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

[jumper]

Nicely shows the two output waveforms out of phase.

WRT to the neutral, yes. That is the way I am analyzing it.

It is my preference since I was taught that way but I learned the other methodology as well, usually more looking at from the primary side though.

 

[Besoeker]

WRT to the neutral, yes. That is the way I am analyzing it.

It is my preference since I was taught that way but I learned the other methodology as well, usually more looking at from the primary side though.

Yes, your diagram clearly shows the primary to be single phase. But I still say the polarity signs, + and - ,have no place on a diagram of an AC circuit. The dot notion works just fine.

 

[jumper]

Yes, your diagram clearly shows the primary to be single phase. But I still say the polarity signs, + and - ,have no place on a diagram of an AC circuit. The dot notion works just fine.

No problem. Which particular diagram? I posted a few.

 

[Besoeker]

No problem. Which particular diagram? I posted a few.

The one in post #530.

 

[jumper]

The one in post #530.

Bes, there are two diagrams in #530.

Center tap or Center tap dual voltage? top or bottom?

 

[SG-1]

I went and snagged the section I wanted from a 2017. Makes two points for me.

Seems like the NEC calls it single phase. 120/208V also.......

310.15(B)(7) Single-Phase Dwelling Services and Feeders. For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (4).

For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of 2 ungrounded conductors and the neutral conductor from a 208Y/120 volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (3).

By NEC standards this definition would make the old 2-Phase with the 90 degree phase displacement single phase.
Now to get Larry to agree.

 

[Ingenieur]

To me, when in series, placing the two dots together, or the two lines together, makes them out phase. There would be zero line-to-line output voltage. The two loose ends could be tied together without creating a short circuit; in that case, in parallel, they would be in phase then.

if the dots are symmetrical, ie, both on lines, or both on common, the vll = 0
and would be in phase cancelling
around the loop the polarities would cancel

 

[Ingenieur]

We all know what the voltage is. That wasn't the question. The question is why you seem to insist that flipping around the first coil is a "180 deg rotation" and not a "reversal". To me, those phrases are synonymous. Either one is as good as the other, and one does not cause the other.

because it is?
never said that a 180 deg phase shift did not invert 'polarity'

 

[SG-1]

I'm doubtful of that. Please explain.

Using the grounded conductor as reference you get the green & blue waveforms with 120 volts to ground.
Change reference to a phase conductor (green or blue) & you get the resultant red sine wave, almost 170 volts.
If you shift the blue or green it could exactly overlap the other, just like a the system with a 180 degree displacement.

 

[Besoeker]

Bes, there are two diagrams in #530.

Center tap or Center tap dual voltage? top or bottom?

The way they are shown connected makes them electrically the same but it is the top one that shows the waveforms of the two phases 180° apart.

 

[Besoeker]

Using the grounded conductor as reference you get the green & blue waveforms with 120 volts to ground.
Change reference to a phase conductor (green or blue) & you get the resultant red sine wave, almost 170 volts.
If you shift the blue or green it could exactly overlap the other, just like a the system with a 180 degree displacement.

Your waveforms are not correct. For the green and blue 120V is RMS, not peak. And how did you arrive at the phase relationship between them?

 

[jaggedben]

because it is?
never said that a 180 deg phase shift did not invert 'polarity'

Glad to hear it.
So inverting polarity shifts phase 180 deg?

 

[winnie]

Your waveforms are not correct. For the green and blue 120V is RMS, not peak. And how did you arrive at the phase relationship between them?

I believe that SG-1 was drawing waveforms for a '2-phase' (90 degree phase offset) 3 wire system.

If I understand his point, under the naming scheme used by the NEC where a two leg 120/208 service is called a single phase service, such a 2 leg 2 phase service would also be called single phase.

I disagree, not because the physics analogy is not correct, but because of how the two are used. Three wires, one grounded, with a phase offset between the ungrounded conductors of either 90 or 120 degrees; seems pretty similar to me...simply that they have different _uses_.

Two leg 120/208 is used for residential applications where ordinary 120/240 single phase might otherwise be used, but where you have lots of such users (apartment buildings and the like) and want to provide balanced loading to the three phase distribution system. You don't need a 'rotating field producing' phase offset.

Two leg '2 phase' is an old system used for industrial applications such as running motors, where you explicitly want a phase offset that can produce a rotating field.

-Jon

 

[LarryFine]

Nicely shows the two output waveforms out of phase.

To me, the two sine waves as shown are out of phase, but shouldn't be. The waves don't jive with the + and - symbols, which are correct.

 

[LarryFine]

By NEC standards this definition would make the old 2-Phase with the 90 degree phase displacement single phase.
Now to get Larry to agree.

I agree that both systems in those NEC excerpts are correct, but 2ph 90-degrees can not be 1ph.

 

[jaggedben]

Using the grounded conductor as reference you get the green & blue waveforms with 120 volts to ground.
Change reference to a phase conductor (green or blue) & you get the resultant red sine wave, almost 170 volts.
If you shift the blue or green it could exactly overlap the other, just like a the system with a 180 degree displacement.

I think this is wrong. What do you mean by 'if you shift the blue or green'? That doesn't make sense. They are a property of the source.

The 'special case' that is the 120/240 system involves the ability to theoretically reverse the scope probes (i.e. invert polarity) on one 'phase' and have it look identical to the other. That won't be the case for any 'phase shift' other than 180deg.

 

[Besoeker]

I believe that SG-1 was drawing waveforms for a '2-phase' (90 degree phase offset) 3 wire system.
-Jon

Then I humbly apologise.
I thought we were discussing 120-0-120 on this thread.

 

[Besoeker]

To me, the two sine waves as shown are out of phase, but shouldn't be.

That's what you see on a two-channel scope.

 

[jaggedben]

The NEC quote does point out that there is some vaugery or inconsistency in industry nomenclature. In terms of physical properties, the 120/208 3-wire system has far more in common with the 90deg '2 phase' system than it does with the 120/240 'split phase' system. The only important difference between the former two is the degree of the phase shift. Both require two primary waveforms to be brought. So from the primary point of view, they are both '2-phase', while the 'split phase' is derived from a 'single phase' primary waveform.

Far be it from me to try to change industry standard nomenclature though.

 

[ggunn]

Yes, your diagram clearly shows the primary to be single phase. But I still say the polarity signs, + and - ,have no place on a diagram of an AC circuit. The dot notion works just fine.

Ah, the difference between absolute and relative polarity. AC does not have absolute polarity but split phase waveforms display opposite relative polarity. Absolutely.