I---At what point do you decide the margin isn't enough? ---
I did the preliminary analysis on two prospective installations were I recommended not to do it - not enough payback, and these were both 24/7 process applications. Surprised me, i though I could make VFDs payoff.
1988, 4000V,900hp, 885rpm, through a hydraulic variable speed reducer to 720 - 780rpm, driving an ID fan. Best AB (Rockwell) could offer was a stepdown transformer to paralleled 480V drives, stepup xfm to 4000V. By the time we went through the two xfm and the drives, the efficiency was not a lot better than the hydraulic speed reducer. The existing control system was a hand wheel on the side of the speed reducer adjusted by the operating engineer (and that is what they wanted) - so there was not much to be gained by having a 4-20ma input. Electricity at this plant was cheap - they needed 150psi and 25psi process steam and they got it by running the 800psi steam through extraction turbines.
1998, 4000V, 3000hp, centrifigul pump, process required about a 10:1 turndown. We might have been able to do it, multiple pumps, VFD and control valves, but I couldn't convince management to give $1000K to try it out - one of them said show us your figures that shows the pay back time, we will want three years or less - go figure.
cf