Why do wires rattle in EMT when some motors start?

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sdob, if current flows in a conductor there is a magnetic field around that conductor.

Place the outgoing and returning conductor of a simple two wire load next to one another and clamp ammeter around both of them the fields cancel their effect on the core of the ammeter, there is still magnetic forces between the two conductors, we may disagree as to when they would be repelling or attracted to one another but with AC current and changing fields they are going to "vibrate" as a result. If DC currerent they would constantly be either attracted or repelling one another. Rectified but not filtered so you end up with pulsing DC might act a little more like AC.
 
synchro said:
Consider two parallel wires separated by a distance R that are flowing equal currents I in opposite directions. The magnetic fields that encircle each of these two wires will therefore circulate around their respective wire in opposite directions (i.e, they'll have opposite polarity). And so as you move away from the pair of wires, their magnetic fields will cancel each other as others have noted. But in the space between the two wires the circulating magnetic fields will be going in the same direction and actually add to each other, not subtract. Think of it like two gears that are meshing and rotating in opposite directions, but right where the gears make contact they are moving in the same direction. Because the magnetic fields created by the wires have the same polarity there will be a repulsive force between them. The strength of that force will be proportional to I2/R by Ampere's force law. And so the repulsion force will go up quickly as the current is increased.

Now I think the rattling is caused when the force is: 1.) sufficiently large to overcome friction, gravity, wire stiffness, etc. that will otherwise keep it in place, 2.) the duration of the force is long enough and it's sufficiently strong to accelerate the wire and move it a large enough distance to cause a rattle, and 3.) the duration of the force is limited, because otherwise the wires will likely just move to another place and stay there (i.e., another equilibrium point) instead of jumping and returning back which I think would be more likely to cause a rattle. A constant application of 60Hz current will create a repetitive force that peaks every 1/2 cycle of 60Hz, but the inertia of the wires should prevent that from causing any significant movement over each 1/2 cycle (probably at most a buzzing noise) . So I'd expect 60Hz to have a similar effect as DC. It shoud be noted that the polarity of the AC current changes each 1/2 cycle, but the direction of the resulting force is the same (repulsive) because the currents are still flowing in opposite directions.

By the way, I found the following that shows waveforms of the surge current from the heat lamp of a fuser in a laser printer:
Click to expand...

This is entirely plausible, respecting the mass of the individual conductors and their resultant resistance to motion.

But as an EE, which I'm a light year away from, am I out of line hypothesizing that when either current or voltage lags or leads the other, it becomes increasingly difficult to achieve the field cancellation effect we strive for by ,for instance, limiting # of conductors in a conduit?
 
sdbob said:
This is entirely plausible, respecting the mass of the individual conductors and their resultant resistance to motion.

But as an EE, which I'm a light year away from, am I out of line hypothesizing that when either current or voltage lags or leads the other, it becomes increasingly difficult to achieve the field cancellation effect we strive for by ,for instance, limiting # of conductors in a conduit?
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And the laser printer doc is interesting. Digesting it now, ty.
 
I have a 1941 Farmall H tractor. When starter motor had problems (jammed gearhead and rotor wouldn't spin), I noticed the conductor from starter switch (they have a heavy duty switch and not a magnetic switch on this model) to the starter motor would move a little when you pressed the switch. Pretty sure it was magnetic effects making it move even though it sort of hung in free air and wasn't intimately close to the frame at that point.
 
Yup. Well any of us that weld at all at one time looked at all the pretty metal dust lined up next to the welding cable on the floor.
 
sdbob said:
This is entirely plausible, respecting the mass of the individual conductors and their resultant resistance to motion.

But as an EE, which I'm a light year away from, am I out of line hypothesizing that when either current or voltage lags or leads the other, it becomes increasingly difficult to achieve the field cancellation effect we strive for by ,for instance, limiting # of conductors in a conduit?
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I question whether reactance matters much. In two wire inductive circuit current will still be out of phase with voltage by same factor in both conductors. It is the current that is causing the magnetic field around the conductor(s) regardless of what phase relationship is to the supply voltage, and same for simple ammeter to function as well.
 
kwired said:
I question whether reactance matters much. In two wire inductive circuit current will still be out of phase with voltage by same factor in both conductors. It is the current that is causing the magnetic field around the conductor(s) regardless of what phase relationship is to the supply voltage, and same for simple ammeter to function as well.
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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.
 
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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"Not to parse words" then I parse words.
Calling myself out here lol
 
sdbob said:
Nope. Contacts don't blow apart in an extreme short circuit. They fuse together. They WELD together.
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Many breakers are designed in a manner that uses the magnetic forces from the high current to help separate the contacts for faster clearing.
testguy.net

Circuit Breaker Contact Arrangements: Straight-through vs. Blow-apart

The flow of electrical energy in a circuit breaker is controlled by a contact assembly. When a circuit breaker is tripped or manually operated, the circuit breaker interrupts the flow of electrical energy by separating its contacts. As the contacts open a live circuit, current continues to flow...
testguy.net testguy.net
 
don_resqcapt19 said:
Many breakers are designed in a manner that uses the magnetic forces from the high current to help separate the contacts for faster clearing.
testguy.net

Circuit Breaker Contact Arrangements: Straight-through vs. Blow-apart

The flow of electrical energy in a circuit breaker is controlled by a contact assembly. When a circuit breaker is tripped or manually operated, the circuit breaker interrupts the flow of electrical energy by separating its contacts. As the contacts open a live circuit, current continues to flow...
testguy.net testguy.net
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What an incredible place to learn. Thanks Don.
 
sdbob said:
Yup. Well any of us that weld at all at one time looked at all the pretty metal dust lined up next to the welding cable on the floor.
Click to expand...
If you weld with DC current at higher settings you can get "arc blow" which bends the arc because of magnetic forces.
There are also contactors and breakers for DC that purposely use magnetic forces to blow out an arc so that the current is interrupted quickly.


[/QUOTE]
 
1) When current flows in a closed circuit, that forces between the individual circuit conductors always tend to push the conductors apart. Doesn't matter if this is DC or AC; if you have current flowing out on one conductor and back on a parallel conductor the net result is that the conductors get pushed apart. This does not require fault current or current flow on conductors external to the normal circuit path; this is just blog standard 'how current works' in a circuit.

2) The power factor of the load at the end of the circuit doesn't change this. All that matters is the current flow. Except for very tiny capacitive effects (tiny at 60Hz power line frequencies and distances the size of buildings; all bets are off at high frequencies or lengths similar to the wavelength of the frequencies involved) the current going out on one wire in the circuit will be balanced by the current flow returning on the other conductor(s) of the circuit.

3) The forces on the conductors scale as the square of the current flow, and fall off with distance between the conductors. If you set things up to be sensitive enough you will detect the vibration in normal circuits operating on a continuous basis.

4) If you have high current flowing through small conductors (closely spaced and light weight) for any reason then you will get more noticeable vibration, but if this were to be sustained for an extended period of time the conductors would overheat and fail. The situation where code permits this sort of high current flow is only for startup transients, so that is where it gets noticed.

5) I am imagining an experiment; take a standard 16ga lamp cord, split the conductors apart and stretch them between two supports so that the conductors are about 1/2 inch apart, 6 feet long, and stretched so that their natural resonance is 120 Hz. Run say 1A of 60Hz AC through the cord and I bet it will visibly vibrate.

-Jon
 
I vote for winnie's explanation as a 100% accurate and complete coverage of the question the OP proposed. Reiterating:

1. The displacement power factor is irrelevant since no significant forces depend on the voltage.
2,. And the fact that the magnetic fields cancel at a distance far from the wires (as a multiple of the wire spacing) definitely does not mean that there is no field in the space between them!
 
winnie said:
1) When current flows in a closed circuit, that forces between the individual circuit conductors always tend to push the conductors apart. Doesn't matter if this is DC or AC; if you have current flowing out on one conductor and back on a parallel conductor the net result is that the conductors get pushed apart. This does not require fault current or current flow on conductors external to the normal circuit path; this is just blog standard 'how current works' in a circuit.

-Jon
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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.
 
winnie said:
5) I am imagining an experiment; take a standard 16ga lamp cord, split the conductors apart and stretch them between two supports so that the conductors are about 1/2 inch apart, 6 feet long, and stretched so that their natural resonance is 120 Hz. Run say 1A of 60Hz AC through the cord and I bet it will visibly vibrate.

-Jon
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I'd take this bet.

Now take a 3 phase circuit, stretch them horizontally, placed one on top of another or side by side, separated by a few inches, run a motor say maybe 10 amps and you may see vibration. You're certainly causing fields because the 3 wires aren't equidistant. We see/hear this all the time in overhead high voltage lines.
 
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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If and only if the two or three conductors were concentric would the field immediately outside the outer conductor shell be zero and would the magnetic forces among the wires net to zero (assuming they are perfectly centered).
As I stated above, the fact that the fields from the conductors cancel at a distance does not mean that there are neither fields nor magnetic forces in the space close to and between the wires.
A clamp-on ammeter around all the conductors will measure net zero flux and hence net zero current. But by your argument you would have to also read zero when you clamped each individual wire. Which does not happen!
 
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.
Click to expand...
Go back an look at the breaker link I posted. They made the current flow in opposite directions by the physical design of the contacts, to help force the contacts to open.
With circuit wires, the current is flowing out and back (in opposite directions) and the forces tend to push apart from each other. The larger the current flow, the more the more force there is to push the conductors apart.
 
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.
Click to expand...

You are quite correct that the net flux encircling a complete circuit is zero, and that in the far field the magnetic fields of a complete circuit cancel out.

However you are missing the near field effects, and they are key to the question.

While the spacing is small, there must be some space between the wires in a circuit. Between the wires the magnetic field does not cancel out, and this magnetic field will act to push the wires apart.

Don't confuse the far field approximation with the near field case.

Jon
 
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