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.

Saturday, 4 April 2015

Tachometer installed

Against a background of fast reducing flows, it's been a busy time with my Powerspout.  Much of the 'busy-ness' of course has been the need to downsize nozzles to keep matched to the falling flow (3 changes in a week), but of greater excitement has been planning for other developments:
  • installing a tachometer to measure turbine rpm
  • running a test with a 'de-finned' rotor to see if power output was improved (it was but only by 1 watt).
  • receiving a reduced core stator from NZ which will allow the turbine to operate at a lower output voltage, and so, hopefully, allow operation for more weeks per year.  The new stator has only 18 poles rather than 42. More about it in a later post.
  • planning with Michael Lawley at EcoInnovation to run tests using ceramic bearings (no grease) and SKF E2 Energy Efficient bearings (greased but more free rolling) to see if they enhance energy output.  Again, more info in a future post. 
The past week also saw the end of the 'abstraction year' and for the first time I was able to complete my return to Natural Resources Wales by doing it on-line.  This was helpful: I had made a mistake in my version of the annual record and this became immediately evident when 'in-putting' the figures to their on-line record because my total was different from theirs.

The final tally was therefore less than I expected it to be: instead of going over my limit, the total was 3,870 cubic metres (8%) under. No reprimand for me this year then !

The project to install a tachometer was born out of a curiosity to know exactly what the 'loaded' rpm of the turbine actually is (see here for previous post where this was investigated using sound frequency), and also to investigate how rpm alters at different levels of power output.  The fundamental issue I'm trying to sort out is: how much of the voltage rise I see at low flow / low power times of the year is due to the shaft turning faster, and how much is due to the inverter imposing less load on the Smart Drive alternator.

In looking for a suitable tachometer, ebay again came to the rescue:  a British-made, 1988 vintage, opto-reflective type of tachometer having a remote sensor. I only needed to devise a way of mounting the sensor tidily in the Smart-Drive housing: 

The sensor needed something to hold it which could be easily bent to get a good position: annealed, thick, copper wire 

A piece of foil stuck on to the rotor rim, which has been painted matt black

Sensor positioned in casing and held in place with Blu Tack

Lead and plug brought out through other vent hole, and stored inside when not in use.

And now the true speed of the turbine is revealed: 968 rpm (+/- 4; at 441 w; 1.79 lps), a figure which is considerably lower than the theoretical optimum speed for the pelton, 1260 rpm, but a figure, nonetheless, which is supported by the frequency of sound coming from the turbine, as mentioned above.

It'll be interesting from now on to record rpm at each level of power output, starting soon with the newly acquired 18 pole stator.  I can't wait !

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