Reduced core stator: Part 2

*For latest news, see note added at end, added 4 years after original blog entry was written.

The performance of the new stator has been almost wholly pleasing.  I made no alterations to the pelton nozzles between changing stators so comparison of one with the other was possible. The flow into the turbine was therefore a constant, namely 1.51 lps. Here are the things I observed:

1. Power: rather than being better, was 8% worse. This was the only disappointment.  I attribute it to the efficiency of the alternator being poorer when 24 of its poles have been cut off:

2. The quality of the power was markedly smoother, presumably because at the lower MPPV, the inverter was better able to fix on a maximum power point and hold to it: (note the scale in the lower trace, which is from the 18 pole stator, is different from the trace above, hiding the extent to which the trace is better.)

3. MPPV settled at about 145 v; before it was around 370 v; current rose proportionately from about 1.1 to about 2.4 amps:

4. Shaft rpm, shown here at 1079, later settled at about 1100 ± 4 once the grease had warmed up and been spread about, higher than the 968 ± 4 seen with the former stator:

5. In keeping with this higher measured shaft speed, the splash pattern from the pelton showed the changes expected. The bottom jet has moved to the left of the centre line whereas before, it was to the right; the top jet doesn't seem to have changed at all:

6. During a 24 hour period, there is still a slow undulation in the trace of power output, being just 7w peak to trough. What the cause of this is, I don't yet understand:

My big hope now after installing this new stator is that as summer approaches and water diminishes, I will be able to continue generation for more weeks than was previously possible.  Before, once flow got down to below 1.2 lps, I had to stop generating.  The excitement will be in discovering how low a flow I can go to.  Bring on the dry weather !

*Update note added 5 years later (21 Jan 2019)

• The hope of being able to generate year round and not stop generating in the drier months has been completely fulfilled: when flow gets to below 1.23 lps that is the time when I change to the reduced pole stator, changing back again as the flows of winter pick up.  For me these changes usually happen in June and November.
• With the reduced pole stator the lowest flow I can generate at is 0.53 lps.
• When the above entry was written, I was using an SMA SunnyBoy which operated in MPPV mode.  The algorithm the inverter follows in MPPV caused the operating voltage to rise at lower levels of generation and, as stated, this was the reason for introducing the cut down stator so operating voltage would be lower.
• Since that time, I've been using an SMA WindyBoy which operates in table mode. This algorithm works differently from MPPV and instead of the voltage being too high at low levels of generation it became too low: the inverter placed such a load on the SmartDrive that rpm (and hence volts) were too low.  It was the low rpm I needed to correct - it was causing the pelton to be operating away from its 'sweet spot' speed thus causing a loss of efficiency in the transformation of 'water jet energy' to shaft rotational energy.
• The good thing I have discovered is that the reduced stator corrects this problem with the WindyBoy just as well as it corrected the different problem with the SunnyBoy. It does this because the voltage put out from the cut down stator is less, and in consequence the look-up table in the WindyBoy commands a lesser current (i.e. imposes a lesser load) on the SmartDrive, and that allows the shaft speed to settle at a higher rpm.
• Final tuning of shaft rpm can then be done by standing off the rotor with packing washers. I aim for 900 to 1050 rpm.