"Good, good, gooood, good vibrations....nah, nah, nah, nah, naaaah, nah, nah, nah... " Oops, sorry, I got distracted there for a moment.
Well, I've got something new to research, because I confess I have no idea what "magnetostriction" is, though
the word appears to a melding of magnetic forces and friction.
Much of the rest makes sense to me, in particular:
once you introduce any form of capacitive loading on the rectifier the conduction becomes very rough, occurring in short intervals when the emf is above the capacitor mean voltage. A battery behaves just as a capacitor and is every bit as bad. The worst case is just at cut in when conduction is discontinuous, current just flowing when the peaks of the 6 pulse ripple are above battery volts.
If I put my ear right against the Sched 40 tube, it really is a rough, drumming of sorts. There are blade noises that can clearly be linked to a one per rev cycle. If I had to guess I'd say this was more like a one per magnet pair pulsation. It does seem less intense as the rpms increase.
With mppt I assume that the input of the inverter is a bank of capacitors
I'm not sure here if you are more broadly including grid-tie applications, Flux, or mean the converter? I think you know that we're not grid-tied. Our system has two, independent stud-mounted diode rectifiers. Each set of six feeds one of two MS Classic controllers. The controllers allow the input voltage to free range up to about ~160-170 VDC, and then buck the voltage down to the nominal 48V bank.
I doubt that I'll take any pre-emptive measures...adding inductors and such, as this is likely closer to this turbine than we'll normally be by 200-300' (not to mention insulated walls and ambient noise of the wind itself). Still, our power curve is pretty linear from cut-in to furling, and I'm thinking it may help to not be quite so steep out of the gate. A bit lighter loading early might help both noise at cut-in and blade matching. I may also try dampening the amplification through the stub tower/tube.
clockman, this is interesting, you've got the benefit or comparison to near identical machines. Our gap is also fairly close, and the coils sectors are identical, so I'm not thinking there is any unnatural imbalance, or anything necessarily wrong. This discussion has been helpful to understand better 'what' it is.
If you think about it we have many of the contributors - rectification, capacitance (batteries), current flow in six-pulse peaks at cut-in (as Flux explained), no white-noise cover of drives, fans, motors - and then, we have a hollow tube connecting this variable frequency, energy pounding device to the ground. It may be then no surprise at all that it springs to life with a "grunt".
Thanks for the feedback.
~ks