h a l f b a k e r yQuis custodiet the custard?
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The idea is to electro-magnetically suspend a rotor in a vacuum. Built into the rotor is a solar cell which supplies a small amount of power into a coil which is also built into the rotor. The solar cell gives a pulse of power depending on the direction of incident light. The magnetic field produced
by the coil, reacts with a fixed magnetic field to produce torque. The rotor spins, limited by drag in the near vacuum, electromagnetic drag in conductors, inductance of the coils, solar cell capacitance, and finally the structural limit of the material in the rotor.
Just a curiosity.
(?) 400 000 rpm with superconducting magnets
http://content.aip....B/v56/i4/397_1.html
Abstract of paper [kropotkin, Oct 04 2004, last modified Oct 21 2004]
Radiometer
http://www.howstuff...com/question239.htm
How the paddles in a vacuum work [Ling, Oct 04 2004, last modified Oct 21 2004]
Multi-axial "inertialess" rotor
http://www.inertialessdrive.co.nz/
Seems to get around the problem of rotors that fly apart - click "About" to learn how [Adze, Sep 26 2005]
"Perpetual Motion" Spinning Top
Solar_20Powered_20G...Motion_22_20Spintop
A similar idea. [Wrongfellow, Aug 17 2012]
Solar powered dancing daisies
http://www.dollartr...-/p317279/index.pro
Not nearly as cool as this idea, but cheap and available [CraigD, Aug 17 2012]
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I have a feeling a know what the limit would be: centripital force, and the tensile strength of the rotor material. |
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Most likely the rotor will fly apart as [TF] pointed out. If that doesn't happen the drag in the vacuum will limit it. What vacuum are we talking about, 1E-3, 1E-15Torr? If vacuum doesn't do it the solar cell may stop working because the doppler effect will shift the incident wavelength well above and/or below its sensitive range. Last, not least the circumference will not be able to go faster than speed of light. |
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I don't get the part about the solar cell. |
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I take it this motor is a DC brushless? |
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[K_sra]: I checked again, and I still cannot see any category that is suitable. This brings to mind the game that I played once as a kid: 20 questions to guess what the other person was thinking of. Is it a product? No. Is it green? etc. Something like that might be quite useful instead of scrolling down a list. |
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[TerranFury, kbecker]: I have a feeling that you may be right about the structural failure. |
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[kbecker]: I think that a standard 1E-6 Torr (TV tubes) would be enough for testing.
About the solar cell - I assume that either you don't understand my description of how to use the solar cell, or why I should want to use it in the first place. |
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The former: The solar cell rotates with the rotor. When the rotor turns to the correct position, sunlight (or other incident light) falls on the solar cell and power is supplied to the coil that turns the rotor. In effect it is a timing system, the solar cell produces power once per turn. |
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The latter: Of course, the rotor could be turned by an external field. But I chose a solar cell on the rotor for a couple of reasons. The first is simplicity and cost. No external components. The second is one that I didn't mention originally, and that is the fixed field could be the earths magnetic field. |
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Finally the rotor would turn with almost no external components. It would be interesting for young science students. |
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[RayfordSteele]: Yes, but I don't think I have seen a dc brushless with power generated on the rotor before. |
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Another potential limiting factor: Slight assymetries of the magnetic fields involved would induce eddy currents in the rotor; the faster it spun, the larger the currents. So, eventually, if it hadn't failed structurally by then, the rotor would melt due to Joule heating. |
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I agree that it would be an interesting experiment, Ling, so have a croisssant. :-) |
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Is the Earth's magnetic field strong enough to use in a motor application? If so, why haven't GE, Westinghouse, etc developed Earth-field motors? Does this (product, experiment, toy) have an application, or is it a curiosity only? |
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Gardnertoo: Theoretically, you probably could. But the operation of the motor would be dependent on its orientation. It simply wouldn't work with its axis of rotation horizontal. Also, I suspect you'd have to use electronics to do the actual field-switching rather than a commutator, since you don't have an easy way to tell how the motor housing is oriented relative to the field lines - how do you tell which side is nearest each of Earth's magnetic poles? So it's probably possible, but also probably not terribly useful. |
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We need an 'other: science' category, or maybe 'other: physics.' |
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Howabout 'other: magnetism?' |
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[TerranFury]: Thanks for the bun. I prefer mine with marmalade. By the way, the input energy to the system is simply the sunlight on the area of the solar cell. Since this is typically only 1 KW per square meter, there couldn't be enough energy to melt the rotor. But you are right that there will be electromagnetic drag. That's how the old trams used to slow down (but not stop, since movement is required for the effect). |
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[gardnertoo]: The field strength in industrial motors (about 1 Tesla) is much more than the earths magnetic field . From my memory, torque is proportional to the field strength (Force = B (Field strength inTesla) x I (Current) x Length of conductor). In my rotor, the torque is extremely small, but it has very little friction or other work to do. So it can rotate, but it won't be able to give much power. I think it could work in the horizontal position, but in my minds eye, I pictured it hanging from a magnetic bearing. |
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Regarding applications: A thing that makes people ask "How does it work?"? A science class discussion tool. I throw this open to the imagination of others. |
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[fogfreak & DrCurry]: Have made the necessary adustments (removed question: " I wonder how fast it would go?"). Thanks for your remarks. |
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Why not make it simpler by using the same principle that those little sun-powered toys use where the solar wind powers the little black and white paddlewheel in a vacuum, but use external magnetic levitation bearings. I'll bet it'd sell. Power the bearings with a solar cell. |
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i don't know much about anything,
really, but couldn't this be a kind
of solar powered gyroscope? i
know those are useful. they keep
buildings upright don't they? [gyro
architecture, 2 fries short of a
happy meal] ha ha. |
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as long as its initial orientation
was known, markings on the rotor
could indicate to a sensor which
way it was facing. |
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either way, i'd love to see it in
schools. the kids need hella good
stuff in they classrooms, yall. |
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This is a rather cool idea for what would surely have great novelty value. |
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I dont really think that super high rpm would be achieved though because the current through the coil has to reverse for every revolution and inductive reactance would limit the frequency that this could occur. |
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The vacuum and the super high revs are not essential in fact I expect you could easily build a working version
based on a slab of polystyrene or a piece of cork tile. Cut this into a circular piece (say 3 inch diameter?) and mount on it your solar cell and the coil. The turns of the coil need to be in the vertical plane. |
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Float this in a dish of water and even the slightest current will cause the 'raft' to eventually align itself according to the earth's magnetic field. |
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To finish the project mount a piece of polarising filter material over the solar cell and arrange another fixed piece of filter for the sun light to shine onto your raft. |
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The polarising filters will effectively turn the current off and on as the raft rotates. |
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A source of the filters would be junked PC VDU anti glare filters, but not the real cheap one. |
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The field needn't change at the same frequency as the rotor's. It could change at 1/2 that frequency, 1/3, etc. (You don't need to push a swing EVERY time it comes back down to keep it going. Especially in this case, where it's levitated in a vaccuum.) |
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[riccoman]: See link. I have one of these at home, and it is the black surface that appears to be pushed, suggesting that the vacuum is not so high.
But if the vacuum was very high, and the suspension was friction free, then it would use the same principle as the solar sail. I expect it would then rotate extremely quickly. |
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[changokun]: Thanks for your support. I believe that kids should be taught how to teach themselves. Richard Feynman is a good example for this (The pleasure of finding things out). |
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[Kiwijohn]: A useful demonstration, but needs enough inertia to rotate the wheel through the unpowered position. Or make it multipoled:>) |
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[TerranFury]: Electronic switching, too? I tried to find out the capacitance of a solar cell, as I think this might be the limit - not successful yet. I think the complete circuit could be regarded as a current source in parallel with (a diode, a capacitor and (inductor and resistance)). With resistances dotted about in series the current source etc.
At high frequencies, the capacitor will pass the solar cell pulses, and the voltage to the inductor will reduce.
However, if the friction of the system is very small, then even a little driving force will still propel the rotor. |
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I have refined my concept somewhat. |
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All that really needs to be mounted on the little raft is a single coil with two solar cells each with a blocking diode. The cells are mounted back to back in the more-or-less vertical plane and arranged to supply opposit polarity voltage to the coil. As the raft rotates the cells will be alternatly exposed to the sunlight and hence the current through the coil and the resultant magnetic field will alternate. No need for the polarising filters at all. |
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Regarding the Crook's Radiometer, these do not operate by solar wind and would not operate at all in a perfect vacuum. The principal is that molecules of the low pressure air in the glass envelope expand when they come in contact with the dark, heated, surface of the rotor. If it were operating on solar wind it would rotate in the other direction. |
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[KiwiJohn]: If in a (more) perfect vacuum would the solar wind effect not then spin it the other direction? |
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[riccoman] Hmmmm, I guess it would if the bearing friction could be overcome. |
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[KiwiJohn]: You've just invented the solar commutator!
p.s. In the link, that I provided, is another link which explains that it has been proven that, in very high vacuums, the radiometer does change direction due to the photons bouncing off the shiny surface. Doesn't a solar sail operate using the same principle, but with a range of particles? |
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Ummm structural failure... ??? No way the area of the rotor will never be large enough to absorb enough solar energy... The trick will be making it light enough to spin at all... |
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Some panels are 80% efishent at certain lazer emited wavelengths! |
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It's all feasible except for “The solar cell gives a pulse of power depending on the direction of incident light,” which is utter nonsense. (Perhaps you don’t know what a solar cell is?) But you don’t need it, anyway. As far as the motive principle goes, this is no different than running a motor with a solar cell. |
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Our washing machine's spin dryer won't spin too quick if the load is not balanced. I know! I'll strap myself to an office chair and spin round and round really fast. *spin* *spin* *spin* *spin* *spin* *spin* *spin* *spin* |
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ldischler; A pulse of power which varies in a sinusoid form through 0 degrees to 180 degrees of the rotation of the rotor. The position of the 90 degrees point is dependant on the direction of the incident light. Tadah! |
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Adze, thanks for a very interesting link. |
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I had thought of simply mounting the solar-cell planar with the rotating mass, but your plan would seem to involve tilting the cell to the optimum angle(?). |
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You might want to slip a capacitor in there somewhere sose you're not wasting the energy when the solar-cell is at its best compared to when the armatures are at their best. |
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I could vote for this if it were a little faster. |
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//but your plan would seem to involve tilting the cell to the optimum angle(?). // |
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No, it turns round with the rotor, but obviously only receives light when it is looking up. |
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