Conductors rattle upon starting

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gregw

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I'm curious if anyone has an explanation on why conductors would rattle in the conduit during motor starting. There is a 300HP, 460V motor with I'm told a soft start (assuming it's a wye-delta), yet apparently the conductors rattle in the conduit when starting. I haven't been able to verify the installation yet, but am told there are 2 sets of feeders to the motor(conductor size unknown, but apparently greater than 380A or 420A otherwise they would have used 500kcm or 600kcm in a single feeder). NEC lists the 300HP motor at 361A / 460V, so is the only valid assumption that the conductors are undersized (even in light of the soft start) and the voltage drop due to undersized conductors causes the increased current and stress in the conductors? I appreciate your insight into this.
 
Conductors move due to magnetic interaction, which is dependent on current, not voltage drop.
 
Agreed, but the only way I can see the magnetic interaction occurring is by voltage drop creating the increased current, asthe soft-start is limiting in-rush current. Is there something more that needs to be considered for conductor sizing in large motors beyond 125% of the FLA?
 
Even with a soft starter there is still a step change in current from zero amps to many amps, and that will produce a step change in magnetic fields. Attraction and repulsion does the rest.

Is the conductor undersized? Don't know, but I would not have thought rattling was evidence to suggest they are, more evidence of inadequate support. Or, more prosaically, sometimes these things just happen...
 
Greg,

Have you read 430.22(C) , The feeder for this motor needs conductors

sized 125% of fla, conductors leaving the 'soft start' , and there will be six

of them, need to be sized at 58% of fla or 209.38amps.
 
gregw said:
I'm curious if anyone has an explanation on why conductors would rattle in the conduit during motor starting. There is a 300HP, 460V motor with I'm told a soft start (assuming it's a wye-delta), yet apparently the conductors rattle in the conduit when starting. I haven't been able to verify the installation yet, but am told there are 2 sets of feeders to the motor(conductor size unknown, but apparently greater than 380A or 420A otherwise they would have used 500kcm or 600kcm in a single feeder). NEC lists the 300HP motor at 361A / 460V, so is the only valid assumption that the conductors are undersized (even in light of the soft start) and the voltage drop due to undersized conductors causes the increased current and stress in the conductors? I appreciate your insight into this.
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magnetic contactors 'chatter' means that the contacts were not as good as they were before. we clean the contacts and do some re-tightening. contactors of motors that are on-off constantly are most prone to this.
 
Rattling conductors.... I've been down this road before! I was in a building doing a TI where everytime the elevator went up it would rattle the 200A 200' feeder. It souded like a frieght train! You could hear it all over the office building - all four floors, even worse in the basement where the feeder was. It became a big finger pointing party.... Elevator guys swapped out soft-start left and right, blaming the EC's that put it in, and the EC's blamed the soft-starts...

Eventually it came down to the oringinal engineer (who died before the project completed) who specified that the conductors be 'transposed'. So thats what eventually happened, they re-pulled it, and it was reduced significantly. You can get conductors special ordered and reeled this way, but real hard to find. I also talked to a guy who said he knew an easy way to do it in the field, but I havent tried it - but sounds as it it will work.

Ever notice that most cables are transposed? (Twisted clock-wise) I used to have the occassion to start 50HP motors on SOJ cord and they would flex like a snake. But in conduit they bounce loose all over the place. If they are twisted as they would be in a cable the magnetic fields that get them hopping acts on all the conductors together and pulls them closer and further apart with the current against eachother alternately tightening and lossening that twist. Also reduces induction to other conductors.

(FYI the twists in say cat-5 cabling also act in a different but simular way in canceling out and reducing induction to other pairs, acting as shielding.)
 
As others have said, a rapid large change in current produces forces F=IL X B. These forces oppose each other and push each other.

I recall doing experimental fault testing where individual conductors where run to the location of the fault, and you could see them separate as they slid apart from each other along the floor.
 
plate said:
I recall doing experimental fault testing where individual conductors where run to the location of the fault, and you could see them separate as they slid apart from each other along the floor.
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Sounds dangerous?
 
this one way you can see the conductor jump around is good example is jump starting large diesel engine you can actally see the jumper cable move around during cranking cycle [ i did see it pretty often ]


the same with larger electric motors or loads do that too.

Merci , Marc
 
Even with a soft starter, the amount of current can be significant enough to make conductors rattle for sure. Think about it; if the motor FLA is 361A, normal stating current (X-Line) will be 6 x 361 = 2166A. Even if it is a Y-delta starter, the starting current is still 33% of normal starting, so 2166 x .333 = 721A; no small potatoes. Then if it is Y-Delta, once it transitions into Run (delta), the current surges just as high as if it were X-Line anyway. if it is a soild state soft starter, the current may be limited to 350% lets say, that is still 361 x 3.5 = 1263A; again, a lot of magnetic force to deal with.

A common practice that helps to avoid this to triplex the conductors prior to pulling them into the conduit. Very few people do that however because it makes them more difficult to pull.
 
Jraef said:
A common practice that helps to avoid this to triplex the conductors prior to pulling them into the conduit. Very few people do that however because it makes them more difficult to pull.
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Though 90's no real problem - sometimes easier. Through an LB barely possible, use large pull boxes.
 
Jraef said:
Even with a soft starter, the amount of current can be significant enough to make conductors rattle for sure. Think about it; if the motor FLA is 361A, normal stating current (X-Line) will be 6 x 361 = 2166A. Even if it is a Y-delta starter, the starting current is still 33% of normal starting, so 2166 x .333 = 721A; no small potatoes. Then if it is Y-Delta, once it transitions into Run (delta), the current surges just as high as if it were X-Line anyway. if it is a soild state soft starter, the current may be limited to 350% lets say, that is still 361 x 3.5 = 1263A; again, a lot of magnetic force to deal with.

A common practice that helps to avoid this to triplex the conductors prior to pulling them into the conduit. Very few people do that however because it makes them more difficult to pull.
Click to expand...



This has been an interesting thread. This "triplexing" in theory sounds like it might help some. Is there any proof as to whether actually does help, and if so how much does it reduce the magnetic forces?
 
It doesn't really reduce the magnetic forces, just contains them so the cables don't rattle. It's very common in the elevator industry where long vertical runs are common, motor starting is frequent and noises are frowned upon.
 
I appreciate everyone's input. It's a good point that the reduced voltage starters still have a significant in rush in larger loads, and the triplexing or transposing the conductors before pulling intuitively makes since based on the magnetic conductor reactions. Thanks again.
 
gregw,

I still think that ALL the conductors from the 'soft-start' have to be in the

same pipe, You did say that two raceways were used?? Did they put 6

conductors in each or 3 in each, I'de bet on 3, the reason ??, is that there

are 300HP motors all over the place, and when they start the conductors

don't repell so hard that they slap the raceway. JMO.
 
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