"We are aware of Bernoulli. From this we have found that the tangential velocity of the runner (at PCD) will be approx. 13m/s. We agree- jet diameter at vena contracta should be between 8 and 10mm. However, according to 'The Micro Hydro Pelton turbine manual', nozzle diameter should be at most 11% of PCD and at least 8% (to limit the effects of divergence of the jet) This suggests that the runner PCD should be less than 125mm. Combining this and the runner velocity we get a rotational speed of 2000rpm. Oh dear!"
This 10% pcd does seem to be in keeping with larger turbines but I never thought it was critical I have never seen these limits on larger turbine design. I do suspect that with tiny nozzle sizes the bucket shape becomes critical and that may be more of a factor with micro hydro. I am inclined to think that the Turgo runner is a better proposition but it does have a higher specific speed and that will probably keep your rpm high but it may tolerate a larger pcd than that normally proposed.
In many ways I am not sure why you are going with this low speed large massive alternator design when you could build a compact and efficient version running at 2000 rpm. If the limitation is self build from scrap with little facilities then I would stick with the lower speed. I can't see that a larger pcd is going to have a disastrous effect on performance but I would be tempted to look at Turgo spoons, I see it being simpler and with the tiny jet and practical size buckets you may end up with the Pelton being hit off centre and running as a Turgo if the division in the centre of the bucket is a large % of the jet diameter.
"Could we sacrifice the quality of the jet (to reduce RPM) by increasing the size of the nozzle?"
No. These turbines convert pressure to velocity at the nozzle, using a larger jet than your flow can sustain reduces the effective head. You will loose very much more than departing from some ratio of jet to pcd diameter.
"Would another option be to wind 60 turns per coil and parallel 2 per phase? At these small turns we will be able to wind the coils nice and thick at least"
Yes but don't do it. Without slotted cores you invariably get significant circulating currents with parallel coils. Just wind the wire 2 in hand with half the turns and connect in series as before. Uses manageable size wire and avoids all the circulating current problem.
"Are we using the wrong generator design for this application- possibly we could use a design with lower flux density in the air gap and get better results. Could we just widen the air gap if we find that the generator causes the pelton wheel to slow too much?"
I wouldn't go for lower flux, you already have a lot of mass in this thing that is restricting your turbine speed to lower than ideal. I would go for smaller discs, keep the flux density at half Br with nearer 650mT. You may need different shape magnets to get enough winding space at the centre and I suspect round magnets would be better. If you went down to 6" discs with suitably chosen magnets you could probably run at 1500 plus rpm.
Yes indeed you can change air gap to get turbine at its correct speed. That is the correct thing to do but don't sacrifice flux and flux density. Just use it as fine tuning.
With high speed you should manage a good efficiency from the alternator and if you get the cut in right ( but a fraction on the low side) you will have precise speed adjustment with air gap setting.
For higher speeds I would use a radial design but I wouldn't be working with lack of basic facilities, you had better stick with axial.
Flux
