Had this in the less frequented therory area, figured it might get more attention here.
---------------------------------------- I am about to have to re-install a feeder (By someone else) for an elevator with a haunting rattle. In-rush current rattles the conduit so badly the it can be heard all over the building. (Not that I think this will help much) It has been suggested that Triplexed conductors (Twisted) would help reduce this noise. Although I am unsure of to what degree it would help, I was trying to explain the benifits of it to our PM today. I was wondering if anyone could confirm anything that should be should be listed, ammended or removed from this list.
Reduction of Eddy Currents (Minor) Reduction of parrelel coupling through induction Conductors "pull together" in in-rush situation as opposed to "rattle" (Mocking sine wave)
I learned about triplexing conductors some time ago, and can not find any good referances about it anywhere.
Thanks in advance, Mark
Mark Heller "Well - I oughta....." -Jackie Gleason
If magnetic induction from the amount of current is causing the rattle, larger conductors might stiffen it, might even slow the accelleration (f=ma) enough to reduce the rattle but the magnetic field would remain unchanged. So it would probably still rattle pretty badly.
I wonder if filling the conduit with expanding foam or epoxy might eliminate the rattle...
[This message has been edited by SteveFehr (edited 04-02-2005).]
Feed is about 175' as the crow flies, but I got down there to map out a path and turns out to be 230+' of 250MCM. And has some exessive voltage drop, down from 208 to 180V at start.
Anyway, this thing has been battled out a few different ways. Elevator guy says its the feed, the electrician who installed the feed says its the elevator. And owner of it is confused and going for what makes sense to him.
Elevator guy suggested the foam filled conduit idea, the same as SteveFehr. But since this feeder heats up in frequent use, and it being a bad idea anyway, I laughed hyserically when I heard it. I should then derate the run for fill at 100%? He also suggested the triplexing idea...
Then theres me: I don't think Upping the size of the conductors to 350MCM is going to do anything except a slightly lower pitched rattle. 350 and triplexed I think will help. But the high in-rush will still be there. Many other people have had various thoughts on it, and range from locked up hydro pump, pump valves opening or closing during improperly timed during in-rush below speed, reversed windings, bad starter, voltage drop, triplexing, etc. no one whats to stick a 43B with 1000A, or 3000A CT's on the main to learn any more about the problem for a few hundred bucks, but would rather spend a few thousand on changing out the wire. And our PM bit on this agaist my judgement on staying away from it. She's pricing it for a regular 350 run in exiting conduit (Probhably too low as usual), and they'll bite. So guess who gets to pull it, me and my crew.
Now on a different note: I have some interesting expeirances with large motor starting. While in the military, we had RO machines with 100HP pumps, which ran rather well on a 60KW gen set, but start-up would rock the gen set pretty harshly. Most of the time we would paralell up 2 60's for start, and use 1 to run. The cable we used for it was 250/4. (FYI most cables are twisted/triplexed.) Which would kind of jolt a bit at start, noticable. We ran out of that once, and used 4/0-4, and it would flex like a snake very noticably for the first few seconds of start-up, kinda wriggle a bit slowly. Ran out of both on occassion, and used THW individual conductors, no conduit as it was a temp feild operation in Somalia, the conductors jumped all over the place. So I figure triplex will help, to what degree, I don't know. As this elevator feeder rattle is like nothing I have ever heard. Imagine a 20A on 12's short circuit in 1/2", 1000%.
Mark Heller "Well - I oughta....." -Jackie Gleason
Re: Triplexed feeders (re-post)#50422 04/03/0507:26 AM04/03/0507:26 AM
e57...according to table 430.250 (2005NEC) a 75HP motor at 208volts has a full load current value of 211amps. So, 211amps x 125%=264 amps for the minumum ampacity of conductors for the feeder to this motor.Table 310.16 places a maximum allowable ampacity of 250mcm at 255amps from the 75 degree column.
So, the original feeder used is undersized according to my math. Figure in the distance of the feeder, the temperature of the area the feeder passes through, and the inrush amperage of a motor that size @ 208 volts and I'm suprised that those feeder conductors don't jump right out of the conduit with the magnetic field induced upon start up.
Now I don't have any experience installation such as this, but a motor this size at that voltage seems a poor design to begin with?
I agree with you that a triplex wire installation will reduce the amount of wire slap the feeder is producing because of the tight wrap of the conductors. But the heat dissipation that you would have with a single condutor installation would be better. Also the 350mcm replacement feeder should help, 500mcm would be better.
1) What size conduit is installed? 2) What is the temp. in the area of the equipment and couduit run? 3) If it was even allowed, how would you fill a 230ft run of conduit with foam
Re: Triplexed feeders (re-post)#50423 04/03/0510:12 AM04/03/0510:12 AM
The derating tables are not so much for insulative effect, as for heating of adjacent wires- if a particular wire is bounded on all sides radiating heat at 60C, then it's going to get hot, just from residual heat from its neighbors even if it has no current. This is not the case in foam fill; you won't have to derate 100%. You'd have to find the thermal conductivity of the foam and calculate your own derating table. Or, rather, let your PE run the numbers and stamp it
With what you're dealing with, it might just end up to be cheaper that way.
[This message has been edited by SteveFehr (edited 04-03-2005).]