Shorted stator

[dlenox]

Last week I was running my 17' wind turbine in low/moderate winds, it was producing some nice output with about 1kw in a 15mph winds.

Did not expect it, but at 4:30am the wind kicked up considerably and for about a half an hour turbine output went up to between 3kw and 6kw.  By the time I woke up and heard the wind and managed to throw the contactor switch it was too late, and it appears that the stator had been developing a short.

The graphs showing watts/amps output and turbine rpm confirmed it as shown below:

Output was peaking out at at around 100amps, 6000 watts, while at below 200 rpm!

The graphs also tell me that the furling still is not working properly, in spite of lightening the tail vane by 20lbs, so I will be finally adding about 2-1/2" more offset from the yaw bearing.

Initially I was hoping that it was the plug/receptacle, but upon further inspection it appears to be the stator.  Using some binoculars I do not see any tell-tale discoloration of the stator, the turbine still rotates in a stiff wind but it does so very slowly.

So it looks like a stator replacement is due.  The last stator I used 14 gauge wire, two-in-hand, with 59 turns per coil.

I am not sure but I will see if I can fit coils wound with 15 gauge, three-in-hand, if so will be shooting for about 52 turns per coil. Worst case is that I use same coil configuration as last time.  This time, no matter which direction I go with the coils, I will be excessively insulating the coils and phase wires from each other to (hopefully) prevent this from happening again.

I know that others in the past have said that the wires in the stator vibrate (causing humming) and I have to wonder that if adjacent coils are actually touching if the vibration could cause a short to develop?

This whole situation is compounded by the fact that the day after I got the 17'er flying again I broke my calcaneous (heel bone) into 3 pieces, and am in a cast for at least 8 weeks...

But during the interm I am accumulating the wire and parts needed to fabricate the stator.

Still learning,

Dan Lenox

[fabricator]

Wow, that really sucks Dan, and I just finished winding my coils tonight, 14ga 2 in hand 59 turns, I'm going to vacuum bag my stator to insure all the coils are totally saturated, hopefully that should preclude vibrating wires.

That is a turn of bad luck, when it rains it pours I guess.

[SparWeb]

I'm really sorry to hear that, too, Dan.  Not being able to get the mill down to see what the problem is must be the most frustrating part.  Hope you feel better soon, too.

Some would feel daunted by all the trouble.  Glad to see you're still enthusiastic!

[willib]

Sorry to hear about your stator and your foot.

Your mill seemed to peak at 4800 watts before the rpm dropped well below what it had been doing. its such a shame after all that work. are you still using the linear actuator for braking ? or did you come up with something else.

Take care

Bill

[dlenox]

Bill,

This iteration I left off the actuator, I use a contactor to short the stator for stopping.  However the stator shorted itself, thus eventually stopping it.

Dan

[dlenox]

For the next stator I plan on using lots of shrink-wrap and rubber tape to insulate the inter-coil connections that get accumulated around the inside center of the stator.

So that the coils do not physically touch I will be putting a thin insulating spacer between them before I pot them in the riberglass resin

The turbine had a pleasant slight humming, nothing excessive. Not sure that vibration is/was an issue but do believe that any humming that you hear is the coils vibrating.

[scottsAI]

Dan,

Stator fault does seem correct.

Yet, do not see where you have not ruled out the rectifier...?

Scott.

[ghurd]

Good Call!

Certainly worth a look.

I kind of doubt it would let it sort of bounce a few times between 0 and 100A like that?

Most of my diodes get smoked all in one shot, with no question about exactly when it happened.  LOL

G-

[DanB]

It will be interesting to see the condition of your stator.  If it's not discolored then you might have just got unlucky with a connection between coils which maybe fairly easy to fix ~ I have had some luck locating shorts by wiring the stator to a battery / pressing on things to find the problem.

Looking forward to hearing more anyhow!  I hope it's an easy fix...

[dlenox]

Scott/Glen,

Rectifier was first thing that I checked as it was easier for me to hobble over to, swapped it out.

A couple of days ago I hobbled out to the tower figuring that possibly it might be a burnt plug/receptacle, but no funny smells/looks. I even went as far as unplugging the plug completely so all there was is the cable coming down the tower to the plug, and the turbine turned like it was still shorted.  I have not dis-assembled the plug, and will do so, but no tell-tale burning smells in the fiberglass enclosure at the bottom of the tower, so this seems to be unlikely.

So unless there is something wrong with the cable (using 24" long kellum grip at top) the only thing left is the stator.

Got stitches out today, they put in a lot more hardware than I originally thought, a total of 6 screws! 3 length wise and 3 width wise, they put me into an air-cast and gave me permission to drive so I'm a little bit closer to a 'normal' life.

The cast is removeable and they want me to daily do simple range movements to get some flexibility back, but still will have the cast for 8 more weeks. It is amazing how quickly you loose muscle mass, my calf looked like a wet noodle after 4 weeks of being in the splint.

Been thinking about stator and I hate to change too many variables at one time, since I will be adding offset from yaw bearing I think that I will make stator same as last time using #14, two in hand and reassemble using the same rotor gap as current.

Maybe next year after I can confirm that the furling finally works properly will I make another stator using #15, three in hand. One of the reasons to wait is that right now I use a .6ohm 6kw power resistor to help overcome stall, and I believe that the total stator resistance will be lowered slightly when winding with three in hand, so I think that it may just compound the stalling issue.

Even once I can walk it will take some time to get the strength level to climb the tower so hoping that the turbine will be back in operation November time frame. Until then I still have the 720watts of solar panels that I mounted on the tracker that I made.

Dan

[dlenox]

DanB,

I will still be making a new stator and plan on replacing existing stator with new one, as that I can do ahead of time on the ground to save some time.

One can hope for a fixable short and will look for it once I get the turbine back into service.

Dan

[Flux]

It is just as well to treat these stators the same as high voltage machines. Most get away with murder, modern wire is pretty good, but it makes sense to sleeve all the interconnecting wires ( or at least 2 of the 3 phases). I would infinitely prefer the normal impregnated glass sleeving to any form of rubber tape.

Heat shrink sleeving should also be perfectly ok.

Again a barrier between coils makes good sense, Nomex seems most sensible here.

Stators always hum and It is unlikely to be the vibration of individual turns, it is likely to be general vibration of the whole stator casting in various modes.

If you wind the coils fairly well then they should be ok. Polyester or vinyl may be thin enough to partly impregnate between turns if the thing doesn't cure too quickly but I doubt that it will do so fully and I also doubt that it will matter. I tend to wet wind the coils with the same resin as the casting so that the turns are all stuck together, I never suspected it was necessary but gives coils that are easy to handle rather than the things that spring apart when you remove the winding former. For big machines it may actually be a good idea rather than just convenience. I suppose superglue does the same thing to some extent but I tend to regard it as a contaminant rather than a useful material.

My suspicions are that you have a short between phases rather than something peculiar to each coil so I suspect the inter coil links or a short between adjacent coils.

You may be able to get some idea of the location of the short by passing a controlled current between each pair of leads in turn and checking the field pattern with a small search magnet or compass. You should spot missing coils which are bypassed by the short.

With a bit of luck you may be able to carefully get into the problem area with a bit of surgery and clear the short. If the whole thing is not obviously fried I think you have a good chance of reclaiming it.

Good luck and if you build another then I agree with your proposals.

Flux

[ghurd]

New Understatement of the Year Nomination?

"I broke my calcaneous (heel bone)"

'Broke' sounds a little lame for the situation!

Sorry to hear it.

G-

[boB]

Do you remember what the stator DC resistance used to be ?

And, what is the DC resistance now ?  Just curious...

Looks like a possible char-broiled stator where the varnish/insulation on the

wire was shorting and un-shorting while it was cooking.

Also, what is your capture intervals ?? Looks like your dashboard system is

sampling at very fine intervals.... I like that.

boB

[Ungrounded Lightning Rod]

While buzzing can come from loose coil turns (like in a long air bubble) striking each other or something else nearby, the stator will vibrate even if it's totally solid.

The force on each coil is proportional to the product of the current through it and the amount of flux being dragged through its turns.  This drag of the mag field against the coils varies cyclically and moves from coil to coil progressively around the stator.  The result is a force on the whole stator that varies cyclically both in strength and direction.  Additionally, the gross amount of torque - drag on the shaft and reaction on the stator - is the sum of the contributions from all the coils and magnets and this varies somewhat with angle.

With a three-phase stator the force almost balances out - but not quite.  (Especially when the genny is doing battery charging so the current waveform is not a nice sine.)  So this residual imbalance shakes the whole stator a bit and makes it hum.

A two-phase (or any even-phase) stator can balance out more closely - almost dead-even if its pole and coil geometry produces a true sine wave and it's driving a resistive load.  A single-phase stator has an enormous load vibration because the whole drag force cycles between zero and max twice per cycle.

(Though three-phase isn't quite as balanced as two or four, it is used because it has a special advantage {espceially for resistive loads}:  For a given amount of copper in the transmission line wiring it has the least amount of resistive loss of any phase count.)

[dlenox]

boB,

Data is actually captured every 1/2 second and averaged across the log period.

My software is currently setup to log data points every 10 seconds, seems like a good compromize on the log data file size, while the software can write to the log file up to every second.

At 10 seconds I find that you actually miss the high points, but it's all a compromise anyway.

Dan

[GWatPE]

Hi Dan,

the graphs you presented show the mill/loading work well up to 40A.  Above 40 A, the windmill is heaviliy loaded, as shown by the plateau in the rpm graph.  The blades are not being stall limited, as indicated by the current continually rising, even though the rpm has plateaued.  

The mill clearly has furling issues.  I would sort the furling out, and make a new stator.

If you intend to harvest the 100 or more amps, then you will need to reduce the stator resistance by at least a factor of 4.  Not likely without more magnet or radical design change.  The physical size of the current stator dictates a certain max power dissipation.  This has been exceeded, and power limiting measures need to be changed.

Gordon.

[Flux]

I see no evidence of furling in the range shown on that graph but I assume it was meant to indicate the fault details rather than machine performance.

Until you see the condition of the stator you won't know whether you have kept within the thermal limits.

I have had serious doubts whether these machines do actually furl for a long time, this data certainly adds to my suspicions. It is certainly mostly stall limited but without special blade profiles designed specifically for stall limiting the power still creeps up, as the power increases as the cube of wind speed and the thing doesn't stall out hard enough to deal with this rapid increase in power.

There may come a point where the blades actually get the thing through stall and if this happens below furling then the thing could pick up speed and get seriously hot.

I think you have published your data somewhere, it would be interesting to look to see if it does ever run into this region where the revs pick up again from that stall condition.

I have never managed to get any furling scheme to hold constant power. All mine drop power when they furl. Unless I see some indication of power drop I suspect there is no furling. With a lot of messing about you can keep a decent power beyond furling but with this simple gravity hinge I would expect a significant fall in power when furling kicks in with a drop in rpm.

Flux

[dlenox]

Flux,

From what I have seen over the last year I would agree with your conclusion that my machine doesn't furl for long periods of time, can't attest to other machines though.  But I have previously seen the tail swing around to almost full furling position, only it did not seem to jerk the turbine around out of the wind path, then seconds later the tail drops and the whole sequence started over again.

This is one reason that I wanted to lighten the tail, figuring that the weight of the tail vane 'pulled' the tail out of furling position.  But it has been such a short period of time since I last put it up, and seems like it did not change things to a large percentage.

At this last incident I did have the .6ohm load resistor in place and while this seems to increase output (and reduce blade stall), it also seems to dramatically change furling.  Without this resistor I have seen the power curve flatten off at about 25mph. But last year even with the power curve flatten when the wind speed got up to 36+ mph the turbine output (and rpm) went up a lot, so there may be something in your comment about the blades pulling through stall

Perfectly honest I don't care about power drop when furling, survivability is more important than short bursts of large amounts of power to me.

More info will be gotten when I can bring down the stator and inspect it. Will be sure to post pictures. I painted the stator white so any signs of heat should easily show.

Dan

[dlenox]

Gordon,

Have known about furling for the last year, that is one reason I lightened the tail vane to 10lbs from about 30.

Don't like to change too many variables at one time and prefer small steps this way I can see exactly the effect that a single modification has.

When I can get the turbine down my next modificaton is to add between 2" and 2-1/2" more in the yaw offset. Hoping that this will finally cure the furling issue.

I really have no interest in generating over 3kw of power, and live in a high-wind area, so more concerned with survivability.

Dan

[willib]

I'm not convinced thats what happened, we will find out soon hopefully ,(foot willing), your graph shows to me that your power was still up there for a few minutes anyway till you shut it down.

i think your stator may have overheated and warped and started rubbing against one of the rotors.

we will see

bill