Why do wires rattle in EMT when some motors start?

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sdbob said:
Not to parse words, but it's the change in current that is causing the magnetic field. My assertion is that in a circuit that's trying to bring a motor from 0 RPM to x RPM, current lags voltage making it more difficult to 'net zero', or cancel out magnetic fields.
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NO!!! 100% NO.
It is the change in current that generates an electric field and voltage differences. A DC current (ignoring the duplicative redundancy there) certainly generates a magnetic field. Look at any DC solenoid or electromagnet.
And two constant magnetic fields can exert forces. Look at the interaction of two bar permanent magnets.

If you do not have an intuitive feel for the forces involved, you need to either do some simple experiments OR correctly apply the force equations in detail.
 
210706-1959 EDT

sdbob:

You have not been educated in electrical and magnetic circuit, and field theory. There are way too many posts presented on this thread to answer a simple question.

If you take two magnets and point their north poles toward each other, then what force and direction do you feel as as you bring the magnets closer together? Do the same test with south poles.

Next mate a north with a south and what happens?

Consider a single isolated wire and pass a DC current thru the wire. What does the magnetic field around that wire look like? There are circular magnetic fields about the wire perpendicular to the flow of current. The direction of the field is a function of the direction of the current in the conductor.

Put two conductors close together, parallel to each other, and have these perpendicular to a sheet of paper. One wire will have current I flowing into the paper, and the other I flowing out of the paper. Put the wires on a horizontal axis. Draw a circle around each wire. Above each wire put an arrow on each circle. These arrows indicate the direction of the magnetic flux. These arrows either butt heads or tails. In either case the force on the wires is to push the wires apart. The greater the current the greater is the repulsing force between the wires.

If the current is DC, then there is a continuous force. If current is AC, then the force is not constant, and varies from zero to a maximum every half cycle. In a 60 Hz system this means the forces are modulated at 120 Hz.

Make a hairpin loop of wire with the two parallel wires very close together. Apply a high current to the loop. What happens?

.
 
GoldDigger said:
NO!!! 100% NO.
It is the change in current that generates an electric field and voltage differences. A DC current (ignoring the duplicative redundancy there) certainly generates a magnetic field. Look at any DC solenoid or electromagnet.
And two constant magnetic fields can exert forces. Look at the interaction of two bar permanent magnets.
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Duh, you're right. Got that wrong. It's the change in current that generates the magnetic fields in an AC circuit. The ones that make wire vibrate.
 
gar said:
210706-1959 EDT

sdbob:

You have not been educated in electrical and magnetic circuit, and field theory. There are way too many posts presented on this thread to answer a simple question.

.
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I haven't been educated in electrical and magnetic circuit, and field theory. Then you go on to explain that like charges repel opposites attract like the tenured professor you are. What an assumptive, arrogant, condescending post.

That's why I haven't posted anything in years.

Enjoy yourselfs.
 
sdbob said:
I haven't been educated in electrical and magnetic circuit, and field theory. Then you go on to explain that like charges repel opposites attract like the tenured professor you are. What an assumptive, arrogant, condescending post.

That's why I haven't posted anything in years.

Enjoy yourselfs.
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Don't let Gar's post bug you. He is very well educated in the physics of the situation, but needs the Dale Carnegie training to go along with it. :) (self referential irony intended)

You brought up an interesting discussion, which is what this forum is for.

Jon
 
sdbob said:
I haven't been educated in electrical and magnetic circuit, and field theory. Then you go on to explain that like charges repel opposites attract like the tenured professor you are. What an assumptive, arrogant, condescending post.

That's why I haven't posted anything in years.

Enjoy yourselfs.
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Don’t take this the wrong way, but some of yours can be seen as arrogant also..

as stated above, it was an interesting question.
Leave or stay, it has “stimulated grey matter”
 
210707-1034 EDT

sdbob:

I am not now a professor nor have I ever been one.

I have never before run an experiment to verify that two wires next to each other carrying the same current. but in opposite directions, would push apart. I accepted that they would because it was obvious.

So for you I setup an experiment on my basement floor for experimental proof of what happens. This was with a piece of #16 Belden hookup wire bent into a loop from 25 ft of wire, loop12.5 ft long. The loop was made with initial wire spacing probably less than 3/8" apart. This loop was powered from a transformer about 16 V open circuit. Applied power to the transformer and this produced a small separation of the wires, an inch or two. Proof that the current forced the wires apart. Higher currents will produce greater forces.

.
 
gar said:
210707-1034 EDT

sdbob:

I am not now a professor nor have I ever been one.

I have never before run an experiment to verify that two wires next to each other carrying the same current. but in opposite directions, would push apart. I accepted that they would because it was obvious.

So for you I setup an experiment on my basement floor for experimental proof of what happens. This was with a piece of #16 Belden hookup wire bent into a loop from 25 ft of wire, loop12.5 ft long. The loop was made with initial wire spacing probably less than 3/8" apart. This loop was powered from a transformer about 16 V open circuit. Applied power to the transformer and this produced a small separation of the wires, an inch or two. Proof that the current forced the wires apart. Higher currents will produce greater forces.

.
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The Bussman factory in St. Louis had a rest rig where they laid two conductors side by side. They would first protect the wires with a circuit breaker.

Then, they would simulate a fault to trip the breaker. I don’t recall any of the parameters, but the wires were big and I believe the breaker was 100A (or more). The wires would violently jump apart-a couple feet or more iirc.

Then they would repeat the test using their current-limiting fuses in place of the breaker. The movement of the wires was barely discernible.

The idea was demonstrate to potential customers how much better fuse protection was than circuit breakers.

This was early 1980s, before fault current calculations became a regular occurrence in most industrial plant settings.

To me, this is the same as your experiment, only on a much larger scale!

ETA: if the wires were in EMT, you certainly would have heard them make some noise in the first test!
 
retirede said:
The Bussman factory in St. Louis had a rest rig where they laid two conductors side by side. They would first protect the wires with a circuit breaker.

Then, they would simulate a fault to trip the breaker. I don’t recall any of the parameters, but the wires were big and I believe the breaker was 100A (or more). The wires would violently jump apart-a couple feet or more iirc.

Then they would repeat the test using their current-limiting fuses in place of the breaker. The movement of the wires was barely discernible.

The idea was demonstrate to potential customers how much better fuse protection was than circuit breakers.

This was early 1980s, before fault current calculations became a regular occurrence in most industrial plant settings.

To me, this is the same as your experiment, only on a much larger scale!

ETA: if the wires were in EMT, you certainly would have heard them make some noise in the first test!
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Doesn't that somewhat depend on the fuse type and design characteristics? A current limiting fuse could lessen such effect. Some breakers might lessen the effect when compared to using some non current limiting fuses.
 
kwired said:
Doesn't that somewhat depend on the fuse type and design characteristics? A current limiting fuse could lessen such effect. Some breakers might lessen the effect when compared to using some non current limiting fuses.
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Surely does. Don’t forget who was demonstrating and what they were selling.
 
retirede said:
Surely does. Don’t forget who was demonstrating and what they were selling.
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I remember in the 90's Square D had a demonstration unit they took to trade shows and other events that was somewhat a similar thing at convincing people they had something. Could interchange several different makes of breakers in the setup, and it had a small replaceable metal link of some sort in a short circuit setup. None the competitor breakers would trip, the metal link burned open every time. The QO breaker always tripped before the link ever showed any sign of getting hot.

What really was happening was their standard single pole 15 and 20 amp breakers had a low magnetic trip setting compared to everyone else and it was low enough this link could handle whatever current it did let through before tripping. If you would have tried one of their "high magnetic" breakers or a 15/20 amp two or three pole instead of the standard single pole - it probably would have burned up the link like all the other competition breakers did.
 
kwired said:
Doesn't that somewhat depend on the fuse type and design characteristics? A current limiting fuse could lessen such effect. Some breakers might lessen the effect when compared to using some non current limiting fuses.
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Yes, and when I was there and saw the same demonstration, they pointed that fact out as part of the presentation.
 
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