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Forum Index : Windmills : Correct blade size for F&P
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imsmooth Senior Member  Joined: 07/02/2008 Location: United StatesPosts: 214 |
This question is probably best for this board as many use the F&P. I am going to carve my own blades and use Oztules method. I've already bought the wood to make the jig. I have a F&P. Let's assume two cases: cogging and no cogging. Power output at 100RPM is 50W. My question is if I properly carve a blade that is 5' (1.52M) long and 9" wide at the root (using a 2 x 10 for carving), and close to 1.5" deep at the root, should it start up easily with this generator? I want to know if I need to glue wood at the root to increase the thickness so I can carve a deeper root. I think this cause more drag at higher speeds, though. Comments from those that have experimented with their own blades will be helpful. |
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GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi imsmooth, a 5' blade will have approx 1W@1m/s, 8W@2m/s, 64W@4m/s, 512W@8m/s, 4096W@16m/s, etc, etc. wind energy capture at the shaft. You have provided rpm/power data that shows a linear alternator output for increasing rpm. This is nothing like the loading curve needed to correctly load the windmill in the wind. On your data, at 100 rpm and 50W, the alternator would provide 100W@200rpm, 200W@400rpm, etc. If you design for the 50W power level, then your alternator will underload at the high wind energy levels. 400-500rpm is probably a good upper limit for a 3m rotor. I await some test data. I would expect that if you design for say 500W at 8m/s with probably 300rpm, then this may be a good place to start. This will overload at the low windspeeds, as there would only be 60W available at 4m/s, and the alternator would be loading to get 250W. You see loading voltage cutin becomes important. This is why I design for the cutin at between 50-75% max rpm, and utilize a boost cct to give current to the load at the low power levels. I hope you are able to get the loading curve sorted to make good use of the Neo magnets. Gordon. become more energy aware |
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Bryan1 Guru  Joined: 22/02/2006 Location: AustraliaPosts: 1460 |
G'day Imsmooth, Well for my first set of blades to suit a 100S I just grabbed some scrap lumber from the scrapyard and began carving. I made my set go anti clockwise and now I've put the caps on I regularly see 500 watts or close to it when a gale comes along. Now just to be safe against the blade tips hitting the tower I tilted the genny mont 6 degrees and this hasn't affected it in anyway. I've already uploaded pic's of my genny here before so I wont put one in here Cheers Bryan  |
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| imsmooth Senior Member Joined: 07/02/2008 Location: United StatesPosts: 214 |
Gordon, I have to retest the 100s stator, but I did redo the 80s. Referring to the unmodified stator/ceramic magnets it appears the power goes up x4 for x2rpm on my neo posting neo-ceramic link I was using a load of 67.5ohms. If you refer to my first test on the 80s using 100 ohms you get this curve first neo curve Here in the beginning there is a x4 increase for x2 rpm. When I redid the 100s stator I have seen that the power doubles for doubling the rpm: the voltage and current do not increase as rapidly as one would expect when the load is close to the match. I further noticed that the load changes as the rpm increases. The impedance of the generator with the added flux from the neos increases. This would explain the lack of the x4 power increase. While this would be a problem with a fixed load, I plan on getting an inverter that can vary its load at 7 or 8 points. This way I can draw little power at lower rpm and increase the load with higher rpm as needed to match the curve. I have never planned to use a fixed load on my generator. What I would like to know is will 5' blades made from individual 2x10s be sufficient to smoothly start rotation, or do I have to make the roots deeper for better startup by gluing two 2x10s and carving? I'm planning on using the former as they should have less drag and it is easier to setup and carve |
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