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

[LarryFine]

isn't that WHY we ground it?
to have a reference and stabilize?

No and yes.

It's not grounded for the purpose of measuring from. I've never seen a rule that one test probe must be connected to the grounded conductor. I place my solenoid tester's probes to various points when troubleshooting.

 

[LarryFine]

There are _two_ phase angles, 120V at 0 degrees and 120V and 180 degrees.

To me, they only resemble phase angles; they're opposing poles, or polarities. If they were generated as two separate waves and merely synced physically, I might almost agree.

 

[LarryFine]

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.

Agreed.

 

[LarryFine]

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.

Doesn't matter; one core, one phase.

Show me a three-phase transformer where all coils share one core leg.

 

[jumper]

standard in power engineering
phasors

We are referring to Wayne’s methodology. Using -1 here instead of 180d.

At basic electrical level you learn what is taught in that all about circuit link and the other one Mivey posted.

I understand what he is doing from getting an electronics degree, but never went into that in applied electricity.

 

[LarryFine]

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

But which one can be shown instantaneously and which one requires the time factor? For 3ph, yes, there are three separate waves, but 1ph has only one, not two.

 

[LarryFine]

V_L1-L2(t) = -1 * V_L2-L1(t). If the voltage V_L1-L2 is 240V, the voltage V_L2-L1 isn't "negative 240V."

Sure it is! That's more accurate than throwing in a "wait a half cycle and check it again."

 

[jumper]

Doesn't matter; one core, one phase.

Show me a three-phase transformer where all coils share one core leg.

One core, two windings in series. One phase.

But i scope it, gonna use the noodle as a common reference. Two phases.

 

[LarryFine]

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

Because it isn't. It's one waveform that you're dividing into two of half the voltage by arbitrarily tapping the source in the center.

It's no different than using two 6v batteries in place of one 12v battery, and choosing to place a voltmeter probe on the jumper.

Now, if you choose, using the battery example with a meter probe on the jumper, to call that two sources at 180 degrees apart, I will still disagree, but I will also see your point.

 

[LarryFine]

One core, two windings in series. One phase.

But i scope it, gonna use the noodle as a common reference. Two phases.

That's not mandatory, it's a choice. If the British end-grounded 240v secondary happened to have an unused center tap, would it change between being one phase or two phases as you moved your probes among the three terminals?

Technically speaking, an open Delta is an excellent example of a true two phase source. It requires two primary sources that are not in phase, whether separated by 90 degrees or 120 degrees.

 

[romex jockey]

I wonder if i'm too old to go back to school now..... ~RJ~

 

[jumper]

I wonder if i'm too old to go back to school now..... ~RJ~

Prolly......j/k

 

[jumper]

That's not mandatory, it's a choice. If the British end-grounded 240v secondary happened to have an unused center tap, would it change between being one phase or two phases as you moved your probes among the three terminals?

Technically speaking, an open Delta is an excellent example of a true two phase source. It requires two primary sources that are not in phase, whether separated by 90 degrees or 120 degrees.

Of course it is a choice.

That is the whole point!

How you pick your reference point determines what results you will get.

 

[wwhitney]

No one ever responded to this thought experiment showing that insistence on using the grounded conductor as the voltage reference leads to odd results. Maybe it was just too out there?

Cheers, Wayne

Here's a thought experiment: say I have a bunch of equipment that operates at 40 VAC. I choose to use a 120V : 120V isolation transformer where the secondary is tapped twice, producing three equal segments and 4 circuit conductors. Call the circuit conductors A-D in order. The voltage A-B, B-C, or C-D is 40V; the voltage A-C or B-D is 80V; and the voltage A-D is 120V.

Now NEC 250.20(B) requires me to ground one of the circuit conductors, but 250.26 doesn't specify which conductor to ground. I can choose to ground conductor A, take it as 0V, and then the conductor voltages are (0V, 40V, 80V, 120V). I think we can agree that in this case legs B, C, and D are all in phase.

Or I can chose to ground conductor C and take it as 0V. Then the conductor voltages are (-80V, -40V, 0V, 40V). Are legs B and D now out of phase simply because I've moved both the earthing point and the 0V reference? No, they are still in phase, their voltage ratio has simply changed from 3/2 to -1.

 

[jumper]

No one ever responded to this thought experiment showing that insistence on using the grounded conductor as the voltage reference leads to odd results. Maybe it was just too out there?

Cheers, Wayne

Too out there.......

Sorry, but it is a rabbit hole and I ain’t diving in.

We is talking ‘bout single phase 120/240 V distribution trannies here!

 

[jaggedben]

I think Wayne's thought experiment well demonstrates that grounding is conceptually arbitrary. Human conventions for grounding and measuring don't determine which mathematics are accurate for describing the physics.

 

[winnie]

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.

Agreed. I was trying to avoid going down the tangent that somehow there was a difference between an actual single coil with a center tap (which might be found on a three phase transformer with a delta secondary) and two coils on the same core connected in series (as is the common case with a 'pole pig'). Heck, the analysis would apply even if 'pole pigs' were autotransformers (which they are _not_!)

-Jon

 

[jumper]

Agreed. I was trying to avoid going down the tangent that somehow there was a difference between an actual single coil with a center tap (which might be found on a three phase transformer with a delta secondary) and two coils on the same core connected in series (as is the common case with a 'pole pig'). Heck, the analysis would apply even if 'pole pigs' were autotransformers (which they are _not_!)

-Jon

I agree also. Larry was also thinking the delta set up. Analysis wise good....if the person reading understands.

 

[GoldDigger]

V_L1-L2(t) = -1 * V_L2-L1(t). If the voltage V_L1-L2 is 240V, the voltage V_L2-L1 isn't "negative 240V."

Of course it is, otherwise V_L1-L2 + V_L2-L1 would be 480!
Actually, of course, the problem is that you are conflating the idea of a coefficient (where the "-" means something) with a magnitude, which must always, by definition, be non-negative.

 

[jumper]

Of course it is, otherwise V_L1-L2 + V_L2-L1 would be 480!
Actually, of course, the problem is that you are conflating the idea of a coefficient (where the "-" means something) with a magnitude, which must always, by definition, be non-negative.

And why is this a problem? It is a sine wave signal so of course it has a magnitude. No?