Homebrewed Electricity > Wind

Once again in 3-part (phase) harmony

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boB:

This growling can happen with axial flux or laminated radial flux or anything.  I've heard it with the Bergey XL.1 too.
What Flux says makes perfect sense.  I've always thought it was at a certain tower resonance but maybe not ?

Either way, a couple ways it could be reduced or eliminated would be with a power factor corrected boost
converter that draws current proportional to AC output voltage.

An inductor as Joe mentioned might help...

An idea I had that ~might~ work somewhat is to just make the controller skip over the parts of the power
curve that cause it to growl. i.e., unload it until the voltage rises some. But then, as soon as the turbine is
loaded again it might slow down and growl again but it would hit that curve point and unload again.
It would be interesting to try !  The wind speed never sits still so it might actually be less irritating.

The Classic is definitely a capacitive input device so inverter or controller, pretty much the same thing
compared to a battery load.

boB

SparWeb:
You've just made me think of an interesting experiment to try.  I recently joined the 21st century, and got a real smaartphone.  I have downloaded a few interesting apps.  Among them are vibration loggers, some of which do an instantaneous FFT to extract the dominant noise frequencies.  Somehow you can get these for free (he says in total disbelief).  I've played with them a bit, mostly in the car (bumpy road vs. highway), but why not tape it to the WT tower for a minute and see what I get?

Something else you can get (also for free) are software that can turn your smartphone into a digital storage oscilloscope (with the use of a modified microphone, but don't forget the voltage divider resistors).  Anyway the sum of this is that you can measure both the waveform and the vibration using just a smartphone.

Knowing the dominant vibration frequencies, and comparing to the dominant frequencies in the output waveform, wouldn't that be informative about the relationship between the two?
Sure, there's damping and resonance to consider, but measurements are necessary before we wonder about that stuff.  I'm going to got try it...  next time it's windy.

I've had a bit of everything as far as vibration is concerned:

-Slight blade imbalance, visible as the tail waving once per rev.
-Generator cogging, because I'm running motor conversions with iron stators.
-Magnet come loose, that's really noisy.
-Free play between hub riding on tower stub.
-Bearing lubricant freezing, making a scrape sound once every multiple of rotations based on inner/outer bearing race velocity.
-Tower found a harmonic until I realized the middle guy wires were loose.

I'm sure if I think about it longer, I'll come up with more.  Currently the only annoying noises are blade tip swoosh, and the excessive number of guy wires I have on the tower.  They actually make more noise than the generator itself.

DamonHD:
I've stood right under the blades of a 2MW and the blade swoosh was much quieter than the whine from the electronics in the base of the tower (inverter from no-gearbox mechanics?)...  No guy ropes there!

Rgds

Damon

Ungrounded Lightning Rod:
IMHO the source of noise that starts at cutin, on a rectified permanent magnet alternator feeding a battery load, is obvious:  Clipping.

As the permanent magnet spins up from stopped, the waveform voltage rises - but there's no current yet.  As it reaches and passes cutin the voltage rises enough that current goes through the diodes, but only at the peaks of the waveform.  Current corresponds to torque resisting the turning of the blades.  So these short pulses of current become short pulses of torque.  They shake the whole mill - by intermittently placing a retarding force on the blades and an equal-but-opposite reaction force on the tower via the frame of the alternator.  Short spikes with long spaces between them correspond to lots of harmonic content, so while the fundamental frequency is near six times the cyclic rate, there's lots of sound energy in frequencies many times that.

As the mill speeds up further the current peaks increase - but also spread out, until they join and the valleys also start coming up.  It never goes away, but the variation becomes less severe.  As the pulses spread out the higher frequency components become less prominent as well - more than making up for the overall increase in frequency of the fundamental.  The basic growl rises in pitch but becomes less brassy.

There are other ways that current and magnetic variations transfer into sound, but torque variations are the biggie for a noise that appears at cutin.  (Among the others are vibrations of the coils from varying mag fields due to varying current applying varying forces to the wires, magnetostriction in the material of magnetic flux paths with cyclic flux variations, and torque variations from pole structures moving past each other.)

= = = =

"Magnetostriction":  When you magnetize metals it distorts their crystal structure and makes them change size slightly.  The effect is small, but very strong.

(I recall a fiction story, half a century ago, where a runaway giant gyroscope - due to loss of field excitation in a wound-field DC motor {lowering the torque but raising the speed where the reverse EMF balances the drive voltage} making it speed up drastically and dangerously - was brought under control by throwing a few turns of wire around a mounting beam and magnetizing it, causing it to expand enough to squash a bearing.  Struck me as somewhat unrealistic and a good way to weld the bearing {or bend the support and fail}, but hey, it was fiction - and a welded bearing would have caused substantially less damage than a disintegrating flywheel in the fictional situation.  It also struck me as a transparent ploy to let the author to explain magnetostriction to his readership.  B-)  )

electronbaby:
From what Ive seen, a higher pole count seems to make this issue less noticeable. It is definitely caused by clipping with regard to the rectifier at cut in. An active rectifier may help, but probably not worth the trouble and expense. An inductor in the DC line would be an easy place to experiment. I notice more noise resonating from tail vanes on these machines (caused by the alternator) more than any audible tower issues, so thats where some attention could be spent maybe. 

RoyR

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