No current flow on a balanced neutral?

[ronaldrc]

Some one said Threads where very cheap so I thought I'd better start another one for this question.

On a circuit like single phase 240/120 volt center tapped secondary where everything is exactly equal on both sides of the Neutral.

How could you prove that there is no current flowing on the neutral?

In other words if I take two magnetic fields that are identical and equal and
of opposite polarity and put them in the core of a CT then the CT. coil is going to generate no voltage although there are two currents flowing in two
different directions that create these magnetic fields.

Could it be that there are two 120 volt series circuits and each half flowing through the neutral in two different directions beside each other. Or do your
think they actually oppose each other and there is no current flow in the neutral?

I not sure myself.

The out come is the same no matter which way, I just thought I would get some other opinions.

 

[Lcdrwalker]

Here is the way that I do it. Take one circuit from each phase and install a receptacle on each one. Make sure that you are using a common neutral and clamp an ampmeter around it. I plug a 500 watt halogen light into one receptacle ang read the current on the neutral. Plug an identical light in the remaining receptacle. If the lights are the same and the lengths of wire the same, the ampmeter will drop to zero.

 

[480sparky]

.....If the lights are the same and the lengths of wire the same, the ampmeter will drop to zero.

Or close to it. Tolerances of various components will most likely keep you from reaching exactly zero.

 

[jim dungar]

Could it be that there are two 120 volt series circuits and each half flowing through the neutral in two different directions beside each other.

With a single core transformer there is only one magnetic flux direction (regardless if there is one center tapped winding or two individual windings). If we call the outsides of the winding L1 and L2 and the neutral point N, then the flux direction (and currents) are fromL1->N and then N-L2, and we see that there are two currents in the neutral that are flowing in opposite directions.

 

[LarryFine]

How could you prove that there is no current flowing on the neutral?

Prove? In theory, math. In reality, metering.

Could it be that there are two 120 volt series circuits and each half flowing through the neutral in two different directions beside each other. Or do your
think they actually oppose each other and there is no current flow in the neutral?

The former in theory, and the latter in reality.

For some light reading: http://forums.mikeholt.com/showpost.php?p=708650&postcount=4

 

[realolman]

With a single core transformer there is only one magnetic flux direction (regardless if there is one center tapped winding or two individual windings). If we call the outsides of the winding L1 and L2 and the neutral point N, then the flux direction (and currents) are fromL1->N and then N-L2, and we see that there are two currents in the neutral that are flowing in opposite directions.

That is what I thought, but there is another thread about a balanced neutral that has me confused.

How does what you just wrote reconcile with that?

Of course you could take two equal 120v loads and put them across a 240v source with no damage, but is that actually what is happening in the balanced loads circuit? Or is it two equal, opposite currents in the neutral?

 

[ronaldrc]

Jim

What I am talking about has nothing to do with the flux of the transformer.
The magnetic flux I am speaking of is the flux around the neutral conductor.

Which would be opposite of each other.

If you put your amp. meter ct coil around the neutral conductor of a balanced neutral that might be several hundred feet from the distribution transformer.

 

[ronaldrc]

Prove? In theory, math. In reality, metering.

Larry

Forget the math, but in theory and reality.

I'm inclined to believe there is current flow and they change to a series circuit when the neutral is disconnected. But I'm not sure?


Ronald

 

[winnie]

Current flow generally means that there are charges actually moving, in the case of copper wires, this means electrons moving from place to place.

When the electrons move through copper, some energy is lost to heat. When you have current flow in the wire, it warms up.

That being the case, if you had two 'equal and opposite' currents flowing on the neutral, I would expect significant neutral heating.

-Jon

 

[ronaldrc]

That being the case, if you had two 'equal and opposite' currents flowing on the neutral, I would expect significant neutral heating.

-Jon

Good point

That would make a good practical expirement.

Has anyone ever tried that or noted that effect ?

 

[charlie b]

That being the case, if you had two 'equal and opposite' currents flowing on the neutral, I would expect significant neutral heating.

But that is not the case, as I am sure you are aware. A better way to say it is that if you try to send two "equal and opposite" currents through the same wire, the net effect is that no current will flow in that wire.

I once typed up, and posted, and explanation of how this works. Let me try and find it, so that I don't have to type it all over again.

 

[gar]

080917-0929 EST

I will repeat a comment I made in another thread.

If you have a wire of some resistance, meaning not exactly 0, and if the voltage across that wire is exactly 0, then the current thru that wire is exactly 0.

I = V/R = 0/R = 0

In this case there are NOT two currents subtracting from each other.

Change your circuit to two separate neutrals and put a current transformer around the two neutrals. The resultant is a reading of zero. But now you have two separate circuits. Each with the same magnitude of current but out of phase by 180 deg with one another. Thus, canceling each other thru the current transformer.

.

 

[charlie b]

OK, here is my previously posted explanation:
- - - - - - - - - - - - - - - - - - -
Consider a single load, powered by Phase A from a 120/240 volt single phase panel. Let?s call its hot leg (ungrounded conductor) ?L1,? and its cold leg (grounded conductor) ?N1.? The current paths are as follows. During one half cycle, current goes from the panel, along L1, through the load, and back to the panel via N1 (the neutral wire for this load). During the other half cycle, current goes from the source to the load via N1, and returns to the panel via L1.

Next, consider a second, identical load, this time powered by Phase B. The current patterns are the same as above, but lets call the conductors L2 and N2.

First, let?s consider these two as being a Multi-Wire Branch Circuit (i.e., they share a neutral ? N1 and N2 are the same wire). What happens is that when the Phase A load is on its first half (i.e., current leaves on L1 and returns on N1), the Phase B load is on the opposite half cycle (i.e., current leaves on N2 and returns on L2). As a result, the currents in N1 and N2 (actually, these are the same wire, for this example), cancel each other out. So for all the world, it looks like current is leaving the panel on L1, passing through each of the two loads in series, and then returning to the panel on L2. A half cycle later, current leaves the panel on L2, passes through the two loads, in series but in the other order, and returns to the panel on L1.

Next, let one of the two loads be higher than the other, so that the two load currents are not the same. Currents in N1 and N2 are still opposite, but not identical. Thus, there will be a net current in the neutral. It is in this sense that the neutral wire is only carrying the unbalanced current.

Finally, let?s separate the two circuits, by giving each its own neutral wire. As far as each separate branch circuit is concerned, its neutral wire carries the same current as its phase conductor. The currents in N1 and N2 do not cancel each other out, because they are separate wires, and each carries its own current. However, look at it from the perspective of the panel. It sees current coming in from N1 at the same time that it sees current leaving on N2. So the neutral wire within the feeder to the panel views these two currents as having cancelled each other out. That is why, when we calculate the conductor size for a panel feeder, we sometimes get to size the neutral wire smaller than the phase conductors.

 

[jim dungar]

Jim

What I am talking about has nothing to do with the flux of the transformer.
The magnetic flux I am speaking of is the flux around the neutral conductor.

Which would be opposite of each other.

If you put your amp. meter ct coil around the neutral conductor of a balanced neutral that might be several hundred feet from the distribution transformer.

The magnetic flux in the transformer determines the direction of the current in the windings. The direction of the current in the windings determines the direction of the line-neutral currents. If the current in the transformer winding is flowing from terminal X4->X1, then it must also be flowing from X4->N and therefore N->X1. This means one load current would flow out of X1 and into N and the other load current would flow out of N and into X4. The result is two neutral currents flowing in opposite directions.

You must be careful how you define your reference points and then perform your analysis. My preference is to use L-L voltages to define my circuit (I start with a single 240V waveform, and then divide it into two:V12 =V1n+Vn2), others use N as their reference (they start with two opposite 120V waveforms that must be combined: V12 = V1n-V2n).

 

[ronaldrc]

OK

Noted

I think everyone knows what I am asking.

 

[gar]

080917-1025 EST

charlie b:

I disagree with your explanation in post #13. There are no counteracting currents flowing in the common neutral because as I stated in my post #12 there is no voltage across the neutral and thus no current.

The current is only flowing thru the two loads. Unbalance the loads slightly and most of the current is flowing thru the two loads and the neutral current is only the result of voltage across the neutral wire.

.

 

[winnie]

I think that the issue here is "What is the reality of the mathematical tricks that we use to analyze this situation?"

Charlie gave a great rendition of equal and opposite currents flowing on the separated neutral wires, so you put them together and now no net current flows.

I agree that there really is no current flow; the question is: what evidence do you have that there really is no current flow at all, as opposed to no _net_ current flow? How to you prove that there really is zero current flow, as opposed to having two equal and opposite current flows at exactly the same time.

I do not need to be convinced that there really is no current flow, but it is fun to think of experiments that would show this. Heating of the conductor is one piece of evidence. I bet you could make hall effect measurements that showed no current flow. Any other experiments?

-Jon

 

[ronaldrc]

That is why, when we calculate the conductor size for a panel feeder, we sometimes get to size the neutral wire smaller than the phase conductors.

Charlie

I think about all of us here Electricians and Engineers understand the theory
of how this is suppose to work and how we can reduce the neutral size.

But bottom line , do you think the current is flowing in two different directions in the neutral conductor?

Or do you think it meets somewhere along the middle of the neutrals wire and the opposite currents stop there and the circuit just conducts around the series circuit ?

 

[gar]

080917-1048 EST

ronaldrc:

Excluding superconductors, if there is exactly 0 volts drop across a conductor, then there is no current flow in that conductor.

Use any current measuring technique you want and you will find no current flow in the neutral. It just is not there because there is no voltage across the neutral.

.

 

[charlie b]

charlie b: I disagree with your explanation in post #13.

No you didn?t. You just said the same thing a different way. I think Jon summarized the situation best:

I think that the issue here is "What is the reality of the mathematical tricks that we use to analyze this situation?"

One such trick is called the ?Loop Current Method.? I can describe the current in the A phase (from my post #13) as ?I-1,? and the current in the B phase as ?I-2.? As each of these two currents flow in the neutral, they are equal in magnitude and opposite in direction. The Loop Current Method tells me that this results in a net zero current in that wire. The actual physical situation (which, I fear, Jon, I can devise no experiment to prove) is that you really don?t have bazillions of electrons flowing per second in one direction, and another equal number of bazillions of electrons flowing per second in the other direction. So yes, you are right, that there aren?t two currents flowing. But I am free to model it that way, since this modeling process is a commonly accepted engineering tool, and I am free to describe it that way as well.