Faroun/Sam,
First, WOW! You really have the VAWT bug bad :-) Me Too !! You have done some fabulous work!
I would be happy to help you with the calculations from your actual set up, and I think 750W @ 30mph wind speed is prolly a good "best case" analysis of your current system, but that is a gut feeling, not math. On your web page you state the height as 12 feet, and it appears the central hub is ~2ft diameter with the end hubs being perhaps 1ft in diameter giving you a total swept area of slightly less than 24sqft. Despite all of the press about RPM, alternator design can be more important in the overall CP. At the end of the day your entire system is judged by swept area AND overall CP. If you have a swept area of X, the potentially available power is primarily a function of the wind speed (ws^3), the overall CP is simply an assessment of how well you have engineered your system.
You have obviously invested a substantial amount of time and effort in infrastructure and your wings. Good for you, but there is much more work to do to produce electricity. It would be a true shame to couple all of your work to a $100 DC motor designed for something else, so lets min-max how you might achieve whatever power goal you might have in mind, and lets see where the math leads us.
Swept Area obviously increases as the radius and the height increase. RPM at any given wind speed is a function of TSR and radius. Alternator output increases as the rate of change in flux increases in the stator coils. The AC impedance in the stator coils increases as the frequency increases. Jumbling all of these factors together leads us to the electricity output. You likely want your output frequency to be 50hz to 500hz, certainly less than 1kHz. For safety reasons you should keep your RPM below 1000. The height of your current project is fixed at 12ft. The radius of your current project is an area I think you might want to experiment with. I realize that your wings are "V" shaped in the vertical plane, but the radius should not be set in stone, especially since it appears you have two sets of them. For this post, I am going to assume your central hub is 2ft in diameter and the end hubs are 1ft in diameter, please give me the actual diameters and I will revise the math.
Following is a rough guide relating TSR/Wind Speed to RPM. I would STRONGLY advise you to take accurate RPM readings at various wind speeds before beginning any alternator design. You can purchase an electronic RPM instrument for less than $20.
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RPM
Radius 7 12 15 20 25 30 50
TSR Feet MPH MPH MPH MPH MPH MPH MPH
2 1 196 336 420 560 700 840 1400
4 1 392 672 840 1120 1400 1680 2800
6 1 588 1008 1260 1680 2100 2520 4200
8 1 784 1344 1680 2240 2800 3360 5600
10 1 980 1680 2100 2800 3500 4200 7000
Assuming we have a loaded TSR of ~4, your RPMs should range from 200RPM to 2800RPM in 7mph to 50mph winds. Obviously you would want to provide enough load at higher wind speeds to limit the actual RPM to < 1000 for safety reasons. In reality TSR should be the variable and RPM should be controlled by the load. With this in mind, your 500 RPM figure seems like a good choice for a first level analysis.
At 500RPM you have 8 1/3 Revolutions per second. To achieve 60hz with no gearing we would need 7.2 poles in our alternator (obviously this is not possible). Below is a table relating AC frequency to coils and magnet pairs based on 500RPM using the simple 1.33 magnet pairs/phase approach.
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Cycles
Per
RPM Coils Magnets Rev Hz
500 3 4 2 17
500 6 8 4 33
500 9 12 6 50
500 12 16 8 67
500 15 20 10 83
500 18 24 12 100
500 21 28 14 116
500 24 32 16 133
500 27 36 18 150
500 30 40 20 167
500 33 44 22 183
500 36 48 24 200
500 39 52 26 217
500 42 56 28 233
500 45 60 30 250
A quick look suggests 16 to 48 magnets would yield 67Hz to 200Hz at 500RPM. One of the truly interesting facets of the VAWT design is the freedom to make your alternator as physically large as you like. In an HAWT the alternator's diameter needs to be kept small compared to the swept diameter, but in a VAWT this is simply not true. Another bonus with VAWTs is that you can alter the physical design of the alternator to a modified radial design greatly facilitating the actual build. Let me explain these concepts in a bit more detail.
In the conventional Axial Flux Alternator design the magnet poles are perpendicular to the radial plane requiring both the coils and the magnets to be:
1) "V" shaped
2) Narrow with respect to the radial distance from the center
3) a real world compromise.
"V" shaped magnets are now readily available in a couple of diameter configurations well suited for HAWTs, but would be very expensive to have special made for a project requiring a different configuration. A large diameter alternator would allow the use of conventional rectangular magnets spaced quite some distance from the center so that the need for "V" shaping would be eliminated.
The alternative to an Axial Flux Alternator is a Radial Flux Alternator. Again the physical build of a radial flux alternator is well suited to the VAWT. Imagine a disk cut from plywood with rectangular magnets mounted to its rim so that the long axis is vertical, parallel with the axis of rotation, and the magnet pole extends outward radially from the axis of rotation. The stator coils would then be placed around the outer radius of the disk and magnets. Obviously a steel backing plate would be required for the magnets, but this could easily be fashioned from a piece of flat stock formed around the plywood. For a dual rotor the magnets would simply "stick down" from the disk and the coils would be placed "in the gap".
In either case the VAWT offers a great deal of flexibility to the alternator design. With this in mind, increasing the VAWT diameter and lowering the RPM becomes far easier for the DIYer who wants more power without gearing. A 60 magnet configuration is very feasible and with 100 to 400 rpm would produce from 50Hz to 200Hz! Assuming a TSR of 2-4, this would allow a RADIUS of ~4ft! An 8ft x 12ft VAWT is 96sqft of swept area offering the potential of 4x the power of a < 24sqft VAWT at much safer RPMs. The additional cost and complexity of building a 60 magnet alternator may have tremendous pay-offs. With a conservative overall CP of 0.2, you might expect 300W @ 15mph, 800W @ 20mph, 1.5kW @ 25mph and 2.6kW @ 30mph!
I would be happy to help in designing an alternator for your existing VAWT and/or a larger diameter VAWT made from your second set of wings. For your existing VAWT the information I need most is free running RPM at various wind speeds. Without this information it is impossible to design a suitable alternator. The other important factors are how much money you are willing to spend on materials and construction and what tools/skills you have. I am assuming from your scaffolding, previous projects and custom wings that you have construction skills, tools and a willingness to spend money on materials, but it would be very helpful in the design process to know the details.
Let me know here or in PM if you want some help with the alternator design. Regardless, Great Job So Far! Keep up the good work!
Fish
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OOPS! You already posted some of the information I was looking for! Sorry....the answer to your question about anticipated power is:
- Code:
Wind Wind Available Shaft
Area Area Speed Speed Power Power
M^2 Sq Ft MPH m/s Watts CP Watts
1.7652 19 5 2.24 12 0.2 2
1.77 19 10 4.5 97 0.2 19
1.77 19 15 6.7 326 0.2 65
1.77 19 20 8.9 772 0.2 154
1.77 19 25 11.2 1,508 0.2 302
1.77 19 30 13.4 2606 0.2 521
Wind Wind Available Shaft
Area Area Speed Speed Power Power
M^2 Sq Ft MPH m/s Watts CP Watts
1.7652 19 5 2.24 12 0.3 4
1.77 19 10 4.5 97 0.3 29
1.77 19 15 6.7 326 0.3 98
1.77 19 20 8.9 772 0.3 232
1.77 19 25 11.2 1,508 0.3 452
1.77 19 30 13.4 2606 0.3 782
Wind Wind Available Shaft
Area Area Speed Speed Power Power
M^2 Sq Ft MPH m/s Watts CP Watts
1.7652 19 5 2.24 12 0.4 5
1.77 19 10 4.5 97 0.4 39
1.77 19 15 6.7 326 0.4 130
1.77 19 20 8.9 772 0.4 309
1.77 19 25 11.2 1,508 0.4 603
1.77 19 30 13.4 2606 0.4 1042
I got so excited about your project I forgot to use the data in your post! Anyway, fixed now, lol.
Wed Sep 01, 2010 3:31 am 
