questions from a newcomer

[AW383]

I am finally getting the time to build a wind generator. Don't have a lot of time so I want to build one right the first time. Have many questions. Assume that I will be building a three phase dual rotor machine similar to the ones posted on this website.

1) does plate that the magnets mount to have to be a magnetic material (ie, mild steel, cast iron)?

2) assuming use of 2"x1"x.5" neo magnet, what are optimal coil dimensions on x-y axis? How much inner and outer coil to magnet overlap?

3) magnets alternate in ploarity when place on the plate? N-S-N-etc...

4) all coils winding directions the same when they are cast into the rotor?

5) assuming dual rotor/magnet system, i presume the opposing magnets are in a N-S arrangment. Are the magnetic fields of the 2 magnets merged. Coil located within this merged zone?

any help greatly appreciated! THanks AW383

[Flux]

If you want to get it right first time, buy Hugh's book and follow it or copy one of Dan's designs here with the same magnets, coils and prop and you will be ok.

Plates have to be magnetic

Magnets alternate in polarity.  Opposite magnet rotors attract as you say.

Coil dimensions and how they are connected depends on other factors that you haven't mentioned, such as wind speed, prop diameter and system voltage.

Note that Hugh's 5 phase design has different winding arrangements from Dan's 3 phase so don't try mixing the 2 designs and don't experiment with arbitrary numbers of magnets and coils.

There is absolutely no reason why you shouldn't get it right first time. If for any reason you need to use something different from the proven designs perhaps because you can't get something, ask before you make the change, someone will help you.

Flux

[force9BOAT]

Flux,

What do the plates have to be magnetic?  How about if magnet sand is mixed in to the resin that is poored into the center of the coils?  I saw magnet sand used on one of the devices shown on OtherPower.com.  

I am also looking to build a dual-roter alternator but I have a wood shop, not a machine shop. I'm wondering how much efficiently I could get back by mixing magnet sand into the coil resin or adding metel rods bent into a loop to the back of the wooden roters.

Thank you,

Rob

[force9BOAT]

Make that, "Why do the plates have to be magnetic?"

[AW383]

I took your advice and ordered the Scoraig book today. Thanks for the other info by the way. I think that I understand most design details now. I am still confused by the coils though. I would imagine that there is some happy medium with their dimensions in relationship to magnets.

The other questions I had were in coil spacing. I see some designs that have coils directly next to each other. Others I have seen have some distance between them. Which leads to yet another question, with a 4/3 magnet to coil ratio, I am assuming that proper magnet to coil relationships will be present for good three phase output.

I am basically trying to draw everything up and figure out what kind of diameters I need to construct. Knowing general magnet and rotor dimensions and assuming that there will be some amount of space between each coil will produce the diameter information I require.

I looked over my front wheel assy that I have. It is a tad different than what is on otherpower website but I can machine things to work none the less. It is off a chevrolet utility truck. I like it because of its large spindle/bearing size. Quit a bit larger than what is on my cars. I plan on lightening it some, it is awfully heavy at present. Its size will just barely fit in the biggest CNC lathe I have.

Sorry, one more question. Would I be able to mill shallow pockets into my rotor so that the magnets fit within them? Or should I forget that and mount on surface like everyone else is doing? Not sure what magnetism differences would exist by having the magnet in such a machined pocket...

Thanks,

AW

[Flux]

To use the full potential of a magnet it should be in a closed magnetic circuit. Any air gap reduces the available flux density so you need to keep the air gap as small as possible. With air gap type alternators you have to have a gap wide enough to put the coils in but anything else that increases the gap is undesirable. Without steel plates backing the magnets the gap will be many times the thickness of the magnet and the flux density will be far too low. Even steel needs to be reasonably thick to be effective, normally over 1/4".

Bits of wire and magnetic sand will help a bit but the gain will be very little.

The argument for magnet sand inside the coils is that it helps carry the flux, Proper laminated iron would do this infinitely better but would introduce cogging and other problems. The sand may be a slight benefit without the severe problems but I doubt if it would be worth it.

When it comes to the return magnetic circuit between the backs of the magnets it would be about useless, there is no substitute for mild steel, even cast iron has to be thicker for the same effect.

You may be able to set your magnets into wood for the strength and use crate strapping tape coiled up at the back to carry the flux or less effectively you could use a coil of mig wire or something but you will need metal working facilities for some of the other construction such as the hub and yaw mount and furling tail so perhaps you could do a deal with someone to do the metal work in exchange for a bit of woodwork that they might need.

Flux

[maker of toys]

RE: milling pockets to hold the magnets:

I don't know, but I mean to find out.  (I finally figured out what to do with the pile of used up fiat brake disks I've accumulated. . .<G>)

 I intend to make three otherwise identical gens, one using shallow slots (ca. 10% the thickness of the magnets), one with islands to fit the magnets, the last flat. . . then I'll run them all on an instrumented stator and get some data. If there is a significant difference, (>5%) i'll then spend some time optimizing the rotors (more slot depth, etc)  

Details will arrive here when I'm satisfied with the data.

that is, if noone else has done that test.  (I havn't gotten around to searching on the subject. . .)

in my gut, I don't think that a DEEP slot is a good idea. it feels like it would probably be a form of magnetic short. . . . My suspicion is that some form of island may have the best performance of all.

-Dan

[Flux]

About the coils. If you use a 4/3 magnet coil arrangement with a sensible spacing between magnets, and this usually works with gaps about equal to magnet width near the centre ( with very long magnets you can squeeze them closer at the centre to keep a better overall spacing, the coils will have holes the size of the magnets and you wind the outside such that they just touch. If you leave a lot of unwound space at the outside you are not using the maximum sized wire for the number of turns.

There is an unsettled issue about the shape of the coils, with wedge magnets the shape will be triangular or trapezoidal with radial legs. With rectangular magnets some still make the coils this way as they are obsessed with maximum volts at all cost. When you take resistance into consideration it is much more likely that rectangular coils will be better in the end but if you have crammed the magnets tight on the inside then a slightly wedge shaped coil may be better.

With round magnets it is likely that round  or oval coils will give a better overall power output than coils with straight radial sides. You will find arguments for both lines of thought here, you will have to decide for yourself, it will only be a second order effect.

Regarding coil thickness, you will get the best use of materials  when the air gap is equal to or a bit less than thickness of 2 magnets. The coils need to be smaller to give you a working clearance. Again a fair variation is possible but don't go thicker and if you go too thin the mechanical spaces needed for clearance take up a large part of your available winding space.

Don,t put the magnets in pockets, there will be a reduction in useful flux. It is reasonable to machine a groove in the disc to give the magnets a lip to sit on on the outside edge to help counteract the centrifugal force but don't go too deep and you can make the groove wider than the magnets so there is clearance at the centre.

Hope that answers your question, I thought you were after details of turns, wire size and connection methods, which I can't give  with the information available but when you get the book it will all be there.

Good luck.  Flux

[ghurd]

I considered a brass, aluminum or stainless pin or 2 in the disk, at the outside of each magnet. Just a thought.

G-