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Author Topic: sizing dump loads  (Read 2448 times)
buzzard
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« on: February 11, 2012, 01:28:16 PM »

Hi, we have a Miniwind turbine 48V with 3m diameter blades and we are now planning to connect it to 2 tristar 60A charge controllers (after working without charge controller, with just the miniwind controller, and realizing this is not good for the batteries). We will give both charge controllers their own dump loads as advised in another thread. But I'm not quite sure about sizing the dump loads. We're not sure about the maximum current the turbine can give, though we're sure it's not over 100A. I was looking at the navitron dump loads and what seems logical to me is to get 4 of these for each controller:

4 x 720w (48V, 15A) = 2880w, 60A

This seems like a good size for each 60A controller. However, considering the fact that really the dump loads will not just be working at 48V, but maybe current may go up to 60A, the dump loads could possibly be too big for the controller (?). I don't seem to find any clear calculations but wattage seems to go up a lot when voltage goes up for dump loads, so what if the current of the 4 dump loads (60A at 48v) may go up well above 60A at 60V and it's too much for the controller?
I must add that the Tristar manual says that the maximum dump load for the controller is 3600W (60A at 60V), so there is a bit of a safety margin... Still the dump loads may go over 3600W if at 60v...or not?

3 dump loads for each controller seems a bit small, as at 48V that's 6 x 720w = 4320W in total, I think the turbine may produce more than this at times.

Any advice would be very much appreciated!
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clockmanFR
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« Reply #1 on: February 11, 2012, 06:56:03 PM »

Tristar Controllers are hardy beasts,  Grin well the normal ones are, the MPPT design seem to be going poof  Huh.

Okay, i have several Tristar controllers connected to the 48v battery bank, i have set the charge rate at 56.8v if my 3.7m dia turbines start to really produce then the controller dumps to my heaters, each is 1.8ohm Resistance.
 However when i get severe gusts then tristar controller 1, will be maxed out and says "alarm" but then Tristar controller No2 steps in to start dumping excess to its own heater, and when there is even more power coming from all the turbines then Tristar controller No3 starts dumping to its heater and so on.

I do not know much about miniwind turbines, but max safety output at severe gust on my Hugh P's designs is 60v at 42a = 2520 watts. But normal running max is 56.8v at 32a = 1817 watts, or 1.775ohms, or roughly you need heaters with a suitable thick wire element that has a 1.775ohm resistant that will soak up that 1817 watts.

However, after your batteries are full, one Tristar controller (its allways one that do most of the work), will drop to float charge and then this does all the work while the other controllers are still set at a higher voltage, so beware, and use some power.

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buzzard
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« Reply #2 on: February 11, 2012, 07:57:09 PM »

Thanks clockman! Smiley

We seem to have found a better option, on Hugh P's website...there's a link to 2.2 ohm resistors which can handle 48v and give a maximum of 2000w at 66v (1636w at 60v, if I calculate correctly), so if we use 2 of them on each 60A tristar (4 in total) we're at 3272w each (at 60v), well below the tristar's maximum of 3600w for diversion loads. extrahappy
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clockmanFR
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« Reply #3 on: February 11, 2012, 08:21:43 PM »

Buzzard, Hugh knows his stuff.

If your calculations are correct then give it a go. Its a interesting experience when your turbine is generating in a storm and your batts are full and your dump loads are going like hell.   surrender

Just make sure your connections are fool proof and your turbine can not run free.    police

My first experience was the incoming cables were getting as hot as the dump loads, so up rated my incoming cables to 2 x 6mm/2 . They still get warm. In good winds i allow the dump load to work until the controller drops to float mode, at this i feel that the batteries are in a good charged state.

Oh yes, just one more thing. when Tristars are dumping correctly they use PWM which by the time it gets to your dump load coil is a blasted buzzing sound, well it is with my air coil heaters. But then you get used to it.

Good luck and do let me know how it all behaves.
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Eleanor
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« Reply #4 on: February 11, 2012, 09:55:23 PM »

Buzzard, have you read the posts by Adam? www.navitron.org.uk/forum/index.php/topic,1849.45.html (it starts off with a different wind turbine)

He has a similar set up with a 48V MiniWind with 3 x 3m blades and found that they produced less than the 6 x 1.8m blades due to the blades flexing and slowing it down in high winds. Page 7 gives information on the maximum output his produced, around 20-25A. I think he's running it on one Morningstar but you'd have to check. I think if you havenít already done so it would be worth getting a DC clamp meter and measuring the actual output at a few different wind speeds.
« Last Edit: February 11, 2012, 10:11:47 PM by Eleanor » Logged

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biff
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« Reply #5 on: February 11, 2012, 10:36:22 PM »

Really good post Clockman,
                Intresting link Eleanor.Makes me wonder how my 6 blade hub will pan out. My visitors came early today so did not get to fly the baby.Tomorrow is another day.Will post pics of maiden voyage.
                                                            Biff
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Eleanor
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« Reply #6 on: February 11, 2012, 11:16:35 PM »

Biff, looking forward to seeing how you get on with your 6 bladed baby. We had the 6 bladed MiniWind and it was able to produce over 2kW from the 1.8m blades. It went like the proverbial clappers (and sounded like them) and I think that's why Adam was happier with the bigger blades. Ours was the 24V version and I feel that the current was too high and the 48V model is probably better. There were some burnt out PMGs but by the time we bought ours they were supplying the "control box". This monitors the temperature in the PMG and shuts the turbine down when it reaches 100C. I found that this seriously limited the output of the turbine as it shut down in relatively low wind speeds but this could probably have been improved by increasing ventilation around the PMG. If it hadn't made such a racket we'd still be using it. The new wind turbine is going up next week and hopefully it will be bearable. We had a Navi turbine before the MiniWind and I never noticed a peep out of it until the day it took off!  sh*tfan
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buzzard
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« Reply #7 on: February 12, 2012, 07:36:08 AM »

Thanks clockman, thanks to the miniwind controller (='control box') the blades never run free, which is great peace of mind. Battery shed is 100m from the house so we won't have a problem with dump load noise.

Eleanor, that's a great link thanks! Yes, we're probably completely oversizing the charge controllers, but anyway, they've been ordered now and it's not a bad thing to be on the safe side. Also further in that topic is says that Gordon was maybe stiffening the blades to give higher output. I do think (guess) we get a lot more than 20-25A, but may be wrong. Yes we should be measuring the actual output, haven't done so yet, but I'm sure the tristars will tell us...
We started of with the smaller blades (2.4m) about 3 years ago I think. We have had the 3m blades on for a long time now, certainly over a year. We're very happy with them, a bit more swishing noise than the smaller blades but ok. We live in a very windswept place (Caithness) and the turbine has been great. We had the miniwind controller from the start and the automatic braking is fantastic (= reliable). Though the brakes indeed come on much quicker with the 3m blades.
At gusts of around 40mph (18 m/s) the turbine starts breaking regularly (overheat protection). Usually we manually brake the turbine when winds start regularly going over 45 to 50mph, as then the turbine goes off-on-off-on and the pole starts bending with the impact every time it goes on-off...so we just turn it off. but we've never taken it down in a storm (as we're always too late, by the time we realize a storm is coming winds are too high to start taking the tower down) and we've had well over 80mph. No problem with the blades so very happy.
Also no problems with the wires getting twisted in the pole, the times we've taken the turbine down for checks it's never really been twisted. What's your new turbine Eleanor?
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buzzard
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« Reply #8 on: February 12, 2012, 07:47:39 AM »

We started of 3 years ago with secondhand batteries (which we more or less killed), but we finally got some money now so we decided to get some good Rolls batteries   Smiley (4x the 12v 357Ah), and decided to get a new inverter (outback vfx3048E, our old one is a kipoint 3000w which works ok but failed to shut down at low battery voltage, and uses a lot of energy when we're not using anything) and decent charge controller(s) to keep those lovely new batteries healthy... We're first installing the charge controllers and inverter, and only then changing the batteries.
« Last Edit: February 12, 2012, 07:57:18 AM by buzzard » Logged
clockmanFR
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« Reply #9 on: February 14, 2012, 09:14:35 PM »

"There were some burnt out PMGs but by the time we bought ours they were supplying the "control box". This monitors the temperature in the PMG and shuts the turbine down when it reaches 100C. I found that this seriously limited the output of the turbine as it shut down in relatively low wind speeds but this could probably have been improved by increasing ventilation around the PMG."

I agree with Eleanor, your PMG is very important.
I see allot of stuff that's rated at nearly double what it should be, but keep running the PMG at full load and it will overheat.

My lot have been tested/load at 1kw -1.5kw for an hour and the PMG reached and settled at 81c.  

See this..   http://board.fieldlines.com/index.php/topic,144539.0.html   for actual evidence.
« Last Edit: February 14, 2012, 09:17:59 PM by clockmanFR » Logged

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