produced by the flow of current will OPPOSE the field which creates it. So, no. In general the flow of current will not produce a field which is self-sustaining.
    In a specific instance, I remember an article on homopolar generators(like your Faraday disc generator), in which they had subdivided the copper disc with spiral cuts from axis to outer edge in such a way that the normally opposing field became a supporting one instead. I'll look for it and post it if I find it.
     
  3. Kermit2

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  4. Salvador1

    Thread Starter New Member

    Jan 9, 2015
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    thanks Kermit , yes cannot remember exactly but I too have learned that the field produced by current opposes its flow otherwise it would be a perpetual motion thing if the field made the current stronger and more current would then make a stringer field which would make a stringer current and so until something breaks down.

    sadly I can only see small outtakes from only three pages of taht book but in one of those outtakes it starts to talk about a homopolar macine that had no field coils so where di they got the field ?
     
  5. studiot

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    Nov 9, 2007
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    Have you considered the shape of any such field generated?
     
  6. Kermit2

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    The google book I linked is a FREE one, check in the upper left corner for the gear symbol. I think that is where you will find a download option.

    Correction: Upper RIGHT corner :)
     
    Last edited: Apr 3, 2015
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the way around field coils is magnets. still dosnt get rid of the drag induced when pulling power from the generator.
     
  8. Salvador1

    Thread Starter New Member

    Jan 9, 2015
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    oh , one more follow up question here while we're at it.

    In a faraday disc when you spin just the magnet and keep the disc stationary no current is produced , but heres a situation , what about if I spin a magnet that has a time, amplitude and/or polarity varying field and still keep the copper disc stationary , can a changing magnetic field have the same lenz effect on a homopolar generator if the field source rotates , would the electrons in the disc be dragged from rim to center and vice versa with the frequency of the field and the power of the source that rotates the field ?

    I know a changing/varying magnetic field can induce current in stationary wires as that is how most generators work, but I am interested can in also make the " drag" effect on electrons , as we normally get when there is a static field and a rotating disc , assuming the coil ormagnet or whatever produces the alternating field physically rotates.

    As far as I see it should do that because a time varying field is a so called " action at a distance " thing and the since it changes and also physically moves in this case theoretically the changes should be dragged along with the field and should then present themselves as radial current , what do you think?
     
  9. Salvador1

    Thread Starter New Member

    Jan 9, 2015
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    maybe to make my question more clear I can sum up and say
    So in the case of spinnig just the magnet assuming the magnet has a changing field , would that have lorentz forces exerted on te electrons in the stationary disc just as in the situation where the disc was spinning in a static DC field?
     
  10. Glenn Holland

    Member

    Dec 26, 2014
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    There has to be an initial source of magnetism that provides a field at a right angle to the disc.

    Yes, it is possible to amplify a small field by feedback from the output to a coil around the original source of magnetism. This is called "compounding" and it was used on the AC-DC motor generator sets for elevator motors. The field of the exciter had just plain iron cores where a minute amount of natural magnetism was present. A feedback loop from the generator output to the field coils produced amplification.

    There was an adjustment for the feedback and the trick was to keep the output stable or maintain "Flat Compounding".
     
  11. alfacliff

    Well-Known Member

    Dec 13, 2013
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    a spinning magnet with disk in its field? sounds like a speedometer to me. the spinning magnet induces curent in the disk which tries to make the disk turn too, in the same direction. you cant have3 a changing magnetic field with a conductor in its field without inducing current.
     
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