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Forum Index : Windmills : Neo Conversion
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| KiwiJohn Guru  Joined: 01/12/2005 Location: New ZealandPosts: 691 |
I venture that cogging is a necessary side effect of an efficient and powerful magnetic circuit which is necessary for a powerful alternator. In my view the solution to cogging is to increase the number of phases and this is demonstrated by the reduced cogging of the new 7 phase machines. This does not reduce the magnetic drag but makes it less 'lumpy'. The F&P has 42 coil poles and although we can reduce this we cant readily increase the number. However from what I see on here people are changing the number of magnets without too much trouble. To get serious about cogging one could fit just 44 magnets and rewire the stator for 21 phases! Putting each coil in series with the one 180 degrees around the stator, 42 diodes would be needed! Reducing the air gap would ultimately reduce cogging (I think) because weaker magnets could be used for the same voltage. |
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| clarence Regular Member  Joined: 27/10/2006 Location: Posts: 63 |
okydoky. now to make things more confusing. I have an idea why my conversion is cogging. when thinking about this, I assumed that making the magnets wider gives more magnet area partially attracting to all the other teeth [36] not in phase, balancing the whole thing. makes sense. BUT.. what I have done is machined about 1mm from my stator teeth to make it fit in my brake drum. I should have machined the drum I know, but I didn't have the equipment. the stator teeth come up as a T shape and the T edges are a couple of mm thick, but my T edges are about 1/2 mm thick now, so the attractions are not as pronounced on the edges of the teeth. There may be a chance that a conversion with 10 mm wide neo's will be ok or at least better than mine. sorry for all the confusion. I really should have thought about this before I machined my stator. btw I am getting 7.2 volts ac, 1/2 amp load from one phase at 120rpm. tooks some pics today. http://www.geocities.com/clarence_horatio_dexter/trailer/sta tor.jpg http://www.geocities.com/clarence_horatio_dexter/trailer/sta tor2.jpg http://www.geocities.com/clarence_horatio_dexter/trailer/rot or2.jpg http://www.geocities.com/clarence_horatio_dexter/trailer/rot or.jpg |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hey Clarence Hope you dont mind but I made those links work better. You can use the Add Hyperlink button when typing a new message to add links 
What you said about grinding off the top of the T makes sense. Its just like Bruce's magnet hub, his magnets were too narrow ( 6mm ). Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| clarence Regular Member Joined: 27/10/2006 Location: Posts: 63 |
no worries thanks Glenn. I wondered why they werent clickable. KiwiJohn, I read about 21 phase idea somewhere, and it makes sense, but maybe there could be an issue with the frequency being too high? does anyone know if this has been tried? |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Clarence, the frequence would actually decrease, the frequency in Hertz (cycles per second) is related to the number of magnet poles that pass a coil pole. Less magnets means lower Hertz, lower RPM means lower Hertz. |
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| clarence Regular Member Joined: 27/10/2006 Location: Posts: 63 |
yep less magnets lower frequency. I cant argue with that logic. I wonder if this 21 phase thing is the way to go with neo conversions? maybe the rectifier diode losses are a bit of a problem? |
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Highlander Senior Member  Joined: 03/10/2006 Location: AustraliaPosts: 266 |
Clarence a bloke at an re shop told me about magnetic saturation, where if you don't have enough metal in the rotor and stator the magnetic force can only reach so high untill it saturates the weakest or lowest metal component of the magnetic circuit. He warned me by adding too many magnets it will only cogg more and not produce any more power and as I can't add metal to the stator your limited by that, the only other thing to do would be add metal to the rotor or use a different rotor. So when I decogged my stator by just rounding the edges and it lost 2.5 amps that all made sense. I don't know if by grinding your stator would make it cogg more but it will lose amps. Brain hurts time for a beer 
I like your use of the brake drum 
Central Victorian highlands |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Clarence, no matter how many phases you use any particular current only flows through a maximum of two diode junctions. Highlander, if your brain hurts try looking up that article by Dr Tom Chalko, you can find it here (6 pages). I really needed to read it a couple of times to come to grips with it (and I need to read it again) but really he does give just about all the information we need and he is talking about F&Ps! |
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| Highlander Senior Member Joined: 03/10/2006 Location: AustraliaPosts: 266 |
Hi KiwiJohn,Thanks for that. just skimmed over that report. Looks like HEAPS of good info there, I can see what you mean about reading it a few times. This is starting to get too complicated. BTW brain hurts even more now, time for a slab. Central Victorian highlands |
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| clarence Regular Member Joined: 27/10/2006 Location: Posts: 63 |
yes kiwijohn, I hear what you are saying. somewhere there may be a compromise between cogging and output efficiency. The ? is how much does decogging degrade efficiency? Is it worth decogging to accomodate less efficient props to enable startup? Is a generator approaching 90% efficiency realistic for wind turbines if the cogging is too severe? I really dont know, but I bet there are heaps of people that have done the sums and are selling their products. I am TOTALLY in the dark regarding f&p, but all I can do is experiment and give the most accurate results possible. I reckon with enough people doing tests with different magnet configs, it will all make sense eventually. |
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| clarence Regular Member Joined: 27/10/2006 Location: Posts: 63 |
today I rectified my contraption. it is 7 phase 6 pole 1mm wire stator connected star. 19 volts dc with a 2 amp load at 120rpm. I thought the voltage would be a bit higher? but I am very happy with that voltage. I wanted a cutin speed of 100rpm coz I am going to use a 12 foot prop eventually. I put a 100 watt globe on its incredibly hard to turn, but thats a good sign. I give it a HUGE heave and lights up the globe very brightly and the voltage reaches about 15 volts. not very scientific tests. man I wish I had a lathe.  |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
After reading Dr Chalko again I took a deep breath and reached for my hacksaw. I cut a 24 coil segment out of my stator leaving (of course) 18 coils in three phases of 6 coils each. I dont have a lathe but I do have an old exercise bike that is really good for messing around. Before I cut the stator I could manage about 50 watts for a reasonable period of time but cogging made it hard to get started plus the whole thing came to a stop quite quickly when I eased off on the pedals. With the new 18 coil configuration I can just as easily make 65 or maybe 70 watts! It is easier to start and does not stop quite so quickly. Read Chalko 5.2 and that is pretty much what he wrote would happen. |
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| clarence Regular Member Joined: 27/10/2006 Location: Posts: 63 |
did you experiment with capacitors like chalko did? that was very intersting work. |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
No, I have not tried capacitors, maybe I can buy some tomorrow, they might be a good thing because my voltage is just a little lower than I would like. |
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RifRaf Newbie  Joined: 15/10/2006 Location: AustraliaPosts: 32 |
kiwijohn have you got a picture of the way you chopped the stator up? this sounds interesting. |
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Pt w/field Matt Senior Member Joined: 24/02/2006 Location: AustraliaPosts: 105 |
you dont have to cut the stator just grind the top of the unused poles down 2 or 3mm,you could remove the wire as well.on my exersize bike i use 6 poles and 3 poles both in star 1mm wire size 14.75 gearing and produce 7-10 amps with a max of 40 amps or 550watts matt down south |
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