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Forum Index : Windmills : Mini, Dual Rotor, Axial Flux Alternator
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fillm Guru  Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Bluesmoke , You are pretty well on the mark , flux lines travel directly from one face to the other , you should have the magnets mounted on thick enough to contain the flux ,behind the magnet ( simple test is a pin will not hold ) if it does then you have flux leakage , the balancing act is getting the distance between to as strong a flux density so the max size copper and min turns can be used, this in turn usually dictates the coil size and the distance between the magnets , from what I have seen you can have them as close as you want . I do not see where or why GV would throw in a picture from some where which seems to be asking a Question . All I can make out is that maybe the diag shown might be using opposing fields where the magnets repel each other , this is/was a method used by Proven with his alternator design. PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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| BlueSmoke Newbie  Joined: 02/05/2013 Location: United StatesPosts: 25 |
Thanks for the link GV. I'll check it out ... always good to have more resources. ... and I'm always delighted to see accommodations for the Linux user. 
Fillm, I noticed that I do have a little leakage due to the (lack of) thickness of my backing plates, they are approx 3.5mm thick, it would certainly hold a pin but it doesn't look like it will be drastic in this regard. I toyed with my bearing/ rotors today and don't quite have them all dialed-in but I'm getting close, at least now everything is concentric and it's just a matter of spacing adjustment to get to a point where I can accurately test my coil(s). I'm using go-kart bearings along with the go-kart sprocket, backing plates for the rotors. They track nicely although I know I should really be using a thrust type bearing (but these should work fine for a fairly small windswept area, I think). One nice thing I'm finding out about this axial flux (my first) is that everything is (or at least can be) quite modular. I hope to dial things in the first time around but it's nice to know that a new or different stator could always be exchanged, bearings replaced easily or changed out for a different type etc. Taking my time on this little project, it's enjoyable tinkering so I'm in no real hurry. (Also have a few home improvement projects in the works now since the weather has turned beautiful here). I'll keep you all posted on progress. Thanks for your interest! BlueSmoke |
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| BlueSmoke Newbie Joined: 02/05/2013 Location: United StatesPosts: 25 |
Hi All, 1) Tested a couple of coils today. Don't really have a means of gauging the RPMs other than going as close to one second per revolution as I could. ____________________________________________________________ _____________ At one cycle per second (60 RPM) I was getting ~0.4 VAC with the 20AWG @ 70 turns. 0.4 VAC * 5 coils per phase = 2.0 VAC (for one phase) 2.0 VAC * 1.7 = 3.4 VAC (calculation for 3 phase) 3.4 VAC * 1.4 = 4.76 VDC (DC output @ 60 RPM) To get my desired 14 VDC for charging a 12 V battery the turbine would need to be turning roughly 3 times the 60 RPMs rate or 180 RPMs. ____________________________________________________________ _____________ There are still lots of variables, for one, I think when I get a stator built and the rotors really dialed-in I can reduce the air gap slightly. This will probably make the most difference of any potential changes. The 0.4 VAC is taken using a fairly cheapo multi-meter so is very likely not very accurate. RPM's are not being counted at an accurate rate. I'll try again tomorrow using a CAP and diode to see what I come up with that way (for comparison purposes). What this does give me is a "very rough idea" of how many RPM will be required to get to my target charging voltage. (Cut-in should be slightly lower). If I were to use either 3 or 6 of the 30" (Air-X or Ametek type) blades (60" diameter) then 180 RPM may not be too far of mark for hitting the range where these become productive and won't stall as easily. 2) Since I was close to the having all 15 coils wound I finished up the 20AWG coils with 70 turns each. 3) I made my stator mold today using 1/4" thick hardboard. (Getting the coils to fit within the right area was a challenge but after making a special coil shaping gig I managed to get them to sit well together and uniformly). The coils are right at 1/4" thick as well, so I'll need to add a spacer layer (should be easy to add) to my mold (middle section) to allow for any deviations in coil thickness and one layer of fiberglass per side. (This should keep me in the range of 5/16 -3/8" stator thickness which is right about where I was hoping). Total air gap is currently slightly over 3/8" now (this can be adjusted, of course). All in all, a fairly productive day. I wish I had more accurate means of testing but think things will work out fine. I have options to use a different battery voltage if necessary, use a buck converter or MPPT controller, use different blades etc. so not all is cast in stone at this point. I'll likely pour the stator tomorrow or the next day and that will be nice to have completed. BlueSmoke |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
With the Air-X blades you would want a high rpm cutin of 180 or above, only draw back is it will spend 80% of its time spinning and producing basically nothing to the battery, still its nice kenetic art to watch. In the example i gave for calculating the coil voltage, i dont think i showed you need to add 1.2 VAC to allow for rectifier diode loss. Then if you use a blocking diode in circuit to the battery you need to add another 0.6 volts to cutin voltage. All depends on your circuit as it can become a slippery slide of losses to rpm real quick. Pete. Sometimes it just works |
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| BlueSmoke Newbie Joined: 02/05/2013 Location: United StatesPosts: 25 |
Hi Downwind, Lol, yes ... I may need to just watch and appreciate the "kinetic art". (My savings from inexpensive neo's can be spent on quality paint). If I'm lucky I'll be able to charge my mouse batteries while I'm enjoying the view. 
BlueSmoke |
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| BlueSmoke Newbie Joined: 02/05/2013 Location: United StatesPosts: 25 |
Hi All, Progress Report: Coil Assembly has all the soldering done and I made 6ea. copper sleeves (for connection points) made from 1/4" copper tubing and soldered the end of each phase to these. Tested the coil phases for continuity (and for shorts) then waxed up my mold, layed in a sheet of fiberglass, poured the resin, layed in another sheet of fiberglass cloth and screwed the mold together tightly all around. Everything fit within my 1/4" spacing (mold depth) so I think I got lucky and will end up with very close to a 1/4" thick stator. I still need to pour resin around my magnets but that should go easier than doing the stator. (I just hate working with fiberglass resin, so I'm glad to be nearly through with it). I'll be working to fab stator mounting brackets soon and try to get a photo or two of my work so far. Arc out PS: Did any of you get to see the eclipse ... how was it? We didn't get to watch it here. |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Some photos would be nice to see. Just remember resin has almost no strength without glass, so add some glass around your magnets before you pour resin around them, use a cheap paint brush to work the resin through the glass to saturate it. If you wait a hour to when the resin is firm you can easy trim any excess off with a craft knife. (leave it too long and you need to grind it off) That implies you have little experience with using fiber glass, and hence my comment above. I doubt your mill will make much power so this might not be an issue, but i would have liked to see some glass in the stator around the coils, a sheet of glass either side is better than nothing, but close to useless in overall strength. The problem is all resins get softer when heated, and coil resistance creates heat which is encapsulated within the resin, now resin is a poor heat conductor so the area around the coils get hotter and softer with the more energy the mill produces, to the point the magnets can distort the stator and even rip the coil from the resin, creating a rats nest of copper wire of the stator. (lots of BlueSmoke  )
Its a bit late now, but information to consider for any future builds. Pete Sometimes it just works |
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| BlueSmoke Newbie Joined: 02/05/2013 Location: United StatesPosts: 25 |
Great information, thank you Pete! This is a good learning exercise and I will certainly take all of these things into consideration on any future builds. As you mentioned, since this mill will likely produce little power I may get away with the minimal glass here. I'll be sure to get some glass around the magnets when I pour those. The stator turned out intact around the coils and mounting points but there are a few voids around the center island and edge next to the contact points. This fiberglass experience was not what I would call a success but I learned a few things in the process. I've filled in those small voids with Bondo although I realize that is cosmetic only and has no strength what-so-ever. A light coat of spray paint on this one for aesthetics and perhaps UV protection and I'll call it a stator. Thinking about the fiberglass process, I can now see where it's the glass (fabric or matt) that has the strength and that the resin is really only there to impregnate the fabric and bond glass layers together (not the other way around). Your comments are appreciated. BlueSmoke PS: Still kind of hate working with fiberglass but probably a little less now that I learned something about it ... it is, after all, another tool (or media) that we have at our disposal to fabricate our creations. |
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