Measuring Iron Loss in a Motor Conversion

[SparWeb]

Flux and Dinges, you were curious, and so was I, so here it is...



Iron_Loss_Torque.pdf



I've tested the 3HP conversion with the "slowly-descending-mass-on-a-pulley" test.  Lotsa fun, a bit time consuming, but I needed something to do while I wait for the glue to dry in my blades...



The idea came up here: 250W Axial Flux Alternator Tests



...And here...



I've saved everybody the tedious detail of how I measured drop height, vertical velocity, RPM, and how I stacked various weights together to apply torque.  The PDF has the results, and I've graphed them in several ways.  The first two graphs chart torque and power against RPM.  You can easily see that the bearings apply a constant 7 in-Lb Torque.  The loss due to iron grows from there.  The other two graphs are the same data, but on log-log scales (because MathCAD is fun!)



To get higher RPM's, I would need a much higher mount.  The weights were dropping rather quickly in the last data points.  (Maybe when I have the whole shebang up on the tower, eh?)  The bike tacho I use for a RPM meter does not respond well at these low speeds, so I resorted to the lines-on-a-stick method with a video analysis.  Sorry, I did say I'd save you the tedious details...



While I was playing around with this, I also shorted the leads of the alternator and put weights on again - heavier ones, of course.  Maybe, in the future, this will tell if shorting the stator is an effective means of shut-down or not.



Anyway, if I extrapolate from the low speed data that I have, I forecast a loss of 300 or more Watts at 500 RPM.  At 150 RPM it takes 40 W (not so bad).  Whatever prop I have had better be supplying about 30 inch-pounds of torque at cut-in wind speed or there ain't gonna be no action!



Should I conclude that a converted motor should be turned slower than an axial flux genny of the same power rating and prop size?



I learned something; hope you all did, too!

[Flux]

I am not sure how far it is reasonable to extrapolate things from such low speeds but your estimate of 40W at cut in seems reasonable.

That is in reasonable agreement of the sort of figures I got with wound fields.

It would be interesting to see how it changes with flux density, Your magnets may be pushing the iron more than some of the multiple round ones.

Thanks for your efforts.

Flux

[dinges]

Interesting measurements, Steven.

I hope to be able to measure my iron losses of the 3hp as well (yes, I know, still no news...). However, I wonder why you measured into a short, as in that case you will be actually generating power. The idea of measuring iron losses is to look just at the iron, not at any generating power.

40W at 150RPM. 300W at 500RPM. Definitely not negligeable. It's the 40W at cut-in (150RPM) that has me worried most. It means low wind performance is not nearly as well as an axial flux, which would, at that windspeed, be generating the 40W as electricity, not heat in the stator. Considering that most windmills face low winds much more than high winds... In low winds, not much power is available anyway so we don't want too waste too much of it.

Interesting. Thanks for the report.

[SparWeb]

Peter,

I had the same concern, and re-ran the tests.  I made a mistake the first time 'round.  The corrected results are below.  My measurement was incorrect and I previously overestimated the torque.  Then I expanded the data range to include more practical speeds.

Iron Loss Torque2.pdf

I also added a wind power curve to put the loss in context.

When you complete your conversion, I'd be interested to know if you have similar results.  Your rotor is smaller, and I bet that will be to your advantage.