Author Topic: Torque value on blade design  (Read 736 times)

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Kyle

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Torque value on blade design
« on: January 06, 2018, 06:42:23 AM »
I am working on an asynchronous induction generator. I have been researching blade design and have not found torque discussed. Perhaps I am not using the correct search terms. My question is. When considering design, don’t you first need to know “how hard” your rotor turns? Then plug in desired rpms, tsr, cut in speed, etc and design from there? I have read about increasing the gap which reduces force required to turn. But, if that is fixed... I am assuming there is a guide that I have not found yet. Also, what is the best way to measure rotor torque - torque wrench?

JW

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Re: Torque value on blade design
« Reply #1 on: January 06, 2018, 10:10:34 AM »
Do some research on cut-in speed

SparWeb

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Re: Torque value on blade design
« Reply #2 on: January 06, 2018, 10:14:29 AM »
That's a very good question, Kyle, and no easy answers.

I recognized that if I went to the trouble of measuring torque while testing my generators, I'd have a power curve that I could count on to match up the blades better.  Never regretted it.
I don't have a guide, really, but I did show how I measured torque during my tests: 



http://www.sparweb.ca/3_Gen_MoCo/Baldy.html

This setup allowed me to measure, directly, the torque on the generator, because that board was the only thing preventing the generator body from spinning while the shaft was driven by the lathe.



No factoring efficiency, losses due to this or that, it's just the simple equation: Torque = Force X distance   and   Power = Torque X RPM
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, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

SparWeb

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Re: Torque value on blade design
« Reply #3 on: January 06, 2018, 10:22:03 AM »
On the other side, the question is "how much torque do the blades deliver?"

There's a graph for that, too :)




The graph above shows three generator power curves (black lines) and the power curve for the blades of one wind turbine rotor, at different wind speeds.  You can see how the blades power curves grow very big with stronger wind, and that there is a theoretical perfect generator curve that will match.

In the example above, the generator power curve on the left is going to "stall" the blades, keeping them running very slow, while the generator power curve on the right is too weak and the rotor blades will run too fast.  The curve in the middle is, of course, Goldilocks.

Actual generator power curves don't quite follow exactly the ideal path, but making comparisons this way does allow you to either select the components you need to match the prop power to the generator power, or to adjust the parameters of the prop that you built so that it does match the generator you have.
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, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

JW

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Re: Torque value on blade design
« Reply #4 on: January 06, 2018, 01:49:58 PM »
Quote from: Kyle
I am working on an asynchronous induction generator

http://www.fieldlines.com/index.php?topic=139034.0

this is the only link that works from the above topic

http://www.qsl.net/ns8o/Induction_Generator.html
« Last Edit: January 06, 2018, 02:10:34 PM by JW »

Kyle

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Re: Torque value on blade design
« Reply #5 on: January 07, 2018, 06:41:13 AM »
Thank you for the reply’s. I have read some on cut in speed. I know mine is 181 rpm. SparWeb - thanks for that info. I don’t have a lathe but my drill press may work.

As to the blade graph. Were those all the same length with different pitch, or same pitch with different lengths or various combinations? Was it all trial and error? What was your starting point and how did you arrive at it?

SparWeb

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Re: Torque value on blade design
« Reply #6 on: January 07, 2018, 11:07:27 PM »
The drill press is good and has some advantages over a lathe.  Particularly, the space you can use to set up to either side of the drill chuck, ie, swinging the table to one side and running the generator with a belt from chuck to genny shaft.  Changing drill press pulleys to change speed gives you the points on a curve.



Anyway, I don't want to get ahead of myself.  And matching blades to generators doesn't have to be very complicated.  Certainly not as complicated as it may have seemed in my previous postings.  Really, it is plenty to focus on reasonable blade diameters to get the power range right, and reasonable TSR to get the blade speed right.  By "right" I mean a close match between the generator and blades you will use.  I went to the trouble of measuring the power curve with about as complicated a set-up imaginable - but I was after a very rigorous answer and I had the resources easily at hand.  I didn't do that for my first two wind turbines, and I won't likely do that again since I don't work at the same shop any more.

Here is a place to start finding out about the power that a wind turbine can collect:
http://drømstørre.dk/wp-content/wind/miller/windpower%20web/en/tour/wres/enerwind.htm

Yes, there was trial and error in my approach - it was by building several wind turbines and learning from them what I wanted to do better the next time. 
By the third WT, I wanted to know more, and started working through the math.  Eventually I had the formulas worked out so that I could generate curves, like the ones I posted above, for any theoretical combination of blades and generators.  I'm not saying you need to do that.  Taking the rough sizing and measurements that others have used is a much better way to get started.

Back to testing generators:  I had a conversation recently with another member that you might find interesting: 
http://www.fieldlines.com/index.php/topic,149388.msg1042987.html#msg1042987

I sketched out a rough set of blade-matching calculations to show what TSR means and how it's calculated, and DaveP68 has been testing generators he makes by converting motors that he gets out of washing machines.
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, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

Kyle

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Re: Torque value on blade design
« Reply #7 on: January 08, 2018, 08:42:31 AM »
Thanks for that wealth of information. It is going to take a bit to process it all.

I do have a question for you on the height of anemometer. In a video from one of the links, you talk about it being different wind speed because it is mounted half way up the tower. If you mount it right on the generator, does the activity of the blades distort the readings or is that the ideal location?

SparWeb

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Re: Torque value on blade design
« Reply #8 on: January 08, 2018, 10:39:23 PM »
Yes, read as much as you like!

The wind at high altitude usually flows smoothly.  Closer to the ground it gets stirred up by buildings, trees, fences - any obstacle messes up the flow.  Not only does it get stirred up, but it is slowed down.  Really even the presence of a perfectly smooth ground slows down the speed of the wind.  So the higher up, the faster the wind, as a general rule. This ignores local spots of gusts, accelerations in certain places between obstacles, etc. but it's a good rule for everyday use.  When setting an anemometer, its height determines how deep down in the mess it is.  You can expect the wind to be faster above the anemometer, and slower below it, most of the time.

There is also the case of setting an anemometer in the wake of a turbine, but that's a separate issue.  That's done when there's no better spot to put the anemometer, and since it's usually manufacturers of large commercial turbines who do this, it's trivial to compute the correction factor for the turbine wake since they have all that modeled by computers for the blades anyway. 

For us mortal folk, putting the anemometer a little lower on the pole and scaling the wind speed a little for tower height is fine.
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, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

mbouwer

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Re: Torque value on blade design
« Reply #9 on: January 09, 2018, 03:16:14 AM »
To be able to build a completely reliable safe turbine
I think a team with various areas of knowledge is needed.
I would like to be a member of such a team.

hiker

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Re: Torque value on blade design
« Reply #10 on: January 09, 2018, 03:55:31 AM »
Team...Fieldlines...would be a great start...😜
WILD in ALASKA

DaveP68

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Re: Torque value on blade design
« Reply #11 on: January 11, 2018, 11:48:49 PM »
On the other side, the question is "how much torque do the blades deliver?"

There's a graph for that, too :)




Hi Steven

I'm impressed with the "Matching a Wind Rotor to a Generator" graph above. Can I ask how you collected the data points?

David
There are realities that if you do not accept, will lead to frustration because you will be spending time on wrong assumptions and the results cannot follow!

SparWeb

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Re: Torque value on blade design
« Reply #12 on: January 13, 2018, 09:19:19 AM »
Hi Dave,

Warning:  the following material contains geekiness which may be shocking to some.

The graph is the result of a self-education I gave myself on a subject called blade-element theory.  If you read this page, you will get more of the context in which I created it:  http://www.sparweb.ca/1_Blades/Aero.html

Once I started making turbines, I found that I didn't really know what matched the power in of the rotor to the power out of the generator.  Threads on Fieldlines written by the late Flux were very valuable to get me started, and I wanted to know more.  I wasn't actually starting from scratch.  In my day job, I'm an aeronautical engineer.  I had been taught quite a bit of aerodynamics in school, so I had a lot to build upon.  What I didn't have was a way to get a hold of a rotor that takes power OUT of the wind.  All you get as an aero engineer is the mechanics of aircraft propellers that put power INTO the wind to create thrust.  There are some crucial differences that made it hard to just open an aircraft textbook and point at an equation or a graph.  If you take the graph of aircraft propeller performance, and try to plot a wind turbine rotor on it, usually you are below the x-axis and to the left of the y-axis, guessing how to extrapolate the lines in a region they weren't meant to.  So I kept looking.

I found a number of excellent technical papers on the NREL website, which were written to teach the theory of WT rotor blade performance.  Then the coin dropped.  I was able to use what I'd learned before and write out the equations of propeller performance start to finish.  I could take the input parameters of blade length, chord, twist, and so on and turn it into a performance curve for varying wind speeds and shaft speeds.  I could control the mathematical model like the TSR is fixed, or the RPM is fixed.

This was the most useful result of that work.  You can find this graph in lots of wind turbine textbooks, but I hadn't found any that did the full derivation before I did it for myself using those NREL papers.


It was RW Hamming who said "the purpose of computing is insight, not numbers".  When I started plotting this graph, I finally had my moment of insight.  The graph actually looks like the lift-curve of an aerofoil, which you might recognize, being a pilot yourself, but facing to the left not the right.  On it you can see where the peak power of the rotor is, where it stalls, where it spins so fast it generates no more power... the perfect graph for power-matching with generators.

Power matching is where the black lines on the upper graph come in.  The black line in the middle isn't a generator's power curve, it's the connection of peak power points of the rotor as the wind speed increases.  If you were to build a generator whose shaft input power follows that curve, you would have built the "perfect" matching generator for those rotor blades.
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, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

SparWeb

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Re: Torque value on blade design
« Reply #13 on: January 13, 2018, 09:43:33 AM »
PS:
While I was writing that, I meant to link to the NREL webpage that I used for sources.
It seems to have been moved behind a user-login page.  Well, it's been 10 years...

The key papers can still be easily found:
"Applied aerodynamics of wind power machines" by Wilson, Lissaman, Walker
http://ir.library.oregonstate.edu/jspui/handle/1957/8140

"Aerodynamic performance of wind turbines" also by Wilson and Lissaman
https://nwtc.nrel.gov/system/files/WilsonLissamanWalker_AerodynamicPerformanceOfWindTurbines(1976).pdf

(strangely I can still find with google the PDF on the NREL's site, but I can't follow links past the login to the page where these documents are listed & linked)
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, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

DaveP68

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Re: Torque value on blade design
« Reply #14 on: January 15, 2018, 01:23:16 AM »
Hi Steven

Nice work and I like your very informative web page and other links useful resources.

Can I ask how measurements made at Altitude = 3000 feet compare with those made at sea level?

For my crude understanding on recovering energy from the wind the amount of energy that can be recovered at 3000 feet compared to sea level will be about 26% less?

David

There are realities that if you do not accept, will lead to frustration because you will be spending time on wrong assumptions and the results cannot follow!

SparWeb

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Re: Torque value on blade design
« Reply #15 on: January 19, 2018, 05:48:19 PM »
26%? I don't think it's that bad. But density is lower... I can work it out...  stand by...
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, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

SparWeb

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Re: Torque value on blade design
« Reply #16 on: January 19, 2018, 05:53:44 PM »
9% less at 3000 feet above sea level.

My "pressure altitude" is 91% of sea level, given the same temperature.  But it's often so friggin' cold here that maybe it comes out even!

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, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024