Magnetostriction

Understanding the origin of the noises made by a machine is important for knowing the machine is in good health. My Powerspout has always made an unhealthy whine a lot of the time, and at the present time when power generation is at its maximum, the noise is particularly concerning.

Listen to the sound:

Although it has made this noise ever since it was commissioned 8 years ago, only recently have I come to understand what its cause is: it is caused by a phenomenon called magnetostriction. 

Magnetostriction is when minuscule changes in dimension occur to the steel cores of the alternator's stator windings as the magnets of the rotor pass over them. The constantly changing magnetic field causes the cores to hum a bit like tuning forks. 

All voltage transformers on the grid network can be heard to have a low pitched hum for the same reason but their hum is based on the mains frequency of 50 Hz; in the case of the Powerspout's alternator the frequency is of the order of 1000 Hz so the hum sounds like a whine. 

I calculate it to be about 1000 Hz from there being 56 magnets, each passing a stator core 20 times per second: so there are 56 x 20 = 1,120 changes in magnetic field every second for each of the 42 poles of the stator; (turbine rpm is 1200 at the present power level of 920 W; 1200 rpm = 20 rps).

The bearings in the Powerspout have run for over 4 years without change and without added grease, and I am particularly listening out for signs they may be coming to the end of their life. The whine of magnetostriction makes this rather difficult but there is a window of opportunity to hear how the bearings are doing when the turbine is shut down and as the rotor comes to rest. 

I only hope I don't miss the sounds of imminent bearing failure if they appear.

Measuring magnetism

In the last diary entry, I mentioned I use two types of rotor: the standard magnetised Type 2 and the more highly magnetised Type 2+. 

Thoughts which have long intrigued me have been: how much more magnetised is the Type 2+ rotor, - and as a separate question, - are the magnets in the 14 'magnet tiles', 4 magnets per 'tile', - are they each of equal strength.

Underlying these thoughts has been the basic question: is it possible to measure the strength of the magnets.

In this blog I want to show how I devised a way to make some measurements using the force generated on a soft iron cylinder held a constant distance away from each magnet. 

The apparatus uses the technology from a kitchen scales of the digital type, the working principle of which is a strain gauge Wheatstone bridge.

The finished test rig looked like this; the force displayed is in grams:

The detail of the 'binocular cantilever strain gauge' looks like this:

The schematic representation of the arrangement looks like this:

The deformation on applying load looks like this:

The wiring diagram looks like this:

The voltage change across the Wheatstone bridge, created by tension and compression of the four strain gauges, is very small; it needs to be amplified and processed for display in the LCD screen as grams weight.

Conclusion:

What did I learn from this little experiment: in truth not much ! The individual magnets of each kind of rotor all seemed to be equally magnetised; the force exerted by each of the Type 2 magnets was, as seen in the picture, around 424 grams whilst for Type 2 + magnets it was around 200 grams more.

One limitation of the system was found to be that the rotors are not precisely circular; this had the effect of reducing the air gap in one place and widening it in another; since the 'pull' exerted on the soft iron cylinder is very dependent on the distance from the surface of the magnet, this limited the reliability and precision of the experiment.

But it was a fun thing to do, - and I had had the 'guts' of the kitchen scales sitting in a drawer for over 10 years awaiting some useful purpose. It was nice finally to discover why I had been keeping it all those years !

My chart for optimum Powerspout operation.

Powerspout owner / operators who rely on a water source that is seasonal will be familiar with the challenge of needing to optimise the output of the turbine when using different sized nozzles for the different seasons.

In this blog, I'm posting the chart I use to help me with this challenge.

For me, the variables are the type of rotor, the amount of rotor packing, and the type of stator. 

When flows are good and a big nozzle is in use, so much torque is produced by the jet hitting the pelton that the more highly magnetised Type 2 + rotor is needed to keep turbine speed down to a speed where maximum power is produced.

At the other extreme, when flows are least and there is a small nozzle, the torque lost in overcoming the attraction between the rotating magnets and the iron cores of the stator is such a large proportion of the total torque being produced by the pelton that the number of poles in the stator has to be reduced from 42 to 18 in order to keep shaft speed up.

Between these two extremes, the chart gives me a system, which I've built up from experience over 8 years, to guide as to when to change from one type of rotor or stator to the other, and as to how much packing to place beneath the rotor for each of the 13 nozzles I employ.

The chart also has a column for efficiency, and this is 'whole system' or 'water-to-wire' efficiency. Numerous factors contribute to this figure, and these include: how good the penstock is; how good is the alignment of the jet on the splitter ridges of the pelton cups; how well cut are the nozzles to give a good jet profile; how much drag effect there is from grease in the bearing housing; how much transmission loss there is between turbine and inverter, (this will vary with the current); how good the efficiency of the inverter is at different input voltages; and how inefficient the Smart Drive alternator becomes by packing off the rotor. 

Peak efficiency can be seen to be 56.8% and this is achieved with nozzle XII, delivering 2.43 l/s at 1102 rpm, generating 723 watts leaving the inverter.

As I write this, the nozzle in use is number VII and the Powerspout is producing 321 watts; for this time of year this is exceptionally good and is a reflection of the rather wet summer we are having so far; unlike most people, I'd quite like it to continue that way !

Rotor packing - the sequel

Early September and I'm well into the driest time of year.  In actual fact, there has been quite a bit of rain but at this time of year it does little to augment the flow from a spring, - which is the source for my Powerspout; I'm presently running on my second to smallest nozzle and generating 136 w into the grid; I've needed to install the reduced core (i.e. 18 pole) stator to achieve this output, something which I was hoping keeping with the 42 pole stator but using rotor packing would avoid.

Dropping down through my nozzle sizes in the past months has given plenty of scope to experiment with rotor packing at each flow level and collect data about its effects. I have written before of how it increases rpm and thereby keeps the pelton operating at nearer its 'sweet spot' speed; I thought this would make for better power output but reviewing all the data has cast doubt on such an assumption.

Below is the plot of watts output to grid vs dc operating voltage.  Remember I am using a WindyBoy inverter which does not use MPP tracking; it simply draws a current from the turbine which is determined by the dc operating voltage; the magnitude of that dc current is reflected in the ac watts output to grid (LH vertical axis); the dc operating voltage varies with the flow delivered to the pelton (horizontal axis).  The plot shows a polynomial curve on which all the data points sit with remarkably little spread. This was something of a surprise.

It was a surprise because the data points were the result of very different operating states for the turbine; some were with rotor packing and some were not.

For each of the data points on the graph above, I also measured rpm; and for the data points arising from the lower flows, I used rotor packing to keep rpm up to above 900.
With rpm data superimposed on the above graph, using the RH vertical axis for rpm, this is the picture:

As can be seen, for those data points where rpm was kept above 900 by rotor packing, there seems to have been no obvious improvement in ac watts over the trend established by there being no packing. 

But perhaps I'm being unduly negative; perhaps without rotor packing the polynomial curve would have been a straight line relationship between dc volts and ac watts, making for less output than I actually got at the low end of the plot.

What counts for me is that to the eye and the ear the turbine undoubtedly appears happier with rotor packing.  I will continue with it.

Rotor packing

The weather has been cool and cloudy here in Wales for what seems far too long. There has been rainfall too, more than usual for this time of year, but none of this has stopped flow to the turbine steadily diminishing.  Today was a day for reducing the size of nozzle to keep flow delivered matched to what is available.

Doing this gave an opportunity.  I'm working in previously uncharted territory trying to make the most of these smaller summer-time flows whilst using a non-MPPT inverter**, and today's nozzle change gave scope to experiment.  

In previous diary posts, I've outlined what the aim is, - to keep pelton speed up in order to have it operate near its optimum rpm where most energy is extracted from the head and flow available. Keeping speed up can be achieved by making the rotor stand off from the stator coils by placing packing washers on the shaft. But the method is one of finding by experiment how much packing produces the best output: too little makes rpm and power output low; too much makes rpm better but reduces alternator efficiency so the end result is just as bad.  Somewhere in the middle lies a best compromise and the challenge is to find it. These pictures show how it can be done:

For these photos, the camera was carefully aligned with the back of the turbine casing so the amount of rotor stand off would be captured along with the rpm on the tachometer.  As can be seen, 8 mm of stand off produced most watts at a speed of 876 rpm, so that was how I left it, but I can't help feeling a tad more, say 10 mm taking rpm just into the 900's, could be better.  

That'll be a job for tomorrow perhaps...***

** It is worth stressing that I am only experimenting with rotor packing because this inverter does NOT use Maximum Power Point Tracking (MPPT) to optimise the power fed to it.  Inverters that use MPPT should find optimum speed automatically and with greater accuracy. They should therefore always give maximum possible power so long as voltage is within the range specified for MPPT tracking.  My own installation probably proved this last year: then I was using an MPPT inverter and with the turbine set up precisely as it was today except for having ceramic bearings and a reduced core stator, the output to grid was 299 watts.  Allowing 10 watts extra output for the decreased rolling resistance of ceramics compared to standard greased bearings, this was 14 watts more than the best I could achieve today (275 watts) with experimental rotor packing.

***... and indeed it was a job done later: an extra 1.75 mm (one of EcoInnovation's special packing washers, specially made for the job) was added to the existing packing of two rubber O-rings. It meant that the 'knob' only turned 9 full turns instead of the previous 10 needed to fully push home the rotor. The effect was to raise rpm to 974 and watts output to 281.  OK, that's a tiny gain over 275 w.  What's important though is that it proves, and calibrates, the method. Very satisfying !

'Low flow matters' revisited

Owners of small water turbines usually get excited in the wet months of a year. That's when their turbines start to perform at maximum capacity.  Yet last December, just when my turbine was increasing its output like everybody else's, I found myself writing in this blog that I couldn't wait for the next dry season.  What on earth was I on about ? - and now that the next dry season is happening, why was it that I was so keen for these dry months to arrive?

The development that prompted my comment was when I started using a WindyBoy instead of a SunnyBoy inverter and came to see that the WindyBoy, quite unlike the SunnyBoy, caused the turbine to operate at a lower voltage at times when flows were small.

This was a startling difference and the plot below shows it.  As can be seen, with diminishing power the SunnyBoy's operating voltage progressively rises, whilst the WindyBoy's progressively falls.

The significance of this was great: up until this time I had always operated with a SunnyBoy using MPPT mode and it had caused me problems in the dry months.

In the first summer, from beyond the middle of July, the operating voltage rose to be too high to keep the turbine running. In the following year, having obtained an 18 pole stator to use in place of the 42 pole one, voltage was kept down and this enabled generation to continue all through the second summer.  But there was the hassle of needing to change stators. My excitement about the WindyBoy came from anticipation that I might in future be able to keep operating without need of having to change the stator.

So how are things working out ? - not quite as promisingly as hoped is the short answer!

The issue centres on speed.  Whilst WindyBoy does keep voltage lower it does so by imposing such load on the SmartDrive that shaft speed is reduced significantly. This effect can be seen in the plot below:

The problem with this reduction in speed is that it makes the pelton operate at an rpm where maximum energy is not extracted from the head and flow available.

One way around the problem is to re-program the 'table' in the inverter, by which I mean re-program the algorithm it calls upon to work, to make it draw less current from the pma* for a given voltage when it is working at the lower end of its range.  This can be done, I think, and I'm beginning to explore how to do it.

Another way which is simpler is to tinker with the SmartDrive.  By placing a packing washer on the shaft so that the rotor is made to stand off a bit when the knob is fully tightened, the operating rpm is lifted back toward the speed at which the pelton functions most efficiently.



Doing this is not, strictly speaking, increasing the air gap as is commonly done with axial flux pma's to adjust their output.  Rather, in a radial flux, "brake drum" type pma which the SmartDrive is, it is degrading the flux linkage between the rotor field and the poles of the stator**. But it has the same effect as increasing the air gap, which is to reduce the voltage generated at a given rpm, or, as happens when feeding the output to an inverter in turbine mode, giving the same voltage at a raised rpm.  

A few days ago when I put in the O-ring illustrated, the rpm was lifted from 829 to 897 and with this, the ac watts output from 449 to 462.  No other change was made which might have confounded these results, and in particular the hydraulic parameters powering the pelton were identical before and after fitting the O-ring.

When I went on to put two such O-rings in, the effect was not beneficial at all: ac watts became worse than with no O-ring despite rpm having risen to almost exactly the theoretical optimum speed for a Powerspout pelton on my site. The explanation for this is that the gain from the pelton operating at its 'sweet spot' speed was outweighed by poorer performance from the SmartDrive.   So the method is not straightforward.  It involves trial and error to find how much packing will benefit pelton performance before that benefit is cancelled by degraded performance from the SmartDrive.

Summer is still in its early stages here in Wales.  It's still too early to say whether my excitement 6 months ago about operating with WindyBoy in the dry months was premature.  I'll see how things go as flows diminish further and continue to play around with rotor packing. 

The experiment I have running in parallel, of operating with variable head, seems to be going OK. It allows a much more 'hands off' approach to changing nozzles as flow diminishes, and significantly, it allows use of all the flow.

The stimulus behind all these efforts is to squeeze as many kWh's out of this 'low flow' time of year as I can, - all in the hope I'll hit my 4 MWh total for the year!

*pma = permanent magnet alternator
** more exactly it is reducing the area of the gap field

Reduced core stator: Part 3 (and final !)

It's a fabulous time of year here.  The arrival of swallows at exactly their usual time and the carpets of bluebells in the wood through which the penstock runs, they both herald spring and the coming summer.  And, of course with summer comes a lack of water and this year we are experiencing that prematurely.

The upside of this relative drought has been the opportunity to put the new reduced core stator through its paces at low flows.  What I have found is that the 18 pole core has made it possible for me to continue generating at flows lower than I was previously able to use, as this graph shows:

But also as the graph shows, the benefit in terms of extra power into the grid over the range of flows where either stator could have been used, is not great: at best probably just 7 watts.

This marginal benefit results from an improvement in "whole system efficiency" with the 18 pole core, amounting at best to about 3% over the efficiency when using the 42 pole core:

The increased efficiency only becomes apparent at flows below 1.4 lps, so for my scheme, that is the point at which I will have to swap over cores.

It also seems, though the few data points scarcely support this, that at flows below about 0.8 lps this improvement in efficiency is lost.  Since the power generated at these very low flows is so small, less than 180 w or 4 kWh/day, it is probably not going to be worthwhile trying to harvest this level of output.

As I write, the turbine is generating 259 w on 1.13 lps.  In spite of the lack of rain, this flow will probably diminish only slowly so there could be another 2 months before flow hits the 0.8 lps when stopping generation makes sense.  

As ever, the anticipation of what actually works out is what makes it all worthwhile.

In search of extra watts

Extracting as much energy as possible from a small turbine is part of the fun of operating one.  

As described in the last post, installing the reduced core stator actually had the opposite effect: power output decreased.  Disappointing as that was, I'm anticipating the time, which hasn't come yet as water flows are still holding up quite well through this past week, when the reduced core will generate more productively than the full core. 

Another investigation to see if more power could be extracted involved using a rotor from which the cooling fins had been removed.  It was supplied as a 'one-off' by EcoInnovation in New Zealand:

The idea was that some energy must be consumed in rotating the vanes of the rotor against the resistance imposed by air, and that by removing them, increased electrical output might be seen. 

The degree of benefit was never going to be very great so a really sensitive way of measuring power / energy output was needed.  

The study involved running the turbine without interruption at constant flow and head for several days, first with the usual, finned, rotor and then with the new, de-finned rotor.  For each rotor, energy output was measured with an Elster A100C kWh meter over a very precise period of time, precise enough to be measured to the nearest second after a minimum of 2 days of recording.  By converting kWh to watt-seconds and dividing this figure by the duration of the recording period in seconds, the mean power output for each rotor was obtained.

Well, the findings were not very impressive:  the study was done on two separate occasions, the first demonstrated a gain of 0.96 W and the second 1.51 W.  So the conclusion has to be that removing the fins is not a worthwhile thing to do.

As a side investigation, the temperature inside the Smart Drive housing was measured with each of the two rotors in use:

This demonstrated that when the fins have been removed the lack of circulating cooling air raises the differential temperature between inside and outside by a factor of 2 or more.  So the fins do achieve their desired effect and removing them could make the Smart Drive compartment too warm, especially if high power outputs are being generated, if ambient temperature is high or if the turbine is situated in the direct glare of the sun.  It has to be remembered that LDPE, the plastic of the Powerspout casing, is a thermo-plastic !

The next investigation into how more power can be squeezed out of a Powerspout will look at using different bearings, starting with SKF's Energy Efficient type of deep groove ball bearings.  Watch this space.

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.

Reduced core stator: Part 1

As mentioned in the last diary entry, I have recently received a reduced core stator from EcoInnovation in NZ. It's now been installed and has been generating for the past 5 days.  In this post, I want to say a bit about it and in part 2, give some observations about how it performs.

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...

Added 14th Oct 2025: a YT video of installing and using the reduced core stator can be seen at: https://youtu.be/CkabaTREILE

Low flow matters.

I measured available flow again this morning.  There hasn't been any significant rain, so I wasn't expecting it to be any better, and it wasn't, - only 0.9 lps.

0.9 lps is 30% of the design flow for my scheme (3 lps) and from the graph in my last post, it is clear that a pelton is still pretty efficient at this part flow.  So why can't I be generating with the 0.9 lps available ?  Why is it I'm waiting until flow increases to 1.2 lps ?

The answer lies in what happens to the output voltage of the SmartDrive at low flows.

There are two determinants* of the output voltage of any permanent magnet alternator (pma): rotational speed (rpm) and load. Thus:

• a decrease in rpm will lower the voltage
• a decrease in load will raise the voltage

Now the first of these, rotational speed, we can discount as causing voltage change: the rpm at which the pelton and alternator turn are more or less constant and determined by the characteristics of the hydro half of the installation.  Ultimately it is net head which fixes the rotational speed and because net head won't change, rpm won't either. For my set up, it is 1,200 rpm.

So any change in voltage has to result from the second determinant: a change in load, and the only load on the Smart Drive is the inverter.

Recognising that the inverter and pma interact with each other is a key understanding. The inverter, far from being a fixed load of so many ohms, is instead a smart device which can change the load it puts on the system. This is seen in two ways:

1.  a steady 'hunting' of voltage as the MPP tracking function of the inverter continually seeks the optimum voltage and current combination.  This can be seen in the following plot where a SmartDrive with nominal output voltage of 220v dc is seen to hunt up as far as 285 v and down as far as 200 v over the period of time the record was made.

2.  a step change in voltage when nozzles are changed to generate either more or less power according to available flow.  Smaller nozzles generate less power and cause the system voltage to rise. This occurs because the inverter, working in tandem with the pma, presents itself to the pma as a reduced load.  

Thus in my scheme, when power out to the grid is high: 713 W, system voltage is low: 295 ~ 315 v dc (the variability being due to MPP hunting as above), but when ac power out is down: 240 W, system voltage rises to 360 ~ 379 v dc.

The effects of these higher operating voltages are several:

• higher transmission voltage so less line loss - GOOD
• higher dc input voltage to inverter reduces its efficiency - BAD
• dc input voltage rises above inverter MPPT voltage range (100 v ~ 320 v) - BAD
• Powerspout's V-clamp starts to dump some of the generated power - VERY BAD

So, with the final point, we have at last arrived at the main reason why 1.2 lps is the lowest flow I seem to be able to operate at, - any less and the system voltage rises to be too high, too much of the time: the V clamp spends so much time dumping power that only a small amount (150 W at 0.83 lps) ever gets into the grid through the inverter.

Here is a record of the power output into the grid at two flow levels: 1 lps (237 W) and 0.83 lps (150 W), showing some dumping at the higher level, evidenced by the occasional, random, downward spikes, and at the lower level, the pattern created by more frequent dumping:

Perhaps I should consider buying a different Smart Drive core for use at low flow times of the year: a core wound so that its output voltage stays around 300 v dc when power generated is in the range 100 ~ 260 W.  

They're available from EcoInnovation at $US 199 plus freight.

And that's what I like about this game, - always something new to consider !  Always a new reason to spend !

*There are others, eg magnet field strength, width of air gap between stator coils and rotating magnets, number of poles and whether connected delta or star, but these are not variables in a Smart Drive pma which is installed and functioning.