blew up another tristar

[cardamon]

So I just blew up my third tristar ts-60.  IT is being used in diversion mode on my 12 volt system with 1kw turbine and 2 l-16's.  I have found out that it does not like my home made dump load which is about 75 feet of thhn submerged in water and the fets short out (permanently) after a few minutes, but I dont know why.  The first two times it happened there were some other variables and along with figuring I had a defective controller I wasnt able to really conclude that the dump load was the problem, but there were some other aspects of the system that were kind of rigged up so after blowing up the 2 tristars I just kept the xantrex C-60 on diversion duty until I got the diversion tank and water heating elements all hooked up and plumbed in.  Fast forward 6 months and the system has been finished and working great with the tristar for months, but I switch over to the old water cooled coil of water load to do some maintenance, and the tristar shorts out after a minute or two - back in goes the C-60 and loves every minute of it.  Ok to sum up:  

1.  load is 75 feet of 16 awg, water cooled, correct resistance/loading verified by connected directly across batt terminals.
   
   
2.  xantrex C-60 loves it, dumps power into it for months.
   
   
3.  Tristar FETS permanently short after a few minutes on it.

   
   
   

The only thing I can think of is that perhaps the wire being semi coil like (its a big wad, but has the memory from the spool), has some inductance and is creating some back emf which frys the fets, and for some reason the C-60 is a bit more rebust in this regard?  Would the different operating frequency (tristar 300 hz xantrex 166 hz) make the tristar more susceptible?  Is this plausible?

[ghurd]

I have to question

"1.  load is 75 feet of 16 awg, water cooled, correct resistance/loading verified by connected directly across batt terminals."

My calculator says that is nearly 100A.

(75/1000x4.016=0.3012ohms, at 28.8V is 96A)

That 96A is a problem in a good 60A controller.

G-

[wpowokal]

I this a grounded system? and if so does the water have a path to ground?

allan

[cardamon]

Ghurd and wpowokal,

It is a 12 volt system, resistance is probably around .25 ohm.  Yes the system is grounded but the water is in a plastic bucket on a wood floor.

[hydrosun]

I recall that the tristar is programmed differently than the C-60. The c-60 has the same setting for connecting between solar panels and the battery and between the batteries and a dump load as a diversion. The tristar has to have a setting changed when operating in diversion mode. I don't know if this makes a difference in this case.

chris

[Flux]

There was something here previously where someone tried to use long lengths of wire as a load and ran into trouble. At the time I think we came to the conclusion that it was inductance of the load that was causing trouble. Your load similarly is probably quite inductive.

Unless you have a scope and can check the waveform for RF oscillation you won't be able to really confirm this. Even if you confirm it it is tricky to sort unless you can get details of the output mosfet switching circuit of the tristar but you could try a schottky diode across the load directly at the terminals or you could try a RC snubber directly across the load terminals.

Possibly with your set up capacitance to ground may also be an issue.

Flux

[cardamon]

I guess Ill just have to not use the problematic load with the tristar.  The only reason I need it at all is when I do maintenance on the water heaters.  Ill pick a calm day or switch in the xantrex.  I wonder if they will give me warranty again....

[SparWeb]

...tristar 300 hz xantrex 166 hz...  Well it's probably isn't helping.

If you're handy with the math, and understand stuff like cable capacitance, try looking at this:

http://www.nexansenergy.com/egy/equip/teck90/electrical_char.htm

I have this suspicion that there is a lot of capacitance "charging" up your cable coils but it's nothing more than a gut feel.  I don't know more than "plug-and-chug" with equations like those.

You could also try looking up datasheets on the FETs in the C60 and comparing it with the Tristar's FETs.  Again, I don't know how easy that is, but there are some guys on this forum that make it look like a snap.

[bob golding]

have you tried checking the current though the load when connected to the batteries? i run my ts 45 at around 26 amps and so far have not had any problems. the  fets are pretty easy to change and somewhere on the morningstar site there are instructions to do it. i have a dead one that i intend to change the fets on,when i get time. the fets are fdp 2532 and cost around 30  uk pounds for a set of 6. i think that is around 50 dollars. the price point drops if you buy 10. you will need 12 for a ts60.

bob

[cardamon]

Thanks, yeah it runs 60 amps right on the money when connected to the batts.  Its just strange how sensitive the tristar is to this issue while the xantrex is not.  Good point about comparing the fets from each one.  Ill see if morningstar will warranty me for a third time....

[BigBreaker]

I had the same thought as Flux.  You probably have some inductance in the line.  You could use a parallel diode and a parallel RC to protect against inductance.  If you already have the snubber in there then a small series inductor (with some known material inductance) would protect against capacitance.

But I don't think it is capacitance because there would always be some resistance in the line to keep the current from surging - think of it as a ladder of small resistors shorted to ground through small capacitors.  The controller is rated to 300 amp surges, so any surge would have to be pretty big.  Also your voltage is quite low, so .5xCxV^2, your energy of capacitance, will always be low.  I don't understand the heating element you are using but many of them are tightly coiled internally to increase the effective length and therefore resistance of the cable.  Depending on the packaging of the element, that may or may not be obvious.  That coiling is going to lead to inductance.  Everything I know about your situation sounds like an inductance problem, not a capacitance problem.  Put a snubber on there.

The only reason not to use a snubber is that they waste energy but that's what you are trying to do anyhow... so slap one on there chief.

[niall2]

hi ....

could i ask a question about adding a snubber ?
i,ve  recently put a c60 on my mill and i,m dumping 48v up to about 750 watts into a 48v water element

i did put a fast diode and a crude what  i think is a snubber  circuit on it as well

the snubber is .22uF polypropelyne cap in series with a 220r resister across the element

the resistor is about 10w

would these values work ? ...even if they were of some help it,d be better than nothing

thanks

[Opera House]

As far as the inductance, I would wind such that the inductance cancels.   Half the coil in the opposite direction and laid on top the other or have two lengths twice as long and connect opposite sides in parallel.

A RC network would have to have a much smaller resistance than 200 ohms.  Probably closer to 5 ohms.  Cap would definitely have to be polypropylene and have to be snubber rated.  Most caps can't handle more than 100 mils of current.  Cheap polyester caps go nuts as the frequency increases.  Five parallel RC networks with higher value resistors is the way to go if you don't.   I used to design industrial snubbers.  In OEM quantities we were paying $4 each for caps that would handle 5A.

[niall2]

thanks opera...

i,ll change the resistor value tomorrow and keep an eye on the cap...dont know if its snubber rated , it came out of an old pc supply
threre seems to usually be one or two block shaped ones in them


niall

[dnix71]

Would twisting the dump wire like cat5 make a difference? Even the individual stands of cheap phone wire have so many turns per foot to a minimize cross talk.

http://en.wikipedia.org/wiki/Twisted_pair

You might want to read this too: http://www.navitron.org.uk/forum/index.php?topic=1685.0;wap2

Do you have solar, too, or just a turbine? Could the batteries be dumping back into the Tristar along with the turbine output?

[Rob Beckers]

Look up bifilar winding of wire, this is what Opera is suggesting, and it is an old method of creating wire-wound resistors with little if any inductance. If you are truly running that Tristar right at the 60A limit for extended periods of time you're asking for trouble. Maybe put heavier-duty FETs in there if you have to replace them anyway.

-Rob-

[boB]

Big load resistors can definitely have a large amount of inductance.   Some of them are wound so that the inductance is much lower, but it does not cancel out completely.

The energy has to go somewhere, especially the energy when the the load turns off.  Diodes can help but they can get hot so should be good ones.

boB