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Forum Index : Windmills : Yet another axial
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Rastus Guru  Joined: 29/10/2010 Location: AustraliaPosts: 301 |
Hi Chris,Hugh,Glenn, Its great to follow such indepth discussion with varied approaches and each party giving the respect due to each other without becoming dissagreable!!!!Cheers Rastus see Rastus graduate advise generously |
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ChrisOlson Regular Member  Joined: 19/01/2010 Location: United StatesPosts: 60 |
Not all all too shrill or dogmatic, Hugh. Frankly when I built the dual stator ferrite generator (The first one), I had somewhat the same feeling. I hesitated to even show it to the homebrew community because dual stator axials are frowned upon as being "inefficient", or whatever. But after testing the single phase and mapping a real world power curve for it with the turbine running way too fast, I knew it would work and match my available shaft power while still meeting the rest of the criteria I needed to get it to fit on the turbine. My design was more of a retrofit than a fresh sheet of paper. I took a successful neo machine and replaced the generator with a ferrite just to see if it could be done without changing anything else. In the end I tweaked it some more by making it more powerful yet, using bigger wire and a few less turns to increase the amp capacity and power output. Then I struggled with the fact that it needed to turn faster. The power curve was now a little too steep for the 3.8 meter rotor, and I didn't want to change the gear ratio again. So that's why I decided to try 4.0 meters with a high tip speed two blade rotor. The 13G was an instant success on the tower - pretty much 10% more power across the board. The cyclic loading of the two blade rotor when it yaws is still a minor problem but I am working on a "fix" for that too, that doesn't use a teetering hub. So that's what I do - test and arrive at results. Then go to my shop and do what makes sense to me based on the results I got. The results may differ from what's in your recipe books, but they are from a different viewpoint and goal so they are probably just as valid. I know you do not like the dual stator stacked design. But it is very powerful in a small and lightweight package that is going to be hard to beat with ferrite magnets in another design. I could stack another module on the output shaft and go bigger yet with it, increasing the power another 50%, and build a bigger ferrite machine without increasing the total weight of the generator more than about 18 kg. So the modular stacked design has some merit in my way of thinking because it's easily and economically expanded for more power, using the same stator mold and tooling that I used build the ones I already have. I also looked at doing a ferrite radial, and I looked at the Seeley motor that oztules has worked on at one point. After kicking several ideas around I decided it was going to be very hard to build a unit as powerful as what I got in the same size and weight. Probably impossible to match the dual stator axial with a radial in the same size and weight with ferrite magnets. Regards, -- Chris off-grid in Northern Wisconsin, USA |
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scoraigwind Newbie  Joined: 23/09/2009 Location: United KingdomPosts: 21 |
hi Glenn, Radial flux has some advantages but making the coils fit onto a curved surface was a pain. I don't really like using cores although they can help to reduce magnets costs and they also help with cooling. I am addicted to the low starting torque and the unlimited current surge of the axial 'air-gap' design. So far as improving on my own designs, there is always scope. I have noticed that a lot of successful alternators such as Chris' geared one use coils with smaller holes than I would recommend. ... I notice that you also emphasise the idea of having a hole in the middle of the coil that is big enough to take all of the flux. I am wondering if that is actually the best advice and considering doing some experiments there. It's true that you will get some inner turns that are being cut by a magnet arriving at the same time as leaving and therefore you will miss out on some voltage during that phase of the cycle, but it might not matter too much since: a) that is a part of the cycle close to the 'zero crossing' where the coil is unlikely to be conducting anyway. b) We are talking about adding some turns of wire that would have relatively low resistance (very short turns) and using space that would otherwise be wasted in the stator. The real question is whether the contribution to the peak voltage (which is broad in the case of a star connected coil) is sufficient to more than offset the extra resistance in the coil. And I suspect that it is. I learned early that narrow coils can give disappointing results but I may not have understood the lesson properly. I suspect that narrow holes are OK so long as the coil as a whole is wide enough. I want to quote a guy called Max or menelaos on the fieldlines board in this context. ...For a long time I as well build with wedge shaped coils now. The theory on what is the most efficient solution differs a lot from what pracical results have discovered... The magnetic fields do definitively look not even close to what can be simulated in various tools...It is more of a mess :-D ...so sticking to the theoretically most efficient coil design does not give as much advantage as we would assume it to do... So I have gone over to making the holes verry narrow, basically as small as I can bend the wire to on the bottom and as big as I can male it on the top without the coils just touching each other on the other side so that I do not have to move them appart on the bottom side where they basically directly touch. Also I found for myself that the coil legs should not be to wide. It is quite a difference if you compare the length of the wire on the first turns with the ones on the last turns...it can be more than double...only adding resistance. So making them about 2/3 of the magnets width is the most I would go for. I better don't tell people how manny stators I have build and then thrown them away after making tests on them as they crowded my tiny workshop and how manny magnets I have messed up...but it was definitively worth it for learning from it. This does not really relate the the ferrit topic but on the other hand side it does because it saves a lot of space which we need for compensating the lower flux of ferrit magnets without getting giant alternators...so with going back to ferrits, those issues become more important... Well, this is my way of approaching the problem ;-) Max It's interesting... Hugh Piggott |
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| ChrisOlson Regular Member Joined: 19/01/2010 Location: United StatesPosts: 60 |
Max is a prolific experimenter and I wish he would post more of his projects. He has done some very good research on the axial air-gap generator, and built some very large machines with them. -- Chris off-grid in Northern Wisconsin, USA |
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| Xmaswiz Regular Member Joined: 14/04/2011 Location: United StatesPosts: 69 |
don't know if this would be of use to you, but found these a while back for $.70 each 4x1x1 inch, shipping might outweigh the cost savings but thought some might want them. http://www.magnet4sale.com/Ceramic-Magnets-C8-Hard-Ferrite-M agnets.html Santa Maria, CA. Noel |
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vawtwindy Newbie Joined: 23/10/2010 Location: IndiaPosts: 31 |
Thats a pretty good price XmasWiz, I wish i can it and ship it to india? Because i was always obsessed with Oztules radial flux design and trying to build them with Ferrites. "Width (in.) 1 Thickness (in.) 1 Magnetization Through 1 " Hope this is through Width? |
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| Xmaswiz Regular Member Joined: 14/04/2011 Location: United StatesPosts: 69 |
if you click the link, it shows a pic of the magnet and the poles, they are through the thickness. They should ship anywhere. Santa Maria, CA. Noel |
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