I don't have a lot of experience with power systems either (though a couple of recent projects have been quite an education by fire), but I'm thinking that given the numbers, it's possible there's some mechanical problem. If the big motor starts up under an abnormally heavy load it may require a surge capability up to many times its rated continuous load. Any chance there could be something mechanically binding, such that it comes up stalled? You may need to look at both motor and generator for not only their continuous ratings, but also compare the motor's startup current with the generator's surge rating. I'd tend to want a surge rating on the generator with a generous margin, because the mfrs tend to be a bit "optimistic" in such ratings. If the load has a large inertia, the generator is going to need a correspondingly large inertia (flywheel). A bigger generator may indeed be the answer - not for the continuous rating but for the startup surge. But first I'd check to be sure there isn't something mechanically wrong with the load, or maybe even electrically wrong with the controller.
As to the voltage conversion, be careful how you're measuring things. I may have this backwards, but I believe 208V normally refers to a delta connection, whereas 240V refers to a wye connection. Picture the voltages of each phase as an equilateral triangle centered at and spinning about the origin. The sides of the triangle are 208V long, and each of the corners is 120V from the origin, which represents the neutral. Thus if you measure one phase of a delta connection (where you're measuring between two of the phase wires) you see 208V, vs. measuring from a phase to the neutral where you see 120V. I lost a factor of two somewhere in the visualization, but the basic concept is correct. Is it possible that you've somehow crossed up the motor connections such that it's expecting a delta connection but you've supplied a wye connection? That too would account for a very heavy current. You might want to try one posting over at the regular PEDA forum, asking for replies to come over here. I thought I was signed up here but never got any posts till I recently resubscribed; there may be others in the same boat. Steve Hendrix In a message dated 2005-12-16 07:17:14 PM Eastern Standard Time, [EMAIL PROTECTED] writes: > We've found ourselves in somewhat of a bind. We've got a PCB router in > house > which is powered from 3-phase 208V. Its a very, very long story, but its > going to be 10wks till our utility supplies 3-phase power. Currently we have > 240V single phase power, so I've looked into a phase converter but we still > need to bring the voltage down to 208V. The other option we've been trying > is to use a generator. Turns out that when the 5hp vacuum motor fires up, > the generator doesn't respond quickly enough to counteract the effect, so we > suck the voltage (and frequency) through the floor. This of course causes > the motor controllers to complain. We've got a 20kW generator already (for a > ~4kW load), so I don't think a bigger generator will fix the problem. Is > there another way around this with a generator? (Ie, could we try to run the > controllers from a UPS or will we kill the UPS?) If we were to run a 3-phase > motor load continuously while we run the router, would it tend to buffer the > effect? Power systems is well out of my league, so any comments you have > would be welcome. > > Thanks, > Darcy Davis > Design Engineer > ____________________________________________________________ You are subscribed to the OT discussion forum To Post messages: mailto:[email protected] Unsubscribe and Other Options: http://techservinc.com/mailman/listinfo/ot_techservinc.com Browse or Search Old Archives (2001-2004): http://www.mail-archive.com/[email protected] Browse or Search Current Archives (2004-Current): http://www.mail-archive.com/[email protected]
