don't use pwm, use your a/d converter to output 6, 7 or 8 I/O pins to turn on or off a bunch of resistors.
size the resistors such that they pull 16, 32, 64, 128, 256, 512 watts or some multiple of this, in this way you can for example dump anything from 16 to 1008 watts in 16 watt steps, with just 6 resistors and 6 mosfets from 6 io pins and almost no code at all.
Interesting idea. Since I don't care about the power above 500W, it could be dumped any which way- the PWM idea is basically to protect the turbine so that it doesn't stall suddenly when a huge additional load is suddenly connected.
So using steps of loads will also work, but with my limited electronic knowledge I'd need a prebuilt circuit that can control the mosfets, like this:
http://arduino-info.wikispaces.com/Brick-4ChannelPowerFetSwitch(will have to replace the fets with higher voltage versions and mount on heatsink)
I don't have a power/V/RPM curve from the turbine manufacturer yet, but using a different 1kW 48V turbine's data. It looks like I'll have to program the windmaster inverter to draw 500W at 350RPM/36V. Since everything above that is lost, the dump loads should be programmed to just maintain that speed. Using 4 x 300W loads they should come on at:
1. 420RPM/46V
2. 480RPM/48V
3. 490RPM/49V
4. Just above that as this resistor is only used when the GTI is offline.
All values to be adjusted in practice since the charts are based on a 2.5ohm constant load, will be quite different with a variable load applied.
In the case of the GTI being offline, it would mean that the turbine would be free spinning till it hits 420RPM, when the first 300W load comes online. Will this be dangerous or detrimental to the turbine health?
I noticed that the GTI has a 5 second wait period between detecting adequate voltage and actually starting to inverter.
Bringing the Sunny Island into the equation, it's designed to work with SMA GTIs which can vary their output based on the grid frequency. So as the batteries require less power, the Sunny Island increases grid frequency from 50Hz gradually up to 52Hz and it expects the connected GTI to reduce its power proportionally. Since the windmaster 500 inverter doesn't support this, it will simply disconnect entirely when the "grid" frequency goes out of range... which I think is at 51Hz.
I'll be controlling loads on the AC side of things so as not to waste this energy, but if that fails then it's entirely possible that the dump load and GTI will be in an on-off switching loop, where the GTI goes offline for a bit while grid frequency is too high, then the turbine spins to 420RPM until dump loads kick in, then GTI comes online, turbine slows, dumps go off and the process repeats.
That's where I think it would be neater to have a PWM system so that everything can be controlled gradually to reduce wear on the turbine.
To avoid this ripple current on the GTI's input, what if I used 2 different 3 phase bridge rectifiers, 1 for the mosfets and dumps and the other for the GTI?