Resistance Dump Load

[freejuice]

Hi Folks,  I combed through the archives before I asked this question about resistance on dump loads and what I gatherd is this......( please correct me if I'm wrong I need some assuranaces )

If my alternator can put out a max of 1500 watts in breifs spurts before the wiring jumps ou of the stator like those novelty spring "snakes in a can"

I should have a dump load such as wound ni-chrome wire, heater elements, etc etc to burn off the extra power that the batteries cannot handle.

 So do I build a dump load that is just below the 1500 watts, so the mill stays "loaded" with some resistance ( keeping it from free wheeling) or do I make a dump that can handle the full 1500 watts?

Also, I found in the archives here, that to calculate resistance is voltage squared divided by watts gives me ohms ( hopefully I'm correct in this assumption) So do I need to calculate the ohms in the dump too or just worry about matching up the watts?

One last question before my head explodes, I see lots of resistors on ebay, some are 300 watts a piece but it appears they have different ohm ratings...what kind of resistors do I need to look for in regards to a 24v ten footer Dan design?

 I'm  also willing to build my own out of Ni-chrome wire, but I need some help figuring out how to calculate the resistance in the wire and converting this information into watts....my head hurts...

 Thanks for any and all help

 Gavin

[imsmooth]

Your dump load needs to handle the maximum watts your turbine will put out and them some more.  Otherwise, it can overheat.

I use a resistor bank to load my wind turbine if the grid it is tied to goes off-line.

My resistor are non-inductive.  I would stay away from wire-wound as you may get inductive kick-back depending on how you set things up.

To get a ballpark of what you need, first determine the voltage your resistor or loading bank will receive.  Say it is 48v.  If you want 480watts, 48^2/480 = 4.8 ohms.

Your load will need to be 4.8 ohms and the total wattage needs to be 480watts.  You will either have to find a singular loading element, or put several in a parallel/series combination to get the right wattage and ohms.

[Flux]

A normal dump load dumps from the battery so the machine doesn't loose load if the dump load is not big enough. If you try dumping from the mill before the battery you open a whole new can of worms.

Ideally it should be able to dump the peak power of the mill otherwise the volts will rise above the set point during peaks. If you have sealed batteries you must prevent this. For flooded cells it doesn't matter for the odd second or two but you still need the dump rated close to the maximum mill output.

The resistors must be capable of handling the dump current at dumping volts and stay within their power rating.

For 24v you will be dumping at about 28 volts. You can use your equation if you wish. Take your 1500W example. 1500 = 28^2 x R this gives R as about 1.9 ohms. The resistors in total have to dissipate 1500W.

If you chose 300W resistors then you would need 5 to handle  the 1500W. You can use them in series or parallel. For the series case you are looking at about 0.4 ohms or in parallel you are looking at near 10 ohms to make the nominal 2 ohms.

If you use the parallel connection you can always add another one if you find the mill peaking above 1500W but if that happens for long you will not need a dump load, I think that is a gross overload for the common 10ft machine and I wouldn't want to see anything sustained above 700W.

Flux

[ghurd]

With my controller,

The load must be limited to 6A per individual fet,

And the loads must be seperate.

And the fet's heat sinks must be seperate (not electrically connected on the same heat sink).

That means each resistance should be 5 ohms, with a power rating of about 200W.

Each will dump about 165W in a 24V system.

It needs repeated 6 times to dump about 1000W.

I do not like working with nichrome.

Something like this, but 6 instead of 10,

a couple more pics of it here,

http://s701.photobucket.com/albums/ww20/ghurd1/Large%20Dump%20Load%2024V%201650W/

[freejuice]

Flux, Smooth and Ghurd,

 Thanks the light is finally coming on...I dont know what I would do with out you folks!

 All the best,

 Gavin