How to size the propeller based on generator specs

[AdamM]

Hi Folks,
My brother and I are working on a small wind generator project.  After taking apart a few treadmills, we've found a PM motor that looks like it has a RPM/voltage ratio that might do for a generator. Now we're wondering how to match the size of the propeller to the motor for best results. I have Ed Lenz's blade designer program, but it seems more geared for determining the specifics of blade shape rather than bigger questions like what size the blade should be in the first place.

This is largely an experimental project, which will be used to charge some backup batteries for when the power goes out. Since the usual cause of our frequent power outages is wind storms, low wind performance probably won't be a major concern.

The motor is labeled 2.25 HP @ 130 VDC / 2200 watts. I believe it is the same model as this one: http://www.ebay.co.uk/itm/150664839024 .  There is no RPM rating on it, but when we chucked it into our drill press, geared for 620 RPM, the open voltage on the motor was around 19v. At around 1100 RPM we measured 33v.

In order to avoid overheating the motor, I figured we should keep the output current below the rated input current. That is, 2200/130 = 16.92 A. So at a battery charging voltage of 14v we are looking at a maximum theoretical output of around 237 watts. We'd like to drive it directly, for simplicity.

I can put different prop values into the blade designer program, and (as well as the detailed blade measurements) it gives an "estimated prop performance" table with figures for wattage and RPM at different wind speeds. Based on some trial and error with this, it looks like a 2 meter 3-bladed propeller with a TSR of 12 and the default values for angle of attack and lift coefficient (4 and 0. would be somewhere in the right ballpark.

Does this sound reasonable to you? Is 12 an unreasonably high TSR?  Does anyone have rough guidelines or suggestions about what the blade size and tip speed ratio should be for a given generator and wind situation?

Thanks very much for your help,

Adam

[wooferhound]

This may help you a lot . . .
http://fieldlines.com/board/index.php/topic,143631.0.html

[birdhouse]

adam-
to me 620 rpm for 19v sounds way too low for cut in on a 12v nom. system.  you'd need some VERY strong winds just to trickle charge a 12v battery.  

i started with a motor that hit ~14v @ ~120rpm.  ended up using it on a 24v nom. system, and it cuts in a little late.  probably be near perfect for a 12v system.

if it were me, i'd scratch that motor, and find a lower rpm or higher volt or both version.  start looking at servo motors on ebay.  don't get scared of the three phase AC, it's easily converted to DC with a rectifier.  

tsr of 12 is getting a little crazy.  seems most here aim for tsr 5-8 (and most probably miss their design tsr by 1-2 at average wind speeds)  tsr is a tricky beast to learn exactly what's involved and how it varies with alt. loading, windspeed ect...

adam    edit: (yea, we've got the same name)

[Flux]

Depends on what you want.  500rpm cut in is way too fast for a 2m prop.  With a very fast 2m prop ( 2 blade and not to Ed's design ) you won't see anything much below 15mph. That may be ok in your storm, but you will most likely come into the storm with a flat battery.

I would be suspicious of the shaft size for a 2m prop any way.

A good fast 4ft prop with tsr 7, will manage tsr9 at cut in and starts to be a bit more reasonable. The smaller prop will restrict the low wind output, but half an amp is very much better than nothing.

If the periods you are worried about are very windy you may well see the 300 or so  watts the thing can handle and it may do what you want as a back up.

I agree with birdhouse ( save confusion with Adams) that there are many better things available but if your requirements are modest, you have a good site and it is back up for windy times then it may be fine.  Just look at some of the stuff Jerry has done with small 4ft size machines to see what is possible.

The way to approach the calculators is basically as you have done, but don't choose a prop with tsr over 7 and make sure you reach cut in at tsr 9 in something near 10mph or a bit lower, that gives you cut in. Then look at predicted power out of that prop at say 25mph and see if it is anywhere near the power you need. Don't expect to exceed 50% of the calculated blade output at best with generator and wiring loss.

Flux

[Jerry]

AdamM.

Take a look at the GE-ECM motor. The 3/4 HP to 1 HP is the best choice. They are lighter then the tredmill motor. They will actuly spinup easyer (no brushes). It will work very well with a 4ft to 5ft blade with TSR 6 to 7 depending of the required cut in and max power you want. Cutin at 5 to 6 mph. As Flux said 300+ watts at 30 mph. I'm convinced there a beter choice for this size machine your wanting to build. There is someone selling these  GE-ECMs here in the classified section. Search this forum for many stories on the GE-ECM. I've used the ECM for years, it a great small genny. It will out preform the SW AIR 404 and all the GM alternator type (WindBlue, Misoury Wind, Hornets) and the like and definetly the tredmill motor. There are some easy mods to peak its preformance listed in the stories here. I've used one for years on my 48 volt home system and on my motorhome for camping trips.

Jerry

[AdamM]

Thanks to everyone for your excellent suggestions!

I put a 1.25 m prop diameter and TSR of 7 into Lenz's blade calculator and it looks like this would cut in somewhere around 15 mph. If I bump the TSR to 9 it would cut in at about 12 mph or so.  

Power output will be lower for sure -- probably staying below the max the generator can handle -- but I'd rather not have to worry about burning up the generator anyhow.

I think I'll go with Lenz's measurements and a 1.25 meter prop for this version. Does that sound like a reasonable (if not perfect) configuration?

The treadmill motor is less than ideal, but possession is nine-tenths of the specifications, to mangle an old cliche. I'll keep the ECM option in mind for future versions -- thanks for the tip about that.  I've been browsing some of Jerry's posts and it looks like he's done some really interesting stuff!

Eventually, in a more permanent location, we'll probably build a larger machine with a home-built generator.

Thanks again for all the helpful replies.

Adam (the other one)


PS: I just noticed I left the other thread I'd started on here back in 2008 dangling after several people replied regarding my bicycle chain questions. Sorry about that, and thanks for the unacknowledged replies.

[ChrisOlson]

Does this sound reasonable to you? Is 12 an unreasonably high TSR?

TSR 12 is a little high for a 3 blade, and I doubt you'll get one to run at that speed very good even at cut-in.  But I'd (personally) try a 2 blader running at 8 TSR or so.  I'm running a 4.0 meter machine with a two blade rotor @ 8 TSR @ 25 mph and it easily matches and exceeds the slower 3 bladers for power.  But it's noisy in higher winds and the blades are showing considerable erosion problems at the tips already, after only a few weeks on the tower.

A little slower turning prop sure saves on maintenance of the blades.
--
Chris

[Flux]

Adam

I think your 1.25m prop will be fine for that motor. If the calculator predicts a wide and thick root section ( they normally do)  i wouldn't waste the timber or effort of carving it for the inner third of the radius. Stick to the calculator values for the outside where the power is produced but blend it out on the basis that you were carring the outer section to the centre in a linear sort of way.

The extreme width and pitch on the inner third seems to work with mppt where you can maintain constant tsr. For any other form of loading I have found it counter productive and the prop is more accommodating to mismatch without it and you will get better speeds at cut in.

Chris,
 if you happen to see this, that has been my experience and fast 2 blade props do need tip protection, although surprisingly even serious erosion doesn't affect performance to any extent, it becomes a mechanical issue in the end.

Flux

[ChrisOlson]

Flux, it was just my thoughts that a two blader might work for this motor pretty good.  You can increase the diameter of the rotor a bit using a two blade vs three, gaining a little swept area, and it can still run faster than what the smaller diameter three blade will run at.  You only capture about 2-3% more of the wind power with a 3 blade than with a two.  And you can make up for that with the increased swept area and still get the speed required for the generator.

It was just a suggestion.  Two blade rotors are often overlooked.  And in a smaller size like this that really needs speed, I think it would work pretty good.

--
Chris

[Flux]

Yes Chris you are right that you could probably use a bit bigger 2 blade prop and get the same speed.  For those trying to work from calculators this would most likely cause a lot of confusion.

The Lucas freelite was 2 blade, 6ft diameter with a nominal cut in speed of 450 rpm and it started doing things at just under 12 mph so even that prop would be ok on the treadmill motor for back up during windy periods. I would be a little unhappy with that size of prop without knowing the strength of the shaft, but a 5ft 2 blade prop would be an alternative to the 3 blade at 4ft.

Starting may be the point where things go wrong, the freelite was a wound field dynamo and there was no load below cut in except brush and bearing friction. I had trouble with starting on permanent magnet alternators, not the cog, that is a much over rated issue, it's the drag from iron loss. The treadmill motor probably has fairly weak ferrite magnets and it may start ok.

The other thing is that for general use a machine that won't start  under 12 mph is a bit of a non event, but for back up in times of high wind, it may not be any problem at all. The power available in winds below 12mph on a small machine is not that great anyway so unless it is very well designed for the low wind s it may not be contributing much except to supply battery self discharge until the thing is needed for the back up when the big wind comes.

The gyroscopic issue that dogs the 2 blade prop is not as big an issue as often imagined until you get to over 8ft diameter so I do agree that it's worth a look at. Don't forget fast props are noisy but blade tip shape can ease things, the number of blades doesn't have much bearing on the noise, the worst case is with a lightly loaded prop running above design tsr and in the region where drag id holding the speed back rather than the mechanical load.

Somewhere on here I posted a diagram of the freelite prop ( back on the old board) and it's simple to make, runs very fast and you could scale the diameter to 5ft or 5ft 6". Works better than the clever things slavishly following calculators for a fast prop.

Flux

[ChrisOlson]

Yes Chris you are right that you could probably use a bit bigger 2 blade prop and get the same speed.  For those trying to work from calculators this would most likely cause a lot of confusion.

Flux, what I was thinking, was that the calculator could probably be used to come up with the basic airfoil shape for a suitable three blade, then use that for a two in slightly larger diameter.  You couldn't predict the results that the calculator is going to come up.  But those results are usually not applicable to the real world anyway - it's just a basic guideline, pretty much.

That's basically what I did when I built my 4.0 meter, but I didn't use a calculator.  I just took an airfoil that I had used with great success on three blade rotors at 6 TSR and built a two blade rotor with it at 8 TSR.  I increased the diameter from 3.8 meters to 4.0 to make up for the 2-3% loss in Cp and it works positively beautiful.  Other than on my large machine I have not come up with a way to "fix" the cyclic loading problem when it yaws, but I'm still working on that.  With a very fast small two blade I don't think that would be an issue.

The thing is, most airfoils are pretty forgiving and will run under a wide range of angle of attack quite well.  And the faster they run, the less the angle of attack is, and the further they get from stall.

So it was just a thought.  From what I figured out using an airfoil that is not supposed to even run at 8 TSR well, it actually works in the real world.  I determined that you can pretty easily get 10-15% more rpm with a two blade vs using the identical airfoil on a slightly smaller three blade.  You do lose a little startup torque.  But once you get a puff of wind to get it turning it stays running down to the same lower wind speeds that the three blade does, it accelerates much faster when the wind picks up, and with a "loose" generator that really needs to spin fast it delivers the rpm's to do the job.

My inspiration to try a two blade rotor was the Eoltec Scirocco, which in independent testing has proven itself to be one of the most efficient wind turbines ever built.
--
Chris

[Flux]

Yes I agree in general, it reaches a point where for a given tsr, it is better to reduce the number of blades than reduce the chord with of the blades of a 3 blade rotor, otherwise you end up with things like the Air machines.

At one time the Germans did quite a lot of work on single blade machines with a balance weight. It is far easier these days to produce low speed alternators than it used to be. I wouldn't suggest going to one blade in this case though , it's a mechanical nightmare .

Flux

[jlt]

     
        To make a 2 blade prop start spinning at a slower wind speed. I put 2 more short blades on them .It also cuts the chattering when they yaw.
    But doesn't affect the rpm. 

         Windchargers with centrifugal air brakes use the same concept.

[ghurd]

Somewhere on here I posted a diagram of the freelite prop ( back on the old board) and it's simple to make, runs very fast and you could scale the diameter to 5ft or 5ft 6". Works better than the clever things slavishly following calculators for a fast prop.

Flux

I think this is it,
G-

[AdamM]

Hmm... Thanks for the interesting discussion. A two blade prop sounds attractive. It would be a good deal easier to build, since we could do away with any hub assembly and simply bolt a single-piece prop onto the flywheel that is already attached to the motor.

The freelite diagram looks like it's got a symmetrical curve rather than a normal airfoil. Is that how the prop is actually shaped?

What would be reasonable TSR for a 5' two blade prop?

Thanks for your help!

Adam