Why do wires rattle in EMT when some motors start?

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So in summary...
Vibrating conductors, with respect to an AC motor starting, are caused by the conductors being forced to 'become magnets not become magnets' 120 times per second. (Sometimes) Violently pulling themselves against the inner walls of the EMT, releasing, rinse repeat. And pulling puching against each other. This is amplified by a motor that starts slow due to reasons such as pulley size or voltage drop. It's amplified by marginally sized conductors. But it's CAUSED by slip.
 
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.
See it all the time on cardboard bailers. Especially the single phase ones. Two #6 in a 3/4” have plenty of room to rattle.
 
So in summary...
Vibrating conductors, with respect to an AC motor starting, are caused by the conductors being forced to 'become magnets not become magnets' 120 times per second. (Sometimes) Violently pulling themselves against the inner walls of the EMT, releasing, rinse repeat. And pulling puching against each other. This is amplified by a motor that starts slow due to reasons such as pulley size or voltage drop. It's amplified by marginally sized conductors. But it's CAUSED by slip.

It's not due the simple fact that the motor is drawing very high levels of current during the event? In my laser printer example we get one noise instead of the rattle because the high inrush cuurrent is gone in a cycle or two. I need to try to find my current probe and capture it with a scope.
 
Transient due to the high inrush current. Same as switching on a cold 1kw halogen lamp.
I've never seen wires vibrate turning on a lamp. Not doubting you, just never seen it.

But take inrush transients for example. Like a transformer buzzing when it's turned on. In a transformer it's not high current causing the initial vibration. It's the fact that the core's mag field is lagging the coil's. BZZZ. Very much the same and very much different.
 
DC is that, direct current. Current stays constant, unlike AC which rises and falls with the cycle.
Yes. And at the top of the waveform, say +120 volts, that's where you'd expect to find the peak current. Then waveforms cancel fields and no vibrations. It's when the current lags voltage. That's what demands the field generation.
 
It's not due the simple fact that the motor is drawing very high levels of current during the event? In my laser printer example we get one noise instead of the rattle because the high inrush cuurrent is gone in a cycle or two. I need to try to find my current probe and capture it with a scope.
High AC current has never caused a wire to become magnetic ever to a large degree as long as the conductor returning such current is in close proximity.
 
See it all the time on cardboard bailers. Especially the single phase ones. Two #6 in a 3/4” have plenty of room to rattle.
But only during start-up, right? You're not saying you have a bailing machine where the wires are rattling when it's running. Only starting.
 
I've never seen wires vibrate turning on a lamp. Not doubting you, just never seen it.

But take inrush transients for example. Like a transformer buzzing when it's turned on. In a transformer it's not high current causing the initial vibration. It's the fact that the core's mag field is lagging the coil's. BZZZ. Very much the same and very much different.

Transformer inrush is different than inrush with a restive load.

I don't have a 1kw halogen lamp but I do have a ton of 500w. I just took three of them, plugged them into a triple tap adapter, and when I plug that into the laser printer's receptacle I hear the wires in the pipe move. I unplugged and quickly replugged it and no noise. I waited 30s for the filaments to cool then plugged it in again and got the same noise. Its the inrush from the cold filaments. Again, its not buzzing or vibrating, its one bang. I think the situation allowing me to get noise with such comparatively low currents is the fact the wires are vertical in the pipe from a jbox up high to the receptacle above the floor so the wires are just sort of floating in the pipe. If there was any twist to them or a horizontal section of pipe I bet I would have no noise.
 
I've never seen wires vibrate turning on a lamp. Not doubting you, just never seen it.

But take inrush transients for example. Like a transformer buzzing when it's turned on. In a transformer it's not high current causing the initial vibration. It's the fact that the core's mag field is lagging the coil's. BZZZ. Very much the same and very much different.
The lamp has "inrush current" as well. Is typically for lesser time than a motor starting up though. The element changes resistance as it heats up and is not a true linear resistor
 
So in summary...
Vibrating conductors, with respect to an AC motor starting, are caused by the conductors being forced to 'become magnets not become magnets' 120 times per second. (Sometimes) Violently pulling themselves against the inner walls of the EMT, releasing, rinse repeat. And pulling puching against each other. This is amplified by a motor that starts slow due to reasons such as pulley size or voltage drop. It's amplified by marginally sized conductors. But it's CAUSED by slip.
The same rattling of the conductors occurs when you turn on a large number of incandescent lamps on a circuit. Pretty happens on any high inrush load.
 
Transformer inrush is different than inrush with a restive load.

I don't have a 1kw halogen lamp but I do have a ton of 500w. I just took three of them, plugged them into a triple tap adapter, and when I plug that into the laser printer's receptacle I hear the wires in the pipe move. I unplugged and quickly replugged it and no noise. I waited 30s for the filaments to cool then plugged it in again and got the same noise. Its the inrush from the cold filaments. Again, its not buzzing or vibrating, its one bang. I think the situation allowing me to get noise with such comparatively low currents is the fact the wires are vertical in the pipe from a jbox up high to the receptacle above the floor so the wires are just sort of floating in the pipe. If there was any twist to them or a horizontal section of pipe I bet I would have no noise.
Someone here can surely explain the mechanism you're describing. I can't. One bang from the conductors feeding a resistive load is certainly different than an inductive motor starting. I can't, off the top of my head, explain why a resistive load would not be capable of cancelling out it's own mag field.

I'm no electrical scholar. I'm just an electrician trying to figure s*** out, lol. Blame it on my Asperger's lol.
 
The same rattling of the conductors occurs when you turn on a large number of incandescent lamps on a circuit. Pretty happens on any high inrush load.

Hi Don! A familiar name after many years. Much respect.

But again, the mechanism that may or may not cause copper conductors to become temporarily magnetic in a resistive load is almost certainly different than a motor starting. And one I can't explain at this moment. It's not as common or severe as the compressor starting in the back of the shop that sounds like Bundy getting the juice every time it starts.
 
Someone here can surely explain the mechanism you're describing. I can't. One bang from the conductors feeding a resistive load is certainly different than an inductive motor starting. I can't, off the top of my head, explain why a resistive load would not be capable of cancelling out it's own mag field.

I'm no electrical scholar. I'm just an electrician trying to figure s*** out, lol. Blame it on my Asperger's lol.

there is still a field around each individual conductor The cancellation effect is surrounding the set containing all conductors of the circuit, clamp an ammeter around an individual conductor of the set and you get a reading, clamp around the entire set you get no reading or with some losses maybe a very low reading.
 
Isn't this the same magnetic effect that blows the contacts apart in a breaker under short-circuit conditions?
 
Wildly shooting a shotgun in the air, i'd suppose any magnetization of copper conductors in a circuit with rapidly rising current due to a resistive load could maybe be attributable to voltage drop, and again current form out of phase with voltage.

But in a motor start it's the opposite. Current is actually dropping as the motor reaches synchronicity (thank you spell check). Slip is dropping too.

Please, chime in all. I know there's a difference, I'm just not smart enough in theory to state it off the cuff.

And this is an awesome mental exercise for everyone in my opinion.
 
there is still a field around each individual conductor The cancellation effect is surrounding the set containing all conductors of the circuit, clamp an ammeter around an individual conductor of the set and you get a reading, clamp around the entire set you get no reading or with some losses maybe a very low reading.
Exactly. So clamp the entire set and you get no reading. No reading means no fields generated/broken down/regenerated. Same with a motor circuit after it's up to speed.
 
High AC current has never caused a wire to become magnetic ever to a large degree as long as the conductor returning such current is in close proximity.

Nope.
The other two wires aren’t returning the same current at the exact same moment.
If phase A is at positive peak the other two phases are at 1/2 negative peak

Triplex (or Qpx) these cables before they are pulled in you won’t have this problem.
 
Nope.
The other two wires aren’t returning the same current at the exact same moment.
If phase A is at positive peak the other two phases are at 1/2 negative peak
A single phase motor, once up to speed, presents waveforms offset by 180 degrees, with both current and voltage peaks being very close to the same. 3-phase they're 120 degrees, but my point is during motor slip, the current peak is offset from the voltage peak.
 
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