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  • Missing eddy currents in copper?

    Discussion in 'The main mechanical design forum' started by yadnom1973, Mar 12, 2019.

    1. yadnom1973

      yadnom1973 Active Member

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      I have a set of little flywheels that have a magnet to break the motion of the system. I have one aluminum and several copper ones all water jet cut roughly to size. The copper and aluminum disks I have are all the same dimensions but bought from three different sources.
      Yesterday I machined the Aluminum disc and three of the copper discs to the correct specs on the lathe.

      I checked the amount of drag caused by these discs some time ago and found the copper was far more effective than the aluminum. Today I needed to check the operating distance of the magnet so repeated this test on the newly machined parts and was shocked to find the three copper discs provided so little drag it bearly registered. The aluminum and the other copper ones worked very well. The drag the magnet courses on the other copper wheels is considerable so the difference is huge, one copper disc brings the whole system to a halt immediately but with the other discs I machined yesterday I can barely feel any drag at all, at first I was unsure there was any until I spun it up faster then I could just about feel some slight resistance.

      I thought the strength of the eddy currents produced was dictated by the conductivity of the metal used so I am now confused as to what happened here. As I said before the copper discs are from three different sources so maybe different coppers. Does anyone know what has gone wrong and if it's the copper which coppers are best for use in this job?
       
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    3. s.weinberg

      s.weinberg Well-Known Member EngineeringClicks Expert

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      don't know a ton about it, but did you specify what alloy of copper should be used?
       
    4. Mark_Armstrong

      Mark_Armstrong Active Member

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      Following this because it sounds interesting and I know nothing about it...
       
    5. Mark_Armstrong

      Mark_Armstrong Active Member

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      Any dimensional differences between the ones that work and the ones that do not work as well?
       
    6. yadnom1973

      yadnom1973 Active Member

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      My understanding is that copper alloys are not copper, they are bronze, aluminum bronze, brass etc. I heard of some different types of copper, deoxidized and such but I thought they were all 99.9% pure and as such the conductivity is very similar, not something that could be noticed in this use case. I've been trying to do some research but have failed to find anything that helps.

      I'm questioning if they might have sold me some kind of bronze by mistake but the colour of the problem discs is identical to the working copper ones, with that distinctive red hue.
       
    7. yadnom1973

      yadnom1973 Active Member

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      6mm to 1/4 inch thick but I've tested plates from 1 inch to 3mm and the difference between them is small compared to the difference between these two discs. the difference in drag between 6mm and half an inch, for example, is hardly noticeable.
       
    8. s.weinberg

      s.weinberg Well-Known Member EngineeringClicks Expert

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    9. yadnom1973

      yadnom1973 Active Member

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      That's a long list of copper alloys I never heard of, Many with conductivity below 40% all the way down to as low as 2% if I'm reading that right. Most of them seem to be Beryllium coppers, whatever that is, copper that doesn't conduct, I picked a few numbers at random and found one with strength and hardness similar to steel.
       
    10. yadnom1973

      yadnom1973 Active Member

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      I'm pretty sure most of these could not really be called copper, including the Beryllium coppers, it's more of an alloy like phosphor bronze or aluminum bronze. Though there's a big grey area there, Tellurium-nickel copper, Phosphorized.
      I'll just have to be much more specific next time I buy it, copper seems to a very broad term.
       

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