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

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Ingenieur

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buffalonymann said:
We call it single phase because that's exactly what it is. One primary winding, one secondary winding - produces just one sine wave from L1 to L2. It doesn't stop being a single phase xfmr because we tap the midpoint to provide 120 VAC.

In fact, with perfectly balanced 120 VAC loads, we don't even need the center-tap; we could connect the loads in series across the 240. Everything but the imbalance is across 240VAC.
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2 sec windings, one dot grounded, the other a supply
each out of phase with the other resulting in 2 phases

so if we have 480 wye and a load across 480 (no neut) it becomes single phase?
but if we have 2 loads line-n and line-n that is 2 phases?

but the same does not apply to the 2 sec coils and neut of 120/240/1?
not 2 phase power but 2 phases, independent
put an R load on one and a Z on the other
is the R ckts pf affected by the others reactive load?
hint: no
 
david luchini

david luchini

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wwhitney said:
Only if VL2-N(t) is a sine wave. So yes, in the special case of a sine wave, take your pick between "times -1" and "180 degrees out of phase." I'll pick the one that applies more generally, "times -1".

Cheers, Wayne
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VL1-L2(t) = -1 * VL2-L1(t). If the voltage VL1-L2 is 240V, the voltage VL2-L1 isn't "negative 240V."
 
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Ingenieur

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wwhitney said:
OK. But other voltage waveforms regularly occur in reality.

Cheers, Wayne
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but this thread morphed from another where I was ridiculed for saying 120/240/1 is comprised of 2 waveforms of differing phase

so that is what we are talking about
and you used sine in you example
???
 
jumper

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wwhitney said:
"180 degree phase shift" is more standard in the classroom than "times -1"?

Cheers, Wayne
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In basic electrical courses, yes.

Later, as all things, when students advance they can understand more.

But in general an average jw will never have to know more than the simple basics for his job.
Most will never even have to learn complex mathematics and imaginary numbers.
 
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jaggedben

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david luchini said:
VL1-L2(t) = -1 * VL2-L1(t). If the voltage VL1-L2 is 240V, the voltage VL2-L1 isn't "negative 240V."
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Um, yes it is. The instantaneous voltage L1-L2 is the opposite polarity, and thus the negative, of the instantaneous voltage L2-L1.

I get that for engineers educated in phasor math, it might feel more convenient to use the same method on split phase services as on three phase systems. One of the points I've been trying to make in this thread is that, for a sparky and the work they need to do, the 120/240 service can be sufficiently understood using only the concept of polarity. It is of significant practical and economic difference that installations on 120/240 generally don't require engineering services for this reason.
 
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Ingenieur

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480 : 120/240/1 xfmr
1 prim winding
2 sec windings

the 480 is comprised of 2 phases
277/0 = 277.13 + 0j
277/120 = -138.56 + 240j

sum = 277.13 + (-) (-138.56 + 240j) = 415.69 - 240j = 480/-30
so, 2 phases combine to 1
so 1 can't be devolved into 2

what if the prim was center tapped
phase to hi, phase to low, neut to center tap
sec has 2 corresponding windings, 2 phases? correct?
what if the sec was 1 winding served by the 2 primary?
 
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winnie

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buffalonymann said:
We call it single phase because that's exactly what it is. One primary winding, one secondary winding - produces just one sine wave from L1 to L2. It doesn't stop being a single phase xfmr because we tap the midpoint to provide 120 VAC.
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jumper said:
Can you show me a real life power distribution tranny with a single secondary winding and a center tap?
Everyone I find is two separate windings ones on a common core with x2 and x3 leads. Whether internal or external.
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It doesn't matter if it is one coil with a center tap or two coils with a jumper. The coils consist of individual turns and the individual turns are electrically in series, and the center tap is simply connected to one of the turns (either directly or via the x2 x3 leads). They are equivalent.

It is a single phase transformer. Everyone here agrees that it is a single phase transformer. There is approximately a single flux coupling all of the turns of the coil (only approximately; remember leakage inductance). All of the turns of the coil develop approximately the same voltage.

That does not change the fact that there are _two_ apparent phase angles, that you can use these _two_ phase angles to do correct math describing the operation of the transformer and the neutral.

The _fact_ that you can do correct math to give correct predictions using phase angles of 0 and 180 is sufficient to make clear that the two phase angles are present. They are related by inversion, they are in some senses the same (zero cross at the same time), and by alternate selection of reference you can change the system to have only a phase angle of 0. But you can still do correct math with _two_ phase angles. We agree not to call the system a 'two phase' system, but clearly there are two phase angles available.

-Jon
 
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GoldDigger

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Ingenieur said:
120/240/1 IS a SINE wave
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In the ideal definition, yes.

And in a totally practical, physical, sense it is a good approximation to a sine wave, with to a better approximation to a wave with only odd harmonics present.
It is sufficient for the result that -V(theta) = V(theta + 180) that only the fundamental and odd harmonics be present in the waveform V.
 
I

Ingenieur

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GoldDigger said:
In the ideal definition, yes.

And in a totally practical, physical, sense it is a good approximation to a sine wave, with to a better approximation to a wave with only odd harmonics present.
It is sufficient for the result that -V(theta) = V(theta + 180) that only the fundamental and odd harmonics be present in the waveform V.
Click to expand...

utilities strive for <5% thd

no need for a Fourier series for steady state
transients are another matter as are ss switching devices
 
jumper

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winnie said:
It doesn't matter if it is one coil with a center tap or two coils with a jumper. The coils consist of individual turns and the individual turns are electrically in series, and the center tap is simply connected to one of the turns (either directly or via the x2 x3 leads). They are equivalent.

It is a single phase transformer. Everyone here agrees that it is a single phase transformer. There is approximately a single flux coupling all of the turns of the coil (only approximately; remember leakage inductance). All of the turns of the coil develop approximately the same voltage.

That does not change the fact that there are _two_ apparent phase angles, that you can use these _two_ phase angles to do correct math describing the operation of the transformer and the neutral.

The _fact_ that you can do correct math to give correct predictions using phase angles of 0 and 180 is sufficient to make clear that the two phase angles are present. They are related by inversion, they are in some senses the same (zero cross at the same time), and by alternate selection of reference you can change the system to have only a phase angle of 0. But you can still do correct math with _two_ phase angles. We agree not to call the system a 'two phase' system, but clearly there are two phase angles available.

-Jon
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Jon, I know this. Buffy is the one arguing here.

Quote from OP link:

Split phase 120/240 Vac source is equivalent to two series aiding 120 Vac sources.

My point was that no 120/240V power tranny is really a single center tapped secondary.
We can analyze it as such, but it a multi winding secondary with 2 120V coils wired in series that uses the center connection as a reference point when comparing and “adding” the two for a 240V signal.
 
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