Magnetic Induction

Never heard of it, but the science behind it makes it an awesome idea. Learn something new every day on this internet thingy.

Isn't that what they call a hysterisis loop?

Are you referring to 'shading coils'? http://en.wikipedia.org/wiki/Shaded-pole_motor
I believe a hysteresis loop is the graphical representation of the magnetic-memory behavior of materials.

No I was thinking of electrical hysteresis such as in ferro devices, but I have never seen this used, and probably using the wrong terminology....

Perhaps I do not understand the drawing.How will the conduit be installed with the wire in the openings?

The reason all phases and neutral plus ground must be run together is to prevent inductive heating of the conduit, unless the conduit is PVC or non-metallic, and to provide a fault current carrying conductor adequate to trip the upstream circuit breaker.The ground is sized according to the size of the upstream circuit breaker.

I have never seen the method you have shown, but then again, I haven't seen everything.Please ask your instructor for his source for this neat tidbit of info.

Thanks

The diagram means nothing to me.

What is that a sketch of?

I believe what we are looking at is a bare wire weaved in and out of wireway cooling slots for greater thermal transference of heat out of the wireway. That is why i found it to be interesting if thats what we are looking at.

What? Like a heat sink on trunking?

If OP could google-pilfer some photos and post them it might help us get onto the same page.

Would someone like to explain the physics of this to me? Current through the long straight wire will generate a circular magnetic field around it, which may induce a current in a parallel long straight conductor (the conduit). Putting a loop of wire parallel to the magnetic field is not going to do anything, is it?

in his original post he is not talking about induce current, he talking about reducing heat. does that help explain it. that is why I guessed it to be heat sinking methodology. But maybe he does think that layout reduces the magnetic field and indirectly reduces heat generated. guess only he can clarify.

I can't say I understand the drawing, but I can see that if you have an unbalanced load, or a non-linear load (such as fluorescent lighting), that you can have significant neutral current. If you run the three phases without the neutral through a conduit, the neutral current does not cancel out the three phase currents and so current will be induced in the conduit. An additional copper loop would act as a transformer secondary, and being of lower resistance than the conduit, would generate less heat.

An additional copper loop would act as a transformer secondary
Only if the coil is at right angles to the magnetic field, which is not the case here. In any case, a low resistance circuit does not generate less heat when the lower resistance is associated with a higher current, since the heat is proportional to current squared x resistance.

My idea was to run the extra conductor through the conduit parallel to the three phase wires. The conduit would be magnetized around its circumference, perpendicular to the wires and the extra shorted conductor. I believe the induced current in the shorted loop would reduce the magnetization of the conduit. This magnetization, not Eddy currents, may be the cause of the heating.

I'm not at all sure why you wouldn't just run the neutral through there, which would accomplish the same thing.

This is a solution I know:

This view is the enclosure bottom plate or entry plate. That is IF it has to be matallic (Thus the question of induction...). otherwise the problem does not arrise.

Just make sure that the conduits are plastic with plastic locknuts etc, OR you use fibre or plastic washers if metallic: to prevent shorting the cut slits.

Also lets the geckos get in to mess with your connections. Perfect.

Well, if you make the slits Wide enough!

these gaps are only necessary if you can't have all the phases in the same hole (for whatever reason).

Also, the Gaps can be closed with any isolating material... don't be difficult!

I still can't make any sense out of the OP.

Lots of good guesses here.....

Yes because there are no opposite fields to cancel each other, GA and very useful and straight forward article.

This does not meet NEC requirements.I have been an Unlimited Electrical contractor since 1987,with 15 years experience prior to passing exam.License is recognized in 7+ states.

Still, I will never have all the answers.

I am now retired, so there may be new things coming out every day,but I cannot find a reference for what your instructor said.

It is forbidden to split phases in a metallic conduit due to heating of the conduit itself.

PVC conduit may be used for this purpose with no problem,so I presume your instructor is referring to PVC conduit or wireway.

Please ask your instructor to clarify exactly the conditions and circumstances of his method.

It appears to me that the #6 conductor could heat up if sufficient unbalanced current

is present.This in itself could present a hazard.

I can see where the loop would act as a shorted secondary to neutralize minor induced currents, but how is the conductor to be passed through the opening unless you drill a hole beside the conduit knockout to allow passage of the conductor,forming a keyhole type slot.

Is he talking about making a single turn of wire around each entering group of wires?

This would make more sense to me.This would become a path of least resistance for induced currents, and if all phases are included in the loop, effectively neutralized.

I am sure it will make sense with more details provided.

Please give as much info as possible.

For the stated purpose, this looped wire does not make any sense:

If each opening, in the metallic enclosure, has one of the phases or the neutral, induced current will flow round the openings. The currents will be AC and usually small, depending on the conductor's load. The prescribed loop might have it's own circulating current and will not prevent the ones in the metal of the enclosures. It might have NONE, depending on how successful it was entwined (!!?).

The only way to prevent circulating currents is to have all 3 phases + Neutral in the same opening OR slit all the opening to form a virtual single opening.

"The only way to prevent circulating currents is to have all 3 phases + Neutral in the same opening"

Which is why, as I stated above, the NEC forbids splitting of phases in metallic conduit.

If all three phases are balanced,you will have no current flow in the loop.

The algebraic sum of 3 phase currents is zero.

Only if the phases are unbalanced will there be flow in the loop.

Try clamping an ammeter around all three phases of a load and see what you get.Or around a hot and neutral at the same time.

This principle is used in GFIC circuit breakers to detect leakage to ground.Both conductors are routed through a loop inductor, and the result is amplified by an op amp to generate a signal to trip if leakage(unbalance) exceeds a designated threshold.

It did make sense to me (sort of) eventualy. With the induced current flowing in the loop oposite to the line conductors

Don't know what you'd do with just a 3ph supply!

It may be better with a few arrows

It's a nice drawing, but would it not be appropriate to make the black arrow 3 times as thick as the other three, to illustrate that whatever combination of current flows is going on in the 3 phases is opposed by the current flowing in the return? The green path ceases to have any meaning (if, indeed, it had one to start with).

If the phases are balanced and no harmonics present there is no nett neutral current.

For gods sake Wal, don't introduce harminics in to it.

Oh No ... her we go. Anybody notice we haven't heard anything from post-n-go WarEagle who originally made the post? Amazing how much conversation an "Ink blot" can spark (nuance intended). After exhausting the harmonics aspects ... let talk about the similarities that can be found between harmonics and Proportional Integral Derivatives (PID), and really do that pencil sketch justice.

Makes a kind of sense if the original image was not a trunking cross section. Ordinary conduit pipe does not seem a problem since single phase circuits are put down them with wires in random configuration - but there are not high ampere turns in conduit compared to hundreds of amps in cables.

Regard trunking etc as a strange transformer core with a single primary turn [representing the resultant of 3 phase primary circuit not in an ideal equilateral triangle disposition] and a shorted secondary. Pardon my artistic skills..........

On the useful rule that in a transformer "the voltage sets up the flux" (back-emf = primary supply voltage, ignoring resistance), I suggest a low resistance shorted secondary = low secondary voltage = low primary voltage = low flux = low magnetising losses = less heating, if the current in the secondary does not cause much loss.

Seems worth a test to me - Send WarEagle to the lab with 2 metres of trunking?

I am not sure "non-metallic" is an essential requirement, non-magnetic applies to brass and some stainless steels. In any case, stainless steel is often used for cable trays and has the advantage of a much higher electrical resistivity than mild steel.

67model

Can a current be induced in an aluminum conductor?Copper?Steel?

Will this current create heat?

Will this heat affect the conductor insulation?

Think about it.

In reply to HiTekRedNek,

Q1 - Yes

Q2 - Yes

Q3 - Only if the heat is enough to raise the temperature much.

Huge quantities of lead-sheathed oil impregnated cable have been used, in single core as well as 3-4 core.

Following is spec. extract of PVC covered cable, note single core has AWA - Aluminium Wire Armour, multi-core SWA - Steel Wire Armour.

PVC Low Voltage 0.6/1.0kV Copper

Voltage Rating : 600/1000 volts
Description : PVC Armoured cables for use in fixed installations in industrial areas,
buildings and similar applications.
Construction : Stranded plain copper conductors, XLPE insulation, cores laid up, PVC bedding, single core AWA, 2 or more cores SWA, PVC sheath.
Operating Temperature : -15degC to +90degC
Core Identification : 1 core Bn or Be, 2 core Bn, Be, 3 core Bn, Bk,Gy
Sheath Colour : Black

Manufacturing Standard : BS 5467

And these cables are used with metal cable glands which terminate and clamp the armour.

Note that with steel the flux density and thus e.m.f induced are multiplied by the high permeability which is about 1000 for steel. Steel also has hysteresis losses - unlike non-magnetic metals.

67model

Sorry guys. I have been away for a while.

A Contractor installed (3) 15KV bushings on top of a metal building. THese bushings connect to the 15KV swgr inside by (2) per phase 500MCM shielded cables (MV-105). The run is short maybe 10ft. The drawings instructed the shop to install a 1/4" alum plate on top of the swgr and cut holes for the (3) bushings due to the magnetic induced heat effect of installing cables AA-BB-CC. There was no room in the chase with the stress cones for ABC-ABC.

They did not install the alum plate as instructed. The stress cones will not allow the cables to be pulled back thru the 3" bushings. I know you must install alum to stop the heating of the metal swgr top cover or plan B you can cut a slice from bushing to bushing in the metal to stop the effect. The system is rated for 1200A. I was going to show this drawing as a possible solution but I have never used it or know anyone who has used it. As I said it was part of a class on the NEC a good while back and I could not find my notes. I was hoping someone might know about this arrangement as a possible solution. The other solution, as stated, was to cut gaps in the enclosure.