A Question of Balance.

[RayDunzl]

If Maxwell were here, I'm sure he could answer this straightaway.

Since he is not, I'll ask those who know him well.

Assume a cable pair, twisted or untwisted or both, AC voltage source at one end, load at the other.

Two cases:

If the cable is fed (as in 120VAC) in an unbalanced manner, one wire swings 170v and the other is near 0v.

If the pair is fed balanced, as in opposing phase +60/-60 (to create the 120vAC across the load)...

How does the 'noise' radiated from the pair differ, or does it?

 

[amirm]

Non twisted wire acts as a dipole. From close distance, it will have this kind of radiation pattern:

Contrary to commonly stated myth, they do NOT cancel out.

However, if the distance between the two conductors is small relative to where you measure it, then it acts like a single conductor with the differing fields cancelling out. This is why it is often said to keep your low voltage wiring 12 inches away from electrical lines (or run them perpendicular).

A twisted pair wire solves this problem in that it will create a double helix radiation pattern which cancels out much more readily than the two parallel wires.

 

[RayDunzl]

That diagram (as I would look at it) shows the fields from current flow, which would be the same in both wires in either of my cases above.

Is there an effect related to the changing voltage, as well, in which the voltage measureable at each cable would be different in the two cases?

 

[amirm]

I thought you were asking the difference between two wire configurations. Was that not it?

 

[RayDunzl]

Well, there's an effect (magnetic) due to current flow, and there is(?) radiation due to voltage potential...

So, maybe I'm more curious about the emissions due to voltage difference between the two (wall outlet hot/neutral 120v and balanced +/-60V).

Does a wire with volts paired with ground radiate a voltage related noise differently than a wire with volts paired with a wire with the inverse voltage (instead of ground potential)?

Maybe I'm asking an invalid question (which can be devilishly difficult to answer).

 

[RayDunzl]

I suppose it is answered here - https://en.wikipedia.org/wiki/Electric_field

Two nice diagrams.

And a nice sound bite: " The electric field combines with the magnetic field to form the electromagnetic field."

 

[Fitzcaraldo215]

Ray - interesting questions. I may be in over my head here, because Physics was a long time ago and I was a EE dropout. So, Maxwell's equations are over my head. Use a grain of salt.

First, I think the main intent of balanced power is common mode noise rejection coming into the circuit from external sources, but internal to the circuit. Whether or not it also offers better performance reducing outward radiation to other nearby devices and circuits is I think what you are after.

I would think the radiation that actually induces outward interference from a variable AC circuit to nearby devices is magnetic, not electrical. But, in circuits, the electrical and magnetic energy are intertwined and directly proportional to one another: they are inseparably electromagnetic. You cannot have one without the other. The higher the electrical energy, the higher the magnetic flux. Although, you can conceptually look at or measure the electrical or the magnetic fields separately.

In any case, in a balanced circuit aren't the legs equal and opposite in polarity, so wouldn't the magnetic flux also be equal and opposite. So, I would think the balanced AC should reduce outbound noise radiation in other nearby devices and circuits, which is where I think you are also leaning.

 

[Speedskater]

Voltage is always measured between two points. In this case the points are the two conductors (Hot & Neutral) carrying the current and thus developing the fields around the conductors. Because the two conductors should be close to each other these fields cancel. But the Safety Ground/Protective Earth should not have any current, it should not have a field.

 

[DonH56]

The safety ground can provide an additional termination path for the EM fields from the other conductors. It also enables a dedicated shield (largely) independent of the signal path.

 

[Fitzcaraldo215]

Yes, but many AC lines and cords have no shield. Agreed, if they do, that might better isolate the AC from induced noise going into the power line and also radiating out from the power line or power cord. Stock power cords are typically unshielded. Fancy, expensive ones are shielded, but so are many much cheaper alternatives.

I wonder how many of the subjective claims about power cords might have some validity. But, it might relate to the simple fact of this shielding and how it might interact less with other wires, including interconnects, in the typical spaghetti mess in back of the system rack.

 

[DonH56]

Some precision test equipment comes with shielded power cords. I have seen them on some medical equipment, and the old femtovoltmeter (leaf type) I used to use now and then. But, we have several million $$$ of test equipment in our lab, and nary a fancy power cord to be found. We do have some racks with 30 A plugs, however. If we could hear from 500 kHz to 50 GHz with a dynamic range of 100~140 dB I am sure we'd need better cords, however.