Quote:
Originally Posted by aerohead
There are 'soft-start' ways to go,where that fractional power-factor penalty inrush can be dispensed with,and the motor will never pull more than it's nameplate current.
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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.