New member of the BS club!

[Volvo farmer]

4' of threaded rod... $8.98

eight 1/2 nuts $1.44

An hour standing in the snow turning wrenches

The smell of burned polyester and lacquer

New wall art for the power shed.

Trailer hub, steel, 40'of tower, wires and controls with nothing to do makes Volvo Farmer a sad panda

Being a member of the Burned Stator club... Priceless!

[David HK]

Judging by the score marks it looks if the stator may have become loose and touched the magnets.

Any more post mortem details?

I trust you will spend Christmas Day and Boxing Day enjoying the delights of the workshop making a new stator.

Happy Christmas.

David in HK

[willib]

Oh man that is a sad sight to see.

we are in the midst of a blizzard at the moment.

sorry for your BS

[ghurd]

Ouch.

Ask Santa for wire!

G-

[boB]

Darned Wind anyway !!

boB

[windvision]

I'm with Dave. Just curious as to the cause, now that we see the effect. Thanks.

[Volvo farmer]

OK, here's the real story.

It was a week or three ago. Really strong wind and gusts. Local news said 100mph gusts, but that's miles and miles away from here. I woke up at 3:30 AM because something was shaking the house pretty bad, it was the wind, stronger wing than I have ever seen here. I was greedy for the power because it had been dark here for a while and the batteries were low. I know now I should have shut it down, but I didn't.

It ran about 3 hours straight through the storm still it started to get light outside. I was seing 20-30 amps sustained into 24V and saw one or two peaks at 50+ amps.

I went out to check things at first light and the wind and gusts were even scarier outside than inside, so I decided to shut it down. I threw the switch in the power shed in what I thought was a 12 amp lull, went outside, and things were still going round and round. Went back inside turned it back on, waited for a lull, shut it down, and this time it stopped.

I was more concerned about the tower than the turbine, checked all my guys and tightened a few turnbuckles. Turned it back on, and it didn't seem to start easily. It did finally start, but I heard a rubbing noise that I had not heard when I shut it  down. So I shut it down again.

The real damage came because I turned it back on, then ran to town for something. The wife called and said, "the wind turbine is spinning again". Well we had a good enough gust to drag all that magnet wire out of the coil, and that was the end of all round-and-round activity.

I'm going to round magnets and the new DanB design of stator. I feel kind of ashamed for not making the new parts myself, because I pride myself in being self sufficient, but I figure the Dans needs beer and gas and smokes money just like the rest of us, so I'm just buying the new improved stuff.

I'm keeping my old rotors though, and already dreaming of using them on a second machine, maybe a machine that I have a lot more hands-on than my first. I'm learning that it's good to be able to repair your own turbine too.

[Dave B]

 That's too bad, I'm sorry for your loss but thanks for posting.

 My thoughts for what it's worth : I would have no fewer than 5 gusetted stator brackets. I would have at least a 5 bolt heavy duty spindle and hub assembly. I would not use polyester for the stator material. I would run a gap of no less than 1/8" magnets to stator and no less than 1/2" steel rotor disks. I would have my outside rotor resting on much wider supports than the nuts only hanging on the lug bolts to help limit the rotors flexing and or the bolts bending. I would be very careful when using the shorting switch without an extremely robust assembly. Shutting it down in higher rpms will bend, twist and torque things out of shape just long enough for the magnets to scrape even ever so slightly and then the countdown to distruction begins. Last, I would adjust the machine to fully furl below it's safe sustained output. Just my opinions but based on a lot of experience and observation.  Dave B.

[Flux]

One thing I keep noticing with these burn outs, one side of the stator seems to be fairly well cooled and the other side is not. We expect a lot of air cooling up there in a gale but in reality I don't think it is that great. When head on to the wind I think the air flow is minimal. When trying to furl and at some sort of angle to the wind one side does seem to get much better cooling than the other.

My guess is that the final crunch came when it failed to stop on the brake, that would have put a lot of heat into the stator. It may have run for months in the brunt out state if the windings had not contact the magnets. If we keep burning out wire rated for 200C then I am not sure that it is fair to keep criticising the polyester, no resin is going to help for long once the wire is burnt out, all it could do is prolong the time before the wire bursts out and rubs the stator.

The real issue is too much power for the size of machine.

I agree that the round magnets are a better choice, I would go that route but rather than run stalled I would add some line resistance to get the thing performing, you will then be forced to make it furl and once you get it to do so reliably then it will be ok in any wind.

Flux

[HaroldCR]

 Why does no one put a cable from the tail to the ground, so the machine can be furled manually  Flexible covered cables are not that expensive, from the tail to the down cable ??

[electrondady1]

   50 amps x 24 volts is 1200 watts

thats what volvo farmers mill is capturing .

so, what would be the result if the alternator was allowed to produce at a higher voltage during the storm.

i'm thinking it would still capture 1200 watts, but at say 48 volts wouldn't the amperage be reduced to 25 amps.

[Flux]

Yes indeed that is true but it needs a mppt scheme to do it unless you can somehow change the system volts in a high wind. Adding resistors in the line to let the volts rise doesn't solve the problem entirely. You need to keep the stator current down, just loading the thing at high voltage without some form of ratio transformation still limits you to the maximum stator current. If you keep the mill loaded to the point where it furls and the stator current is within limits then all is fine but you may not get much of the 1200W into the battery, most will go as heat but if it is in external resistors rather than the stator then all is well.

The whole business of load matching really comes down to the fact that generated volts needs to track wind speed and alternator speed. If you charge a fixed voltage battery you need to alter the transformation ratio. Even simple step changes such as series parallel or star/ delta or star/jerry would go most of the way to protecting the machine but wouldn't be desperately good at keeping the blades on peak performance.

Using the thing totally inefficiently doesn't matter much in most cases and the higher output of a properly matched scheme would be wasted if the batteries were full and dumping, that seems to be the main reason people tolerate this match at low wind and stall approach, fine if you make it furl safely and you don't need the extra power but stall regulation alone is dodgey as you kid yourself it is furling when it really is just sitting stalled and when the big wind comes it pulls out of stall and produces more than it can handle.

Flux

[scoraigwind]

It's basically a 500 watt machine.  And a very good one.  Works well in low winds.  Just needs to furl right.  A lighter weight tail would have been the way to save this machine.

All sorts of very clever cooling stuff could make it into a 600 watt machine maybe, but the extra energy you would get is not as much as the above numbers suggest, because low winds are a lot more common than high ones.

Bigger magnets or a bigger magnet rotor disk would get you a higher rated output if that's what you need.   But the lesson to learn here is that the tail must be right, not that polyester is not good, nor that you have to have your hand ready on the brake switch all of the time. A well governed machine will look after itself and never burn.

[windvision]

Volvo Farmer:  Can you show us the other side of the stator? I have a feeling there was no rubbing on that side. If not, it tells me that Dave B. is onto something with how we spread the load on the rotor closest to the blades. I am curious!!

[Dave B]

 As I have said before and Hugh confirms this size and design is basically a 500 watt machine, period. Push it beyond that and you roll the dice for not if but when it burns out. (the good winds will happen)

 It is well designed as is but becomes light in form, design and function when pushed beyond it's limitations. That is not a fault of the design but rather not understanding it's limitations and that goes for any size machine.

There can be many variables between each machine, system components and location etc. The key for each build is to adjust the machine to completely furl before it's continuous rated output. A wagging tail up and down at 500 watts for this size is not it. Flux continues to try to hammer this (furling or not furling) point home also.

 It's a tough pill to swallow but if you want to run more than 500 watts continuous you will have to build a larger, more heavy duty design all the way around. Price can go up quickly but that's just being realistic. The more realistic about each design the fewer burn outs we will see.

Dave B.

[Flux]

This is another issue but yes with thin discs you will suffer distortion unless you support the blades in such a way that the gyroscopic forces are not taken via the discs.

I have been asked many times about the minimum thickness of discs not to loose flux. My answer has always been that the mechanical limitations are more important. Thin discs will either collapse or distort and you will have much more worries than a bit of lost flux from disc saturation.

Unless you use very thick discs there is a lot of advantage in arranging things so that the blades are supported from other than the outer part of the disc but this doesn't usually cause failure unless the discs are seriously thin. I think Dave B has a point about such disc deformation causing cracks on cast magnet blocks leading to water entry and corrosion.

I make my own hubs and build things backwards so that the blades are supported on the front of the hub and the alternator is overhung on the back but if you use a commercial hub you are somewhat restricted in what you can do.

Flux

[windvision]

Flux: From a design perspective, I think you have a better design by building it "backwards". This takes all the movement out of the allthread that normally holds the back rotor and the blades, and places it squarely on the hub. Thus eliminating that movement.To make this work, we need to extend the axle by sliding it in a tube and welding it. Then move the entire assembly forward to clear the tower and weld.

   As I look at the pictures, I don't think the stator is moving, rather the outer rotor from the torque force of the blades. Look at the difference between the 2 rotors. I find it interesting the hottest part of the stator is also in the same position as if there would be a blade strike.

   I agree completely with the concept of controlled furling. I just think there are some mechanical forces we can minimize to help the machine live thru these high winds.

   

[scoraigwind]

I take on board the possibility that gyro forces might contribute to magnets rubbing the stator, although I don't think it could happen unless the clearance is very tight.  

But I am not convinced that putting the alternator behind the hub helps.  It looks stronger to put the alternator on the back and the blades on the front but in reality I have not seen any benefit.  I have only done it a couple of times, and it made the machine a real cow to assemble, but I could not really see any practical benefits.  

The structure based on allthreads is pretty solid, especially when the space between rotors is filled with nuts and washers.  I have been using 1/2" hardware for a few years but I notice that my early ones with 3/8" seem to be working just as well. I don't like to rest the blades directly onto the magnet rotor in case the gyro forces push the magnets into the stator, but the moment of inertia of the magnet rotor itself is pretty small by comparison to the blade rotor.  The allthreads themselves do not seem to move in any shape or form. If they flexed, they would break pretty quickly.

[MattM]

Ever thought of a sacrificial plate over the magnets opposite to the stator with  an outer rim around the stator/rotor assembly?  You could keep them from damaging each other  with some kind of low-tech non-conductive, non-caustic lubricant.  I'm thinking siliconized gel like the stuff you find in the heels of sneakers.