Why is residential wiring known as single phase?

[gar]

120309-0809 EST

For those who are arguing that we have simply swapped the meter leads I have the following comments:

If you have really done some circuit analysis, then you understand that the sense (polarity or phase) requires picking a reference point from which to work.

Consider a vacuum tube radio. Typically you would pick the chassis (common, ground, neutral, etc.) as the DC voltage reference point. Plate voltages would be positive relative ground or the tube would not conduct. Grid bias voltage would be negative relative to ground or the tube would be driven into saturation, and if the plate series resistance was low, such as a transformer load, then you would also burn out the tube by overheating the plate.

With a typical DC voltmeter the meter has a scale from 0 to a max. There are few zero-centered meters. To measure the grid bias you would put the positive meter lead on ground and use the negative lead as your probe. But you would know and understand that the meter was reading a negative voltage relative to ground.

With an oscilloscope or typical digital voltmeter you would put the common meter lead on ground and leave it there and directly read + and - from the instrument.

You put the instrument reference lead at the point from where you want to make (reference) your measurement.

In the above vacuum tube example you would not describe the grid bias as positive simply because the analog meter read upscale.

Now consider a 12 V battery with a series load resistance of 10 ohms plus 2 ohms. The 2 ohm end is connected to the battery minus terminal. What is the voltage across the 2 ohm resistor relative to the node between the 10 ohm and 2 ohm resistors? Is it +2 V or is it -2 V? What is the voltage across the 10 ohm resistor relative to the same node? Is it +10 V or -10 V?

Back to the vacuum tube amplifier. Put an AC signal on the grid. Synchronize an oscilloscope to the input signal. Display both the AC input signal (grid voltage) and the AC output signal (plate voltage). Are these two signals "in-phase" or 180 degrees "out-of-phase"?

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[pfalcon]

I'm afraid you're math is wrong. If V_1n = V_m*sin(wt) and V_n2 = V_m*sin(wt), then V_2n = +V_m*sin(wt+180)

That made your vectors wrong as well. Using vector addition on the following will result in zero volts, not 240 volts.

-120@0 + 120@0 = 0

It's -120 at the tail, not the head. Correct for the designated reference point.

-120@0 -------------> 0 -------------> +120<0
equivalent
0 ----------------> +120@<0 -----------> +240<0
by moving the zero reference

 

[rattus]

Flip a coin. As arguments for the Sine function, either wt OR wt + PI may be used - but not both.

I flipped a penny and it chose wt + PI.

(wt + PI) is then the 'phase' of V2n.

wt is the 'phase' of V1n.

 

[jim dungar]

If you have really done some circuit analysis, then you understand that the sense (polarity or phase) requires picking a reference point from which to work.

We have chosen a reference: The realtionship between the primary and the secondary winding outputs and the resulting actual connections of them.

 

[rattus]

It's -120 at the tail, not the head. Correct for the designated reference point.

-120@0 -------------> 0 -------------> +120<0
equivalent
0 ----------------> +120@<0 -----------> +240<0
by moving the zero reference

The reference point is the neutral. Quit messing with MY phasor diagram. It is the way I want it.

 

[pfalcon]

For those who are arguing that we have simply swapped the meter leads I have the following comments: ...
If you have really done some circuit analysis, then you understand that the sense (polarity or phase) requires picking a reference point from which to work. ...

You spend a lot of time giving examples that no one is arguing over. The lead direction is important if you're trying to determine why single phase is single phase; for which rattus's phasor diagram has always been incorrect. But for most other purposes the direction of the phasors is not relevant as long as you keep them consistent. Rattus can't seem to get beyond that. For his applications in the field, the phasors can be drawn either direction and therefore he insists on using the most convenient. Fine. Unquestionably it's easier to work with. But if you want to go back to the OP question, then no, you have to get the leads right and phasors right. Different application, different rules. Which also explains why rattus thinks the phase constant keeps disappearing. It's not. It doesn't disappear, it equates at all values and therefore cancels. Axiomatic mathematics.

Take your comment for example: Plate voltages would be positive relative ground or the tube would not conduct.
Ground is an arbitrary term with an arbitrary value we typically assign as zero volts. We don't have to assign zero volts. We could assign 1000 volts. And as long as we use the value consistently all the math will work out and the device will function. Rattus uses his numbers consistently. Therefore they work out.

 

[pfalcon]

The reference point is the neutral. Quit messing with MY phasor diagram. It is the way I want it.

So sad. Too bad. It's a discussion forum.

Yes, it's the way you want it. It also has nothing to do with why single phase is single phase. Your diagram is application dependent.

 

[K8MHZ]

Over 2,000 posts to answer a relatively simple question.

Unbelievable.

 

[david luchini]

It's -120 at the tail, not the head. Correct for the designated reference point.

-120@0 -------------> 0 -------------> +120<0
equivalent
0 ----------------> +120@<0 -----------> +240<0
by moving the zero reference

A vector doesn't have a different magnitude at the head and the tail.

But your other diagram showed a reference at the head for each vector, and it was also incorrect.

-120Vrms @ PI <----------0---------->120Vrms @ 0

120@0 - (-120@PI) = 0

 

[rbalex]

I flipped a penny and it chose wt + PI.

(wt + PI) is then the 'phase' of V2n.

wt is the 'phase' of V1n.

Fair choice.

I flipped the same penny and got

V1n = Vm*sin(wt)
V2n = -Vm*sin(wt)

I flipped it again and got

V1n = -Vm*sin(wt +PI)
V2n = Vm*sin(wt + PI)


Both sets equally valid to

V1n = Vm*sin(wt)
V2n = Vm*sin(wt+ PI)

Edit add: You did read the part about "but not both", right?

 

[rbalex]

Would love to get into this discussion..just no enough time!

HEY - you hijacked Post 2000. That officially belongs to gar

 

[gar]

120309-1036 EST

pfalcon:

Sure I can put the chassis at 1000 V relative to the earth. But for my circuit analysis the chassis is going to be zero as my reference because that makes sense in relation to the vacuum tube circuit I am analyzing. I should have pointed out that my cathode of the tube was at common.

When you look at the characteristic curves for vacuum tubes you find that these are generally defined in reference to the cathode.

In the early radio days a separate battery was used to provide the negative grid bias, and a higher voltage battery to supply the positive plate voltage. Then along came the idea that you could avoid the separate grid bias power supply by using a resistor in the cathode circuit to common with a large shunt capacitor. Would not work for a DC amplifier, but works fine for audio and higher frequency applications. This effectively created a negative bias on the grid without the need for a separate power supply. Use was made of the average plate current in combination with the value of the cathode resistor to determine the grid bias. Still to analyze the circuit using the tube characteristic curves the voltage measurements are made relative to the cathode. So there is still a negative bias on the grid relative to the cathode even though the DC bias on the grid relative to common is 0 V.

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[rattus]

Fair choice.

I flipped the same penny and got

V1n = Vm*sin(wt)
V2n = -Vm*sin(wt)

I flipped it again and got

V1n = -Vm*sin(wt +PI)
V2n = Vm*sin(wt + PI)


Both sets equally valid to

V1n = Vm*sin(wt)
V2n = Vm*sin(wt+ PI)

Edit add: You did read the part about "but not both", right?

Yes, and I thought it was nonsense.

Why don't you respond to my post # 1992?

 

[rattus]

Reference Nodes:

Reference Nodes:

Why is there such resistance to using the neutral/ground as a reference in the split phase system? It is used in the three phase wye without any objection.

It makes sense because for all intents and purposes the ground potential at a point is stable. The strongest argument for using it is that it is common to both V1 and V2. Just like gar's example.

Of course, there is no common in a delta, but even then it is convenient to ground a corner to facilitate analysis--all on paper of course.

 

[rbalex]

Yes, and I thought it was nonsense.

Why don't you respond to my post # 1992?

OK

Two functions exhibit the same phase if and only if their start points coincide. True - if they also share the same period That is (wt + phi1) = (wt + phi2), that is phi1 = phi2. NOT True; true when sin (wt + phi1) and sin (wt + phi2) are equal at a common t_0.

You just can't help yourself moving the arguments indiscriminately in and out of the functions can you?

For sines, the start point is the negative to positive transition. Also NOT true, That may be a convienient convention but isn't a requirement - even from your attenuated defintition. Do you not understand what arbitrary means?

But for the split phase system, V1n and V2n start PI radians apart. Also NOT True - they both start at 0 at t₀ = 0 Edit add: Assumming phi1 = 0 and phi2 = PI

Therefore the two voltages exhibit two phases. Also NOT true since you began from a false premise.

 

[rattus]

OK

Two functions exhibit the same phase if and only if their start points coincide. True - if they also share the same period That is (wt + phi1) = (wt + phi2), that is phi1 = phi2. NOT True; true when sin (wt + phi1) and sin (wt + phi2) are equal at a common t_0.

You just can't help yourself moving the arguments indiscriminately in and out of the functions can you?

For sines, the start point is the negative to positive transition. Also NOT true, That may be a convienient convention but isn't a requirement - even from your attenuated defintition. Do you not understand what arbitrary means?

But for the split phase system, V1n and V2n start PI radians apart. Also NOT True - they both start at 0 at t₀ = 0 Edit add: Assumming phi1 = 0 and phi2 = PI

Therefore the two voltages exhibit two phases. Also NOT true since you began from a false premise.

I arbitrarily choose the negative to positive transition as my start point. Then sin(wt) starts at t0 = 0. The negative to positive transition starts at PI for V2.
The function is shifted by PI radians.

It is silly to say that two waves separated by PI radians, with differing phase constants carry the same phase.

To carry the same phase, phi1 = phi2, then they would be in-phase as well.

What if the separation was PI/2, would you still claim they are of the same phase?

 

[jim dungar]

Why is there such resistance to using the neutral/ground as a reference in the split phase system? It is used in the three phase wye without any objection.

When did I say you cannot choose the reference point you want to use?
The problem seems to be you choosing only parts of the equality: -V2n=Vn2. You have 'an inversion' and 'a rotation', but your analysis seems to only focus on the rotation.

It makes sense because for all intents and purposes the ground potential at a point is stable. The strongest argument for using it is that it is common to both V1 and V2. Just like gar's example.

Potential to dirt (ground) means nothing to me except when I am dealing with issues concerning lightning, however potentials to a common reference plane are extremely important to me.

Of course, there is no common in a delta, but even then it is convenient to ground a corner to facilitate analysis

Choosing an 'always' reference is often very in convenient. There a 'single' common in high-leg deltas, zig-zag or T transformer connections either. Corner grounded delta primaries feeding neutral grounded wye secondaries also have two different relations to ground.

--all on paper of course.

Do what you want on paper. I guess you have missed where I said I do this all of the time, but when I do, I apply V2n=-Vn2 in a consistent manner and I understand how my paper analysis relates back to my real world reference.

 

[engy]

Does anyone think we are making this too complicated?:happyyes:
We need to look at this from a higher level, and understand the real meaning of a ?phase?.
Let?s start without a neutral.
If we reference L1 and look at L2 we have 240V single phase.
If we reference L2 and look at L1 we have 240V single phase.
We don?t have two phases. It?s the same one.

For the same reason if we happen to use the center as out reference, we still only have the one phase, we are just choosing a new reference. Yes, you can make a formula that shows a ?phase relationship? of 180 degrees, but you didn?t change the physics of what is really there.

Otherwise three-phase is really six-phase, and that probably deserves its own thread?

I will bring up my hill analogy. Imagine being at the top of a 240? high hill. If we walk half way down, we still have one hill that we are in the middle of. Not two hills, one up and one down.

 

[rbalex]

I arbitrarily choose the negative to positive transition as my start point. Then sin(wt) starts at t0 = 0. The negative to positive transition starts at PI for V2. So - you get 4 out 5 of your assertions wrong and still can't get it?

The function is shifted by PI radians. No - now you are attempting to shift the starting time for V2. They must have the same starting time - or we aren't talking about a conventional 120/240V system anymore.

It is silly to say that two waves separated by PI radians, with differing phase constants carry the same phase. Not at all - You just don't like it

To carry the same phase, phi1 = phi2, Not necessarily then they would be in-phase as well. Yes - So?

What if the separation was PI/2, would you still claim they are of the same phase? No - but that wouldn't be a conventional 120/240V system, would it?

 

[Rick Christopherson]

Why is there such resistance to using the neutral/ground as a reference in the split phase system? It is used in the three phase wye without any objection.

That's the part that's personal preference that you want to force onto everyone. The only time I would consider using a common reference point is when I'm poking around a system with a volt meter to troubleshoot it. I would never use it for designing a complex circuit or system, because it needlessly complicates nodal or loop analysis. When designing systems, rarely do you have single, simple devices connected phase to neutral. It is a large network of devices, and each node in the system has its own expression.

Even on a 3-phase wye system, a common reference point is applicable only when you have loads connected phase-to-neutral, but fails when you have (or also have) phase-to-phase loads. Why choose a reference point that is not universal for the whole system?

The converse question back to you would be, why do you want to add an extra phase angle when none is really required? A minus sign is a lot easier to deal with than a phase angle (not all loads are reactive).