Supercomputers, fishing for information
paulkoning at comcast.net
Tue Nov 8 11:55:51 CST 2016
> On Nov 8, 2016, at 12:33 PM, Guy Sotomayor Jr <ggs at shiresoft.com> wrote:
>> A VFD is a good option and may be quite economical if you get one of the low cost simple ones. I have one (3 hp model for my lathe) that cost only a bit over $100, though the price has gone up since. (Westinghouse TECO brand.) VFDs specified for single phase input tend to stop around 3 hp, as far as I have seen. Rumor has it that higher power units will also work (possibly with some derating) even though they claim to be 3 phase input, when you feed them just one phase on 2 of the 3 wires. I haven't tried that (but it matches how my VFD is connected).
>> Given that you have a number of smaller devices and that not all might need to run, several smaller converters sounds like a good option, especially if that gets you into the "economy VFD" range.
> Yea, that’s what I’m struggling with. The issue is that the control units power the devices that are connected to them (from what I can tell), so I have to power the entire string as one unit. The same goes for the 3340’s - the entire string is powered as a unit. The string of 3340’s need ~5kVA (I don’t know how that translates to HP). I’m still trying to figure out the requirements for the other strings.
The classic formula is 1 hp = 750 W. But there's more to the picture. Motors draw a steady state current based on the power demanded from them, and the hp rating shows the max that they are designed for. But when starting they draw much more current.
http://www.vfds.com/blog/vfd-buying-guide has a pile of good information, including more about the use of single phase input. It's clearly an accepted practice, and yes, there's derating involved.
Given that your devices are partly motor loads, but certainly not entirely, the startup surge is likely to be modest by motor controller standards. So the key question would be the steady state current vs. what the controller is rated for.
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