Optimum Blade Diameter for Wind Turbine

[Kevortex]

Does anyone have a design criteria or rule of thumb for determing the optimum blade diameter for a wind gen? Is it based on wattage of your turbine? Torque required? Rotational speed required? Cut in speed? What about blade width?  How many blades is optimal? 2? 3? 4? 6? I'll be using a 1400 watt DC output PM generator and I want to try building my own blades (PVC). I'd prefer a VAWT but for this project, I'll try a HAWT.

There are such radical differences in the wind gens I see. Someone has to have a philosophy on this, don't they ?

[wdyasq]

The best thing for you to do is a bunch of research. For your first project I recommend doing it like folks cook .... use a known recipe. Make that, then decide on your next move.

You just asked a question with 7 variables and one constant. A very good math guru can work 4 variables .... sometimes. 5 variables ... only in dreams ... those dreams are referred to as nightmares.

Ron

[Flux]

Determined by the power you want in a given wind speed.

Energy in the wind is fixed for a given wind speed and rotor diameter, the overall efficiency ( Cp, typically not much better than 0.2 unless you do something clever) then finally settles the issue.

Any other way of looking at is backwards.

Flux

[racer]

Flux

"Determined by the power you want in a given wind speed."

Couldn't your first sentence be the power needed to optain the power one desires at a given wind speed.

I have been testing my first motor conversion on my little lathe plugged into a Watt meter. My first reading was the lathe turning by itself to get estimated watts required for my lathe to run at given rpm's. Then I ran my second test with generator running open circuit(volt meter). I noticed a pretty big jump in total watts needed to turn the generator. example; it neededs est. 30 watts of power to turn 180 rpm's and 64 watts to turn 420 rpm's.

My third test connected to 24 volt batteries showed that I get 56 watts output for 450 watts of extra power load on the lathe over free spin usage. 36 volt test showed only 85 watts out for 476 watts in. Will try 48 volts next.

My input watts don't count DC motor and controller loses at loaded rpm's.

It makes choosing the right blade design dificult. My best guess is a TSR 7 6.5 foot cutin around TSR 9 and stalling to TSR 5 for a 48 volt system.

I use "www.alton-moore.net/wind_calculations_english.html" to just estimate blade design.

[CmeBREW]

Hello-

"1400 watt DC output PM generator"

Can you give us more info about this DC generator?  Is it actually a DC motor? If so, what are the ratings?(volts,rpm,amps)

If it is actually a real DC generator, what is the name brand, rpm, rated volts, shaft size, etc.

Most everyone here will say that 3 blades is optimal. (even though, it is no crime to experiment with 5 or 6 blades in a turbulant wind area)  

How are the average winds in your area?  Do you have low winds most of the time?

Also, it would help to know what your system voltage will  be?  We assume 12v  

[Flux]

"Couldn't your first sentence be the power needed to optain the power one desires at a given wind speed."

I can't see any difference between that and what I said. But form the rest of your comments you seem to be working things backwards. You are trying to choose the best blades for a given alternator. When you have made your choice you have to take what you get. If it is less than you wanted in a given wind you are stuck with it.

Many do it this way and if you have no specific figure in mind in the first place then it is ok.

You have noticed that your alternator takes power on no load ( mainly iron loss) and it takes lots of power to drive it on load, partly due to the load itself and partly due to inefficiency.

Using drive motor watts is an indication but includes the motor and other losses and will be more trouble to calibrate than it is worth. Mount the alternator shaft in the chuck, run the other end on a centre in the tailstock and let the body float. Restrain it from rotating with a spring balance. You now have direct mechanical input from torque and speed and you can accurately match the input power to the best blade for the wind speed you want to work at. Apart from fiddling with tsr you have little latitude and you have to take your best match as the power you can expect.

Flux

[racer]

I started with a motor conversion (stock wiring) just to get something going. I didn't have a spring balance so I made a bracket with a lever on it and attached it to the motor base. Measured out 12" from shaft centerline and put a bolt through lever to rest on my electronic scale. Using the formula ft.lbs.*Rpm's/5252*746=watts I can now see that my lathe was pulling allot of watts. Makes my conversion look allot better. Thanks for the help.

[Kevortex]

I'm using a DC permanent magnet motor that is rated at 2 HP, 230 volt, 1750RPM, (amps unknown)7/8" shaft size.

I'm in Iowa and wind speeds run in the 12 mph and up range. Iowa is one of the highest wind energy states in the USA. I'm using a 12 volt system. This is my 3rd wind generator. The previous ones basically used experimental prop sizes. I started constructing a VAWT design to use with this generator but I though I'd get this up and running a bit sooner while I work on the VAWT. And yes, I'll post some pix of the projects.

Thank you everyone for all your help.

Kevortex

[ghurd]

Every drop of info about the motor might help.

It is a decent HAWT motor, by the sound of it.

G-

[CmeBREW]

Do you know what rpm the motor/generator reaches 12v ? ('cut-in')

This is important to know.

Guessing, it looks around 100rpm which is sorta slow for 12v system. It would probably stall with 3 blades. (Probably even PVC)

You might try the longest PVC blades possible (7'dia. from my experience) , and keep adding blades until it gets over Stall in low winds.

Or, later you could try 3 wood blades, which are faster at any given wind speed than PVC, and thus, more power than PVC blades, but you would probably have to move up to a 24v system so no stall in low winds.

You just have to do some experimenting like the rest of us with motors and/or servos as wind generators.  Thats part of the fun.

-Just some possible ideas.