Homebrewed Electricity > Frequently Asked Questions (FAQs)
PMA VS. AXIAL FLUX WINDMILLS
ChrisOlson:
I've seen all of them with burnt up stators. Bergey Excels, Provens (which is actually a type of axial with a toroidal core), as well as homebrew axials. I'm a little skeptical of using core losses to control output of a wind turbine, and I think the most successful ones have used some type of reliable furling mechanism, whether it be side furling or feathering blades. So I think it's a horse apiece and the inherent simplicity and efficiency of the air-core axial is pretty hard to beat.
Yeah, I got plenty of crispy stuff. Collection of 4 AWG crispy wire. Rectifiers with stuff hanging out of them that I don't even know what it is. Stators that were on fire until what was burnable was all gone but they looked really cool on the tower, sort of like a P-51 that got shot down. Got rare earth deposits in the field from magnets that flew off and never found them. Got pieces of blades that I never did find the rest of the pieces for. But overall I think this is the nature of wind power. If it ain't built like a tank it'll break. And that goes for the biggest utility scale turbines too. So I find it a little hard to say a "PMA" is "better" than an axial, or vice versa. It's all in the design and how well you build it to stand up to the elements. Light duty wind turbines do not stand the test of time.
oztules:
"I'm a little skeptical of using core losses to control output of a wind turbine,"
Sorry Chris, I have not made the point properly.
It is not iron loss/core loss or any other loss that controls the output.
It is simply a matter of the amp turns generated in the windings, reacting against the magnetic flux of the magnets ( amongst other things ).
The net result is when the max power is gotten to, the magnetic fields all but cancel out further flux getting into the windings... so it uses no more power... it ceases to increase the load on the blades and they can try to run away. As the rpm increases, the magnetic fields stay in a steady stand off, and no further power is absorbed by the genny, and so no further heating can occur, The AWP did this at 1.5kw... it behaved like a normal alternator, until it got near 1.5kw... and it just sticks there day in day out ( that one is on a 1000 foot ridge, ALWAYS windy.... it cannot crispy critter... it just can't.... not physically possible.... nor can it over power the control electronics, as the output cannot exceed that max point.... everything is protected naturally.
They have 3.7M blades, but you will never see crispy stators from this thing.
It shares the same Hugh furling, and is very very heavy.
When I say inefficient.. I should say it does not use the available wind effectively once the reactance has cut in fully... it just does not use any more wind than it needed to get to that point. It then does not see the next 200mph of wind, thats for the blades to deal with.... the alt cannot break if it goes off wind seeking on it's own, ignoring the furling... blades need to be decent to handle high speed rotation in these circumstances, but the normal furling should handle this in nearly all cases.
Put simply... it is idiot proof..... no other alternator style can claim that as an intrinsic part of it's inheritance. The F&P washing machine motors are of this type as well... cannot be burnt.
........oztules
ChrisOlson:
--- Quote from: oztules on June 02, 2014, 06:52:25 PM ---The AWP did this at 1.5kw... it behaved like a normal alternator, until it got near 1.5kw... and it just sticks there day in day out
--- End quote ---
oz, yes I knew what you were talking about. I just used the wrong terminology. Air-core axial generators also experience a significant amount of reactance at higher rpm's. So it is not unique to the iron (or laminated electrical steel) core radial generators. And reactance must be compensated for in air-core axials when setting the power curve on MPPT turbines to prevent over-speed of the rotor as the flux "bends" more and more at higher output and speed.
You have the old AWP3.6?
I'm somewhat familiar with the African Wind Power turbines and I believe Hugh was involved in their design. I seem to recall some problems with the early ones, but can't remember all the details.
oztules:
Yes, reactance in air cores is very low, particularly if you are using neo's. Ferrites as in your higher speed units will see this more so...... but not enough to protect them at all.
The AWP was an early awp3.7. There are a few stories I have written on here somewhere, and they show what woeful quality control they had in the early days. The machine literally fell to bits because of terrible/ woeful/disgusting workmanship..... the design was brilliant, but the implementation was hopeless at that time.
That said, when we fixed the machine up as per design, rather than the butchery that was perpetrated to this machine originally, it became a set and forget unit.
I still had to rewind it ( all 90 coils!!) because they never sealed the coils properly.... and they corroded badly.... but never any sign of over heat ....except where the corrosion broke down the insulation..... this is a 500v machine, so corrosion from stupid and lax sealing shows up very quickly in a salt air environment. ( island).
The electrical controller was wired with the wrong look alike components, but still sort of worked, never any blowing up though.
So they are the problems with the early ones..... they were real problems with quality control... or any control from the looks.
If I were to buy a commercial mill this would be it by a country mile...... but I would pull it to bits first, and rebuild it to spec. It will then last forever... as it was a few weeks was enough to kill the massive ( they are huge) bearings in it........... over sized shafts made "lumpy" bearings.... that s how tight they were.
.... john
ChrisOlson:
--- Quote from: oztules on June 02, 2014, 08:55:30 PM ---I still had to rewind it ( all 90 coils!!) because they never sealed the coils properly.... and they corroded badly.... but never any sign of over heat ....except where the corrosion broke down the insulation..... this is a 500v machine, so corrosion from stupid and lax sealing shows up very quickly in a salt air environment. ( island).
--- End quote ---
So the AWP uses a step-down three-phase transformer like the Bergey Excel-R? I didn't recall they operated at that high of a voltage. That's pretty cool, really. Morningstar (which I think builds a pretty robust controller) has a 600V MPPT controller that now has wind curve programming capability. I'd love to try one with one of those turbines to see how it works.
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