Doc Watson bit me pretty good

[TimS]

I was out of town for a few days.  It was a beatuiful morning and my wife left the turbine on when she went to town.  A powerful little micro-cell came through in the afternoon.  I got a call that night that the wind turbine got pretty messed up.  My first question was: "Is the tower still up?"

Well, I got back late at night and it was dark.  The first thing I looked at was the doc watson I had hooked up about 6 weeks prior.  It was great getting all the stats.  KwH, peak amps, all that.  Actually, this 17 footer put out a peak of 84.6 A and its a 48 V machine.  Probably just shy of 5Kw.  That was impressive and great to see.   But back to the story.  I got out my ohm meter that night and found that the DC- side of the little doc watson had failed open-circuit.  Darn.  Well, that explains things.  My advice, don't trust these little meters.

I had to search around in the woods.  This section of blade was about 50 feet from the tower:






The force must have been awesome.  The tail flew off the tower and hit a tree about 15 feet up and 30 feet from the tower:

[ChrisOlson]

But back to the story.  I got out my ohm meter that night and found that the DC- side of the little doc watson had failed open-circuit.  Darn.  Well, that explains things.  My advice, don't trust these little meters.

Split the case on the meter and you fill find that what happened is that the solder melted off on one of the negative leads where it is soldered to the kelvin sense pad.

As long as you have it apart, remove the internal shunt and the negative leads, solder a twisted pair (ethernet cable wire works good) to each side of the kelvin sense pad in place of the negative leads, order either a 100 amp or 500 amp external shunt for it, and hook your twisted pair to the new shunt.

Your meter will still be fine and you can then trust it.  You simply overloaded it.  It is not designed for continuous high power with the tiny internal shunt.
--
Chris

[TimS]

Well, the shunt did burn up, but I wouldn't trust this unit anymore.  The negative lead burned off as well.  Here's a picture.

[ChrisOlson]

Well, the shunt did burn up, but I wouldn't trust this unit anymore.  The negative lead burned off as well.  Here's a picture.

Yes, I've had a couple like that.  It normally doesn't hurt the meter itself unless solder spatter has gotten on to the board someplace and shorted something out.  If you remove the damaged shunt and the black leads, clean it all up with electric contact cleaner (be careful to not get it on the LCD) and test it, there's a fairly good chance the meter itself will still work.

You'll see the Kelvin sense pads under the shunt - they are two little eyelet looking things and that's where the twisted pair has to be hooked up to.

With an external shunt, the meter can totally fail and it still won't unload the turbine like it does when you burn out the internal like you did.  With the external shunt the meter just senses the voltage drop across the external shunt and calculates everything like it did before.  The power flows thru the external shunt, which can dissipate a LOT more heat than the tiny internal one.

I have run a Doc Wattson at continuous 70-80 amps with the 100 amp external shunt with no ill effects.  Theoretically you're not supposed to load a shunt at more than about 65% of it's max rating.  But I have done it many times with the external shunt and it has not hurt it.

If you really need big capacity, you can also get a 500 amp external shunt.  But the meter does not read actual with the 500 amp shunt - you have to multiply the amp, amp-hours, and watts x 5 for actual when you use the 500 amp shunt.
--
Chris

Edit:  Kind of the test I have used to determine if the meter will still work is if when it was powered up it still showed the voltage and had the amp-hour/kWh/minimum voltage stored in it.  That means the meter is still functional and the only thing damaged is the internal shunt and negative circuit where the power flows thru.  When I burned up my first one I emailed the guys at RC Electronics about getting it fixed and they told me what to do and what to look for.  The guy told me what happens is exactly what you see there, and just put the external shunt on and it will be fine.

Since then I've fixed another one that burned up, and put external shunts on two more that I have bought, putting the external shunt on before I even used the meter.  I have never had one fail with an external shunt and I have pushed them pretty hard.

Your turbine is certainly a mess, but hopefully that is rebuildable if the generator is still good.  Strong winds with an unloaded turbine are never good.

[jaysicle]

Sorry to see that happen to your set-up.

The information you will accumulate from the failure is worth far more than the new meter.

While there is something (maybe many things) to be said about scraping the burnt toast so that you can butter it, and eventually eat it with your cereal - I also think your new controls (an metering) will be better suited to what your machine is capable of - pre-catastrophic moments.

I agree 4kW burst is pretty cool - even if it is just prior to coming apart. Glad to hear no one was in the way when it all went down.

-Jayson-   

[Rover]

Hey Chris,

"But I have done it many times with the external shunt and it has not hurt it"

nope it shouldn't short out etc, it may however change the readings in the future as you affect the shunt by running it beyond the the "safe" derating factor.

Completely right though it won't short out if using a larger shunt if max output and a shunt size 50% lower than max... (unless it gets spectacular amps of course, or you misjudge the max).

Tim, I'd probably ditch the questionable meter at this point. Probably depends on if you feel like replacing it with a non contact metering system... hall effect , what not.  Least with no contact you can't get the collapse of the main input circuit due to the metering system.

Unfortunately, I don't know of many of the shelf units that way.  Most of us build our own.

[Rover]

I left out that voltage reading would still need to be contact.. but its parallel .. .no biggie.

[TimS]

Chris-  Thanks for the info.  I may try and get it working again as kind of a hobby, but I think I'll go with a little more robust metering that doesn't have just 14 guage wire on it, etc.  It is too bad that the RC spec says its rated up to 100A.  That is a bit misleading.  The main reason I posted this is to warn others about the problem, and if you are using a doc watson I would go with Chris's suggestions if you continue to use it.  The alternator itself looks ok with binoculars (won't be completely sure till I tilt the tower down), so its off to make a new tail assembly and get some new blades.  Now I know that this particular turbine/blade combination can't really survive unloaded.  I've got to admit I always kind of wondered 
---Tim

[zap]

This post should be a sticky under "Wind" and/or "Controls... good info.

[taylorp035]

but I think I'll go with a little more robust metering that doesn't have just 14 guage wire on it, etc.  It is too bad that the RC spec says its rated up to 100A.  That is a bit misleading.

I was always wondering about why they put 14 gauge on these meters.... pushing 60v and 20A for my dyno through it was good, but running the starter motor was sketchy.  If the engine backfired and stalled out the motor, it would peak at 105A.  While assembling the connectors meters, some of my teammates shorted out the doc wattson on the steel table top (why they were doing this, I'm not sure ) and it peaked at 143 A (shorted out a 4s A123 pack).  It still worked after that, but about 2 weeks later, we shorted it out again when a wire broke off our kill switch and landed on the steel dyno stand, which happened to be grounded.  It peaked at 147A that time and when we plugged it back in, the screen was solid black.  Opened it up, but nothing looked burned...

I wish there was a meter that could read to 100-140v dc that was like the doc wattson... and didn't cost more than ~$75.  I spotted another meter that is just like the Doc Wattson and had 12g wire for <$30, but I would really like something that could go higher than 60v.  I know we peaked over 155v dc once on the dyno (33 rpm/v... ).

[ChrisOlson]

nope it shouldn't short out etc, it may however change the readings in the future as you affect the shunt by running it beyond the the "safe" derating factor.

Hi Rover - watch this movie of me abusing a 100 amp external shunt with a Doc Wattson being fed by a 12 foot turbine:
http://www.youtube.com/user/OlsonFarms#p/u/9/VRRBxUrkHyo

I have done this many, many times.  I have a high quality analog ammeter dowstream of the Doc Wattson at the bus with a 500 amp shunt.  The Doc Wattson still agrees with the analog after all the times I've ran it this way.
--
Chris

[TomW]

The one I have on my 2 turbines combined output I have regularly seen 140 Amps through it and no signs of problems so far.

Would like to do the external shunt trick but it never gets to my priority list.

Tom

[ghurd]

Bummer.  Always a sad sight.

That is exactly why I do not believe a fuse is a great idea.
Unless the fuse is massively overrated for the output.

I never quite understood why they/anyone would trust a Doc Watson for 100A in its factory configuration.
"Maximum amps for chassis wiring"
(from powerstream's chart)
#4 wire, 135A
#6 wire,  101A
#14 wire,  32A
Making traces on a PCB that size able to handle 100A sustained seems like a pipe dream.
Not arguing, just amazed.
G-

[halfcrazy]

I never quite understood why they/anyone would trust a Doc Watson for 100A in its factory configuration.
"Maximum amps for chassis wiring"
(from powerstream's chart)
#4 wire, 135A
#6 wire,  101A
#14 wire,  32A
Making traces on a PCB that size able to handle 100A sustained seems like a pipe dream.
Not arguing, just amazed.
G-

I agree here if it has #14 wire than I would not hook it up in anything capable of over 20 amps sustained.

[kitestrings]

Tim,

Sorry to see this.  Lot's of work, then to have this - the moving weak link - take 'em has to be discouraging.  Mother nature can be humbling.  Years ago I had a blade flex a seemingly unreasonable distance at the tip, to come in contact with a poorly located junction box.  Lot's of blade parts scattered about.

On a more positive note, it looks like the tower survived without damage, and hopefully the alternator.  Thanks for sharing and good luck with the repairs.

~kitestrings

[ChrisOlson]

Making traces on a PCB that size able to handle 100A sustained seems like a pipe dream.

They don't actually make a claim of 100 amps sustained for the Doc Wattson - only spike amps to 100.  They claim 50 amps sustained using the three-wire method of connection, and I've found the meters will take that without failure.  They do have a disclaimer that says:

"*** 50 Amps continuous, 3-wire connection. Derate for wire temperatures next to case above 70 Deg C."

They're an excellent meter, accuracy is very good - you just have to use an external shunt with them when using one on a wind turbine.  You can put one on a solar array and push 40-50 amps thru it all day (I have done it) and it doesn't hurt it.  But with a wind turbine if you push 40-50 amps thru it sustained, then a gust comes along and pushes it up to 60-70 amps, the wire and shunt is already hot and it melts the solder on the black leads where they hook to the pads.

Put an external shunt on it and all those problems with heating are gone.  The external shunt you can get for them is a very robust unit and they are not expensive.  I have bought four shunts from PowerWerx for my Doc Wattson meters.  You can get them either 100 amp or 500 amp - I have been using 100 amp ones and have yet to hurt one.  The Doc Wattson meter will read up to 144 amps with a 100 amp shunt (I've tested it):

http://www.powerwerx.com/tools-meters/current-shunt-resistors-100-amp-max.html

--
Chris

[Volvo farmer]

The one I have on my 2 turbines combined output I have regularly seen 140 Amps through it and no signs of problems so far.

Would like to do the external shunt trick but it never gets to my priority list.

Tom

U crazy!