Author Topic: Blades that can help limit rpm  (Read 8640 times)

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Fused

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Blades that can help limit rpm
« on: May 31, 2017, 11:55:09 AM »
I have been considering how I can make a set of blades that will kinda stall out at
high rpm.
I thought maybe a single plane 7 degree angle blade would kinda hold it's self back?
Noise is no issue where I'm at.
I have my own shooting range on back of property,
Any ideas how to build a slower turning blade that gets in its own way with
turbulence? I have been thinking about this a long time and need
advice as to what it would take to limit rpm to 800-900 rpm on a 4 foot dia blade?
Make a lower lift profile on back side of blade?
This would be used on a non furling frame.

Thanks all, your a great bunch now that chain drive dude moved on....

Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

mbouwer

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Re: Blades that can help limit rpm
« Reply #1 on: May 31, 2017, 12:54:34 PM »
Pitch control?

Fused

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Re: Blades that can help limit rpm
« Reply #2 on: May 31, 2017, 02:46:55 PM »
out of my league......

but thanks

Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

Harold in CR

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Re: Blades that can help limit rpm
« Reply #3 on: May 31, 2017, 03:18:42 PM »

 Not so sure it's out of your league, Fused.

I'm about to get full speed ahead on my wind turbine with feathering blade system that anyone can reproduce. I'm just copying an old Jacobs design that is still used on original machines. I'm just cutting down on blade length and spring tensions.

 Will document with photos as soon as I get going. Already have the hub system and converted induction motor to PM design. Blades are also carved and needing epoxy paint which is difficult to source down here. Got angle iron prices for the tower this AM. Forgot to get BIG rebar prices for anchors.

Fused

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Re: Blades that can help limit rpm
« Reply #4 on: May 31, 2017, 03:52:37 PM »
Yes I would like to see what your up to.
I liked the counter weight system but
never really looked close enough into it.

Sounds like you have a plan, love to see it
when your ready.

I used screw in anchors for guy wires.

Now I have a counter weighted tower that pivots
4 feet above ground. It wont be very tall but it will
be easy to play with to drop and change out mills.

Keep us posted!

Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

ontfarmer

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Re: Blades that can help limit rpm
« Reply #5 on: June 01, 2017, 09:28:25 AM »
Quote
    Thanks all, your a great bunch now that chain drive dude moved on....

  That is a good one.  Got quite a chuckle.

Fused

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Re: Blades that can help limit rpm
« Reply #6 on: June 01, 2017, 10:47:29 AM »
I should not have said all that.
i originally joined back in 2008 or 09 and the forum
got to be a 1 pony show where small mills
were not important. I asked TomW to
remove my account which he did.
So all my old pictures got deleted, everything.
A year or so after, I signed back on the forum
but stayed out of participating.

Now, the forum looks fun again, so Im back.
 ;D
Doug
« Last Edit: June 01, 2017, 01:59:58 PM by Fused »
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

Adriaan Kragten

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Re: Blades that can help limit rpm
« Reply #7 on: June 02, 2017, 11:49:15 AM »
If a simple self stalling blade could exist, it was already developed long ago. About 20 years ago I was a member of a group which would design a small 5 m diameter wind turbine which should look like a big one. It should have three blades with elastic blade twisting as safety system. The blade design would be done by ECN which if a famous Dutch wind energy institute. After working on this idea for three month, ECN gave back the job and said that this idea is not possible. I had my doubts about elastic blade twisting directly from the beginning but if ECN can't design it, I think that it is really impossible. Stalling of a blade is possible but then you need real blade bearings, stops, springs and a synchronisation mechanism. The disadvantage of stalling is a sudden increase of the rotor thrust and extra noice. Some advantages and disadvantages of positive and negative pitch control are given in my public reports KD 437 and KD 622 which you can copy from my website www.kdwindturbines.nl.

A blade of a fast running wind turbine is stalling automatically above a certain wind speed if the rotational speed is kept constant. This is because the tip speed ratio decreases at increasing wind speed and reaches a rather low value where the Cp is low at high wind speeds. This situation is accomplished if the rotor is driving an asynchronous grid connected generator. This method was used for the old Danish wind turbines. However, if the grid falls off, the generator load disappears and the rotational speed and the tip speed ratio suddenly increases up to dangerous high values at high wind speeds. So one still needs a safety system which works without the generator connected to the grid.

Fused

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Re: Blades that can help limit rpm
« Reply #8 on: June 02, 2017, 12:30:33 PM »
quote

If a simple self stalling blade could exist, it was already developed long ago.

Good enough.

Simple thinking after building 15-20 sets of different blades tells me to
run a straight pitch blade at a high degree. This alone removes the speed
increase possible by a blade tapering and twist to a low degree angle tip.

I will build a blade as plain as can be this time.
You see, my axial flux machine peaked at over 20a on a 5 foot 6 inch blade.
I do not want that kind of power out of that generator.
I'm just going to reduce prop diameter, and blade design,
It will stall at some point, I just will have to work with it.

thanks
Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

stofanel

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Re: Blades that can help limit rpm
« Reply #9 on: June 02, 2017, 07:15:51 PM »
Airplane propellers rev up to 3000rpm+ with no problems. A well balanced wind turbine should have no problem spinning at at least 2000 rpm if the components are strong enough. No need to slow down the machine.

Fused

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Re: Blades that can help limit rpm
« Reply #10 on: June 02, 2017, 10:52:19 PM »
Im not trying to get max watts, Im trying to lower maximum output.

Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

SparWeb

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Re: Blades that can help limit rpm
« Reply #11 on: June 02, 2017, 11:54:08 PM »
Hi Fused,
It's a reasonable request, and it's feasible, but hard.  You have to choose the generator carefully, if you want the blades to behave the way you want.

Start with your choice of TSR.  The smaller the TSR the slower the blades turn relative to the wind speed.  Use a low enough TSR and you can do what you want - limit the RPM and save yourself the bother of a furling system.

But the thing you have to keep in balance is the generator.  It's the load that keeps the blades from running away when the wind is strong, no matter what TSR you choose.  So make sure you put on a generator that can handle all the power that the wind can throw at it.  To make the calc's simple, take the sweep area of the blades, find the wind power at the disk.  Use the fastest wind speed recorded for your location.  From the wind power, multiply that by 59% for the Betz ratio efficiency, and multiply that by 50% again for the generator efficiency.  If your generator can handle that power output continuously, from the strongest wind the turbine will face, then you don't have to worry about your generator having a meltdown. 

With the generator spec's in hand, find the RPM where that power output happens.  Use that RPM and the wind speed from your calc to get the TSR at max power.  If that TSR is less than 1/2 of the TSR you selected for the blades, then they will probably stall out, and you've got your bomb-proof machine.

I'm on my 3rd wind turbine iteration and I've almost got this situation.  My motor-conversion generators all seem to develop a current limit at high power, preventing the power taken out by the generator from rising as fast as it comes in through the blades, so I'm still happy to have a furling tail, but I know it's not necessary at the wind speed that it's set to act for.

The other watch-out is that wind power rises with wind speed cubed, and generator load rises with current squared.  Don't pick a wind speed to design for that's too low or a stronger-than-average storm could overpower it.

From what I've seen (on this forum and elsewhere) the thing that kills the blades are damage leading to imbalance, and the thing that kills axial-flux alternators is the heat from electric current.  If the blades alone fail, the alternator may survive, but if the alternator has a meltdown, the overspeed often destroys the blades.  So making sure the alternator won't self-destruct when the wind is high is, to me, the most important job.

There is a penalty when you go down this road, and it's the risk of also having stall when the wind speed is low.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
www.sparweb.ca

Fused

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Re: Blades that can help limit rpm
« Reply #12 on: June 03, 2017, 10:57:57 AM »
what Im working with is a 9" dual magnet rotor
I have 150 wraps of #20 magnet wire. 6 coil 8 magnets per rotor 2x1x1x1//2" n50 magnets.
It can handle a 6 foot blade and furl, but as we all know when or if I drop my prop diameter
my furling will need even more wind speed to furl if it will at all.
So I thought I could drop efficiency at the blades, reduce the output and maybe furl

Thanks for your reply Sparweb.

Doug

I'm running a tristar 45, it will handle 30a continuous and my solar peaks at 16.4a 
so you see Im just trying to prevent my mill from cranking 20a on top of what I already have.
I could get the tristar 60, but that will be a long time before I can save that kind of money out of my
disability.
« Last Edit: June 03, 2017, 11:23:04 AM by Fused »
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

SparWeb

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Re: Blades that can help limit rpm
« Reply #13 on: June 08, 2017, 02:54:44 AM »
It's hard get 20A out of an alt with 20 gauge wire. 
You would get a working machine if you made shorter blades for it - you lose on the low-wind side, that's all.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
www.sparweb.ca

electrondady1

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Re: Blades that can help limit rpm
« Reply #14 on: June 08, 2017, 08:52:07 AM »
it may not be what you have in mind but what about a traditional farm type wind mill? being a drag machine it will have an upper limit.
people seem to like the way they look.
« Last Edit: June 08, 2017, 08:56:34 AM by electrondady1 »

Adriaan Kragten

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Re: Blades that can help limit rpm
« Reply #15 on: June 08, 2017, 09:38:45 AM »
A traditional farm mill is not a drag machine. Although it has a very low optimum tip speed ratio, it uses the component of the lift in the rotor plane to generate a torque and therefore it is a lift machine. The drag component in the rotor plane gives a negative torque. The working principle is the same as for rotors with a high design tip speed ratio and the same design theory can be used to design the rotor of a traditional farm mill (see report KD 35). Drag machines use the drag component, so the component acting in the direction of the wind, to generate a torque. Drag machines are explained in my public report KD 416. Vertical axis machines like the Savonius rotor are partially lift and partially drag machines depending on the position of the blade.

mbouwer

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Re: Blades that can help limit rpm
« Reply #16 on: June 08, 2017, 09:42:17 AM »
A few years ago I had a 12 wieker on my mast.
Even at no load and high wind speed no more than 100 rpm
But how to make an appropriate direct drive?

Fused

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Re: Blades that can help limit rpm
« Reply #17 on: June 08, 2017, 11:27:34 AM »
Shorter blades is what Im gong with
I also have a 1hp ecm that will make a few watts and a garbo gen
I will be putting up, just so I can run it through a turnigy 130a meter.
The Turnigy was connected to the mill output 8 years ago.
It was  max amps recorded on my 9" dual rotor.
Thanks all


Doug

edit
the stator was made to bring each coil lead to a brass screw.
I had the mill "jerry rigged"  and not connected in typical star.
Just to help those that understand the many ways to wire a stator.
and how to pull a few extra amps. Cut in was raised slightly but over all
output increased by quite a lot.
« Last Edit: June 08, 2017, 12:41:43 PM by Fused »
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

electrondady1

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Re: Blades that can help limit rpm
« Reply #18 on: June 08, 2017, 12:42:36 PM »
you should show some documentation other than referring to your personal archive to claim your getting lift on a traditional farm mill Adriaan
 
it's a flat piece of wood set at an angle, not an airfoil shape .
so how does it get a tip speed ratio greater than 1 to 1

i must say that since you joined you are using fieldlines to get hits on your own page but you never post the data here.


 
« Last Edit: June 08, 2017, 12:52:55 PM by electrondady1 »

Fused

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Re: Blades that can help limit rpm
« Reply #19 on: June 08, 2017, 12:53:43 PM »
quote

you should show some documentation other than your personal archive to claim your getting lift on a traditional farm mill Adriaan
i don't think so

I agree, the back of a blade made flat will have no lift.

Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

Adriaan Kragten

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Re: Blades that can help limit rpm
« Reply #20 on: June 08, 2017, 02:51:10 PM »
you should show some documentation other than referring to your personal archive to claim your getting lift on a traditional farm mill Adriaan
 
it's a flat piece of wood set at an angle, not an airfoil shape .
so how does it get a tip speed ratio greater than 1 to 1

i must say that since you joined you are using fieldlines to get hits on your own page but you never post the data here.

The whole aerodynamic theory of horizontal axis wind turbine is explained in my report KD 35 which has 62 pages. The content was checked by my former boss Paul Smulders from the University of Technology Eindhoven. The content of this report is not my own invention but it is the summary of studies of Betz and Glauwert and I have used seven other reports written by CWD in the period 1975 up to 1990 to write it. The aerodynamic forces working on a rotating blade element are given in figure 3.2 and and figure 4.4 of KD 35. The tangential force which is the result of Lift and Drag working on the airfoil is given by formula 4.13. This tangential force finally supplies the torque Q if all blade elements and all blades are taken together. The aerodynamic theory is rather complex and I can't explain it on this forum. If I can explain things about wind turbines in some short lines I do it at this forum but if you want to know complex things in detail you have to study my reports (or other reports about the same subject). I own several other books about aerodynamics of wind turbines but for all those books you need to have at least a master degree in aerodynamics to understand them. So I find it not useful to refer to that kind of books. My report KD 35 can be understood with only some basic knowledge of mechanical engineering and mathematics.

Lift and Drag are generated on any airfoil which makes a certain angle with the direction of the relative wind. Even a flat plate can be seen as an airfoil (see report KD 551). Traditional windmills generally don't use flat plates as airfoil but cambered plates but even if they would use flat plates, it is the lift component in the rotor plane which supplies the torque. Even with flat plates you can get a tip speed ratio higher than 1. I have measured a scale model of a wind servo in the wind tunnel which used eight flat square blades and this rotor had an optimum tip speed ratio in between 1 and 2 depending on the blade angle which was varied in between 60 degrees and 30 degrees. The smaller the blade angle, the higher the tip speed ratio. But the tip speed ratio can't be very high because the minimum drag-lift ratio of a flat plate is rather high and for a high optimum tip speed ratio you need a low drag-lift ratio. The effect of the drag-lift ratio on the maximum Cp is shown for different numbers of blades in the figures 4.5 up to 4.11 of KD 35.

So I refer to reports on my own website not to get a lot of hits but only because this is the most practical way to give the information. In fact I don't count the hits at all and I have no idea how often my website is visited since it was created. I only refer to free public KD-reports or to free VIRYA-manuals so I have no commercial interest if someone visits my website. Regularly I get specific questions about wind turbines or generators, mostly from people from developing countries and up to now I have answered every serious question for free.




 

Mary B

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Re: Blades that can help limit rpm
« Reply #21 on: June 08, 2017, 06:13:36 PM »
Farm windmill blades are not flat, they have an airfoil shape


Fused

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Re: Blades that can help limit rpm
« Reply #22 on: June 08, 2017, 06:36:08 PM »
That one certainly has the shape.
Why do people not just gear them up for a generator?

Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

Adriaan Kragten

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Re: Blades that can help limit rpm
« Reply #23 on: June 09, 2017, 03:24:16 AM »
Why do people not just gear them up for a generator?

Doug

These kind of multi bladed traditional windmills are used to drive a single acting piston pump using a cranck mechanism at the tower top. This pump type has a very high peak torque half way the upwards stroke (see report KD 294). To make the rotor start at an acceptable low wind speed, the rotor must therefore have a very high starting torque coefficient. A slow running multi bladed rotor with an optimum tip speed ratio of about 1 has a very high starting torque coefficient because it has a large total blade area and because the blades have large blade angles resulting in large lift coefficients at stand stil position. However, disadvantages of this kind of rotors are that they are very heavy and that the maximum Cp is rather low because of energy lost in the wake rotation and because the blades are a lot shorter than R.

This rotor can be used to drive a generator if there is a gearing with a large accelerating gear ratio in between the rotor shaft and the generator shaft. But for a generator it isn't necessary that the rotor has a very large starting torque coefficient. Therefore you can better use a much lighter fast running rotor for driving a generator. As the rotational speed of such a rotor is much higher at the same wind speed and the same rotor diameter, you need a gear box with a much smaller gear ratio or no gearing at all if you use a slow running generator.

stofanel

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Re: Blades that can help limit rpm
« Reply #24 on: June 09, 2017, 09:29:06 AM »
One can certainly make a lift based wind turbine using nothing but flat plates. A flat plate at a small angle of attack will produce lift.

But back to the topic:

I would recommend just building a well balanced set of blades with enough strength to withstand the centripetal forces of a few thousand rpm. There's no need to worry about burning out the generator. Power is voltage times current and one can always limit the current via simple electronics.

Fused

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Re: Blades that can help limit rpm
« Reply #25 on: June 09, 2017, 01:03:54 PM »
Like I said I've built a few.
Here is what I have to play with.

https://www.flickr.com/photos/124210111@N08/albums/72157672235255003

These are all 8-9 years old, I get a kick looking back at my wind generators.
I do have some experience on motor conversion to PM and the axial.

Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

Adriaan Kragten

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Re: Blades that can help limit rpm
« Reply #26 on: June 09, 2017, 01:34:19 PM »
A high centrifugal force in the blades is not the only risk of a rotor which has no safety system to limit the rotational speed. The gyroscopic moment in the blades and in the rotor shaft increases proportional to the  product of the rotational speed of the rotor and the yawing speed of the head. Both increase at increasing wind speed if the head is kept into the wind by a vane. High gyroscopic moments cause high bending stresses in the blade roots and in the rotor shaft. Another problem is flutter which if a combination of a bending and a torsion vibration of the blade and which is mainly caused by a too low torsion stiffness of the blade. Fluttering starts very suddenly above a certain tip speed and can destroy a blade within some minutes.

About twenty years ago there were about 15 men in my region which had built their own windmill. Every two months everybody came together to visit one of the windmills and I have seen almost all of them. None of those windmills except mine had an authomatic working safety system. Some had very nice pitch control systems but it neeeded an action to put the blades in a safe position. Within some years all windmills except mine were destroyed. The biggest one had seven meter rotor diameter and felt on a greenhouse with much more damage than only the windmill. So my advice is to never build a wind turbine without a proper safety system to limit the rotational speed and thrust. It may take some years but then there is that big storm where the whole wind turbine is destroyed.

Fused

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Re: Blades that can help limit rpm
« Reply #27 on: June 09, 2017, 01:47:35 PM »
Although I've never had a mill fall or a blade break, it wont affect anybody if it does.
It is 125 feet from the house and I ran all wires to my control room inside my house.
From inside I will lock the mill rotation with a switch.
I have kept all my wind turbines at 6 foot or less blade diameter.
I do not need or want anything bigger.
I do understand the extreme forces placed on the tower.
Thanks

Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

Fused

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Re: Blades that can help limit rpm
« Reply #28 on: June 12, 2017, 07:01:49 PM »
project finished, solar(dirty) and yesterday put up the wind generator,

Here is a view off my back deck.
Thanks all for the advice.

solar and wind by Doug B, on Flickr

Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

electrondady1

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Re: Blades that can help limit rpm
« Reply #29 on: June 17, 2017, 07:59:13 AM »
that's a nice spot . needs an old truck up on blocks at the back there.
what's that red stuff growing there on the left?
so. your mill is about 3' dia. and no furling ?
and the alternator is a 9 " jerry rigged dual rotor?

mbouwer

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Re: Blades that can help limit rpm
« Reply #30 on: June 17, 2017, 11:30:48 AM »
Doug,

Is it possible to make those panels sun tracking?
Of course it's also possible to turn them manually during the day.

Rgds. Rinus

Fused

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Re: Blades that can help limit rpm
« Reply #31 on: June 17, 2017, 03:30:26 PM »
The red stuff canna lilies.
Behind that are rows of fig trees.

That blade was 56 inch dia. It now has a 5 foot set on it.

The generator has 9 inch dual rotor with magnets on
both rotors. The original plans used steel bars on one rotor.

I brought coil ends to brass screws.
That way, I could test different ways of
wiring the stator.

I have plans on taking it back to star connection.
Im not looking for maximum output.
I just want a bit of help keeping batteries topped
off each day after using them every day.
Yes, I have furling on this one.
Not sure it works with the shorter blades.
Ill see.

The panels were put in place as they are to
self track the sun. It actually gives me good
power. The panels are mounted on a big barn door
type hinge.
I can quickly fold then vertical in case of hail predicted.

Doug
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.

Fused

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Re: Blades that can help limit rpm
« Reply #32 on: June 17, 2017, 03:42:07 PM »
The generator is shown in this link, along with old
videos testing blades.

https://www.flickr.com/photos/124210111@N08/albums/72157672235255003


Doug

edit
the black and white one at the bottom of the page is the generator Im trying now.
System specs: 480 watts peak solar, 8- 6v duracell batteries at 24v controlled with tristar 45,, wind generator is axial flux
with 5 foot blades

if good guys don't carry guns, only the bad guys will.