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

[ggunn]

Just how many angels can dance upon the head of that pin, anyway?

 

[gar]

180723-2004 EDT

Going back to my transformer example with two isolated equal voltage transformer secondaries.

If L2 is connected to L4, and before connecting L1 to L3 you measure the voltage between L1 and L3, then the reading will be about 0. These two voltages are in phase referenced to L2L4. Essentially zero displacement between them. Since vL1 equals vL3 you can connect L1 to L3 with no fireworks.

Change the connection so L2 connects to L3, then measure the voltage between L1 and L4. Instead of 0 V the reading is 2 times the voltage of one secondary. Now the voltages relative to L2L3 are 180 degrees apart. Don't connect L1 to L4, sparks will fly.

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[jumper]

Just how many angels can dance upon the head of that pin, anyway?

Seraphim or Cherubim?

 

[jumper]

Perhaps it is just too simple. It really is such a basic thing to go back to how a voltage is defined.

.

Yeah, pretty much but this silly merry go round is running again. Crazy.

 

[Ingenieur]

Yeah, pretty much but this silly merry go round is running again. Crazy.

the question is who is riding?
absolutes do not apply here
the example of deriving 3 ph from 2 is a good example
both positions can be 'correct'
but the fact is there are 2 waveforms with 180 deg difference

"Whoever undertakes to set himself up as a judge of Truth and Knowledge is shipwrecked by the laughter of the gods”

 

[GeorgeB]

Let's try another thought ... obtaining full wave rectification by 2 diodes in "L1" and "L2" referenced to the center tap (neutral). One is negative (call it inverted or phase shifted 180 degrees, I don't care) from the other which may help understand things.

 

[mbrooke]

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.

A bit late for me, but well said :happyyes:

 

[buffalonymann]

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.

No, you are mistaken. Current is the work performed. Canceled current means canceled work. Canceled currents would also mean no money for power company. I've told you how it works, I can't make you believe reality

 

[buffalonymann]

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

I wasn't missing or ignoring what you pointed out, I asserted single phase center tapped xfmr is not two phase power nor is there 180 phase shift, and the physics proves this assertion. when people attempt to look at power with a scope they need to understand what they're looking at.

 

[buffalonymann]

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.

the picture represents somebody's misunderstanding electrical theory - I've explained how it works.

 

[buffalonymann]

I've noticed many you don't simply consult electrical theory to determine how single phase xfmr works. A couple guys have the correct understanding, several do not based on everything but electrical theory.

 

[jumper]

the question is who is riding?

No me! Been there, done that. We once had the true super thread on this topic, 2500+ posts.

It got so acrimonious that the mods closed and cyber nuked it. Gone.

This thread ain’t gonna solve it either.

I know what single phase is just as most of the members (including you by all means) do also.

I understand the relationships in respect to the various voltages involved in different systems/networks/services as others here do also.

Now if we could all work out a way to explain it to some newbie without baffling the poor guy to death, it would be great. Prolly ain’t gonna happen.

 

[mbrooke]

No, you are mistaken. Current is the work performed. Canceled current means canceled work. Canceled currents would also mean no money for power company. I've told you how it works, I can't make you believe reality

Watts is work performed. Voltage has to push that current.

 

[Little Bill]

If this can't be discussed without personal comments about others then it will end.
Keep your comments to the subject being discussed.
Also, there could be some infractions given if it crosses the line!

 

[zbang]

When we get back to what I think was the original question, which was something about calling a single center-tapped coil two-phase, I think the answer always comes back as No. It might have two "phases" if you look from the right direction, but most of us know "two-phase" as a different system.

Or look at it this way- I can drive in screws with a hammer, but in common usage it's not still called a screwdriver, and if I can call it one, very few others will know what I'm talking about ("No, mate, that's a hammer.")

It doesn't have to make sense.

 

[jumper]

When we get back to what I think was the original question, which was something about calling a single center-tapped coil two-phase, I think the answer always comes back as No. It might have two "phases" if you look from the right direction, but most of us know "two-phase" as a different system.

.

:thumbsup: x 2.

I missed given you kudos for an earlier post, so you get 2 for this one.

 

[wwhitney]

For the "180 degrees apart" camp, say I have a device that measures voltage versus time between points A and B. And say I have two of them, and I hook up the second one with the leads reversed, so now I can compare the two graphs. What would you call the relationship between those two graphs?

Cheers, Wayne

 

[jumper]

For the "180 degrees apart" camp, say I have a device that measures voltage versus time between points A and B. And say I have two of them, and I hook up the second one with the leads reversed, so now I can compare the two graphs. What would you call the relationship between those two graphs?

Cheers, Wayne

Pretend I am an apprentice in school/work with nothing but a multimeter and know the voltages and wire sequences for the various systems.

And the IEEE and NEC reference sections.

What else is needed for me to work equations and install equipment?

And yes I know complex numbers and math also.

 

[GoldDigger]

The mathematicians would have no problem with any of these questions.
They would start by noting that the phasors constitute a two dimensional space. And any two *non-parallel* phasors constitute a "basis" of that space.
That means that any arbitrary phasor can be formed as a linear combination of the two basis phasors. (That is by multiplying each basis phasor br a scalar coefficient and adding the results.)
Note that two parallel , whether in the same or opposite direction, cannot form a basis.
The rest all boils down to what you can do with common electrical components like transformers to manipulate the phasors.

Sent from my XT1585 using Tapatalk

 

[jumper]

The mathematicians would have no problem with any of these questions.
They would start by noting that the phasors constitute a two dimensional space. And any two *non-parallel* phasors constitute a "basis" of that space.
That means that any arbitrary phasor can be formed as a linear combination of the two basis phasors. (That is by multiplying each basis phasor br a scalar coefficient and adding the results.)
Note that two parallel , whether in the same or opposite direction, cannot form a basis.
The rest all boils down to what you can do with common electrical components like transformers to manipulate the phasors.

Sent from my XT1585 using Tapatalk

Yeah I get it but please explain it in a way that a seasoned apprentice with skills could understand. Nothing beyond that please.

Can we call single phase center tapped tranny two phase? No. NEC wise? Again no.

Can we identify three distinct voltages/signals from certain single phase systems? Yes.

For a 120/240 set up. Two 120v and one 240V. Is this 2 phase? Is this 3 Phase? No to both.

Goldy, you are way smarter than me, these are rhetorical questions but since you are smarter than me, I figure you know that.