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Joined: May 2003
Posts: 2,876
e57 Offline OP
Have a customer that has an elevator that trips the CB on occassion. Tenants hear the conduits rattle when it starts intermitantly. I've been doing a TI in the building and have heard it too. (Very distinctive like a short)

I haven't looked at it yet. I have a feeling that it may be a voltage drop related issue, as many feeders in the core of the building are long and undersized. Might look into it tomorrow, check amperage, and voltage under load. Does anyone think that Voltage Drop could raise the in-rush current enough to rattle like that? Like a short?

Might have to put a recorder on it for a while, I'll let you know what I find out.

Mark Heller
"Well - I oughta....." -Jackie Gleason
Joined: Dec 2003
Posts: 751
Especially with motors.

Joined: Sep 2003
Posts: 650
Voltage drop will _reduce_ the initial inrush current to the motor, not increase it. The inrush current is roughly proportional to the applied voltage.

However the available torque is proportional to the square of the applied voltage, so voltage drop can greatly increase the acceleration time of the load, and increase the 'I square T' that the conductors and breaker need to deal with.

Additionally, once the motor is in the low slip operating region, reduced supply voltage will generally mean increased current flow, at least for AC motors.


Joined: Jul 2004
Posts: 9,954
Likes: 34
I think I would investigate the "rattling conduits" first. You may simply have a short to ground in one of those conduits.

Greg Fretwell
Joined: Jan 2003
Posts: 4,391
I would follow Greg's suggestion test the insulation with a mega meter.

As an example of what Jon was saying a chop saw was starting up fine on an extension cord but when plugged directly into the outlet it would trip the breaker.


Bob Badger
Construction & Maintenance Electrician
Joined: May 2003
Posts: 2,876
e57 Offline OP
Only took a breif look today...

The company that ran the 2 1/2" conduit 195' w/ 3 - 4/0 was there yesterday. (Big Union outfit) (Voltage drop got ruled out. As it was minimal.) Owner wants every opinion he can get, before they dump the money for the third starter. Which is why they asked me to stop by.... Cause the original installer, and Elevator rep had it out, I'll say why in a moment, anyway I'm going to limit myself between them two.

The facts as they stand now... As I know.
Soft start (1/2 and 1/2) 45 HP 6 pole motor on 208. In-rush is really high, really long, getting progresively worse, went OL on my clamp-on today and operating current seems to vary pole to pole. (600A Was just what I had with me today.) Running amperage also varies, and pole to pole. 160-220A

Now here's where opinions differ, and get down right stupid, between installer and Elevator rep yesterday.

Electricians opinon: (I at this point partialy agree without 3-phase monitoring) Is that the 6 pole motor is mis-wired, or motor is failing on one or more poles. As I have smelled the distinctive stench of burt motor in the elevator before, and brought this to the owners attention, I can agree. Especially after getting such odd readings today, however minimal. The other Electrician blames Elevator.

Elevator guys opinion: which I heard from the other electrician, and owner today. Lack of "triplexing" feed conductors. OK good point, but no-one does it, or should I say almost no-one. Doubt it would be the cause of this, or to this degree. His suggestion, replace the feed. Or this, which sent the Electrician into fits of rage, and myself into fits of laughter, was to "break the conduit open at the couplings and fill it with expanding foam". I almost keeled over when the owner told me, and later talked to the electricain who was still enraged from yesterday.

My suggestion: Check motor wiring for crossed poles, and power monitoring to get clearer picture of what is actually happening, The megger on the feed it's a bad idea either, but hard to know if any degradation of insulation is from install or current, as this thing is about to give up the ghost soon. Let met re-define "rattle"... This feed in the basement can be heard up to the second floor. It's an un-godly sound that says emminant failure. Some more information is that a UPS is failing, lighting, and ballasts are dieing everywhere which makes me thing harmonics too.

I'll keep you all posted, on what happens, but I'm gonna step back and let these two battle it out some more.

Mark Heller
"Well - I oughta....." -Jackie Gleason
Joined: Oct 2004
Posts: 72

Joined: May 2003
Posts: 2,876
e57 Offline OP
Anyway it a (Correction) 250kMCM feed, they megged it, it passed.

Myself and the other Electrician both feel that it is a mis-wire of the 6 pole motor. Crossed phase between two windings, making higher amperage on one leg, and partial short on the other two. But we can't find out, as the elevator rep, wont let us touch it to trace out the leads of the motor. Well within his right too, it is his equipment, but he feels its the incoming feed, that is messing up his starters. And the owner, is hesitant to fork over the dough to monitor it right yet, as he has no idea what to think at this point, and is wieghing his options. It's still up in the air....

On Thursday, I had a FLS inspection and recalled the elevators. After the inspection the elevator was "shut off", We thought is wasn't resetting from our smoke and fire door test, but our panel was reset showing no alarms or trouble. The inspection was done and he left. But after wards, the elevator didn't seem to reset.... After ten minutes of running around seeing if it was our system, the security guard came and asked if we had pulled the switch for it, she found the door open. (None of us have a key to the room....) We went down with her show she could show us which switch she meant, it was the disco for it, with no lock-out, but shut off. I wasn't going to turn it back on, as I didn't turn it off. I don't break lock outs on anything especially elevators, although it didn't have a tag. It turns out one of the tenents has a master key that opens that door, and was unaware we were testing that day (We didn't know she was in the building at 6AM...), heard the horns and strobes go off, combined with the "Second coming sound" of the elevator, and thinking she was "Doing the right thing" shut it off. I think they need to take her master key away, but don't blame her as the feed for the elvator runs through her basemment office, and sounds like a freight train on every start of the motor. Which she fully expects to explode someday.

Anyway, like I said, I am stepping back from it, and let everyone else play thier game. I have 8000'+ of #12 to pull in my space and land to finish next week, my hands are full!

If I hear anything else I'll let you know.....

Mark Heller
"Well - I oughta....." -Jackie Gleason

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