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Homopolar Generators

  1. Dec 21, 2005 #1
    Would Faraday's homopolar motor / generator work if it was rearranged to have the magnet as the disc and conductor / coils where the magnet used to be? A brushless version essentially.


    Something tells me no, but I'm still doing rapper impressions at my desk. :cool:

    I here these things are good for generating huge pulses of current at low voltage (current path is a short, thick solid copper volume from the disc's centre brush to it's perimeter, which is in a bath of mercury), I'm wondering if they could be rearranged to produce higher voltages.
    Last edited: Dec 21, 2005
  2. jcsd
  3. Dec 21, 2005 #2
    Okay, so...
    Number 1 doesn't generate a voltage because there's no relative change in a uniform field (The field could be speeding at light speed or not at all and it'd look the same). But what about number 2? You have a 'uniform' object in a uniform field. Or is the disc uniform? E.g. atoms and electrons are discrete, not uniform.
    Last edited: Dec 21, 2005
  4. Dec 21, 2005 #3
    Yes why not? You spin a magnet in a coil, you get an alternating voltage across the coil.

    edit: but you'd need a different geometry than that - trying turning the coil 90 deg. and putting the magnet inside it. Think of it as a bar magnet.

    Spinning things faster will linearly increase the voltage. Look at the Maxwell (Faraday) equation:

    [tex]\nabla \times \mathbf{E} = -\frac{\partial \mathbf{B}}{\partial t}[/tex]
    Last edited by a moderator: Dec 21, 2005
  5. Dec 21, 2005 #4
    This is tricky because it isn't like a normal motor, the field is uniform (unipolar and not modulated), it's like putting a wire in a horseshoe magnetic, adding a current and watching the wire jump. This time, the wire is a disc that can 'jump' continuously because it's on a bearing, causing it to spin.

    I'm sure you could get higher voltages out of the disc by spinning it quicker but I've been reading values like a few volts at ~1000rpm+ so it'd probably have to be rocketing around to get tens or hundreds of volts out of it. Then the brushes would wear down super quickly.

    I suppose a better way would be to stack a whole load of them in series with each other but it'd be nicer if I could do it all with one disc spinning at moderate speeds and without brushes.
    Last edited: Dec 21, 2005
  6. Dec 21, 2005 #5
    The quest for a simple way to get higher voltage from a Faraday disk generator is something alot of people have embarked on.

    What seems the most practical way to me is to put the current through some solid state device that will chop it into pulses which can then be fed to an induction coil to step the voltage up. If you can use it pulsed, you're OK. If not then further devices to smooth it back out would be necessary.

    All the solutions seem to be like this, though. They start getting irritatingly complicated pretty fast. The very easiest ways to increase the voltage: faster rotation and/or increasing the diameter of the disk, don't get you very much.
  7. Dec 21, 2005 #6
    I believe a brushless version is possible, but complicated. You would have to have leads soldered to the whole perifery of the fixed disc. The rotor would consists of at least one magnet, possibly more, which has to be offset. That is, it can't be a magnet the same size as the disk because, as you know, rotating a magnet like that ends up producing no emf. It has to be offset toward the edge of the disk.

    On the other side of the disc would be a magnet of the same size, placement, but opposite polarity, mounted on a freewheeling armature. My thinking is that the magnets would have enough attraction through the disk so that rotating the one side would cause the freewheeling side to rotate right along.
    A magnet on both sides is necessary to keep the field lines at right angles to the plane of the disc.

    I'm just about positive that in this "brushless" configuration, however, what you're going to get out is AC. As the magnets approach a given lead the emf will be going, say, from the periphery on the disc to the center. But as the magnet aligns with the lead the direction of the EMF shifts around to the opposite, then as it departs from that position it shifts back to the first direction. Brushless, high current, low voltage AC. You could plug it directly into a conventional transformer.
  8. Dec 22, 2005 #7
    Hi eeka et al
    The explanation for this strange behaviour is very different to what you think. It’s in fact quite simple.

    First point: You have to keep in mind that an emf is generated when we have a time varying magnetic field but also in case of a non varying rotating magnetic field. The last case applies here.

    Second point you must complete the total circuits, this includes if you draw a field line draw it all the way round from north to south and back to where you started.

    Also draw your electrical circuit complete with brush, wire and voltmeter (in case of generating). Now you have 2 complete paths, a magnetic and electrical.
    This way you will notice that for picture 1 the rotating field lines will enter your electric circuit through the disk but also exit in an opposing way through the wires. This means that 2 opposing emf’s are generated which cancel exactly, result emf=0.
    Redrawing also picture 2 you will now notice that the disk is cutting field lines but there’s no opposing field in the wiring, resulting in emf.
    Do the same for picture 3. Now you will notice that field lines are cutting the wires going to the volt meter, resulting again in emf.
    This is not meant as a rigorous scientific explanation but you have to fill the various different laws in for yourself. You see almost childish but it works!
  9. Dec 22, 2005 #8
    I think you've hit the nail squarely on the head. The observed effects are completely explained in conventional terms by that analysis and you've solved the mystery of the non-rotating magnetic field.
  10. Dec 23, 2005 #9
    I’ve tried to pass on what other people / authors have passed on to me.
    Still, thanks for the complement.
  11. Dec 23, 2005 #10
    You're welcome.

    There was a long thread about a year and a half ago in which several people puzzled over whether or not the field was rotating when you rotated the magnet. A couple of them mentioned that they were aware of people (working engineers, apparently) in the process of developing Faraday generators who were also unsure if the field were rotating or not. This lead to speculation about all kinds of relativistic and quantum weirdness that might allow for a non-rotating field. So, it's a breath of fresh air to see your explanation which explains everything perfectly well in conventional terms.
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