6.48 mm diameter nozzle delivering 0.91 l/s to the runner which is rotating at 1084 rpm and generating 225 watts into the grid at an overall efficiency of 47%.

Thursday, 4 November 2021

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 moment 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 it 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.



Sunday, 3 October 2021

Year end results for 2020-21 water year

Here are the 3 graphs I post each year showing the output of my turbine for the past 12 months; in each, the bold black line is the data for the past year:

1. Daily power output:


There was an early pick-up of generation with rain falling in October, and generation was sustained more or less at maximal generation (917 W) through to March when the weather became much drier; generation then picked up again from mid May, during which month an unprecedented 192 mm of rain fell (usual being 90 mm); thereafter output dropped steadily as autumn dryness set in.

2. Cumulative energy (kWh)



This graph uses the same data as graph 1 but presents it in a different way, this time showing the cumulative number of units of electricity (kWh) generated as the year proceeded; it can be seen the end of year total (5167) is the most units ever generated, just exceeding last year's total of  5132; the FiT payments received for this energy will be £1,338 for generation plus £216 for export, totalling £1,554. This amount more than covers our electricity purchase costs which for the same period will be about £400.

3. Power duration curve 

Again this graph uses the same data but shows it in yet a different way. The graph indicates the number of days the turbine performed at various levels of power output; it can be seen it did not operate at full power for as many days as last year (the pale pink line) but it exceeded last year, and all previous years, in generating for more days at the intermediate output band between 700 W and 200 W. There was not a day in the year when there was an output less than 140 W. 

Conclusion: 

With this being the best of eight years in which my Powerspout has operated, it is a very pleasing result; perhaps the years are getting wetter; certainly the National River Flow Archive has reported that the calendar years 2015 to 2018 were probably abnormally dry years, and the wetter years following are likely to represent nothing more than a return to the long term 'normal'. 

Wednesday, 21 July 2021

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 !