Triple Rotor / Double Stator?

[Jim Nutti]

I've done a quick search for this, and nothing much came up, maybe it was mentioned on the old Discussion board?

Anyway, the title should give you a good idea of what I'm thinking of.

Sticking with the 'Hugh Piggott' plan, with 12 magnets 9 coils, but add another stator and another rotor.

The main reason I'm thinking of this is because of the cost of magnets. If I could somehow make the 'middle' rotor using only 12 magnets to serve both stators, (some sort of cast to hold the magnets in, similar to the stator cast but obv stronger) I figure there would be twice the power available from a triple rotor alternator for only 50% extra cost of the magnets compared to a dual rotor? Therefore getting more power per pound (dollar) of my magnets.

Note: I havent even built a dual rotor yet, I fully intend to build one of these (following the 'Hugh Piggott' plan on your web site to the letter), before I even start on this idea, I was just asking about if it had been tried before / anybody else with experience with these??

Me and my Old Man are both just full of ideas as soon as we get into something, but appreciate we have to get the basics right first. So we are working on a 10 ft dual rotor machine, much as is seen on this web site, we are just trying to get the most out of seemingly expensive magnets!

Another (daft) idea we came up with was a 9 (dual rotors) magnet 27 overlapping coil machine, I worked out it would still be three phase, but would actually end up costing more for the similar power output we'd get (due to the extra wire cost), but that should give you an idea of how much we are thinking, even if we dont have the resources / money /experience!!

Your comments / criticisms are most a appreciated!!

Thank you.

Jim Nutti

[Titantornado]

Must have been a very quick search you did, because in a matter of three seconds, I searched and found the answer to your question in past posts.

But here it is anyhow:  http://www.fieldlines.com/story/2004/6/26/124229/785

[Norm]

   Nope not a new idea it was just talked about

about 60 or 80 postings back...again ...as I

read it Flux and I think DanB pretty well gave a

convincing explanation ...something about

magnetic flux density getting weaker....everything

is a compromise and there is just a thing about

taking something to the extreme....

   Can understand your enthusiasim...

but you really should start with something

smaller ...

    Well wishing you a lot of luck anyways...

      Have fun ....

                     ( :>) Norm.

[Ungrounded Lightning Rod]

The thickness of the magnets equals the thickness of the total gap for maximum utilization of the magnets.  So you get the same amount of copper in the same amount of field with a triple rotor dual stator machine as with two separate dual-rotor rotor machines well-separated on the same shaft.

But if you use the same amount of magnets and copper on just one dual-rotor machine of twice the radius, you get twice the power (and twice the torque load) at a given RPM.  With the same wind and pole size and spacing you get twice the frequency, twice the voltage, and the same current.  (Adjust your turns ratio and/or coil wiring arrangement to trade voltage and frequency as appropriate to match your application.)

This is because the magnets are moving twice as fast past the coils, but have the same force dragging them - on twice the lever arm.

Same expensive magnets.  Same amount of expensive copper.  Twice as much cheap steel in the two double-radius rotors as in the four single-radius ones (assuming you don't skeletonize the region between the magnets and the axle).  Lots more watts per buck.

[Ungrounded Lightning Rod]

With the same wind and pole size and spacing you get twice the frequency, twice the voltage, and the same current.

That's "the same wind(ing of the coils)", not "the same wind (blowing on the turbine)".

[dinges]

http://www.fieldlines.com/story/2005/4/5/162915/1327

And there's much more like this, if you do a proper search.

[Flux]

This has been covered before, but usually using magnets on either side of the centre disc, in which case it is 2 machines of the dual rotor type.You will get twice the power.

You are considering a magnetic circuit in which the centre magnets are common.

You will not get twice the power with a 50% reduction in magnets, so it is a false start.

You are effectively using magnets 50% longer for a dual rotor but splitting the magnetic circuit into 2 series paths. This is not really to your advantage as you need physical gaps between the rotor and stator for mechanical reasons and by splitting the magnetic circuit in two, you have added 2 gaps that are unnecessary and wasted winding space. You would be better off with magnets 50% longer and a single stator. If you must use the same magnets I am fairly sure a single stator with 2 magnets stacked on one rotor and one on the other would do better than your proposed plan and save the difficult problem of a non magnetic rotor to support the centre magnets.

Far better to use 2 larger rotors with 50% more poles and you would gain significantly over either option.

In this specific case I would avoid the 18 pole option, it causes problems, so use 16 poles and have 4 magnets spare or use 20 poles and buy 4 more.

Flux

[ghurd]

I am surprised no one mentioned magnetic poles come in pairs.

So 9 magnets won't work.  8/24 or 10/30 will work.  There are issues with space at the inside and outside, making it better suited to a single rotor.

http://www.windstuffnow.com/main/alt_from_scratch.htm

G-

[nothing to lose]

"Another (daft) idea we came up with was a 9 (dual rotors) magnet 27 overlapping coil machine, I worked out it would still be three phase, but would actually end up costing more for the similar power output we'd get (due to the extra wire cost),"

I missed that too Ghurd,

Your right, 9 magnets won't work, has to be an even number. At first reading I thought he meant something like I was thinking about trying. 12 magnets 27 coils.

 I'll throw away some wire probably on a bad idea, but I am thinking about 27 coils instead of 9, but still 12 magnets per rotor.

If I try it, each coil will be about 1/3 the turns and about 1/3 the thickness, overlapping each other. Kinda like 3 thinner stators stacked  rotated over each other. So every magnet is always under some coils leg even when in the center of a different coils center hole.

Not sure if that made sence as stated, but I geuss the basic Idea is I consider the center of a coil to be a neutral spot where a magnet is not really doing anything, so at any givin time 1/3 of the magnets are in a center hole dead with 9 coils. With 3 9coil stators each 3 phase stacked and rotated there should never be any dead spot.

[ghurd]

Ed used to do that too. Not sure if he still likes it.

http://www.fieldlines.com/story/2004/9/4/145716/3657

G-

[Ungrounded Lightning Rod]

You want to stuff the thickness between the magnets with as much copper as possible to take advantage of your magnets' strength.  If you leave a gap you've lost the power that would have been generated by copper there, and if you widen the gap you've weakened the field.

That means you need to bend the coils so the loops outside the magnetic path are able to pass over or under the bundle from another coils entering/exiting the gap.  Of course this makes it hard to fabricate the stator, since it has to be thin between the magnets and thicker inside and outside their path.  It also tends to restrict cooling air.  (At least assembly is still easy, since you can jack the rotor into a slot as easily as you can jack it down over a flat surface.)

If you leave the coils flat but stacked and widen the gap between the magnets to clear the stacked-coils assembly, you might as well have used the regular, easier-to-fabricate, cools-better design, where the magnets are closer together but the region in the center of the coils is empty due to non-overlap.  Sure some of the mag field is wasted - but it would be either way.

[nothing to lose]

It's probably not a good idea but I was thinking keeping the same stator thickness and the same amount of wire and basically just spreading it out more over a larger area using the 27 coils instead of only 9.

Hopefully they will be alot thinner, but say if the 9 coils were 3/4" thick for a 3/4" thick stator then I would make the coils 1/4" thick for the 27 coils and still have a 3/4"thick stator. It would still be the same airgap between magnets although each coil is only in 1/3 of that gap, but there should always be a leg of a coil over any magnet at all times. You are probably correct and I am not going to gain anything, maybe even loose something.

 Actaully I think I want 0.6" stator not 0.75", I just used 3/4" for simplicity above.