Neutral currents in depth

[mivey]

And it helps if your model ignores the real world relationship between current flow, winding directions, and magnetic flux.

It is not about ignoring those real world relationships, it is about also recognizing other real world relationships.

But, that is the nice thing about mathematical models, you can use them to prove your point.

Aweful convenient, those things.

And yes the currents in the neutral of a single winding center-tapped transformer are 180? out of phase in the neutral even though they are in phase with each other in the transformer winding.

The currents in the two winding halves are not always in phase. Funny thing about that neutral and how it can change a two-wire circuit into something else.

According to the IEEE and ANSI standards for transformer labeling transformers, the top half current is flowing out of X1 towards neutral and the bottom half current is flowing out of neutral towards X2 (of course this top-bottom pattern alternates with the frequency).

That is polarity. It is not the same thing as direction.


add:

I find this resource from Cooper to be very helpful, even though it is written from a utility view point...

Note that they use the word instantaneous direction when referring to polarity.

 

[mivey]

...and have been told by one I have no business understanding electrical theory

What kind of knuckle-head would tell you that?

 

[mivey]

I have stated in previous threads, it is my choice to consider a 3-wire 120/240V system as (2) 120V sources connected in in-phase and in series as it reduces the problems of currents and voltages being out of phase when a neutral conductor is either included or removed from a circuit.

How are the currents and voltages out of phase when feeding a center-tapped full-wave rectifier or a pulse circuit like Besoeker has shown?

 

[mivey]

I cannot believe you are saying that at any single point in time, the actual current in 1/2 of a transformer winding is 'out of phase' with that in the other 1/2.

No one has said they are out of phase in the same direction at the same instant (well for the voltages anyway, but actually, the currents are not in phase with Besoeker's circuit).

The issue is saying that we have to take them to be positive in the same direction.

 

[jim dungar]

Never had a quarrel with you on the directions of the real currents...

Trying to stay on the topic of current, are we agreed on a final conclusion;
In a single core transformer with a single winding primary and a single secondary winding direction, there is a single real world "direction of current" that exists. This 'direction of current' must be considered when connecting windings into series and parallel arrangements.

 

[mivey]

In a single core transformer with a single winding primary and a single secondary winding direction, there is a single real world "direction of current" that exists.

I will agree to that for the most part for instantaneous currents in one direction

This 'direction of current' must be considered when connecting windings into series and parallel arrangements.

Of that there is no doubt. Instantaneous currents and their relative directions are what polarity is all about.

 

[jim dungar]

The currents in the two winding halves are not always in phase.

Could you provide the mathematical formula demonstrating how the direction of the secondary current in a single transformer winding, feeding a resistive load, is not referenced to the direction of the flux in the transformer core? If not a formula, how about an authoritative source?
In previous threads you acknowledged being familiar with the work done by Faraday, Maxwell, Lentz, and Fleming. I know there are industry standards like IEEE/ANSI that relate primary and secondary current directions (i.e. current into H1 yields current out of X1). So what am I missing?


Or are you now discussing the 'phase difference' between inductive and capacitive currents?

 

[kwired]

What kind of knuckle-head would tell you that?

Apparently somebody that loves himself very much

 

[gar]

111122-1034 EST

Consider 1/2 of the secondary loaded with a capacitor, and the other half with a resistor. Assume the leakage inductance is very close to zero. Both secondary voltages will be in phase or 180 degrees out of phase, which ever you choose by your labeling.

The currents in the two halves of the secondary will be out of phase relative to each other by some non-zero integer multiple of about 90 degrees. The transformer flux to support this composite current will be at some odd angle relative to the primary voltage, and the primary input current will match what is required to produce that flux.

.

 

[jim dungar]

How are the currents and voltages out of phase when feeding a center-tapped full-wave rectifier or a pulse circuit like Besoeker has shown?

It has to do with the way the rectifier components (i.e. the diodes) are connected, not the interaction of the transformer primary and secondary windings. I thought you would recognize that?

 

[rattus]

Trying to stay on the topic of current, are we agreed on a final conclusion;
In a single core transformer with a single winding primary and a single secondary winding direction, there is a single real world "direction of current" that exists. This 'direction of current' must be considered when connecting windings into series and parallel arrangements.

Can't we also do this by considering polarity, that is dot to dot for parallel and dot to no dot for series?

 

[jim dungar]

Can't we also do this by considering polarity, that is dot to dot for parallel and dot to no dot for series?

Absolutely.

And in this case aren't you using the dot to indicate a fixed relationship to the primary, based on the physical construction of the transformer(s)?

 

[rattus]

Absolutely.

And in this case aren't you using the dot to indicate a fixed relationship to the primary, based on the physical construction of the transformer(s)?

Of course, but I will leave that to the manufacturer. That is, unless I wind my own.

 

[jim dungar]

Of course, but I will leave that to the manufacturer. That is, unless I wind my own.

So the relationship between the direction of the primary current and that of any secondary currents is dictated by the physical construction of the transformer.

We are free to say, although there are standards, whether the current is flowing in the direction of the dot(s) or away from the dot(s), but what ever we direction we choose applies to all other similar windings, on the same core, at the same time. Of course this is alternating current so any discussion of direction, is simply to establish a reference.

 

[gar]

111123-1113 EST

jim:

For a voltage transformer the dots indicate the voltage phase relationship, not the current. The load in combination with the voltage defines the current.

What are the phase angles of the currents in each half of the secondary when one load is a pure inductor, and the other a pure capacitor? The easy way to view this is in the neutral.

.

 

[jim dungar]

111123-1113 EST

jim:

For a voltage transformer the dots indicate the voltage phase relationship, not the current. The load in combination with the voltage defines the current.

What are the phase angles of the currents in each half of the secondary when one load is a pure inductor, and the other a pure capacitor? The easy way to view this is in the neutral.

.

Rattus brought up the mention of dots, I said there are standards for transformers.

I have not said you cannot, nor should not, use the neutral. In fact I have repeatedly said it is my choice to use the physical connections of the transformers to pick my voltage and current directions, as there are many situations that do not contain a neutral conductor.

I have been trying to keep this thread to currents, as there are enough other ones going concerning voltages.

Take a simple 3-wire convection range, it is predominantly one L-N load of a motor, and another L-L load of a resistor. How does using the neutral point, as a reference for current direction, simplify the analysis?

 

[mivey]

Could you provide the mathematical formula demonstrating how the direction of the secondary current in a single transformer winding, feeding a resistive load, is not referenced to the direction of the flux in the transformer core? If not a formula, how about an authoritative source?

You can continue to refine a question down to the point where you get a very specific answer to a very specific question that can only be answered using a very specific phrase just like you want to hear.

That does little to make you learn or think beyond the very specific constraints you have been thinking in.

But for the record, the flux in the core is not what the load is depending on. The load only cares about the voltage you apply to it. It does not care if it came from a single flux created by a single voltage, a single flux created by two phase-opposed voltages, two different fluxes, or even if there is a coil at all. The voltages are what they are.

Except for some very specific cases, there is no compelling reason to trace everything back to the primary side of the transformer, or further. It simply makes no difference to the load as long as it gets what it needs. We do not have to use the primary to define what we call the positive direction of force. AC changes direction every 1/2 cycle and our loads can use either direction.

In previous threads you acknowledged being familiar with the work done by Faraday, Maxwell, Lentz, and Fleming. I know there are industry standards like IEEE/ANSI that relate primary and secondary current directions (i.e. current into H1 yields current out of X1). So what am I missing?Or are you now discussing the 'phase difference' between inductive and capacitive currents?

The best I can tell is that you are missing that the voltages at most loads are what they are where they are. There also is nothing about the direction of flux that defines what we have to use as a positive direction for an AC signal.

Let's think about the difference between a two-wire circuit and a three-wire AC circuit (a two-terminal source and a three-terminal source). In a two-wire circuit, there is one voltage and one current. Nothing universally defines which direction is positive and which direction is negative. The current out of one terminal is the same current you get back in at the other terminal. Pretty cut and dried.

In a three-wire circuit, we have three voltages. The higher voltage is just another version of the two-wire circuit, so let's focus on the two smaller voltages, especially with an unbalanced load. Even though we have two voltages that are the same, we can have two currents which are not the same. The flux in the winding halves is not the same. We wind up with two circuits that share a common conductor. The neutral makes this a lot different circuit than a two-wire circuit.

Nothing says we have to use the two voltages such that their positive force is taken in the same direction. In fact, with circuits like Besoeker's, we can actually use the two voltages such that the positive force is taken in different directions. They will have positive voltages taken with 180? displacements.

It has to do with the way the rectifier components (i.e. the diodes) are connected, not the interaction of the transformer primary and secondary windings. I thought you would recognize that?

It was a rhetorical question. The point was to make you think about the way we use the voltages does not have to trace its way all the way back to the primary or further.

You are trying to use the direction of flux at a given instant (polarity) to determine the positive direction for the voltage. There is no such thing defined by any laws.

Think about it for a minute: If you say the flux direction defines the voltage directions, shouldn't the reverse be true as well? Do you not see the inconsistency in that logic when I can combine two voltages with a 180? displacement to create a single flux in a single core? The logic you propose would dictate that as soon as the voltages are combined, I would have to re-define the positive direction for one of them.

Don't misunderstand me, I am not saying there is anything wrong with defining both as positive in the direction of the polarity marks like you propose, I am just saying it is an arbitrary definition and is no more right that the choice of taking positive to be away from the neutral.

 

[jim dungar]

Again, I have been trying to keep my discussion in this thread to current 'directions'.

I have been advocating the use of industry standards (i.e. 'dot' and 'X1' terminals) as the basis of choosing references for discussions of center-tapped-transformers and other transformer windings on a single core. I find this maintains a consistence approach when discussing transformers interconnections be they delta, wye, or autotransformer as well as the use of phasors, but maybe that is just an appearance.

Also, I find it easier to converse with laypeople (those with limited electrical background) when I explain two 120V transformer 'outputs' in series.