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Forum Index : Windmills : spacing for coils in axial flux alternato
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| arklan Newbie  Joined: 18/08/2010 Location: AustraliaPosts: 29 |
i have a question about the spacing of the coils in axial flux alternators. from what i understand, the gap inside the coil has to be about the same size as the magnet or bigger. if the magnet passes over 1 side and then starts passing over the other side before leaving the first side, they cancel each other out and drop the voltage. what i see in all these stators is this gap inside the coils, but what about the outside of the coils? from what i knew of the theory of how it works youd need the same gap on the outside aswell? what most coils have, all the coils are bunched up against each other... 
wouldnt this work better? the gap inside the coil is the same as the gap around the outside of the coil... 
the gap inside and the gap outside is the width of a magnet |
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MacGyver Guru  Joined: 12/05/2009 Location: United StatesPosts: 1329 |
arkian Use 1/2 the magnet's width for the inter-coil spacing. This assumes you are doing a 'traditional' axial-flux build with a 3:4 coil-to-magnet ratio. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
My experience in axial flux alternators is limited, which is one reason I'm building one so I can learn more, but what I do know is you need to get as much copper in the magnet path as possible. Your right about the inside space, it has to be slightly bigger than the magnet. If its smaller, some of the current generated will be canceled out, effectively wasting copper wire and increasing coil resistance, reducing the potential output of the alternator. But the coils should have no space between them and the next coil, thats just wasted space. A magnet passing over empty space wont be generating any current. I also think, though not sure, that the width of each side of the coil should not be wider than the magnet. For a given number of magnets, there is a optimal coil size, and therefore a optimal PCD for the magnets. This is where a cad program comes in very handy. But at the end of the day, either of the drawings above will work, the top one will give a higher output. Sometimes you need to make a compromise between the best possible layout and whats actually achievable. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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niall1 Senior Member  Joined: 20/11/2008 Location: IrelandPosts: 331 |
hi Arklan i,ve been looking at the nice diagram you posted and thinking a lot about it ...it seems if you layed out another 3 coils beside it (extended it ) the mag layout might be a bit off...would the edit below be a bit a little different ?....thanks for putting up your diagram Arklan...its getting me to try to think about stuff.... 
ps..and reason my sanity ... 
niall |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
arklan What Gimo says makes sense. Perhaps I should have put "What 'I' use is 1/2 the magnets' width for inter-coil spacing, which is what I did on my ax-fx and merely passed on that information to you thinking I was spot on. Maybe if I were to tighten up my coils' "real estate" my little ax-fx would do better. Then again, it's so small, the difference may not be measurable. Hope I didn't lead anyone astray. Go with Glen's advise here. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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ChrisOlson Regular Member  Joined: 19/01/2010 Location: United StatesPosts: 60 |
Glenn is correct. Your generator layout should look similar to this unit that I just built over the weekend: 
That generator is wound with 12 AWG - don't even attempt it unless you have a conduit bender to make coils with. If you build with large gaps between the coils the generator will "buzz" and vibrate and you'll have stator bolts coming loose all the time and several other problems. This is due the loading/unloading of the generator pole as it wipes a coil leg, then "lets go" and travels across a gap doing absolutely nothing except going for a ride on the merry-go-round until it gets loaded again when it finally gets to another coil leg. The rule of thumb that I use is no more than half the width of the magnet as gap between the coils. However, even at half the width of the pole, the generator will still vibrate bad enough. The generator in the photo was installed on the turbine last night and ran all day in a 20-25 mph breeze on a 10 foot machine and it runs so smooth at 1 kW that all you can hear is a steady hum in the tower. I've run generators with wide spacings between the coils and they buzz enough to cause sufficient vibration in the tower to take the bearings out of your anemometer. The other thing that's important in an axial is having the coil leg perpendicular to the path of the magnet, hence the wedge design coils in the photo. To get optimum efficiency adjacent and opposite generator poles should wipe both coil legs at precisely the same time. You can't just arbitrarily make up coil shapes and dimensions and get an axial to work. Those 12 AWG coils were such a bitch to get right that I still ended up with a slight bow in the coil legs, even after clamping the coil between two wood blocks and beating the snot out of the coil leg with a rubber hammer to straighten it. But it's close enough because it runs pretty darned smooth. Edit: This is what that stator looked like after casting and ready to bolt on the turbine. I cast it yesterday morning and bolted it on the turbine about 8:00 last night. This is a 24 volt unit, 50 turns of 12 AWG, N50 2x1x.5 bar mags on 10" generator rotors for a 10 foot machine. It put out 44.61 amps @ 28 volts today, peak on the Doc Wattson, and ran at 30-35 amps most of the day. 
-- Chris off-grid in Northern Wisconsin, USA |
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