10' smoked in high wind

[mgtd]

Since my mill has been up (about 4 months) we have had very moderate summer winds, however, Sunday Nov. 26th, we experienced gusty high winds and as I was watching/ monitoring my 10' homebrew Otherpower mill, I noticed my La Cross anneometer cups were gone (tail spinning erratically, cups gone). The mill was producing somewhat above 15 amps (meter reads 0-15) into my 48 volt battery bank. The machine appeared to furl once in a while, but not completely.   Next I noticed smoke coming from the genny. Here are a couple Static pic's of the mill, once things have calmed down.  Anything obvious to casual observers?  I'm wondering if my tail is too heavy, or surface area is too small to allow complete furling. The tail material is 1/4" sheet PVC (probably heavier than recommended plywood)  Everything else is pretty much the standard build.  I'm in the process of re-rigging my gin pole and getting ready to lower the 103' tilt up tower for inspection. I'll post what I find, anything in particular to look for. Any help appreciated.

[Flux]

"The mill was producing somewhat above 15 amps (meter reads 0-15) into my 48 volt battery bank."

That is the first problem, how much above 15A. maybe quite a lot.

I doubt that it furled at all unless you saw the tail beyond 60deg. Surface area too small will not prevent furling, too heavy is the problem.

Did it stop working or did it carry on working with smoke coming out?

We shall have a better idea when you get it down and see the damage to see if it was overload or the loss of a phase from a possible rectifier problem.

My suspicion is that your over 15A may have been over 30A, the power rises very rapidly with increased wind speed.

Flux

[Jerry]

Flux is right. It would be nice to know several things at time of burnup.

Like volts, amps and therefor watts?

Its a 48 volt unit but what was the actual voltage at the time?

Wire ga. how many turns? Magnet size and shape? Did I miss disc diameter and stator thickness?

These failur reports are very helpfull in designing future units and avoiding simular problems. Thanks for the report.

                              JK TAS Jerry

[mgtd]

Thanks to those who responded, I will make every effort to answer questions posed. My intent is to build again, stator and rotors.  I'd like to do it more correct this time, and get more than a few months out of it.  None of my comments below are flaming anyone, I'd like some insight.

I built my mill starting about 6 months ago, per instructions online.  After my build cycle was well along I began noticing several deviations/improvements by otherpower commentators ("we don't do it that way anymore") made regarding resins used, casting the lugs into stator mold, stainless straps for magnets on rotors, et cetera.   Is there a concise location for the changes to the online published information.  Would purchasing a hard copy of the instructions be "current".

Will the overheating have damaged the magnets?  What is the likelyhood of me damaging them during removal. The fiberglass resin they are encased in shows discoloration and cracking. Is there a way to test the magnet strength? (without a full scale physics lab)

Answers to questions--

Did it stop working or did it carry on working with smoke coming out?

     Ans.  It was spinning fast, smoking and still putting out current and voltage.  A few seconds later, during a lull or due to mechanical resistance, it slowed down and I used the kill switch to stop.  After that, little to no rotation when kill switch removed, even though wind was still pretty fair.

Like volts, amps and therefor watts?

     Ans.  Battery voltage was in low 50's (52-53 volts, Current was 15a plus, wattage therefore would be between approx 800 w and 1600, assuming current may have been as high as 30a.

Its a 48 volt unit but what was the actual voltage at the time?

     Ans.  Battery bank voltage was in low 50s at smoke point and shutdown.

Wire ga. how many turns? Magnet size and shape? Did I miss disc diameter and stator thickness?

      Ans.  17 ga wire,   140 turns, as per build instructions.  Magnets are 1x2x.5        neo.  12" diameter rotors of 1/4" plate, as per build instructions. Stator thickness was approx. 1/2, per build instructions.

Additional information from Poster---

Rotor/stator/rotor gap was approx 3/32" (varied a little)

103 foot tower, tower drop is 10 ga wire transitioning to 6 ga wire at disconnect, for the 120 foot run to the house.

Rectifiers (full wave bridges-purchased from OtherPower) were not substantially warm to touch. They are mounted on a substantial heatsink.

My tail assembly  weighs 21 lbs.  Is this close to standard. Does a standard exist? The vane material cannot amount to a pound or two either way compared to the weldment.  How does one then "lighten the tail if it is too heavy, and built per specifications", as suggested.

Here are pictures of disassembly

[tecker]

To me It looks like the tail is too small  for those blades .

[MattM]

Thanks for posting these pictures.  It acts as a good warning to everyone around here to plan for the inevitable day that strong winds will come up and treat their turbine with an impersonal degree of violence.

[Jerry]

MGTD thanks for the added information. I gather your alt. is also wired star.

I've noticed a story every few months or so about a simular burn problem with this type wind generator (dual rotor disc alt).

I built me second dual rotor disc windgenerator a year or so ago.

It was small, 8" disc with small .2" thick wedge NEOs. The normal 12 magnets per disc with 9 coils.

I had figured with its small sizes it could be rated fairly as a 400 watt machine.

This ubit has a 4ft blade and NO furling.

In a wind test mounted in the bed of my S-10 PU the wattage #s were much higher then I expecyed.

Even after being pused to 40 MPH the stator remained cold. 30 seconds after reaching 1225 watts I felt the stator cheking for heat. There was none (stone cold).

Latter this unit was installed on the roof of my retail car stereo store.

We had one of our NW winter storms. At 6:00 that evening winds were 60+ mph the littel 400 watt genny was produceing 1600 watts.

We closed the store for the evening. Latter that night the local news (11:00) reported 81 mpg gusts at the Salem airport. The store is less then 1 mile from the airport.

The following morning I expected to find this littel genny scrabled all over the store parking lot.

Not at all, the winds had died down to 15 or 20 mph or so and the littel genny was still flying unharmed and still making power.

Not bad for a littel 4ft gend wond with 250 turns of 22ga. wire with 6 amp fullwave bridges.

Most folks would not expect 22ga. wire to survive. But this unit was not wire star or delta.

I used a fullwave bridge per coil. With all DC outputs wired in perelell.

At 6:00 that evening the littel genny was doing 1600 watts, this was 60+ mph 5 HRs latter the wind was doing 80+ mph. I have allwas wondered whatt the wattage fugure was at 80mpg ?

Many of the Lab test I've done star has most of the time run much hotter then "Jerry Rigged" or the way I wired this unit.

 

Theres a story here.

http://www.fieldlines.com/story/2006/4/12/223514/612.

                    JK TAS Jerry

[Flux]

Yes that is just a burn out from too much output. These things all seem to show the same thing, one side is far better cooled by the wind than the other so at full rating I think the wind cooling is a significant factor in maintaining even 600W. My guess it that you peaked at over 1500W.

I think most of the marking on the magnet rotors is from rubbing the stator, you can probably use those magnet rotors again, if the cut in speed is ok with the new stator then the magnets have survived. I would be inclined to not remove the magnets and start again, the normal cause of magnets coming off is from the rubbing of a burnt stator and you seem to have not done enough damage to worry about ( have a close look but they look ok to me)

Starting again from scratch the stainless band is an extra safety feature but you will probably do more damage removing the magnets than it's worth at this stage.

The issue that I have been worried about all along is the furling, unfortunately I have never built one of these machines so I have no first hand experience but I have seen a lot of comments that make me suspicious.

All my machines furl in such a way that during a storm the actual power drops below the peak at furling point. Comments I have seen here seem to imply that the power continues to increase with wind speed. This is a bit risky unless the furling is at a very safe figure at the furling point. When you hit furling point as the wind picks up, the thing will peak at a certain power but those peaks will be infrequent and the average power will be way below the peak and the stator will be ok.

With increasing wind speed the thing will run at full furled power for longer periods and the heating will increase. On a good clean site ( as you probably have at that height) you may have conditions in high wind where the average power is within a few % of the peak furled power. From bench tests your alternator would not survive 600W continuous without wind cooling. Even with wind cooling I think you are relying on the power duty cycle being less than 80% to maintain 600W. I am convinced that you can't reliably maintain more than 15A at 48v for any length of time.

There is another factor that complicates things and that is how far your blades go into stall. This affects the furling and with long cable runs you may need to furl earlier as being further from stall will increase the power from the blades and will also increase the seeking force preventing it from furling.

As you have fixed the tail hinge angle at 20deg your only option to furl earlier is to reduce tail weight. I see no other method than to use something lighter for the vane ( you can't save much weight on the boom).

I get the impression that under some circumstances the design as originally published is ok but some site conditions seem to push the thing up over the safe limit.

I know that with my machines ( which don't run stalled so it is different) I can't get away with tails anything like that heavy without reducing the hinge angle to a value where errors in having the tower vertical become an issue.

If I was building one of these machines I would make absolutely certain that it furled far too early, it is easy to add some weight to the tail when you know what it is doing. I don't like the idea of finding that it furls way too high in the first storm.

Flux

[sdscott]

Flux is correct; wind cooling is a significant factor in storm survival.  Notice the slightly better cooling that the top-most coils received; indicating that the air movement was better here, at least?  I believe that complete potting of the rotors is a major mistake, as it leaves only the mechanical air gap on each side of the stator for air movement and cooling.  It is far more beneficial to half-pot (or not at all) the rotors/neos, aiding in more efficient air movement around the stator surfaces.  The resulting "centrifugal fan-action" caused by the rotating magnets will also be greater if the system is given a supply of air at the low pressure side (hub).  I have achieved this by cutting a hole in the center of the blade assembly. (No structural integrity compromised.)

Also, as Flux has pointed-out on numerous occasions, aim for early-furl in design; do not try to squeeze more power out of a given generator design, or it will cost you in the end.

[tecker]

That really badly burned spot looks like the star connection. The coils  next to the the over heated zone were pulled out of the melted area by the rotor .

[sdscott]

The front face of the stator is not burnt quite as bad as the rear face; suggesting, as you might expect, less air flow for cooling at the rear (because of the hub as seen in the first photo, maybe?).

[Blewsage]

Hi MGTD, Thanks for all the great info and pics! Sorry to read about the meltdown! I think I recognize your mill; didn't you start out with fiberglass blades? If so, what happened to them? I'm currently gathering the parts to start my own 48v 10' mill. Consequently, I really find this info valuable, and I can't thank you enough for sharing your experience.

So, where are you with this? From reading your post it sounds like your 1/4" PVC tail weighs only a couple of pounds. So, it doesn't sound like you have much to work with there. I'm kicking around what Flux has mentioned in regards to furling earlier, and wondering if lessing the angle of the tail to 15 degrees might help? I'm also wondering if widening the stator/rotor gap slightly would improve cooling, even at the expense of a little performance? Can I ask what your cut in speed is with 140 turns of 17 AWG?

Thanks again for your post!

[mgtd]

Blewsage--yes, I started out with the China fiberglass blades and then carved my wood blades due to feeling I would get better blade speed and performance with lighter weight blades.  I think I accomplished that--to the point of destruction.  The China blades are mounted to their hub and hanging from the ceiling of my shop.  As far as where I am--Ordered 16ga wire to make a new stator.  Going to perform a number of mod's to increase airflow over stator, including cutting a hole in the center of the blade front plate as suggested by sdscott), slightly opening the center holes of the rotor plates (as much as practical), slight turning down of the hub's bearing area circumference (to remove rough casting and allow a little more airflow), and since there appears to be some room around the hub's outer flange, I'm considering machining and placing a few small blades to move air on the flange's circumference.  All of the above is purely speculation at this point, but I believe everything mentioned can be accomplished without changing the strength or mechanical integrity of the components.  As I progress, I'll post follow up pictures.  

Now to you and everyone else following this saga--I originally built using fiberglass resin as dictated in the online procedure--since then I've read that different potting compounds have been used and I need the sources and brand names to locate some that may be better.  Here in rural NE Missouri I have to travel an hour to get to a Menard's et cetera.  The vinyl ester and ATH (aluminum trihydroxide) is what OtherPower is currently using.  

Thanks for the input and stay tuned for updates.

[Blewsage]

Thanks for the info! I searched the board for info on ATH; one search suggested US Composites at www.shopmaninc.com as a source. I repeated the search and can't seem to pull up the post to thank/give credit to whomever it was supplied the info. So, whomever it was, thank you! I just order some Vinyl ester and ATH from them.

I also read some posts about about casting the rotors leaving part of the magnets exposed to create a fan effect. My magnets don't have a hole in the for securing; so I hope they'll stay on the rotor. I'm considering using some JB weld to glue them to the rotor before pouring the vinyl ester.

Another post recommended leaving a hole in the front of the prop hub to allow air to enter.

I appreciate all the recommendations, and hope to learn from others expertise and experiences.

[ghurd]

The size is out of my league,

BUT

JB Weld.  Enough is plenty.  It has magnetic properties.

Too much runs up and all over the magnets.  It can make a heck of a mess.  It is a bigger problem with conversions (round magnet mounting surface) than axials.

Keep an eye on the magnet rotor for 12 hours, and have 1000 toothpicks and a roll of toilet paper handy to remove the excess JB Weld.

G-

[Blewsage]

Hi MGTD, wondering how the rebuild is going? Read your post on, "Tail size" you stated you were modifying your rotor. Was wondering if you were considering using a dremmel tool or router to make groves in the resin between your magnets to promote air flow? I've got most of the welding done on my 10 footer. I've offset the spindle to 6.5" instead of 5". I'm hoping this will promote earlier furling. I've also made the tail abit lighter. I can always add weight later, but taking it off is a little more difficult.