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Forum Index : Windmills : 3 or 4 Bladed Anemometer?
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| pntrbl Newbie  Joined: 12/07/2008 Location: United StatesPosts: 32 |
My current project is anemometer related and while I had planned on using 4 blades, I've seen numerous 3 bladed examples lately. 4 blades will be easier in terms of keeping everything perpendicular, but I could step up and do three if that's any better for some reason. I have a Spindexer that'll do 120 degree increments nicely, but I'll be down to a hand file for a flat surface to mount and locknut the arms to. Not insurmountable in Alu tho. Was gonna use square stock for 4 blades and then turn the corners off after doing the mounting holes, which would be dead easy. But I've never been one to take the easy way out. Thanx for any and all advice. SP |
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Gill Senior Member  Joined: 11/11/2006 Location: AustraliaPosts: 669 |
G'day pntrbl, I try to keep up with all anemometer threads but your reference to 3 bladed ones has me at a loss. By blades I understand you to mean propeller or fan blades as opposed to the vertical axis cup type. All examples of fan types I have seen use multiple blades. The converted computer fan covered by this site has around 7 blades as does a hand held unit I own. In spite of this, I see little to no compulsion to use any specific layout for your project. Perhaps you could tell us a little about it as you state it is "anemometer related". I'm guessing a meter to clamp onto an animals bum to measure fart volume for greenhouse gas prediction??????? OK, maybe not. But a little more info might allow us to make more helpful suggestions where possible.   was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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oztules Guru  Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Pntrbl, If you are using a digital reader, then perhaps less blades is better, as this will likely get higher rpms for the same wind with less cups. This will give you better resolution which I think causes problems at low wind speeds if the rpm is very low for you digital types. ..........oztules Village idiot...or... just another hack out of his depth |
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| pntrbl Newbie Joined: 12/07/2008 Location: United StatesPosts: 32 |
Please substitute the word "cups" for blades on my previous post. Still learning the correct terminology so please bear with me if I sound like a fool. 
Windmills are still a new obsession and it seems to me the 1st thing to do is find out how much wind I have to work with. So I'm building a pair of anemometers. Pictures are worth a thousand and this is what I've got done so far. 
That's one of the rotating shafts and the pointed thingy is out of a GM V8 distributor. I'm hoping that will mess with a hall-effect glued to a small neo enough to give me 8 pulses per revolution. I don't own an oscilloscope but I hear WinScope is fairly accurate at low frequency measurements so Plan A is to use that to set the hall-effect proximity. I'll need a clipping diode to avoid frying my sound card but I understand any 1N will do. The bearings are miniature Class 5's. The housing, hub, and arms are still hiding in some chunks of Alu out in the shop, and the cups are 50mm stainless steel coffee scoopers from WalMart. I also just got my 1st PicAxe to learn on. It remains to be seen if I'm smart enough to come up with some long term data logging capability, but I'm gonna try! So that's where I'm at. The number of cups and the total dia of the cups are both still optional. Any advice on the best solution is greatly appreciated. SP |
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GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi SP, looks like Gizmo's photo fix has broken. I still ensure my photo posts are <400 pixels wide before posting. Gordon. become more energy aware |
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| Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
SP, OK , I'm with you now. I honestly don't see much difference between 3 and 4. So long as it spins freely. Your having 8 pulses per rev is good for very low wind speeds and/or low sample duration. Does Winscope log to a file or is it in real time only? was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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| pntrbl Newbie Joined: 12/07/2008 Location: United StatesPosts: 32 |
Does the pic come up gigantic here at Homegrown for some reason Gordon? At 800x600 it's standard issue straight from Photobucket which displays quite nicely here at home and, as far as I know, numerous other websites. I know I hate it when I have to scroll all over the place to see something and have no wish to inflict that on others. SP |
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| pntrbl Newbie Joined: 12/07/2008 Location: United StatesPosts: 32 |
Thank you for your response Gill. I'll stick with Plan A for once and go with 4. Just recently it seems like all I've seen are 3 and I got to wondering if I was missing something ..... The WinScope is just a display type of software that uses your soundcard for input. From what I hear amplitude and the exact waveform are not entirely accurate, but at lower frequencies it does switch at the right time. I should be able to verify the hall-effect is working and also calibrate mine against a borrowed hand held anemometer, but that's all. No data logging. Data logging is gonna depend on me learning how to use a PicAxe. SP |
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| Dinges Senior Member Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
Whenever I build something I try to look carefully at how professional manufacturers of equipment build their products. Then I copy, or 'improve'. For as long as I can remember anemometers are made with 3 cups. I have never seen one with 4 cups (that doesn't mean they don't exist though). Not sure why the 3 blades but there's probably a reason I don't know. Unless you know for certain 4 blades works as well (or better) in your application, I would personally stick with what everyone else does. Until I had seen evidence that other methods would be actually better. An outer-hex nut has 6 edge surfaces, all at 60 deg from eachother. Maybe you could install a large nut somewhere (you'd just have to cut thread on your shaft to screw the nut onto) and then attach the cups to the nut edges ? That would ensure your cups would end up nearly perfectly at 120 deg. from eachother. |
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| pntrbl Newbie Joined: 12/07/2008 Location: United StatesPosts: 32 |
Hmm. Food for thought and Thank You for that gentlemen. I'm building two because I want one at 30ft for a baseline to compare the other one to at various heights. Since fussin' in the shop is one of my favorite pastimes anyway, I'll solve the problem by building one of each! Once we have a winner it'll just mean a new hub and potentially another arm and cup to convert the loser. What about diameter guys? The cups are 50mm. Just under 2". The hub has yet to be designed but if speed's the cure should I keep the total diameter as small as possible? SP |
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Bernie the Bolt Regular Member  Joined: 26/10/2006 Location: United KingdomPosts: 45 |
I am sure oztules is correct that 3 cups will spin faster than 4 in the same wind. I have looked carefully at my commercially made 3 cup design and think it is more aerodynamically efficient as a 3 cup than a 4. When one cup is full of wind at 90deg to the wind direction the other two are not, whereas in a 4 cup design one cup is in the best position to slow the rotation when its opposite is full of wind . Have I made that clear ?!x?x Bernie the Bolt I'd rather be sailing! |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
There is a very interesting, IMHO, page about anemometers om Wiki at http://en.wikipedia.org/wiki/Anemometer which mentions the 3 and 4 cup question and also factors relating to accuracy. |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
In IMHO, your HO is correct.... good wiki that. 
.........oztules Village idiot...or... just another hack out of his depth |
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| Philski Newbie Joined: 16/04/2009 Location: AustraliaPosts: 1 |
HI all, and Greetings from windy Tasmania i did read. The 3 cup anemometer is the international standard for wind speed measurement. 1 working cup and 2 not. against, 1 working and 3 not. cant find the link though sorry. nice job on the small block anemometer too. Phillip |
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