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Question about 7 Foot turbine
By arwenvaughan, Section Wind Posted on Wed Nov 04, 2009 at 01:33:43 PM MST
Has anyone made the Homebrew Wind Power 7 footer, but with 2x1x0.5 neos?

I have built two of the standard Homebrew Wind Power 10 foot turbines and have a lot of left over parts from that process. I was thinking about building the 7 footer that "The Dans" discuss in the Sizing It Up or Down section of the book. Since the parts are pretty much interchangeable and I would like to make a smaller 12 volt machine, I thought this might be a good idea if someone has done the testing. I have a bunch of the 2" x 1 x " 0.5" neos left over and would like to use them instead of buying the more expensive (and powerful) 2" diameter round neos that the design calls for.
Does anyone have any experience testing this single magnet rotor 7 footer with these magnets and a different stator? I know it would produce less power and is not the best use of magnetic material, but that is OK in this case. I am wanting to build a 12 volt machine using all these parts if possible. I could use the second steel rotor disk (without any magnets) on the other side of the stator as well if someone has some numbers on this arrangement. If not, I'll just bite the bullet and order the 2" diameter round magnets. I not really wanting to go through the iterative testing to get this right if nobody else have tried this out yet.
Cheers,
Arwen

Question about 7 Foot turbine | 12 comments (12 topical)

Re: Question about 7 Foot turbine (3.00 / 0) (#1)
by DanB on Wed Nov 04, 2009 at 04:50:50 PM MST

Hi Arwen ~ actually I have made a couple that way and it works out fine, you need to run the airgap pretty tight and I think itll be fine.

in the worst case, you could build with the smaller magnets and put a blank rotor (just a plain steel rotor on the front) - that would get you right about back where it wouldve been with the larger disk magnets.

Re: Question about 7 Foot turbine (3.00 / 0) (#2)
by arwenvaughan on Wed Nov 04, 2009 at 07:43:44 PM MST

Thanks Dan! I thought you must have tested that arrangement out at some point. Cool, I'll just make the stator using 48 turns with 2 strands of 15 gage wire, like it says in the 7 footer section of the book. I'll be ordering a set of the 1/4" flat steel parts from you all then real soon.

Cheers,

Arwen

[ Parent ]

Re: Question about 7 Foot turbine (3.00 / 0) (#3)
by Flux on Thu Nov 05, 2009 at 12:23:01 AM MST

I suspect you will get much better results if you add the second disc with no magnets as Dan suggested.

It will probably work without but it will run fast. Much depends on your blades but fast things tend to be noisy and it will be very responsive in high winds and you will have to watch the furling.

Flux

[ Parent ]

Re: Question about 7 Foot turbine (3.00 / 0) (#5)
by arwenvaughan on Thu Nov 05, 2009 at 06:47:35 AM MST

Thanks for the comment Flux. Yeah, I really like the idea of using the second steel disc with no magnets. It seems like a cost effective way to get more power out of the alternator.

I have heard that using a dual rotor increases the magnetic flux by 4 times, so what does adding just a second steel disc do? Does it double the magnetic flux? Is there a rule of thumb for this? Also, can you confirm that the second steel disc does indeed need to spin with the other magnet rotor and can't be stuck directly to the other side of the stator?

[ Parent ]

Re: Question about 7 Foot turbine (3.00 / 0) (#6)
by Flux on Thu Nov 05, 2009 at 09:13:18 AM MST

Roughly for the same air gap the single rotor with spinning disc will have about half the flux of a dual rotor.

Single rotors with no return disc have no really defined flux path. The parts of the coil near the magnets will see something like half the flux of a dual rotor but the back part of the coil will see much less.

In the common case where the stator thickness is about magnet thickness the it seems that you get about half average flux as long as the stator is very close to the magnets, this dimension becomes very critical.

Dual rotors with stator about magnet thickness and a practical clearance gap typically have a flux density of 600 to 650mT depending on magnet grade. With little mechanical clearance and a similar stator the single rotor probably manages about 250mT.

A single rotor with spinning disc and practical mechanical clearances will get you something like 450mT. You need a stator a bit thinner than magnet thickness for best results. As the power goes up as something like flux squared there is a very big improvement. If you must use a single rotor with no return path keep the stator very thin or it adds a lot of copper and resistance with nothing useful generated in much of it.

It is absolutely vital that you spin the flux return disc. If you keep it stationary you need high grade strip material properly laminated. It will cost more than the extra magnets of a dual rotor and you will still have iron loss.

If you use a fixed steel plate you will just make an eddy current brake and it will never start.

Flux

[ Parent ]

Re: Question about 7 Foot turbine (3.00 / 0) (#10)
by arwenvaughan on Fri Nov 06, 2009 at 11:57:40 AM MST

As usual Flux you are a wealth of knowledge...thanks for the info. So it really sounds like I should be alright using this setup. I guess the only question is getting the stator winding (gauge and # of turns) exactly right for this arrangement. I'll probably just stick with the "48 turns per coil using 2 strands of #15 AWG magnet wire" as outlined in the Homebrew book. Wouldn't you know it, I have 11 lbs of #17 AWG left over but no #15.

Cheers

[ Parent ]

Re: Question about 7 Foot turbine (3.00 / 0) (#4)
by jlt on Thu Nov 05, 2009 at 04:21:23 AM MST

I have built a 6ft two blade using the 2x1x 1/2 mags i used a automotive water pump for the bearings and shaft.the rotor came off from a trailer disc brake. it is 10" diameter. I am going to install it on a 4 leg tower. that held a winco 6' machine. The tower is not strong enough for a 10 ft turbine.

Re: Question about 7 Foot turbine (3.00 / 0) (#7)
by brokengun on Fri Nov 06, 2009 at 05:28:06 AM MST

I am running a 7 footer right now with those magnets. I do not have a disk in the back and I agree that the air gap needs to be very close. I really need to decrease the air gap to reach charging voltage at lower RPM.

I have 12 magnets with 9 coils and I'm actually using the Otherpower 24 volt stator which will produce 12 volts in this setup.

Re: Question about 7 Foot turbine (3.00 / 0) (#8)
by arwenvaughan on Fri Nov 06, 2009 at 07:12:34 AM MST

Oh cool! This is another good data point. Do you recall what gauge wire and number of turns you used in this stator? Did you use the Otherpower 24 volt stator that the book recommends using with 1"x2"x0.5" magnets (#14 AWG w/ 70 turns) or the 2" diameter magnets (#15 AWG w/ 90 turns)?

Thanks!!

[ Parent ]

Re: Question about 7 Foot turbine (3.00 / 0) (#12)
by brokengun on Sat Nov 07, 2009 at 06:20:23 PM MST

http://www.forcefieldmagnets.com/catalog/product_info.php?cPath=22_30&products_id=183

9 coils, 66 turns of #14 gauge. I'm not certain if this is exactly what you want but that is what I am running. I am using the rectangular 1"x2"x.5" magnets, yes.

[ Parent ]

Re: Question about 7 Foot turbine (3.00 / 0) (#9)
by Fused on Fri Nov 06, 2009 at 09:21:15 AM MST

Ive just built and tested a Single rotor vs dual rotor with "Fake" magnets.
I have a 1/4" thick, 9 inch dia rotor with 8 magnets. 2x1x1/2" N42
The second 9 inch disk has steel 1/4" thick 2x1 blanks mounted in the same location as the magnets of the first disk.
My stator is 7/16" thick with 6 coils of #20 wrapped 150 times. Wired star. 24v system.

Measuring phase voltage in AC:

Single rotor

400RPM = 18V ac
1200RPM = 52V ac

Dual rotor with "Fake" magnets installed

400RPM = 22.7V
1200RPM = 68.8-71V

Id say the second steel plate is worth about 30% increase.
Hope this info can help.

Fused

Re: Question about 7 Foot turbine (3.00 / 0) (#11)
by willib on Fri Nov 06, 2009 at 06:17:16 PM MST

I am a big proponent of test coils especially on a new design.
once you make the test coil , then you will know what voltage and current you can expect from a finished stator .

Carpe Ventum (seize the wind)

Question about 7 Foot turbine | 12 comments (12 topical)

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