Quote from: gyulasun on October 01, 2013, 01:25:47 PM
Hi TinselKoala,

Yes but either using laminations or ferrite ring the eddy current issue could be minimized.  Of course machining either the air gap in the ferrite or forming the stator from laminations surely makes tinkerer's life harder...   

Gyula

The laminations would have to be parallel to the slots, through the thickness of the material, not the flat way parallel to the ring itself, to reduce eddys much, I think. A nonconductive ferrite might work, I don't know if they heat up when subjected to a changing field or not. The ferrite core pieces from a CRT flyback transformer, which is split, with tiny thin spacers in the split, might work for a trial even though it isn't exactly a "ring" shape.

Indeed it makes it very hard .

In the first picture you can see how I ignorantly started cutting the laminated toroid from the outside to the inside. This caused the toroid to open up with great force as it had nothing to hold it in.

In the second attempt I started cutting the toroid radially, the material is holding its own now, that is until you cut all the way through. The hot glue trick in the second one was also a lesson learned quickly. Since the core is not glued together, as you cut radially you start loosening the inner/outer windings of the toroid. This starts out very Innocently but becomes a royal pain in the ass as you go. So either use epoxy or hot glue to hold the laminates, near the cut, together is a smart move.

Ideally I would hit alibaba.com and ask some Chinese manufacturer to send me a perfectly cut toroid .

I also have some ferrite toroids, this seem to show the effect of low vs high permeability quite well. They don't need an air gap as the bridging piece is attracted quite well to the toroid. However I have no clue about the properties of these specific toroids, but I would guess that the permeability is too low and they also seem to have an awfully bad retentivity, they are permanently magnetized in some parts just by having had neo mags close to them at some point in time.

I would also like to add that there are many different takes on this concept I just picked the one that I thought was most practical to build.

Below are some variants. The first uses motor laminates the kind made for slotted motors. Here the rotors is on the inside to prevent eddy currents.

The second is a bit more special in the sense it has one air gap and uses the toroid itself as the generator. So the coil is wound around said toroid. Again since there's a higher permittivityon one side the flux will predominantly flow to one side untill the rotor hits the 90° mark and again you'd have a max to 0 flux scenario which would generate a voltage. However as far as I know the generated toroidal current should have no effect on the spinning rotor.

Lots of food for thought

Hi Folks,

For those wishing to attempt a build these ideas, I list some toroidal core offers. Unfortunately the cores with  reasonable OD for such setups are not really cheap.  Most of the cores below have a know permeability.

At Surplus Sales of Nebraska: http://www.surplussales.com/Inductors/FerToro/FerToro-1.html   these are Amidon cores:

(ICH)  MM-T225-6  OD 2.25ˇ h= .55"  Mix 6 yellow    6 USD
(ICH)  MM-T520-2  OD 5.2"   h= .8"    Mix 2 red       49 USD

EPCOS manufactures also larger toroids:
 
OD/ID/h in mm:   87/54.3/13.5  N87 TOROID,  Mouser Part No:      871-B64290L0730X087  18.96 Euro
                           73.6/38.8/12.7  77 TOROID,  Mouser Part No:      623-5977011101           16.68 Euro
                          102/66.0/15.0 N30  TOROID   Mouser Part No:      871-B64290A84X830     26.88 Euro

At Farnell (Newark in US and Canada) there are also EPCOS cores like B64290L0084x87,  102/65.8/15  at Newark 30.26 USD

or Ferroxcube cores like TX87/54/14  3C90    21.90 USD at Newark
or                                   TX80/40/15   3C90   19.59 USD at Newark

Of course on ebay you can also find some  (OD 2.25" and 2.9") toroids at seller 'alltronics' for instance or others.

I have found an interesting and unusual ferrite cutting "method" albeit it is shown for ferrite rods, it is rather a 'braking' method because it actually brakes the material. But I think the full air gap instead of the partial airgap may not make much difference in the operation of such setup.  Nevertheless, it is a risky method for sure because of the cost of the larger OD toroids.
This is the link to Clanzer's video on ferrite rod "cutting": http://www.youtube.com/watch?v=cUWn5m8ASC0

But in fact as broli wrote above, ferrite toroids may not need cutting at all in these setups.

Gyula

Since I'm located in Europe I ordered mine from Magnetec.de:

1 x M-028 = 100,00 EUR
1 x M-071 = 70,00 EUR
1 x M-454 = 70,00 EUR

Hopefully cutting them won't be as painful as the sillicon steel, mentally and physically :p. A good saw blade would be handy.