Hello Ross: I own a Whisper 900 which was made by the same folks that made the WhirlWind units. I bought a 3ph transformer from them that is made for their larger 3kw whisper rig for about $600 that was a few years back. That was a few years back. The Whisper series is now turned over to yet another company.
We run this 3 times oversized transformer on our Whisper 900 plant. With our plant, it's maximum wireable voltage is 48v, And we use the transformer to get to 24v or 12v our choice. It'll also do 120v to any over the lower voltages. Elliet Bailey (sp) Founder of WhirlWind told me that the larger transformer is much more efficient then if I had ordered the smaller one made for the Whisper 900. The point I'm trying to make is, if you have the proper transformers, your loss will be so small you won't even know it. At the time the reason I went with the larger transformer, is I could use it if I ever migrate to a larger wind machine. I wish my Whisper-900 were a 120vac unit, line transmission would be even better that way.
If you have enough iron in your transformers, they will keep their power factor, and transfer power at just a few hertz. If your blade is just barely rotating, you will be getting power. That's what your wind unit is after all... is a rotary transformer!
I'm not sure most welders would do the job ? A lot of them have output voltages as high as 90v so this would give you very little step down. Also the duty factor is inportant. You wan't 100% duty factor and most welders don't do that but at the lowest settings, and some don't even do it on the lowest settings. They just don't have enough copper or steel in them.
As far as purchasing the transformers, I think you can do much better than that. If you can't find the transformer just the right size, you can always parallel and series them just like batteries to get your proper output (within reason that is).
Here's an example here is a 120v to 12v 25amp output transformer for $20.00
(See URL at the bottom of this posting ) When I insert it here, it makes the rest of the posting in such a fine print no one can read it !
I notice they have 1890 transformers in that particular model... haha
All of the transformer 115v inputs would be wired to each of your WhrilWind's 120vac outputs.
You would have to wire two of the transformers output's in series to get your 24v output. So two of these transformers would be good for a maximum of 25 amps at 24v output.
Because 4000 watts divided by 3 = 1,300 watts per phase divided by 24v you will need transformers capable of delivering at least 55.5 amps per phase. Three sets of these transformers ( 6 transformers in all ) would give you 75 amp capacity per phase while you only need 55.5amps. Over kill is good, as they run cooler and more efficient.
6 transformers X $ 20.00 = $120.00 per phase X 3 for 3 phases would = $ 360.00 for total transformer cost.
This way you could re-wire the transformers easily to obtain not only 24v but even 12 volts and 48 volts.
Also you don't need to start with that many transformer sets to get into charging your batteries, if you have a means of regulating your unit from over speeding and shutting it down from the 120vac side. (If you have a control box) If you don't you would need to use all the transformers so you would be able to shut it down from the 24v side and you would need something like the Enermaxer charge controller or something similar to keep it regulated. ( I own a couple that I'm not going to use because of equipment changes that I might sell cheap)
There are probably other surplus sources for even larger 120v to 24v transformers out there, you will just have to look around. What I mentioned above was just something real fast off of the top of my pointed little head ! haha
Also Russ, If you have quite a transmission distance from your tower to your batteries, or if you plan on up grading to higher voltages, I encourage you to keep your 120v unit. But if you are planning on 24v only, I think I can get my hands on a mint 4,000 watt WhirlWind low voltage unit which I think (I'd have to check) is re-wireable for 48v, 24v and 12v. I might be interested in swapping with you as I'm now trying to set up a 120vdc system here. You would only need to swap just the generator head & you could keep all the rest (I think.)
As far as re-configurating the wiring on your current unit, If you could catch up with one of the original WhirlWind dealers, they might be able to tell you, or at least refer you to someone that can tell you.
Last but not least option, you could take it to someone and have the generator re-wound with the correct wire size to get you to 24v which is easy to have done at almost any motor repair shop. They will have to cut a coil & count the number of turns and then check the square mill size of the wire (or weigh it). Then they basically will re wire with a 5 times larger diamenter of wire of the same weight. (someone correct me if I'm off here please.)
John II
http://www.surpluscenter.com/item.asp?UID=2004122717434785&item=15-1103&catname=electric