Windmill's First Storm

[SparWeb]

After checking the weather forecast for the day, which predicted light winds for the next 24 hours, I raised the tower with the mill on it, partly because I wanted it ready to set up the charging system tomorrow, but mostly because the grass needed cutting under there!

So it was up with the wires dead-shorted together, and the 20 kph gusts that came up hardly turned it at all.

Well, that was 3 hours ago...  and the weatherman got it wrong!

The gusts are now well over 50 kph (by feel, sorry no actual measurement equipment up there) and against the dead short, the mill is turning - fast!  I wish I could measure RPM, but the shorted wires aren't going to give me a frequency reading on my multimeter without opening the short-circuit.  I don't dare!

Fortunately, the tail furls, and boy, is it ever furling!  The blades drastically slow down once it furls, then the tail rights itself, the whole rig swings left, right, speeds up, and the cycle repeats.  It is a relief that it works, but what a way to find out!

The situation is under control, for now, because the windmill doesn't seem to be going so fast that it will burn up the stator.  It's well past dark, so I will post back in the AM whether we make it through the night or not!

Steven

[Flux]

You should have had the batteries ready, then there would have been no wind for at least a week.

If you are not ready to see the results you are waiting for, "Mother Nature" gets confused and tries its best to destroy your efforts.

Glad you got away with it, but the fact that you saw it furl makes me think that it must have been turning fairly fast at some point.

Under normal conditions the thrust from a heavily stalled prop is fairly small and it should furl much earlier under normal operation.

Looking forward to your results.

Flux

[hiker]

awful quite out there---he must be busy makin repairs or pickin up the pieces..

or maybe just sleepin in after a long night of babysettin his windmill... :}

[SparWeb]

Sorry to have kept you waiting!

The storm was brief, and had mostly died down by 1:00 AM.  This morning there was no apparent damage, and it's been quiet all day.  

This morning I finished assembling the control box (the bare essentials, anyway).

I will hang the box on the barn wall, with the battery box underneath it.  The component on the far side of the box is the 3-phase switch.  I have a  shorting plate on the contacts, so I can short the generator to brake it, like I did last night.

The braking force is apparent, when I short the leads, but it does not stop the blades.  I didn't really know what to expect.  Should it not be turning at all, Flux?

The prop could be too big...  When I built it, I had myself believing that I would cut the tips down to about 7' diameter.  Once I had it assembled, though, I found I didn't have the heart to put a saw to it.

Another concern is that in the year since building the stator plate, my reading and learning has led me to believe that 18 gauge wire was a bit too light.

It's first, so I didn't expect it to be optimal.  I am very pleased that it is safe - that the furling mechanism works at a reasonably high wind speed.

So much fun!

[methanolcat]

         Looks really good sparweb, glad to hear everything was still intact after hours of hard work I'm sure.

        I don't have mine up and flying yet but I can imagine the feeling of pride you must have.  Good luck.

        Methanolcat

[Ungrounded Lightning Rod]

The braking force is apparent, when I short the leads, but it does not stop the blades.  I didn't really know what to expect.  Should it not be turning at all, Flux?

Assuming you don't mind me answering the question...

The braking force comes from the magnetic field.

The magnetic field comes from the current.

The current comes from the voltage pushing it through the stray resistance.

The Voltage comes from the cutting of the coils by the flux.

The cutting of the coils by the flux comes from the turning.

So it has to turn SOME to get the braking force that keeps it from turning LOTS.

If your genny is big enough to keep the blades in stall the mill will just keep facing the wind and turning slowly.

If your genny is not big enough the blades will start to speed up - which will increase the attachment and the lift forces on the blades in positive feedback until it's spinning merrily.

But (presuming your furling is working right) the increased lift forces from the spinning lift-type blade will then cause the mill to furl.  That will reduce the wind on the blades, which will let the drag from the current slow it.  For some operating conditions this will make the air detach and the blades stall again, and the positive feedback will make it stall enough that the force will drop and it will unful.  Rinse and repeat.

If your furling is set right to protect the genny from burnout in normal operation I think it it should protect it even better in this shorted, oscilating mode.  With a charging system in place the slowdown once it furls will be toward cutin speed, not toward dead-stop.  So you probably won't see this oscilation.  Instead the mill will be running partially furled.  This will continuously generate enough current to generate enough force to furl it, rather than doing this only part time.  So if the mill turns away from the wind rapidly, it will have a similar amount of current (and heating) during shorted mode but only part time due to the oscilation.

But the mill will run faster in the charging case, because it has to get moving faster to produce enough current to produce enough drag to make it furl.

[Ungrounded Lightning Rod]

Is the tail vane unpainted particle board?  If so (even if it's exterior grade IMHO) it should be primed and painted before you put it up for real.  You don't want it to come apart into wood chips and leave you with no tail.

I'd have put the rectifiers on the side of the box with the heatsink fins vertical to aid convection cooling.

[SparWeb]

I observed many of the things you describe, and I follow your reasoning.  I've connected the batteries and it can charge them now (just a couple of Trojans right now).

Naturally, the wind died...

Last night I was seeing the cycle of furl, slow, unfurl, track back and forth, accelerate, and furl again.  "Rinse and repeat", as you say.

If my thinking is correct, the furling responds solely to the thrust load from the blades.  Hence, it will furl in different conditions, because several variables can add up to the necessary thrust.  It doesn't have to be one specific wind speed, unless you keep all other conditions fixed.

So... the shorted connection forces the steepest rise in current, like saying "amps per RPM".  This runs the blades at their slowest rate.  In my case, they may be going fast enough that they won't stall.  Which tells me that the generator is too small for the blades.

Then I re-connect the leads, so that they're charging the battery, and there is some more resistance, now.  The rise in current is less steep (less "amps per RPM"), and the mill will run faster.

Looking up and seeing the mill furl, when shorted, at 50 kph means that under battery load, the furling may not begin until, say, 70 kph (44 mph).

I'm going to go back to the tests I performed on a lathe and see if I can gain some insight.  

[SparWeb]

Never expected to get the size right on the first try!  There's a balance between weight and surface area that I just guessed at.

I would rather use a nicer material for the permanent tail.  Even if I paint that OSB, it will still look lumpy in the sun.

PS, I live in an arid climate.  I have a horse shelter with this type of board on the roof.  It's been there, unprotected, with virtually no decay for years.  The stuff must have a lot of preservatives in it already.  Your point is still valid, but nothing beats wood scraps for quick-and-dirty parts.

[hiker]

nice lookin mill....

that partical board does get a little spongy when it gets wet--looses its stringht as well....glad to here it survide the night,,,,,,,

[Flux]

ULR has covered this pretty well.

It will always turn when shorted as there is no braking torque until current flows. You have the diode drop to overcome before there is any current at all. The braking torque rises rapidly as soon as current flows.

With an over powerful alternator such as Dan uses, it will crawl slowly when shorted and the prop will remain heavily stalled. It will produce too little power to overcome the braking effect and also the thrust developed will be small and it will need a lot of wind to reach furling thrust.

With a less powerful alternator, the speed will start to rise as the wind picks up and if you reach a state where the prop develops more power than the alternator can brake, you loose control and the thing speeds up and dissipates the total power in the winding.

If the prop reaches a reasonable speed then the furling thrust increases and it will furl at some point. If you are lucky you may furl at a short circuit current that doesn't damage the winding but it is a risky area.

When operating normally the thrust should be at its maximum and furling will be at the lowest wind speed. You will also be running at a lower current as you are producing the power into a much higher voltage so under normal charging conditions you should be safe.

If the alternator is on the light side it is far safer to let it run and furl normally in high wind than attempt to brake it. I think you are on the border line and you seem to have survived a storm when shorted but normally I would let it run.

The other extreme is with no load, a most undesirable case. The prop will run away until its losses cause it to produce enough thrust to furl. This will be at a high wind speed. It will at least be noisy and frightening and if it is strong enough it will survive, otherwise it will shed blades or the gyroscopic forces will be too great and the rotor will chew the stator or the blades will hit the tower.

Good idea to experiment with the tail before finalising it. Make sure you have real wind and it does properly furl before making the decision. I think many have thought they were furling just because the tail was lifting off the stop a bit.

Flux

[SparWeb]

ULR and Flux,

Thanks a lot for the info.  I've been watching it today, with light but gusty wind, and it looks like it could use a larger tail surface after all.  It seems to be hunting the wind direction a little too much.  Just when it get up to a good speed and charging a few amps, it starts turning out of the wind (thrust overcomes tail's stabilizing force).

Looks like I'll be relying on this tail, so I'd better make it a good one!