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 !

My chart for optimum Powerspout operation.

Powerspout owner / operators who rely on a water source that is seasonal will be familiar with the challenge of needing to optimise the output of the turbine when using different sized nozzles for the different seasons.

In this blog, I'm posting the chart I use to help me with this challenge.

For me, the variables are the type of rotor, the amount of rotor packing, and the type of stator. 

When flows are good and a big nozzle is in use, so much torque is produced by the jet hitting the pelton that the more highly magnetised Type 2 + rotor is needed to keep turbine speed down to a speed where maximum power is produced.

At the other extreme, when flows are least and there is a small nozzle, the torque lost in overcoming the attraction between the rotating magnets and the iron cores of the stator is such a large proportion of the total torque being produced by the pelton that the number of poles in the stator has to be reduced from 42 to 18 in order to keep shaft speed up.

Between these two extremes, the chart gives me a system, which I've built up from experience over 8 years, to guide as to when to change from one type of rotor or stator to the other, and as to how much packing to place beneath the rotor for each of the 13 nozzles I employ.

The chart also has a column for efficiency, and this is 'whole system' or 'water-to-wire' efficiency. Numerous factors contribute to this figure, and these include: how good the penstock is; how good is the alignment of the jet on the splitter ridges of the pelton cups; how well cut are the nozzles to give a good jet profile; how much drag effect there is from grease in the bearing housing; how much transmission loss there is between turbine and inverter, (this will vary with the current); how good the efficiency of the inverter is at different input voltages; and how inefficient the Smart Drive alternator becomes by packing off the rotor. 

Peak efficiency can be seen to be 56.8% and this is achieved with nozzle XII, delivering 2.43 l/s at 1102 rpm, generating 723 watts leaving the inverter.

As I write this, the nozzle in use is number VII and the Powerspout is producing 321 watts; for this time of year this is exceptionally good and is a reflection of the rather wet summer we are having so far; unlike most people, I'd quite like it to continue that way !