Transport of DC current requires 2/3 of the amount of copper for certain copper losses because for transfer of AC current, a wire in used only 2/3 of the time for star rectification (see my report KD 340). If you use a 3-phase generator rectified in star as brake, the braking torque is maximal if you make short-circuit in between the three phases and the star point. So you need a cable with four wires from the generator to the short-circuit switch which should be mounted at the tower foot. If the generator has no external star point, and if you therefore can short-circuit only the three phases, the maxiumum braking torque is only about 75 % of the braking torque with the star point short-circuited too (see measurements given in report KD 78). The higher breaking torque for short-circuit including the star point is caused by circulating higher harmonic currents in the winding. The 3-phase rectifier must be mounted directly after the rectifier, so also at the tower foot. From the rectifier, a long 2-phase wire goes to the battery charge controller and dump load and these components must be situated as close as possible to the battery to minimise the voltage drop in between the voltage controller and the battery.
Thanks very much! The papers you have referenced are excellent. Here is what I gathered: In terms of maximizing short brake, my turbine only has three wires so unfortunately no star braking (75% braking torque at best). The rectifier is best at foot of tower so as to save on wire. The charge controller is best near the batteries to minimize voltage loss.
Here is my problem: My 47 inch (1.2m) diameter turbine is 220 feet (67m) from my batteries. My turbine came with all three functions contained in one box/unit: 1.) the over speed/over current shorting brake (and short switch); 2.) rectifier, and 3.) mppt charge controller
are all in one box! I can't separate them! I can't go buy a new generator or controller at this point.
So I am faced with a choice - Where should I place that unit?
The first setup I put the over speed brake/rectifier/charge controller unit near the batteries using 12AWG (3.3mm2) wire from the turbine the entire 220ft (67m). Everyone said the controller needed to be near the batteries, and that is my experience with solar. It seemed all good until the wind got strong and the brake was too weak, and it burned up. (I was following manufacturer's recommendation with that small wire gauge too, by the way - go figure!)
I was worried mostly about the over speed brake being too weak and burning it up again, and being unable to separate the brake from the rest of the unit (with rectifier/controller), I put teh unit at the foot of the tower thinking I am maximizing braking torque. I also installed 6AWG wire (13.3mm2) from the generator to the unit (and from there to the batteries as well).
One question is if the mppt controller function over that distance? That is a major unknown. I don't know if it will sense the voltage wrong in the battery at that distance and mess up the output. I suspect it will not work right. If it isn't able to charge right at that distance, then I will have to move the unit to the batteries and hope the over speed short brake function will work at that distance from the turbine, now that I have thicker wire installed.
So my questions are: 1.) whether, with an mppt controller, I can place it at a distance and it will nevertheless charge the batteries? And 2.) If I have to make the choice to move the whole unit to the batteries, whether the shorting brake is likely to be sufficient at that distance, given the wire gauge now used? (e.g., how do I figure shorting torque over distance given a certain wire gauge?)
Any thoughts you might have as to the choices I can make with the equipment I have would be appreciated.