History lesson over but I thought it necessary to look at what has gone before.
We now have the biggest leap in magnet development since the introduction of Cobalt in about 1920.
We non have the capability of producing fields of strength equal to or greater than the electromagnets in much smaller spaces and with no consumption of power. Low speed without enormous size is a reality, winding turns are reduced and this combined with the effectively longer air gaps, lack of iron poles and the high field strength needed to demagnetise neo means that we can build alternators that do not have a curve that tails off with load.
Another feature of neo is that it is possible to still produce good flux density in substantial air gaps and this re-opened the possibility of reverting back to the air gap designs of Ferranti and Siemens, abandoned over 100 years ago.
For normal alternator design the things are designed to have maximum efficiency where they normally work ( near full load) and using iron cores has become the standard design. For us the iron core has certain features that make it less desirable. All iron has losses, both eddy current and hysteresis, the eddies can be reduced to low proportions but hysteresis is a loss at molecular level and increases with field strength and we cant avoid it.
To make the best use of iron cores the coils are wound in slots and this causes us more problems. This is the source of the cogging problem, there is also a loss from the sudden change of flux from tooth to tooth.
Now that neo can produce significant fields in practical gaps there is a real case for not using iron in wind generators. There can be no iron loss and cogging does not happen.
Now the elimination of the field supply, removal of iron loss and the absence of cogging and the ability of a prop to start in winds below 5 mph gives us a chance to think about extracting power from the low wind speeds.
With wound fields there was no real chance of supplying the field loss below 12 mph.
Now we have the perfect solution, why do we have a problem? Because we are greedy, the more we get, the more we want.
In the past we couldn't do much with low winds so we wound the alternator to work from about 12 mph upwards. We were on the part of the cube law curve that was not far removed from a straight line over the range we used ( say from 12 to 28 mph).
The perfect alternator has no slope, output rises vertically above cut in speed, but we can't make that and with about the best we can do the slope of a practical machine is a good fit to the wind power curve and man was happy.
Now the greedy one comes along and decides to harvest the small amount of power now available at say 6 mph, not a lot but useful, and by about 8 mph it is significant,
He winds his alternator to cut in at 6 mph and still tries for a high efficiency, now what happens.
At this point we really need curves but as Yet I haven't produced any but I think you can visualise a curve like a parabola but exaggerated. From 5 to 10 mph the curve has a very small slope, then between 10 to 15 mph the slope becomes about twice as steep and beyond about 15 mph it rises sharply. At low winds it is near horizontal but at the other end it is near vertical.
The efficient alternator has a steep slope and if we fit this at the 5 mph point it will be parallel to the curve at say 25 mph. Now we are in trouble, by possibly 8 mph our alternator will be producing the sort of power that the blades would like to make at say 15 mph, we do fairly well because the prop has some latitude in operating speed but we are pushing our luck at 8 mph and by the time we reach 12 mph the prop has given us all the slack it can offer and is now dragged down to a tsr where it is working well below peak power. We have hit stall and with wind speeds above this the prop falls so far off the cube law that we see no significant increase in power
To take an example, an 8ft prop could produce close to theoretical power from 6 to 10 mph and then level off to a maximum of say 150W for winds up to 25 mph.
What has our greed for low end power cost us? Say that was a 12v mill and we now connect it to a 24v battery. Cut in will now be raised to roughly 12 mph and we are up on the steep part of the power curve and the alternator line is now much more on the power curve ( with the same slope but along the power curve not just parallel to it).
We may pick up a bit of power at about 10 mph and by 12 we are coming on song. At 28 mph with the same prop and alternator we should be in the 1kW region.
That is the issue, be greedy at the bottom, pay the price at the top. That is why I keep trying to stop people going for desperately low cut in. For normal wind areas you suffer a serious price if you aim to cut in below about 8 mph. You will not often see the 1kW winds but if you kill your performance in the 15 mph range you will come off badly.
Can we be greedy and have it at both ends? yes but it doesn't come easy. That is what this is all about.
Time for a break and perhaps a few curves.
Flux