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Forum Index : Windmills : Rotational Speed
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| rmancini Newbie  Joined: 25/10/2009 Location: United StatesPosts: 10 |
Hello All, I’ve been lurking about for some time now, absorbing what I consider a wealth of knowledge from all you folks. I seem to be left with several questions that I hope someone can shed some light on. A little background: My mill started out with 3 blades at about 40” long. This configuration allowed the mill to rotate in the slightest breeze – BUT – I never got all that many RPMs and only about 4-5v in a stiff wind. I then went to 4 blades at 25” long that afforded a much higher rotational speed and about 11-12v also in a stiff wind. My next experiment was 6 blades at 25” long. No real change in speed or output. My question is: Would I get more RPMs with 3 blades at 25” long. I’m wondering if there is some sort of relationship between the number of blades and the expected speed. Rich Laissez les bons temps rouler |
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GWatPE Senior Member  Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
You would expect more rpm for half the number of blades. Loading becomes more critical, as there will be a tradeoff for rpm against torque. A small mill like this will be difficult to optimize this way. I think the main problem is the gen needs too high an rpm. Gordon. become more energy aware |
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| willib Newbie Joined: 24/09/2009 Location: United StatesPosts: 14 |
The way i see it , and i could be wrong, is the diameter of the blades determines rotational speed. So what would 5 blades give you over three? I believe it would more closely track the intended RPM without 'slippage' . according to Warlock's blade calculator if you were to change the number of blades from 3 to 5 , the RPM remain constant and the power output remain the same also. The thing that changes is the chord of the blade it gets smaller as the number of blades increases. thats the way i interpret it anyway. Bill |
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| birdhouse Regular Member  Joined: 27/01/2009 Location: United StatesPosts: 63 |
rich- rpm with wind turbines depends on many variables. diameter, number of blades, blade design, torque of alternator, and wind speed. the basic rules of thumb are: high rpm: less blades, high tsr, small diameter, low torque and high wind speeds. low rpm: opposite of above. look at water pumping turbines. they have many blades and a large diameter for very high torque slow speed applications. drop down to three blades at 25" and maybe your rpm will increase. maybe a three blade 30" ? depends on the torque required by your alternator. this is called matching the blade to the load. it is a tricky business, especially if you don't have accurate measuring devices for rpm, wind speed, ect. if your using pvc blades, try to round the leading edge and make the trailing edge more knife like. this will allow them to run through the wind more effeciently. do you have more details of the alternator or blade material or battery bank ect? this may lend some clues to folks more experienced than i to help you get better results. good luck! birdhouse i pee more than once before flushing, and don't have to flush at all up at the ranch! |
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| rmancini Newbie Joined: 25/10/2009 Location: United StatesPosts: 10 |
Hi Birdhouse 
Pic1 - 8" sched40 PVC blades, quartered. I didn't round the leading edges. Will probably do that next. Pic2 - Motor: 130vdc, PM. Being direct drive, I may be expecting more RPMs than what is possible. Good point about the water pumpers you see in the midwest, lots of blades and they spin really fast. 
Pic3- The mill with the 6-blade configuration. Rich Laissez les bons temps rouler |
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| Dinges Senior Member Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
Rich, This is a must-read document on blade design; it will answer your question, and many more you didn't know you had yet: http://www.scoraigwind.com/wpNotes/index.htm Direct link to the pdf file: http://www.scoraigwind.com/wpNotes/bladeDesign.pdf Generally, smaller diameter means more RPM (but less power). There's also a thing known as TSR that determines how fast the blades will turn at a given windspeed. It's all explained by Hugh in that introductory pdf file. Peter. |
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| rmancini Newbie Joined: 25/10/2009 Location: United StatesPosts: 10 |
Peter, I can't thank you enough for the link!! It is a lot to absorb and it may take me a while to digest it all - BUT - I now have information that I didn't have when I made the blades. I already noticed the relationship between the length of the blade and the tip size. It looks like I have to trim mine a little more and round the leading edges and tips. I also may go back to the three-blade configuration only this time the blades will be shorter. At the very least, I now have a new direction to go for experimentation. Again, my thanks. Rich Laissez les bons temps rouler |
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Perry Senior Member Joined: 19/11/2009 Location: Posts: 190 |
Hi Rich. It would be good to know what rpm that 130 volts is rated at. Should be printed on the motor. That will give you a decent idea what rpm your rotor needs to turn at in order to hit your charging voltage (probably 13.5 volts). Once you know this you can size the blades correctly. I think the best trend would be to go with a smaller diameter, thin bladed, 3 blade setup. Perry |
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Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi Rich, I am entering your thread a little late, but have a few comments to offer. It would seem that your generator was designed to operate (as a motor?) at a much faster speed than your mill can drive it. One clue to this is its diameter, which looks relatively small from your pics. I also note that you are only seeing it produce about 4-5 V when driven by your mill (presumably in low winds). You can, of course, get more speed out of a smaller set of blades. The problem is that, for a given wind speed, the amount of power able to be extracted varies with the square of the mill diameter. If you halve the mill diameter then, in the same wind, you will only get 1/4 of the power. As a general rule, the fewer the number of blades that you have, the higher the TSR that you can operate at, and hence the faster the mill will be able to spin in a given wind speed. Bear in mind though, that the blades need to be designed for a specific TSR to operate efficiently, otherwise they will not be effective. For power generation in particular, more is not better. Short of using a different (read larger diameter and better optimised) generator, the best way of coaxing more power out of your mill is to come back to the original 3 blades, and use some sort of boost circuit. I suggest that you have a look at the MPPT thread in this forum. Some form of speed step up, such as a gearbox, is also possible for your existing generator, but it does increase the losses, and will make starting the mill harder. Regards Don B |
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Dupa Newbie Joined: 09/12/2009 Location: United StatesPosts: 27 |
Newbie here with a little test and tune insight regarding this thread. I had the same problem I ended up with a good compromise with the blade design below. I cut from 12" diameter tubing into six blades (3 for each of my 2 towers. The tips need to run off into a narrow and very minimum pitch to achieve good rpm. I am running Fanuc servo's @ 35amp dc (real time output) with this blade design. I am not a tech wiz at blade pitch hence I had a few outright failures but the current blade I am using works terrific (thank you wind god). I can easily get 200rpm and up (sometimes scary sounding) under full load at about 10-15 mph wind. I have even ruthlessly short the three legs from my motor at high rotation during good wind and it barely slowed it down. It just kept cranking. Well all said; someone here may have a good tech explanation but take a look at my set up. It may give you some insight. Blades: 4ft Overall dia: 9ft Blade tip width 1.250" Cut from 12"dia tubing (don’t know what pitch this makes) 
I is a injenear |
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| rmancini Newbie Joined: 25/10/2009 Location: United StatesPosts: 10 |
Thanks all for the inputs - I guess you can teach an old dog new tricks! After reading the posts and mulling things over I plan to go back to the 3-blade configuration with shorter blades and see if it makes a difference. Our winds have died down here in the Pacific NW and have been replaced by a "Deep Freeze". This will give me time to reconfigure the hub / blades. I've got some machining to do to make the changes. It's funny how when you WANT wind, there's none to be had!! Again, my thanks - I have in fact learned a thing or two. Rich Laissez les bons temps rouler |
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JimBo911 Senior Member Joined: 26/03/2009 Location: United StatesPosts: 262 |
Dupa You might consider the PVC extrusions or aluminum blades that you can purchase on this forum. I know that many of us are using them including myself and find them very efficient and super quiet. You will have to make changes to mount them. Jim |
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