Quote:
Originally Posted by thingstodo
The in-rush is lower when you do soft-start. But you need to over-size the motor since the torque delivered (compressors are under some load, although not 100% load, when they start) is 30 - 45% of rated torque on reduced-voltage start. I have never seen a soft-start that claims to limit current to nameplate values. Do you have a link?
We used VFDs to start and switch to across-the-line afterward, where the variable speed is not required.
500 HP is as high as we go with 600V equipment. At 460V, you would need to drop to maybe 400 HP. The currents get too high and it's too hard to cool the equipment with air only. There are water-cooled industrial VFDs but they are quite expensive. When looking at the whole system cost, they might be worth it though.
You'd likely have to bump up to a 2000V motor for the 500 - 1000 HP range to make the VFD reasonable to cool. But there are other issues with the 2000V VFDs. Nothing that money can't solve, but still kinda pricey. The VFDs don't use silicon any more ... or they chain different stages of silicon switching together. More hardware, more complexity, more cost.
Using VFDs gets you into harmonic issues though. Then you need to add harmonic filters at your 2KV or 15KV distribution level. And they are kinda pricey as well.
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I'm not a double-E,and the information was from my college text on electrical engineering and electronics.We never had a lab for that class,so it's all theoretical to me.
All the texts had similar equations,basically showing power triangles,with Pythagorean-like trig. functions for the resistive,inducive,and reactive legs, and knowing the running power for a load,one could solve for a power factor of unity,and say,apply the proper capacitance to achieve unity.
My oldest brother worked around electronics with Los Angeles County for 30-years.He had to do this sort of thing.
We were talking about the 'Green New Deal' over the phone,and while he's not necessarily for a command and control government,he does think load -shedding efficiencies are a great thing.He specifically mentioned zero-penalty soft-start,as did my college and US Navy electronics text.
When I was doing air-ballancing and hydronics,many clients had gone to SIEMENS,solid-state-electronic controlled variable-speed air handlers.My brother says there's far better technology out there now,compared to what I'd seen.
200-ton chillers all had variable-pitch inlet vanes,which allowed zero-load starts.
Cooling tower fans and pumps had all been optimized for a power factor of 1.
Soft-start pump technology was coming in.(this was 1986)
Scroll-type compressors,I understand,are soft-start,and even in domestic HVAC systems,offer variable-speed compressors for heat pump/AC,as well as soft-start air handlers.
I don't think I've ever worked around anything over 440-VAC ,3-phase.
500-hp.that's impressive!