500amps in parallel wires -resistance change?

[mjune]

Can direct current running in opposite directions in parallel wires increase the apparent resistance of a system? This is a real life problem that I’ve noticed on some electroforming equipment we relocated.

Prior to the move the wires were separate. Now they are located in a conduit together.

Using 500amps in two 4/0 cables per polarity running 70 feet in the conduit. 4/0 (or 0000 gauge) is a stranded 0.460” copper conductor with ~0.73” outer diameter with the insulation.

Before the equipment move the system ran at about 16 volts. After the move it is running at about 22 volts.

Can the magnetic fields created by the wires be interfering with the current flow?

If there is an effect, how can it be calculated?

 

[NascentOxygen]

Hi mjune! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Is this smooth DC, or just rough raw rectified AC?

 

[mjune]

DC is from welding power supplies. There is a possibility that there is some AC signal... but typically when this happens that electroforming operation has issues.

 

[NascentOxygen]

The inductance of that length of cable has been increased with the proximity of the cables, so with current not smooth, the reactance has higher voltage loss.

 

[Bob S]

The inductance of that length of cable has been increased with the proximity of the cables, so with current not smooth, the reactance has higher voltage loss.

The inductance between the two parallel cables decreases as they get closer together. Also, inductance is not a contributing factor in dc voltage drops. From Smythe, Static and Dynamic Electricity, 3rd Edition, page 440:
$$L=\frac{\mu_{o}}{\pi}\cosh^{-1}\left(\frac{b}{d} \right) \text{ Henrys}$$
where d is diameter of cable and b is separation (This does not include the additional internal inductance inside each cable.).
If the cables are confined to a cable conduit, they probably will get warmer, and their resistance goes up. The 4/0 copper wire resistance is about 0.05 ohms per 1000 feet at room temperature, so 500 amps dc flowing 140 feet implies a 3.5 volt drop.

 

[NascentOxygen]

The inductance between the two parallel cables decreases as they get closer together.

That's a good point you raise. Reactance would decrease, not increase.

If the cables are confined to a cable conduit, they probably will get warmer, and their resistance goes up. The 4/0 copper wire resistance is about 0.05 ohms per 1000 feet at room temperature, so 500 amps dc flowing 140 feet implies a 3.5 volt drop.

OP may have an opportunity to confirm this at switch-on after a prolonged off time. Is the voltage seen to creep up by 3 or 4 volts over a period of minutes?

Another factor at such high current levels, in close proximity the current density across each conductor may not be uniform, effectively reducing the cross-sectional area of the conductors and increasing the resistance seen.

 

[Bobbywhy]

We know that current flowing in the same direction in two parallel conductors results in attraction, and the converse, when flowing in opposite directions, the conductors will repel. From this I speculate that the increased voltage drop (increased resistance) may be caused by magnetoresistance...just as the OP suggested. I found this:

“Resistance is caused by collisions between charge carriers (like electrons) and other carriers or atoms. An electron moving through a perfect crystal of metal at a temperature of absolute zero will experience no collisions, so the crystal would have zero resistance. Imperfections, however, do exist, and temperatures above absolute zero cause the atoms to vibrate out of their lattice locations. These vibrations and imperfections cause collisions, increasing the resistance of the crystal.

Applying a magnetic field can also increase the resistance of a material, since the magnetic force on the moving charges will tend to increase the number of collisions between charges. This dependence of resistance on magnetic field is called magnetoresistance. Magnetoresistance is proportional to the strength of the magnetic field, with a larger field producing a higher resistance.

1. The force on a current-carrying wire in a perpendicular magnetic field is due to the force on the charge carriers moving through the wire.
2. This force on the charge carriers produces an increased resistance as the charge carriers collide more with atoms in the wire. This phenomenon is called magnetoresistance.
3. As charge carriers (electrons) collide with atoms in a metal, the deflection of each charge (and thus the resistance in the metal) depends on the spin of the electron.
4. The spin-dependent scattering (and thus the resistance) will increase if adjacent magnetic domains are aligned in opposite directions. This occurs in the absence of an external magnetic field.
5. The spin-dependent scattering (and thus the resistance) is smallest if the magnetic field through which the charges pass all points in the same direction, which is accomplished by the external magnetic field of a bit.
6. This spin-dependent effect is much stronger than the effect of regular magnetoresistance, and so is called giant magnetoresistance. It is, however, a completely different effect.”

http://www.rpi.edu/dept/phys/ScIT/InformationStorage/magres/magnetism_b.html

 

[NascentOxygen]

I speculate that the increased voltage drop (increased resistance) may be caused by magnetoresistance...just as the OP suggested. I found this:

Thank you for presenting these details. Magnetoresistance could indeed provide an explanation, though without the opportunity to investigate we're unlikely to be able to apportion degree of responsibility to one cause or another.

 

[Bob S]

Thank you for presenting these details. Magnetoresistance could indeed provide an explanation, though without the opportunity to investigate we're unlikely to be able to apportion degree of responsibility to one cause or another.

Magnetoresistance is not the problem. I have run higher currents in copper cables in very high magnetic fields.

 

[Nugatory]

Is the conduit made of a conducting material? That will make a big difference if there's enough AC component on top of the DC.