Mini CNC Blades

[taylorp035]

The mini windmill achieved about 30 watts in its initial trial run and managed about 25 watt hours in a span of half a day of ~20 mph winds.  30 watts was at about 2300 rpm, but I plan on doing some bench testing to get the performance curve of the alternnator

It seemed to cut in a medium wind, I would guess 12-ish mph.  I want to reduce the air gap so it cuts in just a bit lower.  It's at least an 1/8" right now, maybe even 3/16".

If the gap was bigger, peak performance would be better (~TSR of 5 at 30W assuming 30 mph).  I was pleasantly surprised it would hold 5-10 watts in a nice breeze.   It starts spinning at probably 1 or 2 mph winds thanks to the axial flux alternator, which is very refreshing considering the dozen other windmills I've made that don't want to start under 15 mph.   I did the math and this windmill is 1/15th the wind swept area of my 7 footer, but made more power than the treadmill motor ever did (7.1 amps vs. 6).

[taylorp035]

Tonight I did some bench testing on the mini alternator.  I then reduced the air gap from ~1/8" to 1/16".  This should reduce the cut in RPM by ~15%.  You can also see how much better the power was at the same speed.

My alligator wire leads are pretty bad, so you will note some extreme differences in current depending on how I had it hooked up.  I was also making sure I didn't go over 10 amps on my Fluke, so I switched to a 60 amp gauge.

It will definitely perform better in low winds, but I'm betting it will not do as well at higher speeds since the TSR will start to be below 7.

*Edit - changed power equation to use m/s instead of mph

[SparWeb]

I'm betting it will not do as well at higher speeds since the TSR will start to be below 7.

Depends what you mean by "well".  It's producing power, but it seems to be turning faster than is safe for the wood. 
Looking at your speed numbers, IMO having a TSR going down below 4 or 5 is better.

I'm not getting thru to Youtube today, so if you're recorded it, please forgive me.  I'm curious how well your clutch is working.

[taylorp035]

Sparweb - I'm fairly confident the blades will hold together.  I watched it spin completely unloaded in some 30-40 mph wind gusts and I'm pretty sure it was north of 6,000 rpm.  They are really, really light weight and small in cross section.  I'm certainly not going to stand next to it though at those speeds.  Once the generator is hooked up, it spins much slower.

The clutch is for the 7' Christmas windmill.  It's still not back up due to the cold conditions and feet of lake effect snow preventing me to drive a 4-wheeler back to the tower.


I did some super heavy Excel work that I've been dreaming up in my head for a few years.  I wanted to exactly predict the RPM of the blade by knowing it's CP vs. TSR curve and the generator power vs. RPM curve.  About 4 hours later, I finally got it working and it works quite well.  It show the compounding effects of the squared power rise of the generator fighting the cubic power of the blades vs. wind speed, while obeying the TSR efficiency curve.

The values here are my best guess for how the Mini CNC blades were performing on it's original outing.  I'll have to plug in my new numbers for the tighter air-gap + better wiring curve for the alternator.  It sure looks like I'm not at the top of the CP curve, so making the air gap bigger might help a lot as well.  I was hoping to lower the cut-in speed, so I'll have to try both.



Ignore the choppiness of the power curve.  The way I calculated it has discrete values and it will drop a value if the numbers don't match up very well.... the real power should be the imaginary curve at the very peaks of the points.
I also chopped off the RPM plot.

[kitestrings]

Cool, you got it to spin in both directions...seemingly at the same time  .  Just kidding, but I've always loved that wagon-wheel effect.  Reminds me of "Westerns" when I was a kid.

Looks good, but do be careful.

[Astro]

What size wire did you use on the coils?

[taylorp035]

What size wire did you use on the coils?

17 gauge wire.  So currently it's wired with three sets of coils in parallel, so it can pump quite a lot of current out of a small package.  I ran it at 37 amps on the drill press and it didn't even get warm.  My weak link was the 25 amp single phase rectifier that was quickly getting hot.

[Astro]

What size wire did you use on the coils?

17 gauge wire.  So currently it's wired with three sets of coils in parallel, so it can pump quite a lot of current out of a small package.  I ran it at 37 amps on the drill press and it didn't even get warm.  My weak link was the 25 amp single phase rectifier that was quickly getting hot.

 I wonder how it would perform with smaller wire and thus more winds? I don't think those tiny blades are ever going to catch enough wind to support a 20 plus amp draw, so..... I have to wonder what it would do with smaller wire, more winds and a more reasonable (for it's size) 8-10 amp load?

[taylorp035]

I wonder how it would perform with smaller wire and thus more winds? I don't think those tiny blades are ever going to catch enough wind to support a 20 plus amp draw, so..... I have to wonder what it would do with smaller wire, more winds and a more reasonable (for it's size) 8-10 amp load?

  It would probably run better, especially at higher wind speeds.  The issue with it right now is the alternator is way too powerful for the blades, so I can either have a lower cut in speed or I can have high speed power.  I plan on trying the low cut in this week, but I expect to add more air gap later on to try and get it to the peak of the Cp curve, which should allow it to make 40-60 watts easily.  It made about 30w in 30 mph winds last weekend, but I think I can make it do 30W in 20 mph winds if I get the TSR up from 4-5 to a much better 10.

With some thinner wire, it would not ramp so quickly in current.  On the drill press and with the heavier wiring, it would rise almost 5 amps for every volt I was over my cut-in voltage with the 1/8" air gap and 6.45 amps with the 1/16" air gap.

[MattM]

Maybe fire up the CNC for a center section piece to extend your diameter another foot?

[taylorp035]

Maybe fire up the CNC for a center section piece to extend your diameter another foot?

I would love to do that if my mini CNC mill was any bigger, but this was as long as I could make one blade while also cutting the center hole for the shaft.  I do have a set of 3 blades that are twice the diameter that would honestly be a much better fit for the alternator.  I might end up going that route in the future.  It will depend if I just wire it into the charging circuit of the big windmill or if I decide to handicap myself with the 10A limit of my other charging circuit and have it totally separate.... even at 10A, I think the mini windmill with the right amount of air gap on the alternator will exceed that with no problem.

[JW]

Maybe fire up the CNC for a center section piece to extend your diameter another foot?
Modify message

I run solidworks 2016

isn't that function a sweep 

[Astro]

Sounds like you have a very good handle on it. I was just thinking that 5 amps or so at 5 volts or about whatever your chart posted said is steep. You are trying to push a lot of current with low volts. But you are doing it. And amperage is what charges batteries. But I do think a touch smaller wire would make your voltage go up if for no other reason then a couple more winds in the same amount of space. Yeah you would trade some amps for those volts, but I think it would be an easier project. But as a friend used to tell me "if it were easy women and children would be doing it". I like the challenge part as well.

[MattM]

Maybe fire up the CNC for a center section piece to extend your diameter another foot?

I would love to do that if my mini CNC mill was any bigger, but this was as long as I could make one blade while also cutting the center hole for the shaft.  I do have a set of 3 blades that are twice the diameter that would honestly be a much better fit for the alternator.  I might end up going that route in the future.  It will depend if I just wire it into the charging circuit of the big windmill or if I decide to handicap myself with the 10A limit of my other charging circuit and have it totally separate.... even at 10A, I think the mini windmill with the right amount of air gap on the alternator will exceed that with no problem.

I was thinking you could make a new center section that would mate to both blades.  Your old holes could be for bolts to hold them on the new centerpiece.

[taylorp035]

Yeah, now that you guys mention it, I could make a center bar that makes it a bit bigger in diameter.  Since I glued the two blades together with a giant lap joint, I would have to cut them apart.  But I could definitely CNC cut a center section so it stays relatively balanced.

[MattM]

Any updates?  It's been quiet for about a month.

[taylorp035]

Any updates?  It's been quiet for about a month.

I've been doing testing with different air gaps.  I started at 1/8" total and then dropped it down to 1/16".  Then last night I changed it to 1/4".  Each step was about 20% change in voltage and almost doubling the power at a given RPM based on my bench testing.   It's fairly clear watching it that the blades are at a pretty low TSR (~3-4) once the wind picks up a decent amount, so the efficiency is pretty low.

Here's my estimated power curves vs. wind speed.  1/8" graph is on the left and the 1/4" graph is on the right.  The power calculation is a bit choppy due to the discritization of the calculations... the power should be a smooth curve at the very peaks of all the points.



Here's the blade power vs generator power ratio plot.  It is based on TSR and MPH.  You will probably have to click on the image to make it full screen.




I'm expecting this to make a lot more power now with the 1/4" air gap since the blades will be at a more optimal TSR once it gets above ~15 mph.  Previously it put out some good power in a light to medium wind (say 10-15 mph), but didn't exponentially go up with higher wind gusts as you would expect.

[MattM]

It sounds like your 1/16 gap produces the best power.  Did you add resistors to tune the starting point (cut in) for generation?  If you set a threshold where it can generate decent power at a more optimal windspeed then it may be better.

[taylorp035]

I forgot to post my bench testing curves of the 3 different air gaps.  I haven't had a windy day yet to try the 1/4" outside, but I suspect it will perform much better at higher wind speeds since the alternator won't be over-powering the blades so much.  This is predicted by the blue lines in the side-by-side plots in my previous post.

Here's the bench test values -


EDIT - I should also note these power curves are with my terrible alligator wire leads that have a relatively high resistance.  When I skip these and use an old school 60A ammeter connected directly, the power goes up 2-3x.  I recorded 22 amps at 2180 RPM with the 1/8" air gap and 37 amps at 3000 rpm (@ 4.2 volts, so 92 and 155 watts respectively). This is yet another reason why I need a huge air gap to not over power the very tiny blades.

[MattM]

How are you locking down voltage at 4.2 volts at all RPMs?

[taylorp035]

How are you locking down voltage at 4.2 volts at all RPMs?

All of my battery packs that I charge are made of 18650 or 21700 li-ion cells in parallel.  The one pack has (39) 3.2 Ah 18650's and the other has (99) 4.6 Ah 21700's.  They are wired in such a way that I can split them into 3 sets and then plug them in series for a 12.6v (max) pack.  Then I run my inverters off of them.  The smaller pack has a 6 pole, double throw switch that does this with one flick of a switch.  The bigger pack has two parallel sets of 10 gauge wires going to some 45A Anderson Power Pole connectors that either go in parallel for charging or series for the inverters.  I have the connectors aligned in a way that I can't screw it up and accidentally short things out.

I've done this since it eliminates the need for balancing any cells and it lowers the RPM I need for my windmills to spin.  The obvious downside is it triples the current, which is less of a concern when you're dealing with small windmills.  My 7' windmill is set up to do 40A continuous and about ~70 amps for a few seconds.  If I upgrade a few things, I could probably push that to 60A continuous and 120A peak.  The mini windmill is limited by my 25A single phase bridge rectifier at the moment, since the coils are wired 2 in series and 3 in parallel (2x3)... so 3 parallel 17 gauge wires ought to do 40-50 amps without getting too hot.

Also, I didn't want to have to deal with a wire going down my tower or a slip ring, so I just charge my batteries on the windmill itself.  And since it's reachable from the ground, it's easy to swap out the battery in a few seconds.

[taylorp035]

Ever since I broke my mini blades and glued them back together, they've never been the same.  So I've decided to put a different set of 3 blades on that are double the diameter (~44.5" diameter).  I also have rewired the alternator so all 6 coils are in series for the single phase setup and reverted to my 1/8" air gap spacer.  I'll try it out tomorrow in some 10-12 mph winds.  With the blades being 2x the diameter and the alternator being 3x the voltage per RPM, it should be a bit more satisfactory in terms of power production.  It should also not stall so bad and actually push the alternator some.  Based on some quick DeWalt drill testing, it appears cut in will be in the 400-500 rpm range.

In other important news, I bought a farm that has 100x better wind resource than my house in the woods.  It's been fun to put the mini windmill in the front yard where there's no trees from the west/north.  Eventually I'll get my other windmill set up in a better position on a ridge on the new property.

[MattM]

Very nice.  How well did they naturally balance?

[SparWeb]

Welcome to country living!
Worth every minute, IMO

[taylorp035]

Very nice.  How well did they naturally balance?

Not very well. In the first image, you can see the added weights I put on the front of the hub.  The blades were carved probably 10 years ago and were not very good in terms of being evenly sized.  I need to re-balanced it now that I took the hub off the shaft and reassembled it since it shook a bit during a ~6-8 mph breeze today (maybe 300 rpm).  It unfortunately wasn't windy today.  I suspect I'll have some better weather later this week.

Welcome to country living!
Worth every minute, IMO

It's just around the block from where I live now.  I bought a 51 acre farm for the land so I can split it off and build a house next year up on a pretty hill.  Hopefully keeping ~30 acres and selling the rest (including a house and large barn).  The house and farm needed a lot of clean up, so a few 1000 hours of labor will go into it this spring/summer/fall in preparation to re-sell it.  Hopefully I can get most, if not all of my money back and net a really nice piece of land 4 miles from the major shopping area in these parts.  The plan is to build an all electric house with solar.  I'll continue to have my fun windmills, especially since there's no trees from the predominate wind directions for a mile or so and it over looks a giant valley (southwest, west, north west and north).

[bigrockcandymountain]

Congratulations on buying the farm.  It sounds like your setting yourself up pretty well for the future.  That is always fun.

One piece of advice on the electric solar home.  If you are building new, go super insulated, no thermal bridges, and extreme airtightness.  Buy excellent windows.  Basically build to the passive house standard or something equivalent. 

Hopefully yoy get some power out of the new setup. 

[DamonHD]

Wow!  Sounds like a plan!

Now experiencing a little bit of jealousy...  %-P

Rgds

Damon

[SparWeb]

no trees from the predominate wind directions for a mile or so

You got it made.  Here I am with walls of them 70 feet tall!

[taylorp035]

The plan is to go with thermally insulated panels (essentially plywood walls with 8-12" of foam in between), so there will be no thermal bridges.  Also looking at the triple pane casement windows as well that will be really good.

So I took the mini windmill to the farm a few times over the last few weeks and have been dissapointed in the speed at which the blades spun.  It never 'took off' like a good set of blades should do.  I decided they were stalling quite badly.  Even in what was a pretty good wind (15+ mph), it was struggling to get over 300 rpm.  I saw it charge the battery for a few seconds once (~400-500 rpm).

So tonight I measured them and they were at only ~3-4 degrees and they weren't all quite in the same plane.  I decided to change them to 5.5 degrees and made them more even.  We shall see how it goes.

Before


After

[MattM]

Your pitch is uniform across the whole length, making the sweet spot to produce rotation variable with speed.  I have a strong feeling your blades take too long to get up to a good rate before it turns out from the wind.  Little angled flaps of metal to extend the pitch down at the root/rotor will make starting up that much easier, to decrease time to reach its useful speed.  A little more tail length or area to keep it in the wind longer.

[SparWeb]

Expanding a little on Matt's point, even the TSR is variable in most cases.  So that moves the sweet spot too.  Meaning, the point on the blade having the optimum incidence for the conditions at any instant may shift toward or away from the tip.

The root has only about 15-20 degrees incidence, which isn't a lot to get them started. 

[taylorp035]

I took it out for a spin tonight, and it did perform better than before.  It's still struggling to get to the cut in RPM in anything much less than what I would call 15 mph winds.  I'm going to bring the air gap back from 1/8" back to a 1/16".  My previous bench testing showed a 15% difference in voltage, so that should make a medium difference.  I think I want to CNC cut some ~14-15" long blades (vs 22" blades right now or my original 11" CNC blades).  I could also take the current 22" hand carved blades and remove one blade.  I'm not sure if that would significantly improve the RPM though.

I could also choose to hand carve a 2 blade design out of a single piece of wood.  I'll have to give it some more thought.

[MattM]

I would do two things:  (1) mark a circle at the root and add in 10-12 small flaps of metal to act as blade extensions, (2) increase length or size of tail to get it to align with wind more quickly; before giving up.

Flaps of metal at the root - spread evenly around a circle - is a simple way to boost start up quickness.  They can be small or as big as you want.  I could fit sixteen 4" wide by 4" long squares on a 12" disk.  They can be 26 gauge (a very common roofing stock) because its stiff enough since they don't spin fast nor need to support any weight.  (26 gauge is 3/4 pound per square foot.)  Aluminum in the .032 variety would be easy and would amount to mere ounces added.  Simple solution to implement, but they get you going from a dead start quickly, especially if you can butt them up all at the center to form a pinwheel look.  I'd keep the angles under 45 degrees, just try to be consistent across all the flaps.  Honestly, flaps are so much easier to fabricate than thick root angles in the wood itself, I'm surprised nobody else uses them.  You can buy a bar folder at Harbor Freight for $10 to put hems on the leading edges and to get consistent bend locations.  Keep it simple.

You should still mill more blades because that's cool stuff.  You can get fancy as your experience increases.