visual effect of capacitors


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oztules

Guru

Joined: 26/07/2007
Location: Australia
Posts: 1686
Posted: 09:32am 05 Jul 2008      

Gill,
Perhaps we need to stand back and see what we have done here.

If we currently have a well matched system (blade diam and tsr to alternator), capacitors will have no benefit at all, and will only serve to stall the blades so that they cannot work at all.... so where to next?

If we wish to take advantage of low winds (as an example), we could carve out some 4m blades, with a tsr of around 5. This is easy to carve to get this tsr, and will be quiet as well.

If we did this, to get our 350rpm, we would need a 34 mph wind to get 3A out of the stator Gordon and Dennis were testing(@24v) ie 75watts. We would have available some 7000watts to play with at the shaft, so efficiency is not the problem.... cut in speed is.

Now we add the capacitors, now at 34mph, 350 rpm, we are getting 11A @24v . So we have gained some advantage. The capacitors were giving the "system" a marked gain. We still have 7000 watts available so power is a non-issue.

Now it is clear that this example is both unlikely and silly in the component match, but it gives an indication of how the cap loading effects performance.

In less extreme cases say 3m prop tsr 5, we get our 350 rpm @22mph. Without caps, we get 3A@24v (about 75watts), or with caps we get 11A@24v or approx 260watts. Now shaft power is about 1300 watts, so there is plenty to spare.

In this system you need either gearing to speed the mill up (messy and lossy), increase the tsr considerably (harder and noisier), add caps, or some workable form of mppt (or Maximiser).

In this instance the caps are the best practical course of action. We have 5 times the power available to use if we can, so efficiency is of no consequence. We have power to burn, just difficulty in getting the revs to use the stator effectively (matching it).

Thats where this capacitor system has some serious advantages to "no caps, higher rpm, smaller blades (to achieve those higher rpm), lower low wind performance (because we need smaller blades to get cutin)".

But, we can't use it to improve a matched system, because your concerns now take precedence. (where is the extra power coming from).

So I guess it is like everything else in the world, different horses for courses. In some cases it will be a clear winner, in others, will be burdensome.

One other advantage which seems to be available, is that the cleaner waveform should improve power factor some, and reduce circulating currents in the alternator, as the harmonics from rectification are reduced significantly. I am blindly guessing that with the improved wave form, efficiency will be better than original anyway into a rectifier/battery scenario (and it could be bollocks too... your "power in /power out" testing is required here)


Just my take on it, it has a place in the scheme of things I feel, particularly if you want more power from low winds by increasing blade diam, but without rewinding/rewiring the stator.



..........oztules
Village idiot...or... just another hack out of his depth