I was looking a bit closer at size configuration and construction of a resistive load bank for our turbine. I've been thinking the best match would be 3.4 ohms per phase (2 - 6.8 ohm in parallel), 6 resistors total. They would be controlled ahead of the rectifier; wired in wye. They're capable of about 800-1,000 watts each, and if my math is right should look something like this:
(2 in II, 6.8 ohms)
Vline Vph R, ohms I watts (tot)watts (per resistor)
80 46.2 3.4 13.58 1,882 314
90 52.0 3.4 15.28 2,382 397
100 57.7 3.4 16.98 2,941 490
110 63.5 3.4 18.68 3,559 593
120 69.3 3.4 20.38 4,235 706
130 75.1 3.4 22.08 4,970 828
140 80.8 3.4 23.77 5,765 961
150 86.6 3.4 25.47 6,617 1,103
In an earlier post
http://www.fieldlines.com/index.php/topic,148112.0.html Chris had mentioned using overhead door springs, so I wanted to compare this option, because they are definitely cheaper. In his case he reported about 3.7 ohms (also wired in wye), with the resistance nearly doubling when they get red hot.
Okay, so a spring for a 120# door is about 4 lbs. Ours has steel spring wire that measures about .15" in diameter. With steel/alloy at about .284#/in^3, this translates to a length of about 66'. And, this is close to what ours works out to - roughly 170 turns of 1.375-1.5" diameter coils. This was the easy part, estimating the resistance is what I'm having trouble with.
An on-line properties table lists CU at 1.7x10^-7, and steel alloy at 5.94x10^-7. This would suggest that the steel might be 3.5 times higher in resistance. If 7 AWG (.1443" dia.) CU is .4982 ohms /1000', does it make sense that my .15 spring steel might be 1.72 ohms/1000'?
If so, a single set of three in wye would yield only .111 ohms /ph, or line resistance of .224 ohms, and even if it doubled under load it would still seem to be way low for our application. What am I missing, clearly something?
Thanks for any help.
~kitestrings