I'm going to jump in on this thread, from an on going personal experience. I have two 1000 ton Trane Centra-Vac's. (lead lag/weekly rotation) My building just completed it's 8th year of operation. My chilled pumps have always been on VFD's and my Chillers have VFD's. For the life of me, I didn't understand why my tower pumps were soft start using auto-transformers tapped at 80% then to 100% about 10 to 15 seconds later. So here we go, 125 hp vertical pump, slamming into a 40% open triple duty (instant head). Pumps were within temp specs but extremely warm to the touch and running full load of (going from memory) about 139 amps. This was all done at start up to maintain Chiller Condenser Delta P. (Chiller VFD's and Vanes taking care of Load)
My background is residential. I only have 8 years in building plant management. Back to my pumps......
We installed VFD's on our tower pumps. Working with my programmer, we installed pressure differential transmitters on the chiller condensers. (no more changing failed paddle flow switches for me) Opened the triple duty's to 100% open and now controlling the pumps to maintain condenser delta P. (demand option referenced of course)
This is the result. My pumps are running somewhere around 68 amps. My pumps are cool to the touch. We're saving in the neighborhood of $24,000.00 a years in operating cost.
This again is my experience in one building. We have added VFD's to supply and relief blowers and have seen a 12 to 14% reduction in operating cost even running at 100%. Our resident master electrician was so baffled, he contacted the VFD mfg and talked to their engineering department. Their explanation seems quite simple. The difference is going from utility to "conditioned" power. I can testify that what I've been told in VFD seminars has become reality. If you can reduce your motor to 80% with a VFD, you reduce your operating cost by 50%. I'm not an engineer and I can't explain all the why's and what for's but I can tell you that it works. The proof is in the electric bills.
I'm a firm believer in VFD's. Installing them are a plus in my book even if you run at 100%. Work with your programmer, work with the engineers and I'm confident that you won't regret installing the VFD's. It opens the doors to a lot more opportunity to meet demand and save energy/money at the same time.
Just my humble opinion.