Why do wires rattle in EMT when some motors start?

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I see this all the time in older elevator setups where the feeders jump when the motor kicks on. Just ran across one the other day that kept making one of my apprentices jump as well.
 
Because electrons slam into each other on the outer ring to cause the flow of electricity. So there is always vibration, it's only noticed during high current draws or ground faults. Other than that who cares? Just do your job correctly and leave those little guys alone in the conduit, what happens between electrons stays between electrons. ;-)
 
DH Electric said:
Because electrons slam into each other on the outer ring to cause the flow of electricity. So there is always vibration, it's only noticed during high current draws or ground faults. Other than that who cares? Just do your job correctly and leave those little guys alone in the conduit, what happens between electrons stays between electrons. ;-)
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When they all get together and push in the same direction, you do have to brace the wiring sometimes.
You just don't need to question their motives. (Like why they push in the same direction even when the current reverses.)
:)
 
sdbob said:
Negative. If you have current flowing out on one conductor and back on a parallel conductor the net result is Zero. In a perfect world EXACTLY zero, every time. Try it. Clamp your ammeter around a piece of romex and you know the result. If the net current is zero the magnetic field is zero. If the field is zero the copper wires do not become magnetic therefore cannot attract or repel each other or the conduit.
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Wrong. The net flux enclosed by the loop may be zero but this doesn’t mean there aren’t magnetic fields present within the loop causing the conductors to exert forces and repel. The magnetic fields are not uniform and not perfectly canceling. The conductors would have to be occupying the same space and hence have zero current for perfect cancellation.
 
One thing that wasn’t mentioned was the momentary DC offset and asymmetrical current during motor starting which not only produces larger currents and forces, but an exponentially decaying DC repulsion (in addition to the 120-Hz pulsations from the AC oscillations). This in connection with non-uniform forces acting along the length of the conductors due to the circuit geometry; i.e. random spacing and lay of the conductors and the reaction from the magnetic raceway (which also produces repulsive forces back on the conductors) as well as other restoring forces like gravity and tension, in addition to the movement of said conductors are all part of the dynamic system to produce noticeable vibrations.
 
I have BROKEN 250 MCM due to high current. (4 megamp)
Have often seen wire 'rattle' (seen = visually observed) during motor start (wires laying in tray, obviously wires in conduit not visible)

The magnetic forces are the cause

Also, put a high current thru a piece of copper tubing and you can turn it into a solid bar, collapses in on itself, eh?
 
junkhound said:
I have BROKEN 250 MCM due to high current. (4 megamp)
Have often seen wire 'rattle' (seen = visually observed) during motor start (wires laying in tray, obviously wires in conduit not visible)

The magnetic forces are the cause

Also, put a high current thru a piece of copper tubing and you can turn it into a solid bar, collapses in on itself, eh?
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Magnetic pressure, indeed. Also used for electroforming as an alternative to explosive forming.
 
sdbob said:
Ok, so how much current is needed to make wires rattle? Some of us have worked on some pretty high amp current circuits. None of us have ever seen wires rattle. This is the most common incorrect answer.
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Why do we need to add additional conductor support in some higher current equipment?

Why do they have beefier bus bar supports when there is higher available current?
 
sdbob said:
Ok, so how much current is needed to make wires rattle? Some of us have worked on some pretty high amp current circuits. None of us have ever seen wires rattle. This is the most common incorrect answer.
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None of us? I most certainly have. I did a 500HP gang saw once and the wires in the conduit made a HORRIBLE sound whenever it started.
I also laid out the conductors for a 125HP pump on the ground for testing the pump, the wires jumps away from each other when we started it (I already knew what was going to happen, the pump guys didn't, it was fun to see them jump!).
 
sdbob said:
Ok, so how much current is needed to make wires rattle? Some of us have worked on some pretty high amp current circuits. None of us have ever seen wires rattle. This is the most common incorrect answer.
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Agree with jraef above.
Ive closed in a substation recloser on a bolted fault down line and the overhead wires on the circuit danced quite wildly.
It’s been proven many times high current makes wires dance and move wildly.
 
Hv&Lv said:
Agree with jraef above.
Ive closed in a substation recloser on a bolted fault down line and the overhead wires on the circuit danced quite wildly.
It’s been proven many times high current makes wires dance and move wildly.
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Conductors of a true DC circuit may not rattle because there is no alternations in the current but can still move. I seen it happen on my old Farmall H tractor with the conductor to the starter motor when the starter had mechanical issues and therefore was pulling higher than normal current when attempting to start the tractor.
 
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