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Forum Index : Windmills : Power output in relation to RPM
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| KiwiJohn Guru  Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Herb has quite rightly (I think) pointed out that a permanent magnet alternator increases output proportional to RPM whereas the wind turbine produces power that increases to the cube of the RPM. So what is needed is a generator that behaves more like the windmill, little or no output at low RPM increasing at a cube rate as RPM increases. Herb has mentioned that a series excited generator would do this, but I dont have one of those lying around. 
So, at the risk of starting a whole new diversion but keeping within the tradition of TheBackShed by using what might be easily to hand might I suggest that a torque converter be considered? Even small ones must be available at the scrappers now from small cars though I am not sure how difficult it would be to mount one on a mill. If I understand correctly the torque converter would allow the mill to start without turning the alternator at all then it would take up the drive at something nearing a 2:1 torque ratio and as RPM increased it would bring the rotor up to full blade RPM? The viscous coupling used on some cars' cooling fans is not a torque converter though they might be useful as a means to overcome cogging? |
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SparWeb Senior Member  Joined: 17/04/2008 Location: CanadaPosts: 196 |
Okay I'll bite first, but only because I feel like pointing out that there is nothing inherently wrong with the TSR varying through the power curve. There are many ways of optimising the generator load so that it matches the blades' wind power capture, like field-would generators and such, but the flip-side is that the traditional approach is to just change the optimum TSR of the blades! Pitch-control is the most common means. Those who have fixed-blade windmills (the majority of us home-builders) just aim for a rather high TSR at cut-in, normal TSR somewhere in the preferred working zone, and hopefully a slightly lower than optimum TSR before furling comes in to protect the system, otherwise you will see the progression reverse to increasing TSRs at higher wind speed leading to overspeed, despite the furling. That would happen in a situation where the blades over-power the generator. Mismatches don't usually last long (  sounds like the voice of experience!)
If Mr. Flux was on this board, he would bring up something like MPPT, which also forces the windmill to match the wind's cubic power curve. It takes a windmill whose genny would otherwise be to big for the blades, cuts the load down in low wind speeds, and progressively eases in extra load in higher winds until full advantage of the over-sized genny can be harnessed in strong winds. I probably haven't covered half of the inventive ways that people have to squeeze that last drop of energy out of the wind. Ultimately it has to be reliable, and be possible to hoist up on a tower, too. Do you know if torque converters are actually efficient at transferring power? How much slip is there..? Steven T. Fahey |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Torque converters have 100% slip at very low RPM and when they do begin to transfer power there is torque amplification which is at a maximum of almost 2:1 falling to 1:1 as RPM increases. |
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| Robb Senior Member Joined: 01/08/2007 Location: AustraliaPosts: 221 |
Has anyone here toyed with the idea of making a F&P with an automatically variable magnet section? ie magnet rotor slided back and forth like a fly ball governor. |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Robb, yes, someone on here experimented with that, I think they even made a test rig. |
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fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Arr , a topic that is a little more up my ally , torque converters require fluid to be shifted internally , a car T/C requires a pump and of course transmission fluid , this might serve as a slight engineering problem to mount on the front of a wind generator. Then there are the viscious couplings (fan hubs) which still need fluid to be moved into a minium clearance area , this is usually done by the metal coil on the front heating up , no torque increase in these as they are designed to give minimal load to the engine . I remember a while back there was someone posting pictures of moving hubs in and out to try and maintain higher RPM , it comes easy to say , but try starting to build it and the $ will start to accumulate quickly .. The only viable torque converter for a wind turbne is variable pitch blades which are achievable , this will allow high torque in low wind and low torque high speed in high wind and can even go into self limiting , I have toyed with this and have drawn many scribbly designs on the best way and continually come back to the same conclusion , KEEP IT SIMPLE , we now can minimize clogging by twisting the poles (SIMPLE), we can use caps and voltage doublers to better the output in low wind and increase the load and output in high wind (SIMPLE). To get even better start up with dual stators then having a floating blade hub that winds back in reverse in wind under 5klm works very well and is fairly easy but not necessary . The whole thing is I see it is to have the best blades that we can afford and be able to load them correctly by knowing what is power needed to drive the gen @ a given speed and then furling before the point of no return...Hopefully as time goes on and as the test data is posted on the power needed to drive the F&P we will be able to come up with the best blade for a different applications as well as the best cap sizes for doublers etc PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Sparweb, If Flux were here he may point to his matching the load thesis (for want of a better word). One of his more salient approaches was to wind for half the desired results... ie 24v for a 48v system. If you do the graps of power and wind speed, it turns out that the two graphs correspond reasonably well from about 15mph onwards. So, providing we can do something about boosting the output before 15mph (Gordon seems to be the man here), then the power part of the cycle (>15mph) will be well matched anyway... problem solved. Easy when you say it quick 
..........oztules Village idiot...or... just another hack out of his depth |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
It might, but the torque converter in my ute is probably no bigger than a F&P. 
The 'pump' is, I believe, one of the two rotors shown at the bottom of the picture and is part of the outer shell of the unit and the complete thing is that curious metal doughnut at the top of my picture. The doughnut would rotate with the blades (in a car it is usually bolted to the flywheel). There are coaxial splines on the back end, the outer spline is fixed and does not rotate so that would need a fixture to the windmill frame, the inner spline would drive the alternator. The fluid circulates from the TC into the transmission and through the cooling system in most cars and duplicating that would not be easy. However handling 150kW in a car is much more than we would see in a windmill and I dont really know what the effect would be of not circulating the oil at all, just how much power could be transferred before overheating became a problem? Is a torque converter a great idea? Well I dont really know but if I was interested in finding out I would look further than allowing myself to be discouraged by mechanical mounting problems. |
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| fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
John, The Doughnut, is made up of 3 elements as you can see from the picture , left to right is - Pump , Stator , Turbine , it is not bolted to the flywheel as is a normal clutch , the T/C is splined into the front of the transmission this then allows the stator to be held stationary ( hence stator ) while the pump splines to the flywheel and the turbine impellor conected to the trans input shaft and filled with oil usually by a small pump internally within the trans .The T/C pump directs oil flow via the stator onto the turbine impellors giving the fluid coupling and torque multiplication, the internal galleries to allow oil flow through the T/C are in the splined shaft it fits onto and require seals to seperate the flow , then there is T/Cs with lock up clutches then the stator is mounted on a sprag bearing and the input and output are locked together as direct drive and the stator can turn with the internal oil . One problem will be holding the stator stationary while the blades turn the pump (doughnut) would require the stator to mounted to the pole on an arm with an output shaft through it which you can then connect a gen to then block the internal galleries and fill with oil and it is that simple , post some pics when its completed  PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Just one little point, and it is hardly relevant on this particular forum but my trusty Ford workshop manual tells me to: "Remove the bolts securing the torque converter to the driveplate and lever the torque converter back into the converter housing". I did actually take the precaution of looking at a few TCs on the bench before writing my earlier post.  |
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GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi Phill, A torque converter has moving fluid and requires seals. The friction on the sealing may stall the blades at low windspeeds. Gordon. become more energy aware |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
I am sure there are a million or more reasons not to consider something. |
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| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
I am not planning on a hydraulic solution to an essentially electrical problem. Gordon. become more energy aware |
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| Robb Senior Member Joined: 01/08/2007 Location: AustraliaPosts: 221 |
How about switching phases in/out and/or cycle switching. |
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| Jarbar Senior Member Joined: 03/02/2008 Location: AustraliaPosts: 225 |
Or how about using two rotors on the same shaft one as the generator and the other as the electrical torque converter.Now as usual I'll admit my electrical inadequacy but if the torque unit in the pairing had a variable loading as I have read is used for braking this same type of loading could be used to engage the rotating prop to the generator.I imagine it could be easily and quickly adjustable maybe referenced against the blade RPM at any given time or mapped with some more high tech thingamajig computer device.Slip rings might also come into play as I think the stators will be rotating in this arrangement.This way Kiwijohn and Gordon both get a look in.If it could work???? Anthony. Think I'll get my flak jacket on. "Creativity is detirmined by the way you hold your tounge".My Father "Your generation will have to correct the problems made by mine".My Grandfather. |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Gordon, if you dont like the idea that is quite OK by me however in the spirit of TheBackShed I would not be discouraging others from thinking about it. Anthony, two rotors on the same axis was used on many light aircraft in Europe for control of a variable pitch system, as far as I know it worked well. |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Kiwijohn, Whilst we are in out of the box mode, would it be useful to consider eddycurrent style of clutch... ie use an axial style rotor attached to the blades, facing an aluminium disk attached to your gennie. That way, the prop could do it's own thing, and drive the al plate by the differential rpms. I suspect this could give you your torque converter without the oil or impellers. I have no idea really, but the eddycurrents certainly have some serious drive capability from some tests I did with my rotors when playing with them whilst testing different coils. I found that moving a copper plate between the disks was difficult to say the least. Something to ponder perhaps. .........oztules Village idiot...or... just another hack out of his depth |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Oztules, the eddycurrent clutch would certainly give a means of allowing the blades to get into their operating speed range before the load came on and certainly is an interesting concept, elegant with no maintenance parts, electrical contacts or oil seals to worry about. I dont think there would be any torque amplification though. I have pondered using a F&P as a clutch, the idea was rotating rotor and rotating 'stator', there would be no external connections, no slip rings or such like just a centrifugal switch that would short the stator coils as RPMs increased. I think Austin(?) played with an electric clutch at one time, if I recall correctly it was two rotating discs with iron sand packed loosely between them and when a voltage was applied the little bits of iron all joined hands and produced the drive. Yeah, it is all a bit outside the box but sometimes the best ideas are found there. |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
I admit I am not sure of the torque angle either. But we do slip the clutch to get a car going, and if we feel like killing a clutch, we can move off in 3rd gear by using high engine revs, slipping clutch and driving away from stop.. is this torque conversion of sorts?... can't get my head around this at the moment.... But if the prop is doing 500 rpm, and the plate is trying to catch up with the slip, it feels a bit torquy... hmmm not sure at all. 
...........oztules Village idiot...or... just another hack out of his depth |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Oztules, if the prop is doing 500 rpm and the rotor only 100rpm then I think the prop would be producing 5x5 (i.e. 25) times the power it would directly connected to the rotor, so plenty of scope to waste a bit of heat there and still come out way in front! |
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