May I ask a question about the single vs two phase stuff

[winnie]

A single phase transformer is powered by a single phase primary, when the primary current is flowing in one direction, the secondary current flows in the opposite direction. Polarity is as follows L1 is positive L2 is negative simultaneously N is negative with respective to L1 and positive with respect to L2. A sine wave would show L1 to N starting positive and N to L2 starting positive at the same time; in phase.

There is only one current flowing through the transformer. If L1-N was 180 degrees out of phase with L2-N the currents would buck, they don't. The reason the N carries the imbalance between L1-N and L2-N is because the remaining loads are now a series circuit and their currents flow from L1 to L2

The above is correct as far as it goes, what you are missing or ignoring is a language issue, not a physics issue.

You state 'L1 to N starting positive and N to L2 starting positive'; if one makes the perfectly reasonable choice of using N as the reference for your voltage measurements then the statement would read:
'L1 to N starting positive and L2 to N starting negative'

Using N as the reference reverses the sense of the measurement, but does not change the actual physics of the circuit. It is merely convention that the N terminal be used as the reference for the voltage measurement.

Selecting the N terminal as the voltage measurement _inverts_ the measurement of the sine wave. It does not create a new sine wave. It does not delay the sine wave by half a cycle. It does not change _single_ current flow at all; it simply inverts the measurement.

Now the final terminology issue: An inverted _single frequency_ sine wave is indistinguishable from a true 180 degree phase shifted sine wave. Not simply indistinguishable in the measurement, but also indistinguishable in the maths. Thus it is perfectly reasonable and common convention to call this apparent inversion a
180 degree phase difference. Do the vector math for the neutral current either way, as long as you select your current direction reference to match your voltage direction reference. Both approaches are internally self consistent.

The above does not change the fact that there is only a single current phase.

It simply means that calling one leg of the circuit 0 degrees and the other 180 degrees is common convention, and _works_ mathematically. Since the center tap is generally the grounded terminal of the transformer, and ground is commonly used as the voltage measurement, this is by far the most common convention.

Note, however the requirement for a pure single frequency sine wave. Add harmonics to the picture and the statement that inversion is indistinguishable and equivalent to a 180 degree phase shift no longer holds. If you continue to assume that an inversion is the same as a 180 degree phase shift you may get incorrect results in your calculation. But it is true for single frequency situations, and that is often good enough for AC power distribution.

-Jon

 

[winnie]

They do if by buck you mean cancel.
As the N is the common point it is usual/logical to reference the L1 and L2 alternating voltages with respect to that. And they are most certainly 180° out of phase.

Going to disagree somewhat. Consider an AC source that has second harmonic superimposed on the fundamental.

Compare the original, its inverse, and the signal delayed by half a cycle of the fundamental.

The signal plus its inverse sum to zero.

The signal plus the delayed version do not.

Once more than one frequency is involved, inversion and 180 degree phase shift are different.

Since you can decompose any periodic signal into its sinusoidal components, inversion results in an apparent 180 degree shift of each component individually, whereas a shift of 180 degrees for the fundamental gives a different phase shift for each harmonic.

-Jon

 

[Gary Glaenzer]

Going to disagree somewhat. Consider an AC source that has second harmonic superimposed on the fundamental.

Compare the original, its inverse, and the signal delayed by half a cycle of the fundamental.

The signal plus its inverse sum to zero.

The signal plus the delayed version do not.

Once more than one frequency is involved, inversion and 180 degree phase shift are different.

Since you can decompose any periodic signal into its sinusoidal components, inversion results in an apparent 180 degree shift of each component individually, whereas a shift of 180 degrees for the fundamental gives a different phase shift for each harmonic.

-Jon

:thumbsup:

Well said ! Considering a single sinusoidal signal, "inversion results in an apparent 180 degree phase shift".

And that is where the confusions starts...........

 

[Besoeker]

Going to disagree somewhat. Consider an AC source that has second harmonic superimposed on the fundamental.

Compare the original, its inverse, and the signal delayed by half a cycle of the fundamental.

The signal plus its inverse sum to zero.

The signal plus the delayed version do not.

Once more than one frequency is involved, inversion and 180 degree phase shift are different.

Since you can decompose any periodic signal into its sinusoidal components, inversion results in an apparent 180 degree shift of each component individually, whereas a shift of 180 degrees for the fundamental gives a different phase shift for each harmonic.

-Jon

The fundamental voltages are out of phase by 180°. The currents may not be even without considering harmonics.

 

[ggunn]

Going to disagree somewhat. Consider an AC source that has second harmonic superimposed on the fundamental.

Compare the original, its inverse, and the signal delayed by half a cycle of the fundamental.

The signal plus its inverse sum to zero.

The signal plus the delayed version do not.

Once more than one frequency is involved, inversion and 180 degree phase shift are different.

Since you can decompose any periodic signal into its sinusoidal components, inversion results in an apparent 180 degree shift of each component individually, whereas a shift of 180 degrees for the fundamental gives a different phase shift for each harmonic.

-Jon

Thank you. That's what I have been saying all along.

 

[winnie]

And now I get to disagree with myself (A long shower will do that to you...)

I have been saying that an inversion results in an apparent 180 degree phase shift, implying that it is not a real phase shift.

For a single frequency sine wave, an inversion is indistinguishable from a 180 degree shift in time. Since the two sine waves are the same (inverted vs time shifted), there is no way to say that an inversion is _not_ a 180 degree phase shift.

-Jon

 

[jaggedben]

And now I get to disagree with myself (A long shower will do that to you...)

I have been saying that an inversion results in an apparent 180 degree phase shift, implying that it is not a real phase shift.

For a single frequency sine wave, an inversion is indistinguishable from a 180 degree shift in time. Since the two sine waves are the same (inverted vs time shifted), there is no way to say that an inversion is _not_ a 180 degree phase shift.

-Jon

I disagree somewhat. If in the real world there is any asymmetrical distortion then inversion will tell you accurately what is going on whereas a phase shift will not. They are only indistinguishable when working with ideal mathematics. When hooking up a scope one had better understand how to think of it in terms of polarity and inversion (not least so that you can hook up your scope without causing a short!).

I'd put it this way: If one used L1 (or L2) as common reference for one set of measurements, and then switched to using N as common for a second set, one would be confusing oneself by thinking that changing the scope probes caused a 'phase shift' to exist that wasn't there before. This isn't 3 phase power, and I think that for beginners it is less helpful to describe split-phase as having a 'phase shift' than as having two components that are in series and 'in phase'. Just my opinion on language, but the math is the math.

 

[drktmplr12]

I guess I'll post it again - one person upload a picture depicting his two phase theory and a few others claiming to be engineers agreed with him. You can see the picture in the Zero Sequence CT thread.

lucky for me, i don't need you to recognize that i am an engineer. the picture was intended to show the waveforms of L1-N and L2-N are not in phase with one another.

its been stated many times that phase can only be with reference to some other point. zero must be defined somewhere. We could move the center tap 75% up the winding and call that zero. L1-N and L2-N would still be out of phase.

the primary is one phase (H1-H2) and we split it by tapping the secondary (L1-N & L2-N). is the result two-phase power in the sense of power engineering? no. i don't think anyone here is making that assertion. it is two sinusoidal waveforms that are out of phase. therefore, two phases, with respect to the secondary neutral. there is no arguing this point. it is a fact. our air conditioner condensing units and air handlers are still single phase motors.

 

[jaggedben]

... the picture was intended to show the waveforms of L1-N and L2-N are not in phase with one another.

...

I'm not even convinced that 'in phase with one another' has an agreed upon meaning that doesn't include having lowest and highest voltage amplitude at the same points in the cycle, regardless of polarity. The whole problem here, in my opinion, is people trying to use various permutations of 'phase' to mean something more specific than what that word is consistently understood to mean.

 

[LarryFine]

Please don't get another thread closed by getting personal.


Let's start by using one end of the secondary as the reference (ground) instead of the center tap, and call the tap L1 and the far end as L2. Any argument as to phasing or polarity now? We're all in phase now, right? Only the magnitude of voltage is different. If this were DC, as per my battery analogy, we would be discussing polarity. To me, that it's AC changes nothing.

With two batteries in series, and measuring with a DC voltmeter, one would place the black lead on the negative terminal, regardless of whether testing either one or both batteries, right? The center-tapped-neutral argument is like keeping the black test lead on the center-point of the two batteries, and then wondering why one battery seems to have a reversed polarity.

 

[K8MHZ]

Please don't get another thread closed by getting personal.


Let's start by using one end of the secondary as the reference (ground) instead of the center tap, and call the tap L1 and the far end as L2. Any argument as to phasing or polarity now? We're all in phase now, right? Only the magnitude of voltage is different. If this were DC, as per my battery analogy, we would be discussing polarity. To me, that it's AC changes nothing.

With two batteries in series, and measuring with a DC voltmeter, one would place the black lead on the negative terminal, regardless of whether testing either one or both batteries, right? The center-tapped-neutral argument is like keeping the black test lead on the center-point of the two batteries, and then wondering why one battery seems to have a reversed polarity.

I have tried to use the battery analogy before and it didn't go over well.

 

[Besoeker]

Let's start by using one end of the secondary as the reference (ground) instead of the center tap, and call the tap L1 and the far end as L2. Any argument as to phasing or polarity now? We're all in phase now, right? Only the magnitude of voltage is different.

Agreed. You would have two voltages in phase and of differing amplitudes as you say. But that's not how it is generally implemented. You have 120-0-120. The two 120V are of equal amplitude but in anti-phase. By 180°.

 

[wwhitney]

Agreed. You would have two voltages in phase and of differing amplitudes as you say.

Yes.

But that's not how it is generally implemented. You have 120-0-120.

That's not an implementation issue, that's purely a point of view. You are selecting the center tap as the zero point and insisting on positive voltage values. You could as easily say "we have -120 - 0 - 120" or you could select one end of the transformer secondary as the zero point, and say you have "0 - 120 - 240".

Cheers, Wayne

 

[Besoeker]

Yes.


That's not an implementation issue, that's purely a point of view. You are selecting the center tap as the zero point and insisting on positive voltage values. You could as easily say "we have -120 - 0 - 120" or you could select one end of the transformer secondary as the zero point, and say you have "0 - 120 - 240".

Cheers, Wayne

But that's not what you have.
Neutral is the common point to get 120-0-120 which is what you have. It is the logical point to make measurements with reference to. And plus or minus signs mean little for alternating quantities.

 

[jumper]

There is no north or south. There is no right or left. There is no coming or going.

Until you decide which direction you like.

You get to decide these once you stand in the spot you like. It is your world in which to create voltages. You create the reference point wherever you like.

What!!! You trying to bring some basic common sense to this here cat fight!

It will never work...unfortunately.....sigh....

 

[Gary Glaenzer]

Agreed. You would have two voltages in phase and of differing amplitudes as you say. But that's not how it is generally implemented. You have 120-0-120. The two 120V are of equal amplitude but in anti-phase. By 180°.

OK

I'll bite, just for the fun of it:

WHERE and HOW is the 180 degree phase shift generated ?

 

[ggunn]

There is no north or south. There is no right or left. There is no coming or going.

We control the horizontal. We control the vertical.

 

[wwhitney]

Neutral [..] is the logical point to make measurements with reference to.

No, it is simply a choice.

And plus or minus signs mean little for alternating quantities.

I'm pretty sure all your calculations will be off if you randomly start multiplying waveforms by -1.

Cheers, Wayne

 

[Besoeker]

OK

I'll bite, just for the fun of it:

WHERE and HOW is the 180 degree phase shift generated ?

It isn't a phase shift. It just a phase relationship between the two hots and the neutral.

 

[Besoeker]

I'm pretty sure all your calculations will be off if you randomly start multiplying waveforms by -1.

Cheers, Wayne

Why I would I?