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Forum Index : Windmills : An improved "decogging" solution?
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| AllanS Regular Member  Joined: 05/06/2006 Location: Posts: 67 |
I read somewhere of a clutch containing a fluid that turns viscous in a magnetic field. Anyone know about these things? They might do the same job as a centrifugal clutch but never wear out... |
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| KiwiJohn Guru  Joined: 01/12/2005 Location: New ZealandPosts: 691 |
That viscous clutch sounds like what the Japanese (example Toyota) were putting on car fans in the 70's, not sure about the magnetic aspect of it though. What is really required is a torque converter but I am not sure where you could find one that would operate at windmill speeds. |
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Trev Guru Joined: 15/07/2006 Location: AustraliaPosts: 640 |
Hi all, To have such a problem, you can't have enough driving force from the blades, that is they are not big enough for the load you you have. Or the load is too great for the driving force. My composite fibreglass blades have my f&p (not decogged)turning in such light winds that the voltage is not high enough to charge into 12v batteries. Of course its not long before it does charge. With the stator rewired in a 7 pole delta the voltage comes up quickly. 60 rpm it starts charging. Like Glenn says, its no good having a 700w turbine if it doesn't start. The secret is to match the driving force to the load. AND extract from the available wind what is posible. Trev @ drivebynature.com |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
The problem, as I understand it, is not the *LOAD*, but the starting torque required to overcome the attraction between the magnets and the (iron/steel) poles. At zero speed, there would be zero voltage produced and thus zero current will flow so no load. Does having such large blades reduce efficiency and/or cause significantly higher noise at higher RPM? Exactly, which is why we're talking about various ways to either decog or otherwise minimise the starting torque requirements. I think that's the secret to getting the most out of the system at a given windspeed. The issue of getting the darn thing to START is, I believe, unrelated (although blades with lots of area will probably develop a lot of torque and overcome this problem, they don't make blades as thin as practical for no reason!) |
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| Trev Guru Joined: 15/07/2006 Location: AustraliaPosts: 640 |
Hey Ross, You are right in talking about electrical load, but the magnetic field is also load. Friction is also load. any resistance is load. Again, you're right, its the starting torque that needs to be matched to what ever the load is. If the prop is too small, then decogging will make the power plant smaller, less load, and produce less power. If the prop is a little bigger, enough to drive without de-cogging, then it will produce more power. Yes, larger blades do tend to have more noise than shorter ones, because of the greater tip speed. I don't have much noise from my long ones though. Noise is developed mainly from un-equal wind pressure along the length of the blade, that is, the blade doesn't have the correct twist. As for efficiency, my 3 blade, 2.9m diameter machine would start up sooner, and produce a higher charge rate at any given wind speed, than would the 6 blade, 2.1m diameter machine. I don't claim to know everything, and there is more to the equation than I have mentioned here, but I beleive this is the basics though. Please correct me if I'm wrong. The reason for thin blades is to minimise the wind disturbance and reduce the effect of slowing the wind flow. This should also reduce the noise, the primary reason for narrowing toward the tip, which has the greatest speed. But if a thin blade won't make it turn, then what good is it? Any how, more experiments will happen some time, perhaps I will learn some more. The point here is, decogging is not the only answer to get them turning. Trev @ drivebynature.com |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Hi, Thanks to Glenn for setting up this forum because Fieldlines does not have too many F&P contributors as the F&P washers do not stand on the streets for road-side rubbish pickups in the USA! As ecoinnovation talked of "floating hubs" plus rounding off of the poles as the solution to cogging in their commercial units I have experimented with a floating-hub solution and succeeded. Prop is the Latham-suggested 3m or 10' dia. PVC for the large starting torque required. If interest is shown for the floating hub, then I can post the images on this board, too. This is how it works: Bolt on prop hub hits a spring-loaded lever connected to the F&P shaft and as long as there was separation between bolt and lever, it will wind up the spring and get the torque necessary to start full rotation. Need to balance the two props and setup a larger tower, etc. AND get my Picaxe going for my odd 36VDC system before I can come up with performance figures. Have noticed that some F&P stators no longer have enough "meat" at the poles to allow rounding to reduce cogging (and power). Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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windstuffnow Newbie  Joined: 30/06/2006 Location: United StatesPosts: 31 |
All this fuss about the F&P just plain festered my curiosity to the point I had to buy one. I really don't have much time to play with it so it will go on my shelf for the time being. Just playing with it and working out the details it has 42 coils and 56 poles which means about 167 coggs per revolution. It's wired as a basic 3 phase and it seems to me that replacing the magnets with 14 poles would reduce the cogging considerably. Another thought was to remove the magnets and install 58 poles to offset the difference. The main problem is the magnetic poles line up with the iron fingers to closely in small segments with 56 poles. 58 would help but wouldn't solve the problem just offset the connection enough to reduce it. It shouldn't cause a major problem in output although there will be an area of canceling. One other thought would be to machine the stator back about .030" if it were going to be used as is then only slightly taper the entry and exit. If you wanted to rework it you could rewire it for 62 or 64 poles and that would take care of it. I haven't measured the diameter of the drum yet so I'm not sure how many poles you could actually get in there. I'm hoping to have a little extra time this winter to play around with it. Looks like fun! . |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hey Ed Glad to see you have a F&P. Interresting little beast arn't they. The cogging is only a issue for high speed HAWT's, with little starting torque. Big blades with lots of torque work good, as does your Lenz vertical. We like the F&P because down under they are easy to come by, can make over 300 watts with no modification, and cost bugger all. All the hard work is done, no need to glue magnets, wind coils, etc. And the stator ( 250mm, or 9.84 inches ) is well made and air cooled, so they can pump out close to 1kwatt without the heat problems of the home made axial designs. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Good news! Newer F&P motors no longer suffer from cogging. Have a look at the Ecoinnovation web site in NZ, there Michael Lawley is advertising commercial mills with the new cog-free F&P motors. Also a Chinese-made mill with "pulltruded" constant pitch blades with no taper. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
My brain hurts when I try to work this out but with 42 stator coils and only 3 required to give a useful voltage it might be practical to fit magnets in positions to make a 14 phase alternator. Each phase being 3 coils. Of course this means a lot or rectifier diodes but cogging would be greatly reduced, down to about one fifth of what it is with "14 coil" phases. |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Forgive me if I'm wrong (I'm good at code, not so good at windmills yet!) - the cogging isn't a function of the load, rather one of magnetic attraction to the pole-pieces? Therefore, regardless of how you wire the dang thing, the cogging will be an issue because of the way the magnet wants to "stick" to each of the steel/iron pole cores? The only way I can see of reducing the effect (apart from increasing the distances, or electrically balancing (well, neutralising) the cores (which would require power)) would be to incrementally stagger them so there was no position where the rotor went from a (nett) magnetic attraction to a (nett) neutral point. Ie, if you had enough magnets and poles to ensure that the magnetic attraction across the entire machine was uniform across the entire angular rotation. |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I spoke to Michael from Icoinn a couple of years ago and he mentioned then the new F&P had no cogging, and like mentioned above, it looks like he's using them in his new windmill. This means it wont be long before we start to see these new hubs on the rubbish pile. Michael did mention something about uneven magnet spacing, and I ventured into this here... http://www.thebackshed.com/windmill/forum1/forum_posts.asp?T ID=222&PN=4 It may even be worthwhile to buy one of these new hubs ( I believe the stator remains unchanged ) new, might cost $100 or so dollars, but means a cogless windmill. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
[quote]The only way I can see of reducing the effect (apart from increasing the distances, or electrically balancing (well, neutralising) the cores (which would require power)) would be to incrementally stagger them so there was no position where the rotor went from a (nett) magnetic attraction to a (nett) neutral point. Ie, if you had enough magnets and poles to ensure that the magnetic attraction across the entire machine was uniform across the entire angular rotation.[/quote] Errrrr, I think thats what I was saying Ross, re-distribute the magnets so the minimum number of coils are going through the maximum flux positions at any one instance, in the standard configuration 14 poles are aligned at once. |
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| brucedownunder2 Guru Joined: 14/09/2005 Location: AustraliaPosts: 1548 |
Very Interesting news. I 've heard about this new hub some time back ,but forgot about it. Wonder if Michael would be kind enough to "fill us in " --- Might send him a greeting and ask??. I'm sure he would know ,as he is the main guy getting these "discarded" units from F&P. Maybe we can reposition our existing magnets once we see the new rotor-or maybe the magnets are a different shape??--Anyone know a friendly F&P service outlet that would let you photograph oner of the new magnetic hubs???. Be great if we get over this "cogging" . bug. Bruce Bushboy |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
If you do email Michael, ask him if he knows what model numbers the hubs come from. I was going to send him an email later this week, but you might beat me to it. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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