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Forum Index : Windmills : An improved "decogging" solution?
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| RossW Guru  Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Guys, I've been looking at this F&P, and turning it by hand, and can see the issue of decogging. It's a doozy! I have a cunning plan to virtually eliminate the issue, I think! It works like this. Turn the jolly thing around, put the magnetic rotor on the FRONT of the mill where the blades are. Using a light spring at the BACK of the unit, and the shaft set such that it can actually slide backwards and forwards an inch or so, let the spring push the rotor and the blades "into the wind" just hard enough to disengage the rotor under no-wind or very light wind conditions. Thus, the turbine can get away with virtually no cogging at all. As the wind increases, the force will push the blades and the rotor in the direction of the wind and increase the magnetic coupling between the rotor and the stator. Is this additional complexity that need not be there? Or is it a simple way to let the mill kick off in light winds with virtually no load? |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
You sound like someone who has just got their first F&P motor and has all these idea's.  Rest assured, its all been tried before. Ross your regarded as a endless supply of useful information, so its kinda funny to see you in uncharted waters.
Good idea, but..... The attraction between the stator and hub is very strong. Once I slipped a stator into a hub, and needed a couple of screwdrivers and a mate to separate them. If you had a spring strong enough to push them apart, no amount of wind would be enough to compress that spring and engage the hub and stator together. But it proves your thinking .. Decogging the stator is proven to help reduce cogging, and I think JacquesM-method has promise ( http://www.fieldlines.com/story/2006/8/12/213549/997 and http://www.thebackshed.com/Windmill/FORUM1/forum_posts.asp?T ID=222&PN=2, but someone needs to check the maths, the angles MUST be spot on. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
*blush* Thanks.... I think! lol. As long as it proves *SOMETHING* beyond "I'm mad"! Is the low-speed starting enough of an issue to persue? Because the logical extension of it is to combine the comments of one of the other chaps (gearing up 2:1 being beneficial, 3:1 would make it too hard to start) - what about using the blades and rotor as described, with some either mechanical or hydraulic advantage? Like the master/slave cylinder of a discarded car clutch to provide the mechanical assistance you indicate? The decogging using offset-magnets is nice, but requires skills, materials and machines not available to most of us. The other technique - grinding off the sides of the steel square pole-ends - surely they're square for a specific purpose. Is this method of "decogging" a trade-off? If you grind off some of the core, do you reduce the output? |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hey Ross [Quote]Is the low-speed starting enough of an issue to persue?[/Quote] That depends on what sort of windmill you have. For my HAWT, 2.2 meter diameter 3 blade windmill, its a big issue. A standard stator with no decogging would not start in low winds. Using figures from memory, the windmill would not start until the wind speeds were over 30kmh. But once going, it would continue to make good power in winds as low as 15kmh. So if the winds for the day never went over 20kmh, the windmill would never start, when it could be making power. My stator is now decogged. Its still is a little hard to get going, about 20kmh, but its a big improvement. Yes there is a small trade off in power, a couple of %, but the gain is more watt hours for the day. Bruce's big windmill with the 1:2 belt drive and a Neo magnet converted hub refused to start. But when he wound out the hub to reduce coggnig, it started and make over 500 watts. With the hub out his possible power is less, he could be making over 700 watts with the hub in, but a windmill making 500 is better than a windmill that "could" make 700 watts but never starts. My VAWT is a different issue. Its a Lenz type and has lots of starting torque. It starts a normal stator easily in low winds, so I dont need to decog that one. I did end up using a decoged stator to reduce noise. The vibration from the hub was very loud in those large wings, and decogging reduced the noise to an acceptable level. But this is more of a design issue with my windmill. [Quote]grinding off the sides of the steel square pole-ends - surely they're square for a specific purpose.[/Quote] A "square" end to the pole is slighty more efficient, and as the F&P was designed as a motor and not a generator, was manufactured with square poles. Wind generators like the Air-X use rounded magnets ( same effect as rounded poles ). Fun stuff. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| AllanS Regular Member  Joined: 05/06/2006 Location: Posts: 67 |
Why not have a little anemometer measuring wind-speed. When it's blowing fast enough, use the generator as a motor to get the blade turning, then Bob's your uncle. |
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| brucedownunder2 Guru Joined: 14/09/2005 Location: AustraliaPosts: 1548 |
Hi . I'd reckon this is 1 of the most 'thought about" subjects . Bloody hell, even when I'm out walking each morning ,5 K, ,I think "how can we get these buggers to start in light wind ? "-(i see my windmill from the track where I walk.) I'm almost convinced that the guy that comes up with the electronics to just pulse the thing from the control panel will be on a winner. I've thought of a relay thingo that when the actual electrical pulse gets shot up the line ,this relay blocks the retifier side off for that moment. I've been watching my mill sit without moving for 5 days now ,even though there is quite a bit of occasional wind. Seems like the pulsing electrical device would be the go . Jim in NZ was developing one of these "thingo's" wonder if he's listening ,lol ? . I would'nt have this thing permenently connected , just maybe a "attachment" that can be "push-button" operated when you think there is sufficient wind , me head hurts Bruce Bushboy |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
With the turbine blades free to rotate on the shaft you could use a centrifugal clutch that would engage after the blades have started to turn. |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
OK Bruce - we touched on this many months ago IIRC. Just musing out loud here for a moment. Feel free to pull me up anywhere :) 1. Small electronics box to "synthesise" a 3-phase signal back out to the genset head. 2. A full-blown anemometer is probably overkill for the application. Simpler would be to use two thermisters, pass a small current through both, have one exposed to airflow, the other not, and a comparator to measure the relative resistance of each. (Using self-heating of the thermister, if there is no airflow, the exposed one will track the protected one, if there IS airflow, the exposed one will be significantly cooler). Doesn't have any moving parts, isn't sensitive to direction. 3. The circuit would monitor the output from the turbine, while (a) no AC output and (b) wind detected, periodically fire a burst to the turbine. 4. The "burst" above would need to be a few cycles at least. The dwell period will probably depend on the length of the blades and their mass (ie, how hard it is to get them moving). 5. Done "properly" we would need to provide both positive and negative half-cycles to each phase, but that may seriously complicate the design. (I'll think about this) 6. We don't want no stinking transformers or overly complex bits! Glenn, do you have a simple, easy and cheap thought for "high-side" switching of powerFETs? I'm thinking that a small PIC or low-end ATmel microcontroller would be the go. I think we'll need 6 outputs to control high-side and low-side of a 3-phase 'H' bridge, if we had two A/D inputs we could do the wind-speed detection on-chip. Another A/D and we could monitor battery voltage (don't want to do this until the batteries are completely flat!), and another input to determine if the mill is rotating or not. The code to sequence the outputs isn't that hard (load a bit-pattern and shift, re-inject bit 6 into bit 0, repeat). Using H-bridges means it could be left connected to the mill, when the FETs are "off" it would be inert, and the bridge rectifier should eliminate the need for freewheeling diodes to protect the FETs :) Sounding feasible? Fingers crossed, it wouldn't be terribly difficult to construct either. Thoughts? |
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| AllanS Regular Member Joined: 05/06/2006 Location: Posts: 67 |
You wouldn't need the motor to get the blades turning. You'd just need to supply enough torque to balance the cogging. How about an electric clutch? If the blades have a flywheel attached, this would give it enough oomph to turn the generator over. Even in a light wind, once the blades get up the revs, engage the clutch and dump the flywheel's energy into the generator. On and off, all day long. |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
I was thinking just that while I was in the car going somewhere this morning. The other part of my thought was that (a) I don't want to saturate the core or magnetise it, and (b) long dwell times at high currents make for large FETs. I was thinking of PWM and if it was worth the effort (but if we use a PIC or similar it's trivial) - and a comment from years back about speed control for slotcars, where PWM gave SPEED control but because there was the full voltage, the motors were (it was claimed) developing full torque even though the speed was low. Perhaps PWM *IS* worth the effort if we can get all the torque we need! The clutch from a car airconditioner compressor might be suitable. Especially if a belt or chain drive for some gearing-up were being employed also, although I have a feeling somewhere in the back of my mind they took an almighty whack of amps to run (>5A) which might be more than the mill generates in some cases! |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Something to note. When I used a standard stator ( not de-cogged ), I would grap the drop wire and flick it around untill it hit a blade. If this sent the blade in the wrong direction, the windmill would soon change direction and take off. It didnt matter which direction is started in, so long as it got past that uphill "cog", it would keep going. Imagine you have a little car and you are driving up and down valleys. Say your car has a sick motor ( the wind ), and the hills are steep ( the cog ). You stopped at the bottom of the valley for a pee ( windmill stopped ). You try to drive up and over the hill, but you dont have enough power. How do you get over the hill? You reverse up the hill behind you as far as you can, then drive forward as fast as you can. You still have the same power( the wind ), but the momentum you have gained from the hill behind you gives you enough to get over the hill in front. 
Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| adelaide Regular Member Joined: 24/03/2006 Location: AustraliaPosts: 46 |
u can fix the problem on the outher side add some root (big cord) extenshon bits out of tin / plastic like i have done for 3 years now , folow angle of blade calk and bit wider all over seems to work exalent cos i can start 1 of my mills with a direct drive fp and (2 to 1 ratio) hose clamped car air con cluch on a second non decoged fp . will start both at resonable speed , feell bit sorry for u felow experamenters stuck at that solved point pls try and lets move onto better things like hi rpm 100 p at 12 v think it puts out 112 amps  chagin over 1000 rpm mmmm ratios/ big prophelp to make progres or radio and vitamin b |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
I will try this one again, "centrifugal clutch".
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| AllanS Regular Member Joined: 05/06/2006 Location: Posts: 67 |
I reckon a centrifugal clutch would wear out pretty fast. It'd be slipping a lot of the time, the blades spinning fast enough to activate the clutch, but not fast enough to turn the generator. |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
[quote]I reckon a centrifugal clutch would wear out pretty fast[/quote] Maybe, you would have to select something suitable for the application. |
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| dwyer Guru Joined: 19/09/2005 Location: AustraliaPosts: 574 |
Well I think Allan is got wrong ideas so look at chainsaw,bushcutter,whipper,go-kart and minibikes all have centrifugal clutch do last awhile engine go first and centrifugal clutch fail last any way there are electic clutch on webs site that can be modilfy for windmill use http://www.emtmotor.com.au/brake_clutch.htm 
Dwyer the bushman |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
The difference is, a chainsaw you start and leave at an idle, then when you're ready to start cutting run at virtually full speed. You also don't drop the chain onto a log and THEN pull the throttle. Ie, there is a very short transition period, and its done with virtually no load. The windmill will spend quite some time hovering around the clutch cut-in point, and will have load present. Finally, chainsaw, minibike etc clutches typically run at speeds MUCH higher (where the centrifugal forces are vastly higher) than a windmill, so the force holding the clutch pad is substantially less. |
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makourain Senior Member  Joined: 19/04/2006 Location: Posts: 111 |
get the thing that stops the seatbelt from coming out if you pull on it too fast. |
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| AllanS Regular Member Joined: 05/06/2006 Location: Posts: 67 |
The seatbelt thingy would be ok except for the jerk when it engages. The poor blades would fall off after a while. If you had a decent flywheel connected to the main shaft, this would reduce the stresses on the blades, but the sudden decelerations would still present a problem, I reckon. |
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| makourain Senior Member Joined: 19/04/2006 Location: Posts: 111 |
there would be a way to soften the blow surely  |
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