This new stator has just 18 poles rather than 42 and it looks a bit odd at first sight. The unused poles have simply been cut off and the remaining ones connected in three strings of 6. The wire terminations from both ends of each 6 pole string are available with connectors attached, thus enabling the three strings to be connected in either delta or star configuration. The core carries EcoInnovation's designation 60-6s-1p-6wire whilst the original stator was 60-7s-2p-star.
The reasoning behind replacing the original stator with this reduced version is that at those times of year when water is less and power output consequently less, operating voltage (MPPV) rises to be too high if the full 42 pole core is used. For an account of why the voltage rises, and how generation is curtailed because of it, see this previous post. By taking away some of the poles, the voltage output will be lower.
To know by how much lower, EcoInnovation ran the core on their test bed and supplied the following information about its performance at different speeds and with different connection configurations:
From this, I chose to connect it in delta configuration so as to get the lowest MPPV I could (150 v at 1000 rpm). I chose this option to improve the efficiency of the inverter: as can be seen from SMA's data for their SB1200 inverter, shown below, reducing the MPPV from 320 down to 110 volts should add about 2 % to the inverter's efficiency. Since I would be reducing from 370 down to 150 v, I was hoping for about the same improvement.
I didn't have any worries about increased transmission losses at this lower voltage because the cable is generously over spec (5 mm² copper) and this stator will only be used when power generation is at its lowest limit, with only small currents flowing.
Installation of the reduced core went like clockwork and within a couple of hours. everything was ready to run:
Initially, I had a few anxious moments as it seemed as if the inverter didn't want to connect to the grid in spite of receiving an open circuit voltage registering 370 v. The unloaded shaft rpm at this voltage was a previously unseen high of 1487 rpm. And this was just with the bottom jet open, delivering 0.83 lps. I'm left wondering what unloaded rpm would rise to with greater flow, but that, I decided, was an investigation for another day.
Scratching my brain as to what could be not right, I walked up to the house, which is where the inverter lives and just as I got there, the green winking "waiting" l.e.d. changed to constant illumination. Hooray, - grid connection ! - and then I had to hurry back down to the turbine to turn on the other jet before the load of the inverter caused the pelton to stall.
And since that moment, it hasn't stopped...