New build- 20 foot diameter variable pitch windmill

[windy]

I have a question regarding the thickness of the rotor for my build. I have seen 1/2" thick rotors used on 20 foot diameter mills but have also read that the thickness of the rotor should be the same thickness as the magnets. I will be using 24 -1.5" x 3" x .75" thick neo magnets on a 24.375" diameter steel plate. If I use the .75" thick plate the rotor weighs in at 82 lbs each. Could I get by with using either 1/2" or 5/8" plate. I am trying to reduce some weight.
Any comments or suggestions?

windy

This is my design of the rotor.

[windy]

I forgot to note that on my design the three holes inline with the spokes are used to hold the rotors apart.

windy

[dbcollen]

there is an incredible amount of force pulling those rotors together, I would not trust 3 studs to hold it apart.

[joestue]

for 1 inch wide magnets you might be able to go with 1/4th inch steel plate without losing much flux.

the reason why is simply that half the magnet (.5 inches) at .7T flux density concentrates into 1.4T through the steel backing. however, that's only looking at it two dimensionally. the magnets are on the outer edge of the disk and you've got a slightly longer path on the inside for the flux to flow through as well.

but 1/4th inch disks are probably not stiff enough for a 20 foot wind turbine, but they would be if some additional ribs were cut and welded.. but you'll have to grind them flat after doing that, lots of work to save 20 pounds. 3/8th plate would work.

[SparWeb]

I'm also concerned about the cut-outs around the hub.  There's a lot of torque, thrust, and twisting (the gyroscopic kind) going on there.

[windy]

Thanks for all the replies.

I redesigned the rotor and using a 1/2" plate, got the weight down from 82 lbs to 53 lbs per rotor. I was concerned that a 3/8" plate may flex together from the pull of the magnets.
I got the design from http://www.otherpower.com/20page1.html. The link to all the pictures now work. Thanks Dan.

Below is my new design.
windy

[windy]

This is a better picture of my design.

windy

[midwoud1]

Hi Windy

How do you make your variable pitch blade system ?

In a storm you can not stop a 20 foot with passive pitch and a furling tail.

Or make it with actuator control on the moving tail like Kitestrings did.

Rgds. Midwoud1.

[windy]

midwoud1

Thanks for the reply. I have been following your active pitchcontrol posting since you started it.

I plan on using a 12 volt 500 lbs push/pull linear actuator to control the pitch mechanism. I need to deal with freezing temps during the winter and figured I need a little more push/pull on the actuator. I am using a sun gear with three gears pitching the three blades. It works good in the shop. Only time will tell if it works on the tower.

Will post some pictures of my assembly.

windy

[SparWeb]

Hi windy,
That looks better.

An idea that may help with the weight problem is a design that Kitestrings came up with a few years ago:



This was a mock-up before KS built the real thing.  Maybe it's enough to give you the idea.  I remember seeing a more hi-res picture, and the finished article, but this is all I could find, going back through many pages of old postings.  Hopefully Kitestrings will notice this post and contribute a better picture, or maybe you will have more luck than me searching. 

[midwoud1]

Windy. 
No need for an extra overpowered actuator. Let the blades pitch in the center of gravity.
And use rubber sealed ball-bearings to avoid water inside .

Rgds . Midwoud1.

[aytekin]

Merhaba ben de Türkiye den...
Forma yeni katıldım.  Cok güzel çalışmalar var. Ölçülerinizi kullanarak ben de rüzgar türbüni yapmak istiyorum. Ama ben yeniyim. O nedenle bol bol resim olsa çok iyi olur. Teşekkürler.


Bol rüzgarlı günler....

[MagnetJuice]

Foruma hoşgeldiniz.

Bu sizin için başlamak için iyi bir yer.

https://otherpower.com/20page1.html

Bazı sorularınız varsa buraya gelebilir, ancak bunu yapabiliyorsanız İngilizceye çevirebilirsiniz.

Google çevirmeni buradan kullanabilirsiniz.

https://translate.google.com

Ed

[DamonHD]

Could we please keep all discussions on the forum to English, to maximise usability (and to make moderation plausible)?

And yes, welcome to Fieldlines!

Rgds

Damon

[MagnetJuice]

I'm sorry about that, I should've known better.

Here is his posting and my answer in English so others can easily follow:

Hi, I'm from Turkey ...

I just joined the forum. There are many beautiful works. I want to make your wind turbine using your measurements. But I'm new. So it would be great if there were lots of pictures.

Thanks

XXXXXXXXXXXXXX

Welcome to the forum.

This is a good place for you to start.

https://otherpower.com/20page1.html

If you have some questions, you can come here but translate it to English if you can.

You can use Google translator here.

https://translate.google.com

Ed

[aytekin]

Yours MagnetJuice ...
Form is very successful and very fun. Good information. I'm happy to be here. I'm following.
Thanks...

[aytekin]

This is a better picture of my design.

windy

Mr. Windy.
your project looks very nice. But would you give more information about the project?
how many coils?
size measurement?
Magnet disc diameter?
stator size?
coil size (leg width-thickness and height)? as...
Is your project done? the pictures are very good.
thanks...

[DanG]

https://translate.google.com/translate?sl=en&tl=tr&js=y&prev=_t&hl=en&ie=UTF-8&u=drupal.otherpower.com%2F17page1.html&edit-text=&act=url

https://translate.google.com/translate?hl=en&sl=en&tl=tr&u=otherpower.com%2F20page1.html

[Adriaan Kragten]

In my free public report KD 78 (see www.kdwindturbines.nl), I give the measurements for a PM-generator for different load conditions. One is for different constant voltages which can be compared to battery charging. Another is for different resistances as load which is what you get if the wind turbine is only used for heating. If you compare the measured Pmech-n and Pel-n curves, you see that there is a strong difference. For a constant voltage, power is only generated above the rotational speed for which the unloaded voltage is equal to the adjusted voltage. This facilitates starting of the rotor. For a resistance as load, power generation starts directly from stand still position.

The required mechanical power for a resistance increases about quadratic to the rotational speed but the generated power by the wind turbine increases according to a cubic line if the rotor runs at its design wind speed. This means that if the matching is correct at moderate wind speeds, it will be too strong at low wind speeds. So at low wind speeds, the rotor will be loaded too much by the generator, thus slowing down the rotor to a very low rotational speed. This problem can be solved by disconnecting the generator and the load below a certain rotational speed and connect the load again at a higher rotational speed. Another option might be to increase the resistance at decreasing rotational speeds but this might be difficult if fixed resistors are used. 

[Adriaan Kragten]

The disadvantage of the method given in my previous post is that no power is generated at low wind speeds and that the wind turbine is accelerating and slowing down at moderate wind speeds. I have thought about using a wind turbine for heating only and I now think that there is a more elegant way to load the generator such that the rotor is turning close to the optimum tip speed ratio for all wind speeds.

As I told in my previous post, the problem of using one fixed resistance as load is that this load has almost a parabolic P-n curve and that the optimum P-n curve of the rotor is a cubic line. The point of intersection of both curves depends on the chosen resistance. The higher the resistance, the lower the rpm belonging to the point of intersection.

Next it is assumed that the 3-phase current coming out of the generator is rectified and that only one resistor is used in the DC line. The resistance is chosen that high that the point of intersection of both curves is reached at a rather low wind speed of about 4 m/s. For this choice, the generator will not be able to slow the rotor down to almost stand still at low wind speeds. However, at high wind speeds the generator load will be much too low and the rotor will therefore run at a much too high tip speed ratio and so at a too low Cp. This problem can be solved by adding a voltage controlled dump load which keeps the voltage constant above the value which belongs to the point of intersection. The P-n curve for a constant voltage is much steeper than for a constant resistance and this makes that an acceptable matching is also realized for high wind speeds. So at low wind speeds all power is dissipated in the resistor but at medium and high wind speeds the power is partly dissipated in the resistor and partly in the dump load.

A 27.6 V, 200 W modular voltage controlled dump load is given in the manual at the bottom of the list with KD-reports on my website. The maximum power can be increased by connecting several dump loads in parallel using only one voltage controller. The voltage level can be increased by connecting several voltage controllers and dump loads in series.

To visualize what I mean with the different curves, I have taken the VIRYA-1.8W rotor as an example. The P-n curves of this rotor for different wind speeds and the optimum cubic line are given in figure 4 of KD 664. The estimated Pmech-n and Pel-n curves for 26 V star are removed from this figure and replaced by a parabolic line which is intersecting with the optimum cubic line at Vd = 4 m/s. The modified figure is given in the attachment. For wind speeds higher than 4 m/s, the P-n curve of the load will lie close to the optimum cubic line if a voltage controller plus dump load is used.

[Adriaan Kragten]

The determination of the starting behaviour is rather difficult in the P-n graph because all P-n curves of the rotor go to zero at n = 0 rpm. The starting behaviour can therefore be researched better in the Q-n graph. Determination of the Q-n curves is done in the same way as the determination of the P-n curves only the Cq-lambda curve has to be used in stead of the Cp-lambda curve. The optimum cubic line in the P-n graph corresponds to the optimum parabola in the Q-n graph. A parabola in the P-n graph corresponds to a straight line through the origin in the Q-n graph.

As an example, I have made the Q-n curves for the VIRYA-1.8W rotor including the optimum parabola and a straight line for Vd = 4 m/s and added this graph as attachment. The Q-n curves are only made for wind speeds of 3, 4, 5, 6 and 7 m/s. In this attachment it can be seen that the Q-n curve of the rotor for V = 3 m/s intersects with the straight line for Vd = 4 m/s. The lowest point of intersection lies about at a rotational speed of 37 rpm which means that the rotor is not really starting at V = 3 m/s. The Q-n curve for V = 4 m/s has just no point of intersection with the straight line for Vd = 4 m/s, so the rotor will certainly start at V = 4 m/s.

[windy]

I know this is an old post, but I started it so I will see if I can finish it. After a shoulder and a knee replacement and other old guy health issues, I feel good enough to continue the build. I have the magnets installed on the rotor, and am in the process of winding the coils. I wound a test coil with 10 rounds and at 200 rpm's, I get 3.16 volts AC. I have 24 poles and will be using 18 coils wired star.

I have a few questions. The first is, what would be a safe operating rpm for a 20 foot diameter mill? I had the rotor up to 400 rpm on the test stand and there is no vibration. Of course, that is without the blades on.

The second question is, what would be the AC voltage between the phases at 200 rpm's? I think I know the answer, but just want to double check.
3.16 volts AC x 1.74 x 6 coils in series =32.99 volts AC between phases. I am hoping to wind the coils with #14 gauge wire, two in hand, 100 turns, which will give me 50 turns of #11 gauge wire. 50 turns/10 turns of test coils =5 x 32.99 volts AC=164.95 volts AC per phase. I am trying for around 240-300 volts AC, but that all depends how high I can safely run the rpm's.

[Adriaan Kragten]

I have checked your calculation of the unloaded AC voltage in between two phases and this is correct if the winding is connected in star and if the rotational speed is 200 rpm. If you rectify the voltage with a 3-phase rectifier, you will get an open DC voltage which is a factor 1.35 higher (if the voltage drop over the rectifier diodes is neglected). The open voltage increases linear with the rotational speed. The loaded voltage is much lower than the open voltage and how much lower depends on the resistance of the load. The lower the resistance, the larger the current and the lower the loaded voltage.

The rotational speed is given by formula 4.8 of report KD 35 if the rotor is perpendicular to the wind and by formula 7.1 of KD 35 if the rotor is turned out of the wind by a yaw angle delta as the result of a certain safety system. So the maximum rotational speed for a certain rotor at high wind speeds depends on the delta-V curve of the safety system but also on the generator load. Don't think that you can limit the maximum rotational speed without a proper safety system because there is always a wind speed for which the rotor torque is higher than the peak torque of the generator.

[bigrockcandymountain]

In my opinion, 200 rpm would be a good maximum for a 20' diameter. 

It is approximately tsr7 at a wind speed of 20 mph.  After that, the safety system should start to limit rpm. 

If the hub and blades are really well made, you might go a bit higher.