Starting to build a 7 foot turbine

[elt]

I've decided to make a 7 foot wind turbine as a learning effort before I make a bigger one.

Hughs booklet gives some factor-of-two relationships for design mods; since a 7 foot mill has half the power of 10 foot mill it seems reasonable to base the 7 footer on the 10 foot one...

I'm assuming that the design TSR on the 10 foot blade is 5; a 7 foot blade with a TSR of 7 will have twice the RPMs, so to have the same cut-in (wind) speed, 7 mph, I want the cut-in RPMs to be twice as high so I double the cut-in RPMs by halving the number of turns.

The coils on the 10 foot design are 75 turns of #17, two in hand, that means

37.5 turns of #17, two in hand.

The 10 is designed for 24 volts; I want mine to be 12 volts so halve the number of turn and double the wire sice = 18.75 turns of #14, two in hand... I'll call that 19 turns.

(Maybe I should add a turn or two since the voltage drop for the diode isn't divided by two?)

Since the 7 foot gen makes half the power, reduce the "wires in hand" or wire size by a factor off two... so the coil design is 19 turns of #14 wire (or 19 turns of #17 wire, two in hand.)

That leaves the question of how to make an alternator with half the power of the 10 footer. All I have is a guess here: make the alternator like the one for the 10 footer but only use one set of magnets... one of the things I haven't found yet is the relationship (power output) between gens with two rotor of magnets, two rotors with magnets on one jsut rotor, and one rotor with magnets...

I'll appreciate any corrections and advice given. So far I have the blades carved and balanced and good part of the yaw bearing and furling tail built. I haven't done anything (but paperwork) on the hub and stator mounts pending more confidence in the alternator design.

Hi. I'm new to the site. I like to build things; I build telescopes for a living. I don't expect to get off the grid but we lose power here several times a year (in western NY) and there's only so many hours of gas for the backup generator that I feel comfortable keeping around but every few years we can get an ice storm that takes out the power for a week or more. The state wind maps claim 15 mph in Winter and Spring, 8 mph in Summer and 12 mph in the Fall so it looks to me like there's some good energy up there waiting to be caught. Eventually I'd like to have enough self-made power to run the house lights and, when needed, the furnace.

 - Ed.

[willib]

cool telescopes !

"one of the things I haven't found yet is the relationship (power output) between gens with two rotor of magnets,?, and one rotor with magnets..."

what i have witnessed with my test gen is two rotors with magnets will get you twice the power of one.

[Chagrin]

A lot of simplicity in the design, too. If you don't use two magnet rotors you still have to figure out a way to achieve a good flux path through the coils and get stuck trying to make a goofy laminated steel disk or a massive steel plate.

[luv2weld]

Ed,

Welcome to a strange disease.

You didn't say what your information source was (the downloadable book or one of Hugh's books on paper). The June 2005 version of Hugh's Axial Flux Wind Turbine includes plans for a 4 foot, and an 8 foot version, as well as 10 and 12 footers. It looks like our host is offering the 2004 version, but since I haven't read it, I can't be sure. Maybe  Dan could tell us if the plans are the same in both versions.

If not, you can use this to find it on ebay.

http://cgi.ebay.com/How-to-Build-Wind-Turbine-Generator-Hugh-Piggott-6-2005_W0QQitemZ320014724704QQi
hZ011QQcategoryZ42907QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Or use this to buy it direct from Hugh.

http://www.scoraigwind.com/?=Return+To+Merchant

The best $20 you'll ever spend.

Ralph

[elt]

Thanks Ralph,

I got the June 2005 version and am referencing the 10 foot design in there.

I'm a little jealous of the folks that (seamingly) can do the winding and magnet stuff in their heads; the "rules of thumb" are a big help adapting a designs but I haven't found some rules that would help with magnets such as what happens when you go from n35 to n40 or n50 and what happens when you double or halve the number of magnets... another one I guess at, what happens when you change the rotor diameter. I'm guessing that it's similar to the rule on doubling the RPM, that they are both relative to the linear speed of the magnets over the coils so that 2x the diameter (more accurately, 2x the radius of the center of the magnets) equals twice the voltage. But like I said, these are guesses. I'm surprised, though, that's there's not not a speadsheet published for exploring these parameters. While I don't understand the papers I've googled from university sites, it does appear that to be soomthing that can be calculated.

 - Ed.

[electrondady1]

elt, i'm into vawts,and thats like making it up as you go along but it seems you are makeing assumpions that could render the project useless. since there are detailed step by step instructions on how to build an 8' dia. why build a 7' unit . and guess about cut in speeds magnet size and number and mags coils  turns and conductors?

[elt]

Hi electrondady1,

While I imagine that it's possible that I will make an unusable generator, I am hopeful that some kind soul that sees mistakes in my posts will correct me.

 - Ed.

[Flux]

"I'm assuming that the design TSR on the 10 foot blade is 5; a 7 foot blade with a TSR of 7 will have twice the RPMs, so to have the same cut-in (wind) speed, 7 mph, I want the cut-in RPMs to be twice as high so I double the cut-in RPMs by halving the number of turns."

Your logic is right but I question your assumption. I am not familiar with Hugh's 10ft machine, but I would be surprised that the tsr is as low as 5 ( my guess it is 7) can you confirm.

"The coils on the 10 foot design are 75 turns of #17, two in hand, that means

37.5 turns of #17, two in hand"

This is not logical, if you halve the number of turns then you can double the cross section of wire.

You are right to halve the turns for 12v. This will again allow you to double the wire cross section.

You end up with an alternator that is far too powerful for the 7 ft prop. You could do something crazy like wind it with wire that is much too thin to raise the resistance, much better you could add resistance externally and have a robust winding that you would never burn out. Most people go for better use of materials and use less magnet to produce a less powerful alternator.

You could do this with a single rotor, but you would need to do test coils from scratch as the results of a single rotor depend drastically on air gap and stator thickness.

You could manage 7 ft with 8 poles ( 16 magnets dual rotor) on  smaller discs, but watch the hole size in the stator to make sure you have room for the hub or mounting studs.

As you may have gathered from this, when you change one thing you mess up everything else. As you are new to this and lack experience you would make life much simpler if you built the 8ft machine rather than try your ideas of modifying the 10 ft one.

If you really must have 7 ft you can come up with some ideas and I will check them before you go too far. Alternatively the 8 ft alternator will work with 7 ft blades if you widen the air gap and perhaps use more line resistance than usual.

Flux

[elt]

Hi Flux,

I've read many of your posts and realize how much you try to help everyone; thank you very much. I'm not married to the idea of a 2 meter blade; it's more that at the top of the page I indicated that I was undertaking "a learning effort" by attempting to scale the 10 foot design. Sure, I could build the 8 foot gen or even buy one; but what would I learn from that?

I appreciate that you write "when you change one thing you mess up everything else" but I'm still believing that these things can be designed as opposed to just build and then see what happens. I don't understand the design process (yet!) but from the few EE papers I've read it seems that implementations come reasonably close to design that a design effort isn't fruitless...

This is not logical, if you halve the number of turns... That was based on my interpretation that the 7 foot blade would spin twice as fast as the 10 footer in the same wind, so to get cut-in voltage at the the same windspeed, I figured that you would need to half the number of coils to to make the voltage the same at twice the shaft RPM.

You are right, I probably did misinterpret the TSR of Hugh's blade designs. He does use "7" in his examples but when I use ALton's blade designer, I can't get Hughs numbers for drop unless I diddle the angle of attack or other parameters. Well, if you diddle the other numbers, you can put in a wide range of TSRs and get his 2mm drop at tip... it's another case where "just do it" might work but doesn't help me understand.

I've only found one formulaic example of someone scaling the magnets. He scaled "power" linearly with the surface area of the magnet face and some other factor on the thickness of the magnet...

If I may ask a question on scaling: the cited 2x1x.5 N35 magnets are readily available as N40, N42 and N50. If you used N50 magnets, I assume that makes the gen more "powerfull" in some way. Would a 500 watt gen become a 50 / 35 x 500 = 700 watt generator (needing 1.4x more wire weight) and a 1.2x (square root 50 / 35) larger blade?

Thanks again,

- Ed.

[Flux]

"This is not logical, if you halve the number of turns... That was based on my interpretation that the 7 foot blade would spin twice as fast as the 10 footer in the same wind, so to get cut-in voltage at the the same windspeed, I figured that you would need to half the number of coils to to make the voltage the same at twice the shaft RPM."

You are quite right about halving the turns, what is not logical is keeping the same wire size and having lots of empty winding space that you could use to improve efficiency.

Normally if you double the speed you can use half the turns of wire with twice cross sectional area, this will give 1/4 the resistance.

You are right about blade calculators, you can get different figures by changing the default settings.

Yes these things can be designed fairly well, but I am not sure that the power out figures from blade calculators will help you much until you have experience of the way power and tsr behave with typical blades. The alternator lends itself to fairly accurate design but again experience counts for quite a bit.

The higher energy neos definitely give you more power or what it really comes to, they allow you to use a larger air gap for the same flux density and that lets you use less turns of thicker wire.

Unless you get to the stage where you can design things pretty exactly you might not see that much benefit. Unless you use electronic load matching you will need to get to grips with the trade off between alternator efficiency and prop power out. Stronger magnets on a given design will help if everything is right, but to make best advantage you will need to change windings and prop speed or diameter. Normally most people go for too low a cut in speed and resort to increasing air gap to get best matching. If you adopt this method you end up with similar results but with wider air gap and don't see the benefit of the better magnets.

Without precise measuring instruments the difference will not be very obvious, measuring wind speed is so troublesome that changes in the wind will mask significant changes in performance.

As a rough guide you can say that if you double the flux density you get 4 times the power for a given speed, but you can get drastically different flux densities from a given set of magnets by altering the configuration.

If you are seriously interested you will learn fairly quickly. You will develop your own design methods and they will be based more on experience than published papers, which will only apply to one specific case.

Flux

[elt]

Flux wrote: If you are seriously interested you will learn fairly quickly. You will develop your own design methods and they will be based more on experience than published papers, which will only apply to one specific case.

Understood. Experience comes over time. For now, then, hopefully I can borrow on yours and boards' and get somethinge built that works well here...  Maybe one of Hugh's generators is the perfect fit but I don't think that just "build one" and see is the best way to find out. Maybe there's nothing special at all going on at my site but I can't afford to hire a consultant or engineer to find out.

The things that I assume to be considerations are

I have a 24 foot free standing tower and I figure that I can put a hub up at 30 to 31 feet. The wind loading of the tower is spec'ed at 3 feet above the top so I have to wing it a bit but for a hub height higher than that. I feel fairly good that I could put a 13 foot blade on it, though, for just the shear size of me working on it, I'd feel more comfortable with an 11 or 12 foot blade.

My winter and spring wind averages 15.5 mph. Scaled down to a 30' hub height, that gets about 12 mph with 70% of my power coming from 11 to 16 mph wind. (That range is from a state-published local survey. It shows almost twice the power in that range than the wind power spreadsheet models; I assume it's a local effect...)  It also means (I think) that I'm happy to furl early and cut back on the wind loading on my tower. So, is there some optimization for more watts or better efficiency at 15 to 20 mph, that sounds good to me because I don't expect much total power here at speeds below 9 mph.

The biggest hesitation that I have in building Hugh's 12 foot turbine, at least according to the tables in the booklet, is that despite having more coils and magnets, it produces the same power as the 10 footer (at 17 mph)... if a 10 footer gets 500 watts at 17 mph, wouldn't I expect 44% more from a 12 footer? Not to mention that the power formula in the booklet and other places indicates that the 10 footer ought to conservatively make 600 to 625 watts at 17 mph.

I understand that some folks are building gens too big for their blades and may not using their magnets efficiently but it seems to me (an inexperienced person reading Hugh's booklet) that his 12 footer has too much blade. I'm sure it works fine but it just seems to me from "theory" (meaning if I believe the numbers that I want to believe and not the ones that I don't) that an 11 foot blade, which is well within my comfort zone, ought to be able to get more about 750 watts at 17 mph.

Please! Nothing that I've written is meant to be critical of Hugh's or anyone elses work; it only shows how a new guy can get confused.

So that's where I'm at... is it reasonable for me to expect 750 watts at 17 mph from a 11 or 12 foot blade?

 - Ed.

[elt]

The biggest hesitation that I have in building Hugh's 12 foot turbine, at least according to the tables in the booklet, is that despite having more coils and magnets, it produces the same power as the 10 footer (at 17 mph)... if a 10 footer gets 500 watts at 17 mph, wouldn't I expect 44% more from a 12 footer?

... so I wrote to Hugh and asked him. He said "I checked and the windspeed for the 10 footer is wrong.  It should be 8.6 m/s  around 19 mph" [for 500 watts.] Whew! That makes sense from a mathematical point of view though the watt-monster in me had hoped that the 12 footer would be the one making more power rather than the 10 footer making less (at 17 mph.)

- Ed.

[Flux]

You may struggle to get about 750W from a 12ft machine at 17 mph as normally built.

If you are not worried about winds below 9mph as you say, then you could reduce the turns and use thicker wire and keep the speed up. That way you would be fairly certain to increase your output in the 17 mph region. With care you may be able to get what you want.

I wouldn't get too excited if your wind is from some published figures, you really need the long term figures for your site at 30 ft where the tower is.

If you miss your target and didn't get your 750W until 18 mph you would never know without pretty exact power measurements.

Few people here have the time or facilities to produce exact power curves of their machines. Most at best rely on spot readings from an anemometer near the blades.

This is a useful guide but only regard it as so.

Site conditions will have more effect on power capture than the design of machine.

Bigger the rotor, more the energy capture, no way round that one.

Flux

[elt]

When I ran the numbers, I got that by adjusting to a 9 mph cut in, I can go one wire size larger and keep the coils the same size (same volume.) The result would lowering the coil resistance by 30%

I'm not quite sure how to apply that to the power generated, is the wattage gain just the result of lower resistance, something like this...

As designed, the 12 footer is a 24 volt system and gets 500 watts at 17 mph. That's roughly 21 amps. Watts loss to resistance = 21 x 21 x .6 = 264.6 watts in the original coils and 21 x 21 x .42 = 185 watts. I guess that means that about 80 more watts (and a cooler stator,) for a total of 580 watts total going down to the battery.

... or do you figure it some other way?

 - Ed.

[ghurd]

Ed, a bit of a twist to it too.

V=IR. 21 x 0.6 = 12.6V

And 21 x 0.42 = 8.8V

So at 21A output, the alt needs to make 3.78V less. Meaning it can make 21A at a lower RPM than it might first appear.

Maybe that is oversimplified, but you get the idea.

G-