The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Friday, 9 September 2016

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.

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