Hi Scott, I hope all is well.
You wrote:
I was just talking about a booster circuit [...] sorry to confuse.
I didn't know whether I'd given you bad data or what, but it was the SMPS simulators that were confusing me. They first left me with the impression that duty cycle was the be all and end all of concerns but now I think of the circuits in terms of power and energy transfer and I think I understand them a lot better.
[...] therefore the blade is at 2/3 of upstream velocities.
!!! I was surprised to see the huge difference. We has some moderate wind this morning and when I turned on the MPPT circuit the unload blades were spinning at 150 RPM. When the booster settled down, they were running at 100 RPM... that's (within my ability to measure) exactly the ratio you cite!
It does not appear to me that the mill/booster is very efficient at 100 RPM. It only makes about 10 watts (vs 15 or so predicted) and with the slower PWM I'm using now I can't control the mill finely enough to extract the 3 watts that the circuit running at 64 Khz could. (Of course, that circuit toasted itself so I'll give up the 3 watts to keep that from happening again!) Once the loaded RPM gets up to about 50 watts, though, the efficiency seems much better and the booster gets the power suggested by the wind-power calculator.
Of course since I don't know what the wind speed really is I don't really know what's going on. So this morning I bit the bullet and ordered an anemometer cup set from our host. (I am still excited about the solid state anemometer but I know I'll get the spinning one up and measuring sooner.) ...it's not like I and many other newbies weren't told early on that you have to know your wind speed right at your mill to know what's going on. So, I guess I'm at that point...
I think this will help understand how the MPPT controller should work.
What do you think?
Well, I think that will help with half the problem. Having the MPP curve for your mill helps with charging the coil but the state of charge and load on the batteries will effect the discharge time. Optimum may vary by 25% but I assume that the MPPT would optimize the difference and the actual error could be half that and might not be worth worrying about.
BTW: In "manual mode" I've seen that a fixed setting optimized for about 75% cut in power is "pretty good" all by itself. I'm greedy and will do better but I have alread scavenged an old PC power supply for parts for "the $5 (non) MPPT power booster" ;-) That's farther down the project list though...
To use some numbers loosely, if the cut in is 12mph, then by 18mph the Cp is now maxed.
I get the same number; that seems right to a first order. As the current raises, so do the losses in the alternator coils and diodes. I only bring that up because I've seen from your other posts that you'd like to make a mathematical model. I think that you have to add a few lower order considerations to the "2/3" number depending on how precise you want to be.
BTW: With regard to running at slower PWM speeds, I wrote:
>[...] that sounds audible
It is quite amusing to hear the booster play a little tune as it hunts for the MPP and then reach a steady tone as it finds it (until the wind changes again.
The inductors have bolt holes through them and I did drill the PCB for the bolts but I haven't put them in. Should I bolt them down, maybe put some silicone under them too or just let them sing?
Thanks again,
- Ed.