Why do wires rattle in EMT when some motors start?

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sdbob

Senior Member
I've searched for the answer to this question for years on and off. What is the mechanism that causes conductors to rattle when a motor starts? I've seen many people guess. Some come close. I know there's some bright engineers here. So come on, let's lay out the explanation here because it doesn't exist elsewhere on the internet.

Things it's not:

High current
Undersized conductors
'Unsupported' conductors. (Saw that answer somewhere)
Bad motor
Loose conduit fittings or splices
Chattering starter contacts
F = I ( L x B )

We should all know why they rattle in a ground fault under the right conditions... but why when a motor starts?

Just trying to stimulate grey matter here... Thank you for your thoughts, they all contribute to more knowledgeable tradespeople worldwide. I haven't posted in years, so forgive the sudden intrusion.

Bob in San Diego
 
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sdbob

Senior Member
Assume a 1-phase motor. Place 20 millions amps on A phase and the 20 million on B phase cancel the fields for the most part. Not completely, but certainly enough to mitigate rattling.
 
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sdbob

Senior Member
In a ground fault we know that fault current follows multiple paths back to source, contributingto, or frankly causing the imbalance / magnetic field generation / rattle.
 
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sdbob

Senior Member
LarryFine said:
Actually, that's what it is. The current creates magnetic fields around the wires, and they interact and move the wires.
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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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sdbob

Senior Member
K8MHZ said:
I would hazard to guess that the reactive load does not constitute a suitable environment for a perfectly canceling magnetic field.

Just a guess, though.
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Smoking hot. But almost all motors are a reactive load all the time.

I actually thought this would take hours. We're minutes away.
The Mike holt brain trust never fails to impress.
 
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sdbob

Senior Member
Besoeker3 said:
How high?
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I don't know. 3 amps is high relative to 0.00003. Has any electrician actually looked in a gutter and seen wires vibrating wildly because of current? (besides when a motor is starting) No. Negative. Doesn't happen ever.
 
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sdbob

Senior Member
I've seen that video. This is a result of ground fault current returning through a path other than immediately adjacent to the conductors.
 
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Todd0x1

Senior Member
Location
CA
We've all seen the videos showing cables in a cable tray repulsing from each other during a fault event, I think its like that but on a much smaller scale.

I have a laser printer next to my desk. It is plugged into a receptacle with surface mounted EMT going straight up 15 feet. When I print something as soon as the fuser turns on to heat (high inrush load) I can hear the wires move in that pipe.
 
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sdbob

Senior Member
Go back to the reactive load theory. Draw a pair of sinewaves representing the voltage values of a motor circuit across time. Draw another representing the current values over time. Visualize the current sinewave as a string and pull on one end.
 
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sdbob

Senior Member
Todd0x1 said:
We've all seen the videos showing cables in a cable tray repulsing from each other during a fault event, I think its like that but on a much smaller scale.

I have a laser printer next to my desk. It is plugged into a receptacle with surface mounted EMT going straight up 15 feet. When I print something as soon as the fuser turns on to heat (high inrush load) I can hear the wires move in that pipe.
Click to expand...
The laser printer phenomenon is separate.
 
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