11 footer, round 2in magnets, need coil help

[stoichiometry]

Hello,

I was wondering if I could get some help?    Over the last three months, I've read a lot on turbines, learned to weld, and am about 50% of my way into this project (what fun!!). 

I have made the metal parts, carved blades for a 11 foot turbine,All that is left is to wind coils and mold the stator/magnet rotors and of course pour some concrete when the ground thaws.  In the homebrew book, there is an 'insert' with information on winding a high efficiency stator using round magnets.  It suggests 'two in hand' of AWG13 and 29 turns for 12V. [12 v  is appropriate for my  remote cabin]

Last night, while I was at my shop winding my first coil, I thought about it's shape.  I  built my coil winder to homebrew  spec, but then I got to *thinking*.  Somewhere I read that the inner hole should roughly match the shape of the magnet.  It dawned on me that my coils are nearly a perfect match to rectangular magnets and a poor match to the round magnets. 

I then stumbled upon this discussion board,  found a myriad of opinions regarding round magnets and am now a bit concerned.  In my search, someone said, " First off , a 9 coil 12 pole machine with 2" diameter magnets should have a 14" diameter magnet rotor. Not 12." and many others.   Hence my concern ...


So -- here are my questions.
(1) Has someone built the high eff. 12 volt alternator as suggested in homebrew book?
(2) How did the 29 turns of two-in-hand 13 AWG work?
(3) Did you change the shape of the coils?
(4) Is it advisable to use tapered magnets instead? [the round ones are pricey]

Thanks for any help.  I'll post pics soon!

Kindly,
S

[DanB]

this one should be fun.... 

[jlt]

Do you live in a high .or low wind area ? The 2inch round magnets will give more volts pr turn of wire.lowering the resistance making less heat.  but won.t Make any more power in low winds than the smaller mags. 12 inch rotors will work if you already have them.The 14 inch rotors will give you a lot more room for your coils. so they don't have to be so thick.      11 ft rotors will turn about 10% slower than 10 ft blades. So you will need 10% more turns to keep the same cut in rpm.

[ChrisOlson]

How about this for fun?

You can switch to cheaper N50 bar magnets that won't have as much overlap between phases (exciting two phases at once with the same generator pole) and wind the stator with 50 turns of 13 AWG, one in hand, and wire it delta instead of wye.  About the same resistance, same ampacity, less copper mass in the coils to retain heat, slightly higher cut-in and flattens the power curve a bit to get a better match over a wider rpm range, fits in a smaller diameter (10"), takes a little more attention to detail when building the generator.
--
Chris

[Flux]

I suspect the round magnets were substituted for the rectangular ones to give a bit more flux to give a larger margin of safety. The change is certainly effective but once you depart from the common rectangular magnet blocks they tend to be costly.

It is a one off cost so many may not worry about it. If you are building to the lowest cost I am sure you can get the result with a few more rectangular blocks at lower cost, you can also go up to the top grade neo and not increase the number but once again the N50 may cost more than using 16 of the cheap readily available N40 blocks.

Once you start analysing the various shape magnets for most cost effective design rather than fix a simple problem the I feel sure that you will find that the most cost effective coil shape is basically the shape of the magnet, round for round magnets, trapezoidal for wedge magnets and rectangular for rectangular blocks.

I know someone will come up with the old long coil legs and magnets cutting radial coil legs at right angles and all that lot but in reality a coil is a closed loop of wire and you generate emf by flux linkage and the maximum linkage will come from a loop the shape of the magnet, you just can't have active bits and dead bits in a coil.

To optimise thing you will also need to change the spacing between magnets to get the most cost effective result.

So back to your question, yes the round coils with the non ideal spacing and non ideal coil shape will give you a very good machine and it will be better than the basic rectangular block design. it will not be the cheapest way to get the result unless you can find a cheap source of the round magnets.

If you are worried about the cost then there are better ways but you need to do your own design work. If you accept that the cost of the alternator is a one off one and is small compared with the total cost including batteries tower etc then it is easier to accept that it is a good and effective way to get what you want. Everything in wind power is a compromise including cost.

Flux

[stoichiometry]

OK -- I'll try to provide some "fun".

JLT - My cabin is about 10 miles from here and get about 10% more wind.  According to homebrew, it looks like 29 turns (2 in hand), so you are suggesting another three turns = 32?   http://www.saiawos2.com/KOGS/WindSpd.htm

Chris - I already own the more expensive magnets.  Are you proposing I buy some cheaper ones to 'play' with and save the round magnets for a bigger project?  I am sure this won't be my last turbine.  I am aware of the expense ... but so far, my cost is guestimated to be less than 10% the cost of a commercial one.  And I am having a ton of fun.  

Flux-  I am not sure I can change the shape much and have it fit .  I wound my first coil last night and, while not perfect, fit kinda nicely between the 20 degree lines.  If I went with a round coil, it'd be wider at distance closer to the center of the circle and I am not sure they'd fit.    DO you think the wedge shape coils  (2 inchs at top and 0.75 inches at the bottom) proposed int he homebrew book would work ok with the round magnets?  

I appreciate the help!

CHeers,
S

[ChrisOlson]

Chris - I already own the more expensive magnets.  Are you proposing I buy some cheaper ones to 'play' with and save the round magnets for a bigger project?

No, you just said your coils are a better match for 2 x 1 bars so I offered one possibility that I have built in quite a few generators, using the bar magnets.  I guess I misunderstood that you already have the discs.

DO you think the wedge shape coils  (2 inchs at top and 0.75 inches at the bottom) proposed int he homebrew book would work ok with the round magnets?  

I'm not flux and I'm not too good at playing him on TV.  But, yes, that will work.  You'll get some cancellation where no current will flow in the coil due to excessive overlap of the generator pole on both coil legs at once, which reduces overall efficiency of the generator.  But it will still work if you use a wye configuration winding.  In delta that layout will have severe phase mis-match and drag very hard with no load on the generator, making it difficult for the prop to reach cut-in speed.
--
Chris

[stoichiometry]

Chris et al,

THanks.  I will wind up some 31 turn coils (2 in hand) and see if they fit.  Would it hurt to make them a bit thicker?  My coil winder is 3/8 thick, I can easily make it 1/2 inch and probably get away with a little less wire and slightly narrower coils.   What is the effect of making them a bit thicker?

Kindly,
S

[Flux]

I misunderstood your question I think. I thought you were going to alter things to better suit the round magnets.

If you are at the stage where you can't alter anything then I would leave the coils as they are, there is no point in messing with the shape, the shape called for is perfectly ok.

Turns depends on your wind speed at your site, if very low then stay similar. That design is quite stiff and for a high wind site a few less turns will let you get a better match without having to open the air gap or add resistance but as it stands it stall protects well and may be less tricky to control in high wind, but if you do manage to furl it properly with a bit more speed you will get better overall output.

If you make the coils thicker than I suspect you will get much the same effect as you are infact opening the air gap ( it's distance between magnets, not stator thickness). With thicker coils you could use a bit thicker wire and there may be some overlap in the characteristics but again you are introducing new factors that I really don't recommend for a first build.

I keep advising people to stick to designs as published for a first attempt so you have something that will work first time. When you know what you are doing you may be able to tweak things to your advantage, but changes in one thing can have far reaching effects in other areas so be careful.

Flux

[Flux]

I had to go out and that last post was a bit rushed so I may be able to add a bit more.

You should be able to get the coils in as advised, if you run out of space it won't matter if you leave out a turn or two or you could make the coils a bit thicker to get the same turns in. I wouldn't alter the shape much but if you are very close there is no harm in squeezing the coils a bit more elliptical to get them in.

Coming back to some points in your original question, there is a lot of room for better results from those magnets and larger rotor discs will certainly use them a lot more effectively BUT anything that makes the alternator more stiff or efficient will make it less suitable for the proposed blades. if you were contemplating a change to larger blades then changes to magnet spacing and coil shape would certainly get you more power for magnet cost.

Once you start making these changes you are aiming for a different and bigger machine and there are many other factors such as alternator offset, tail size and a lot of other factors that have to be considered.

Even if you build a design to the book there can still be other factors that can trip you up as the designer will not have control over the length and size of cable and other factors so a machine that has been tested and optimised to work well on one site may behave differently on another and some of these changes are far from intuitive. Against all common sense, using a longer and lighter cable run can actually increase power output with some designs and drastically alter the way it furls and it could burn out. Another more marginal design could go the other way with a long cable and perform worse and overspeed with a big reduction in output.

It is natural to be reading all about coils, shapes magnets etc but nothing is set in stone, there are large variations that work well. Much less is written and far less is worried about concerning other factors of a machines behaviour but in many cases they have far more effect on the total power produced than any theoretical tinkering with the alternator.

Wind your coils and get in something near to the specified winding and you will have a good starting point. You will learn a lot by observing its performance and making changes to air gaps etc to get the best out of it. If you say you intend to build other machines then a lot of what you read will make far more sense once you have seen which things make a real difference.

Flux