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Transfer of AC through rotating capacitor

  1. Dec 7, 2014 #1
    Hi, I need to transfer electricity to components that are rotating on a shaft but the power source is stationary with respect to the rotating shaft.
    the electricity is AC. So I had an idea of a rotating capacitor , two round plates , one stationary then a stationary dielectric attached to the plate and then the other rotating and attached to a rotor.the other end of the shaft would be the return path with a conductive bearing or something similar.

    Assuming the plates , especially the rotating one have good geometry and and tolerances are kept close , could I achieve significant power transfer through such a capacitor if I use high frequency AC sine or square waveform ?

    Also since the rotating plates are round the charge density and potential difference across the capacitor doesnt change whether the plates are rotating or not right ?
     
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  3. Dec 7, 2014 #2

    Doug Huffman

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    You do not want current through a mechanical bearing! Why not slip rings?
     
  4. Dec 7, 2014 #3

    NascentOxygen

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    You won't be able to make a large enough capacitor for this to be practicable. Inductive coupling might be more feasible: a rotating coil and a stationary coil, with their cores almost touching.
     
  5. Dec 8, 2014 #4

    sophiecentaur

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    Yes. Slip rings and carbon brushes will run and run for years and years without wearing out. It's a very well established method and just relies on some skilled construction and good bearings.
     
  6. Dec 8, 2014 #5

    Doug Huffman

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    Ever wonder about ship's screw-propellers on a shaft through the pressure hull turning in the ship's hull magnetic field. It is a real problem, the bearing damage is real and the solution is a grounding slip ring.
     
  7. Dec 8, 2014 #6
    thank you folks for the interest , ok I agree current through a bearing is not that good of an idea , ok lets put the slip rings aside for a while , they are tested and been around for a long time.

    NascentOxygen , you said that i could not make a " large enough" capacitor for this to usable for high energy transfer to the rotor.
    I did some simple calculations for a parallel plate capacitor and got someting like this , for example , if i have two plates or discs with a distance of 2mm which would be filled with the dielectric and if the plate area is about 0.1m2 and the dielectric constant of my dielectric about say 5000, then i got a capacitance of about 2.1 uF , not that bad for a high frequency AC or is there something i can't see here ?
    imagine i need to get a few KW through from my stationary source to the rotor, I use a capacitor and an inductor to form a LC circuit adjusting the inductance and if my source is high frequency something in the range from 50 to 100 khz then should it be possible ?
     
  8. Dec 8, 2014 #7

    Doug Huffman

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    LOL The need is so great that cost and reliability are not obstacles to be considered.
     
  9. Dec 8, 2014 #8

    sophiecentaur

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    What sort of material did you have in mind for this? You may have been looking at relative Permeabilities and this has put your estimated Capacitance out by a factor of nearly 1000. And at least some of the gap between the plates would need to be air, too.
     
  10. Dec 8, 2014 #9
    Doug Huffman what were you reffering to ? I already agree that current through a bearing is a bad idea and a slip ring would be much better and I would use one if needed. I said lets put it aside just because I was more interested in the matters that are yet unknown to me like the capacitor thing.


    well I just visited wikipedia for starters. and there I found the table of a few of many dielectric substances. a number of 1000 was not even the highest on the chart so I devided that it's in use and possible.


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  11. Dec 8, 2014 #10

    sophiecentaur

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    The common substances that I can find are around 1 to 5 dielectric constant. The reason I questioned your estimate is because a 1uF capacitor usually has a very close spacing (<1mm) and an area (wrapped up in a roll) of many cm2. The dielectrics used are Paper, plastic. etc. I guess it may be a good idea for you to check that the substance you found is practical in that application.
     
  12. Dec 8, 2014 #11
    the spacing in my case and probably others has alot to do with the dielectric material properties i guess , if i can get away with having a thickness of 0.5mm or less of the dielectric for a needed capacitance then i guess the overall spaciong between the two plates can indeed be less than 1mm.

    Hmm you say constants from 1 to 5, but that sounds very low as 1 is the reference and the permittivity of vacuum if i'm correct.
    I;m looking at a chart of some exotic or not so exotic alloys or plastics and materials and I found that tin telluride has 1770K which sound pretty much , then lead magnesium niobate with a constant of 10 000 which is even more surprising and found some studies that have been done with this material for using it in capacitors.

    these are just a few examples but since im relying on high frequency I guess dielectric constants with such high numbers wont even be needed something around 500 to 1000 would do just fine since with the area I would have and spacing it would make a capacitor with about 1uF capacitance but I guess at a frequency of 50khz for example even 1uF is enough , well depending on the circuit of course.
    What do you think does it sound atleast reasonable?

    Oh yes I forgot , yes I could use induction to transfer the energy needed to the rotor but space is limited so i guess such a capcitor could take up less space and use less metal than a coil and its armature , ferrite probably.
     
  13. Dec 8, 2014 #12

    sophiecentaur

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    You have to be careful about the choice of material. I cannot find (after five minutes) much information about these exotic substances you have found but the internal losses are very relevant here. What you need to look into is the Complex Dielectric Constant, which will include the Loss (resistive) component at the particular frequency you want to use. You need to ask yourself why it is that capacitors are made the way they are. For high values (electrolytic and Supercapacitors) the technology is very advanced but will not suit your application. You are stuck with much more 'regular' dielectric materials, I think - or else you could buy capacitors off the shelf, which include tin telluride and lead magnesium niobnate. Having read about that substance, I get the impression that it is not available off the shelf and nor is it nice to work with. Think "Engineering", for a project like this.
     
    Last edited: Dec 8, 2014
  14. Dec 8, 2014 #13

    NascentOxygen

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    For a practical dielectric you'll probably be looking at one of the plastics, polyester or PTFE. One of the metal sheets would be constantly rubbing on it. But if you are talking about transferring kW of power, then the required voltages would be unrealistic.

    Do you have access to the end of the shaft? It might be possible to use an off-the-shelf motor as a rotating transformer for inductive coupling. But I think you'll find that practical considerations will dictate the use of slip-rings.
     
  15. Dec 8, 2014 #14
    again thank you guys for replies , much appreciated.

    the exotic elements i mentioned I did just for some reference and example.I could probably get away with much less.
    you are right Sophie , when you say think engineering. And as for engineering since plates and dielectrics are thin and dont take up much space in the " long" direction rather than the " wide" one , I can also have multiple plates in pairs the rotating ones atached to axis and electricaly connected with the stationary ones inbetween , then i could also effectively use both sides of a disc for capacitance.

    I'm searching for materials and other stuff that could be possibly used , while doing so i found out that distilled water has a constant of about 80, which seems interesting , ofcourse too bad its a liquid.Anyway if the water is pure enough or clean i should say and its electrical breakdown properties are high enough could it be used in a capacitor ?
    I'm asking this just of curiosity. Like a water filled parallel plate capacitor.

    But back to the topic , I found out that there is a material called barium titanate, and although im not sure and certainly it hasn;'t been used in wide comercial devices i guess , google mentions many research and some manufacturing of thin films and special capacitors using this material.
     
  16. Dec 8, 2014 #15

    sophiecentaur

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    A multiplayer capacitor like the old variable Cs in radio sets would certainly add capacity. Spacing and vibration ( end float) could introduce problems, though.
    The answer to this would really depend on whether this is a thought experiment or a serious enterprise. If you really need a result then you go for established techniques or else set up and fund an R an D department.
    That idea could well find acceptance if it were developed by a firm with the right facilities.
     
  17. Dec 9, 2014 #16
    so are you saying the idea could be usable ? Well it would use much less metal and take up less space than a inductive energy transfer (coils, armatures and coils again)
    another thing here would be efficiency , but as much as I know about capacitors , they block DC and let ac through and if my frequency is high enough and the capacitors capacitance is high enough for that frequency also then each half cycle can fully charge and then discharge the plates so theoretically almoist all the input energy should get through right ? except for conductor losses and some other ones probbably , should 95 to 97% efficiency be a good guess for such a capcitor transferring energy ?
     
  18. Dec 9, 2014 #17

    sophiecentaur

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    What you have written here demonstrates enthusiasm - which is a good thing- and an ability to think laterally. However, what you have to say about Capacitors and the way they work - including the Efficiency of such a system, shows that you are not really in a position to guarantee success in this venture. You could well end up spending an awful lot of money because of a decision you make which is based on your knowledge. (That would be a very un-engineering thing to do)
    I just don't know where to start in putting you right about the function of Capacitors in a circuit and how you can feed a load via such a system (do you know about Electrical Impedance and Matching?). The nearest thing I can compare your idea with would be radio transmitting antenna design - and I think you would accept that is a very complicated topic. You want to know whether 95% efficiency would be a reasonable estimate. That question just can't be answered until you know the Capacitance that you could achieve, practically with one of those exotic materials. Have you read about the problems of producing them and the physical properties - not to mention the cost? This is all so pivotal in the project that you will need to spend a long time on research and costing for that aspect alone before you can be sure that the dielectric constant you will have to accept is more than around 5.
    Ceramic capacitors are used for various applications and they are very compact - but that will be because you can make the spacing of the plates very small - with no air gap, of course. They are very well suited to many applications because of their small size and associated low parasitics. The nature of the ceramics is that they are brittle and hardly suited to the vibrations and shocks that you get on a rotating shaft. Your basic idea sounds very attractive and you have to ask yourself (OMG, how many times have I made this comment?) whether all the big boys have just failed to think of it and whether you can be the only person to have had the idea. I don't think I'd be over stating things to say that pretty much all the potential novel inventions that can be made to work outside the high tech industry have probably been invented. At least, the ones that haven't are very much at the high risk end of things. To me, your idea represents a massive money pit - which is why I would warn you against getting to much practically involved with it.
     
  19. Dec 9, 2014 #18
    No worries , havent spent a dime yet , that's exactly the reason why I'm writing here and also thinking to myself and reading, to understand whether something is just probable or also possible.

    You are correct , many everyday down to earth inventions have already been made and weare using them and in the last decades we have just improved on their parts or design and materials used but the very underlying principle of operation has stayed the same as with internal combustion engines which have been obsolete for quite a while so with electric motors , and various things.
    But if I would fully listen to your advice then basically , why think anything at all? taking a few cold beers and watching stupid tv shows, thats certainly easier and cheaper than a prototype of something that might or might not work. :D
    Sure the idea is interesting yet not so promising due to problems with materials.

    As for the dielectric it would be put on the stationary plate/plates, so the only small air gap would be between the dielectric and the rotating plate.In modern high tech electric motors the air gap between the rotor and the stator is pretty tight too , certainly below 1mm , so in the end if one can find a good enough dielectric with a high constant why would something else here be a problem ? I guess the main thing about this idea is the dielectric , since the plates are rotaing and need a small air gap and the surface area is limited a strong dielectric is needed. or extremely high voltage which may not be practical so again i guess it comes down to dielectric.

    I didn't fully understand why you say that impedance etc here is part of the problem , i can't see why. the capacitor and the load at the shaft can be compared to a switch mode power topology where one uses a small capacitance capacitor and a few turn wire around a ferrite core then match them to get the resonant frequency and it works, have been implemented probably a million times why here it should be different?In other words everything except the rotating capacitor thing here is a common high frequency physics engineering thing.
    I mean I'm askign this partly because i like to explore different ideas, sure with a high frequency the inductive coupling coils will not be huge as mains 50hz coils are so inductive energy transfer could aslo be used and in the end i would get someting like a 1:1 rotating transformer , just that the capacitor idea is a fun thing to think about and if possible could be a improvement.
     
  20. Dec 9, 2014 #19

    NascentOxygen

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    Have you indicated the rotational speed of the shaft?
     
  21. Dec 10, 2014 #20
    well lower something between 1000 tup to how many the device can withstand , which would be a result of the materials used the geometry of the materials and the diameter of the rotor itself.most probable guess around 3000rpm.so if the capacitor plate device could work both the stationary and the rotating plates would need to have excellent tolerance and weighting.
    Im thinking about the inductive coupling version too , as that would use avaivable materials and for high frequency be also small enough. Yet anyway even for the inducive coupling the smaller the air gap the better correct?
     
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