7.5 hp conversion questions

[windy]

A few years ago, I built a 3 hp motor conversion with a 10 foot diameter blade with a TSR of 7. No matter what I tried, I could never get it to furl properly. In 20mph winds, it would speed up, furl out of the wind, blade would slower down, tail would drop and then speed back up. It just kept repeating this. I tried changing the tail length, decreased and increased tail area, but nothing seemed to change. So I built a 7.5hp conversion thinking that the blade was to big for my 3 hp motor. The 7.5hp motor just stayed in stall all the time no matter how windy it got. So, I built a 16 foot diameter TSR 7 blade and that lasted until the wind gusted up around 30 mph. Broke one of the tips off before I got it down. It started to do the same thing as my 3hp conversion was doing. Speed up, fully furl, blade would slow down, tail would drop, and then speed right back up. Finally the motor overheated, and started to runaway. I have a Tristar TS-60 controller with the excess wattage going to a water heater. I am not sure if I ruined the motor, but if I did, I will not try another conversion. I ordered Dan's wind turbine book and will try building a axial generator. Are motor conversions harder to control than the axial generators? Has anyone else that built a 7.5 hp conversion have this problem?

 Here is what my setup is.

16 foot diameter blade on a 7.5hp 3ph motor conversion

motor offset-9 inches

tail area 15 square feet, 14 pounds of .062 inch aluminum sheet

tail length from tower to end of tail-8 feet.

 Just can't seem to figure out what I am doing wrong. If anyone reading this that built a 7.5 conversion and have it working, could you please post what diameter blade you are using and the tail dimensions. Or anyone with any suggestions would be greatly appreciated.

windy

[Flux]

Motor conversions are not efficient enough to run stalled with normal size blade setups. You managed to stall yours with blades far too small but when you fitted more suitable blades it got out of stall again.

The axial machines run fairly stalled above about 15mph and the top speed is a lot lower. This does significantly change the furling characteristics.

I have never seen a machine do what you described but I have heard of others having this trouble. The fact that you have had 2 machines do the same thing makes me think it is site related and you have something locally causing turbulent wind. Your 9" offset should be ok for a 16ft prop. If you have used free running bearings for the yaw rather than crude pipe on pipe this would make the effect worse.

Furling is not a precise process and my machines show little signs of power limiting until the tail gets beyond 45 deg.From this point onwards the power drops quite a lot and they hold in this state until the wind drops considerably before pulling back into the wind. For some reason probably related to your site yours seems to over furl most likely from a quick gust and then when the gust has gone it pulls straight back into the wind.

I expect the axial will show the same tendencies but it will probably be masked by the fact that in normal operation it will be controlled more by stall than area reduction from furling. As long as you never get out of stall you may never see this strange happening, but if at any time a gust pulls it through the stall point it may well happen.

If this is site related I think something to damp the yaw would help but it is difficult to do. As I said basic friction from less refined bearings may help ( unless you already had this). Adding additional friction may damp things a bit but you really need viscous fluid damping not friction. Although I am reluctant to suggest you try it, a damper on the tail mechanism may cure the obvious symptoms but it will slow the furling and if it is caused by local gusts you may have high surges.

In my part of the world the higher the wind the more turbulent things become and although I have not seen what you describe the actions of my machines do become quite violent. Not the lazy progressive turning away from the wind with tail still  straight down wind as the textbook seems to imply. Certainly there is no real limiting to constant power that you would hope for.

Flux

[SparWeb]

Hi Windy,

I'm sorry to hear you have had so much trouble with the motor conversions - my 3HP convy is still churning away.  I think it's a problem of matching the loads.  I noticed some differences between what you've built and what I have, so I'll throw my 2 cents in.

Maybe start with my comparison chart:  http://www.sparweb.ca/Wind_Turbine_Comparison.htm

My 3HP General Electric motor-conversion has stock wire, and 12 magnets, each 2"x1"x1/2".  It is turned by an 8-foot diameter prop, with 3 blades, and the TSR is roughly 5 or 6.  Since you used a 10-foot diameter, faster prop, I think your problem was 55% more power than necessary.  Even when partially furled, there is still an inflow of air that keeps mine turning.

The similarities don't end there.  I also have a converted 7.5 HP motor in my garage.  I haven't flown it, but I have bench tested it using a lathe and I have a power curve.  Using Hugh Piggott's blade design calculator, and a bit of my experience with the 3HP, I'm looking at a 12-foot diameter prop (not 16 like you have).

Maybe all you need is to do is saw about 18-24 inches off the tips of your current 7.5?!

I wouldn't seriously do that without thinking carefully, and considering a lot more information, first.  But that is my hunch.

Good luck.  If you feel like sharing more info we can delve into more detail.  Nothing wrong with choosing to build an Otherpower 10' mill, but it's sad to see so much work go up in smoke.

[frackers]

I have found that a lack of tail area shows itself by the mill running 30-40 degrees from the wind direction at low wind speeds due to the tail not exerting enough force to keep the mill into wind until the tail itself is running at a fair angle to the wind (despite setting the tail to genny angle to about 110 degrees).

I've just upped my tail size (but went thinner to keep the weight the same) and am awaiting winds greater than 15km/hr (cut-in speed on my mill is about 10km/hr) but the low wind performance is already much improved.

I'm hoping the the seeking problems I have had with my 3m axial will go away with the bigger area.

[Flux]

With this method of furling there is always a tendency for the thing to run at an angle to the wind right up to furling point.

Obviously a large tail area reduces this but it never goes away. There has to be an error in the tail direction relative to the wind to produce any force.

If you have a seeking problem then it comes down to too small an offset relative to prop diameter. Again this seems to be related to the type of blades and also to the loading. Increasing the offset also worsens the tendency to run at an angle to the wind so the whole thing is very much a compromise. It is tempting to reduce offset to save having such a big tail but there comes a point where it ceases to furl.

I don't think tail area will cure the furling and unfurling, I think this is far more related to some local thing screwing up the wind.

Flux

[Ungrounded Lightning Rod]

You should be able to damp a furling oscillation like that by putting a fluid-friction device (like a mini door-closer) on the tail to slow its swinging.

Try a magnet near a copper plate or disk.  (Or a horseshoe overlapping the side of a sheet of copper as if it were "pinching" it.)

Should only take a TINY bit of damping to kill the oscillations.  Too much damping might excessively delay the furling in a strong gust.

[Ungrounded Lightning Rod]

By the way:  How many blades on the oscillating turbine?

My impression of such an oscillation is that it's a result of the yawing being fast and the speed change of the turbine being slow.  Like this:

 - Turbine speeds up.

 - Faster turbine has more drag, resulting in more furling force.

 - Mill furls very quickly.

 - Turbine is still spinning madly.  Lift continues to provide furling force even while furled.

 - Turbine gradually slows down.  Lift goes away.

 - Mill quickly unfurls.  But turbine is still slow so little furling force.

 - Turbine gradually speeds up.

It'd think this might happen with a lot of mass in the genny and light blades.  (Also:  A two-blade might furl very quickly during the part of the rotation where the blades were near vertical.)  Insufficient load on the genny output could also encourage it to keep spinning once furled.  Was there a load attached?

Making the furling a bit more gradual, to let the turbine slow down as the mill furls, should kill the oscillations.  And it's easier to calm the tail (a control) than it is to calm the yaw itself.

[Flux]

I am in a better position to answer your question now. One of my small machines installed on a very poor wind site has shown this tendency. Previously there has never been enough wind in the right direction to make it furl.

It was built with the intention of using a 5ft prop but it got fitted with a 5ft 6" one to get a little more out of it in this crazy site. The offset is marginal for the increased blade size and I just assumed it wouldn't furl. It will probably never see enough wind for this to be a real problem so I let it take its chance.

Today there was some very strong and fairly turbulent winds and I saw it furl and also saw this over furl and shut down process.

The tail is a conventional version with pipe on pipe hinge and it has a fair bit of friction. Also the stop is such that the tail can come truly parallel with the prop.

It takes a lot of wind to see anything happen, the seeking force defeats the furling until the wind gets really violent. It then does furl and over furls quite drastically once the seeking force is defeated. Normally it slows very considerably which is to be expected with this arrangement and beyond about 60 deg of tail movement the restoring force bringing it back into the wind is falling. It ought to hold at least constant force ( which my normal furling schemes do).

Occasionally with a violent gust it goes into furl and slows down a lot, then some turbulence flips the tail right to the full furl position parallel with the prop. immediately the tail pulls the prop at right angles to the wind and it stops.

it ought to go straight back into the wind but with the reduced restoring force and the friction of the hinge reducing the restoring force even more it does not attempt to get back into the wind until the prop actually stops and the gyroscopic force no longer tries to hold it in position at right angles to the wind.

My conclusions are that the offset is marginal causing the seeking force to hold it into the wind until the wind speed is far too high. Once it breaks away from the seeking force it wants to drop to a much lower power level and this is aggravated by the falling restoring force and the tail pivot friction. Then given a sudden flick of the tail from turbulence it gets out of the wind entirely.

I believe that if the tail stop was set to something like 80 to 85 deg it would still drop to very low power levels but would never get into this stop situation.

Based on this machine I would say it needs more offset and the tail stop ought not let it go right to shut off. I don't think any form of damping would help in any way.

There is almost certainly always going to be some of this tendency with a machine that drops power drastically on furling especially with this hinge arrangement where the restoring force falls towards full furl. Reducing hinge friction probably helps avoid some of this reduction of restoring force as I have another machine using an inclined hinge but with decent pivots on the tail hinge. This also drops power on furling but never stops and as far as I can remember the tail does go right to 90deg as it is used as a backup shut down mechanism and the tail has to go pretty well to 90 deg to shut down.

Hope this helps but none of this has been proven to cure the trouble.

Flux

[Ungrounded Lightning Rod]

I think the problem may be more the hinge friction than the seeking force.

Hinge friction means the tail sticks in position and delays furling until the force becomes strong enough to break it free - after which it flips too far.  (In this case, to the stop.)  If it had been swinging freely it would have moved earlier and stabilized at a lower angle.  Similarly, when unfurling a sticking hinge will delay the unfurling then move it too far as well.

Negative feedback plus delay = oscillation.  That hinge should not stick at all - just sit calmly at the stops or swing smoothly when between them.

I'd say try lubricating the hinge and getting it to swing freely., then try again.  You might find you're OK.

Lube up the yaw bearing, too.  You can get the same thing there.  There should be FLUID friction - even a LOT of it - but not DRY friction with its oscillation between static and dynamic friction coefficients.  (Fluid friction allows motion even with very small forces - damping it but not bringing it to a full stop until the force goes away completely.  Dry friction forces the joint to stop in the wrong position and delays motion.)

[Ungrounded Lightning Rod]

(Which is not to say that you wouldn't get an oscillation from the seeking force, like you described, from a low enough offset, even if the bearings are really sick.  But I'd bet sticky pivots would make it happen at offsets where the mill would still furl correctly with slippery ones.)

[windy]

 Thanks for all the comments and suggestions. It wouldn't be hard to increase the offset. I could put spacers under the motor, but how much more offset should I try? Could it be the way I am carving the blades? As far as anything sticking, the generator assembly rotates on a bearing and the furling arm  is greased, so nothing sticks. There is very little friction on either spot. The tower is 70 feet high and the wind was blowing from an open field. No trees or anything that should have cause any turbulance.

 I still don't know if I damaged the motor, but from the picture posted, it doesn't look good. When I got it down, it was still rotating and when I shorted it out, it sparked on the contacts so there might be a some life left in the magnets. The motor was so hot you couldn't touch it. Resistance still checks the same on each phase.

 If you never had to take down your wind mill in strong winds, concider yourself lucky. I never thought a 5" sch40 pipe could wiggle the way this one did. Imagine trying to stand up a wet noodle. With the tip of the blade gone and turning at I would guess, 700+ rpm's, that pipe was shaking from top to bottom. Had to use a tractor to pull it over, as the wind kept trying to stand it back up. I knew if I didn't get it down, it was going to come down. I had all the cable clamps double nuted and I noticed that there were nuts laying on the ground after I got it down.

 I seen the pieces of blade starting to shed off, so I knew I was in trouble. I didn't have to go to the gym to get my heart rate up! But, one way or another, it will fly again.

Broken blade, motor overheated and varnish leaking out.

Front view.

windy

[Flux]

From your description you are certainly furling at far too high a speed for the alternator capability. Once you get over the peak of the prop power curve you are virtually trying to furl an unloaded machine and that never seems to work.

If you can get it furling enough to drop the prop power before you run out of alternator loading you may have a better chance. Your offset seemed reasonable so I think the better thing to try would be reducing tail weight or reducing the hinge inclination angle ( or both). It would also help if you can hold the tail restoring force later so it doesn't drop so much at full furl. If you have it set at 45 deg in the horizontal plane it may help to bring it to 60deg. This will encourage it to come into furling early and will hold the restoring force higher when it finally does furl.

I am not sure what effect the large chord width of the blades does near the root, you probably need that for starting but I have never had much luck with blades like that and they may seek the wind more.

Sadly I suspect your motor is at the end of life but it may run a bit longer if the magnets are ok.It would almost certainly have been happier and easier to control with smaller blades.

Flux

[SparWeb]

Quoting myself: "...Maybe all you need is to do is saw about 18-24 inches off the tips of your current 7.5?!"

I see now that this already started happening on its own - I wasn't trying to make light of the damage.  I've had my own heart-pounding moment with an accidental open-circuit runaway, but luckily no damage.

[tecker]

Go with a wide straight cut on the trailing edge . That's nice work on those blades but the conversions don't need a lot of rpm to make voltage and the small tips are out of area match when the turbine turns just a straight cut on the trailing edge will keep it out of stall when it furls .

[windy]

tecker

Is there a good blade calculator that I can use to shape the blade with a straight trailing edge. All the ones I have seen use a similar profile like the ones I carved.

[tecker]

Just use Hugh's blade Calculator or the one at Other power .Just a straight cut on the trailing edge will give lift as it furls.

[SparWeb]

For the record, the comparison chart has been updated:

http://www.sparweb.ca/Forum/Wind_Turbine_Comparison.htm