Single Phase Theroy

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rattus said:
However, it is incorrect to imply that this is a 2-phase system because the extra "phase" is created by a simple inversion--not a second generator.
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If you took two individual, but synchronized 120v 1ph sources, and connected them "in phase" in such a way that they'd form a 120/240v source, like we get from two 120v windings in series, I'd still call that single phase.
 
buffnitup said:
These voltages are 180 degrees out of phase with each other.
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I disagree. If they weren't "in phase," they couldn't add to 240v. They are of opposite polarity, from the neutral's perspective.

Not sure why they are opposite from each other but...
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Because you're using the mid-point as a reference. Just like a pair of batteries in series, the ends are of opposing polarity.
 
wasasparky said:
Now that I think about it, are you half way up the hill? or half way down the hill?
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Depends on where you are. ;)

Expanding on your analogy, what's the difference between hills flanking a valley and valleys flanking a hill? Your perspective.

If we look at a sine wave from the zero-crossing point perspective, we have two polarities, hills below us and valleys above us.

If you look from the top of a hill (pos. peak), we only have valleys, but if you look from a valley (neg. peak), we have only hills.

From either peak, we would only see single slope directions, up or down. But, what changes because we relocate our perspective?

Nothing, in my opinion.
 
rattus said:
I said V1n and V2n are separated by 180 degrees. This is easily demonstrated with a dual channel scope. I would not measure Vn2 with a scope because V2n would be shorted to the EGC.
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In Europe, yes, but not in the USA.

But you are wrong on the neutral. If there is no neutral, then the subscript "n" is meaningless and we can only measure V12 and V21 which are also separated by 180 degrees.
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That's what it resembles, yes.
 
rattus said:
You are half right Jim, but that is not what I said. I said V1n and V2n are separated by 180 degrees. This is easily demonstrated with a dual channel scope. I would not measure Vn2 with a scope because V2n would be shorted to the EGC.
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You stated a true fact, where you kept a common reference probe/point.
I stated a true fact, where I kept a common relationship between my isolated probe sets.

But you are wrong on the neutral. If there is no neutral, then the subscript "n" is meaningless...
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Are you saying the neutral connection changes how my (2) single phase voltages are connected, or just how your choice of neutral as a reference affects measuring them?

...and we can only measure V12 and V21 which are also separated by 180 degrees.
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If you only have points 1 and 2, how can you measure (2) voltages?
 
rattus said:
... and we can only measure V12 and V21 which are also separated by 180 degrees.
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jim dungar said:
If you only have points 1 and 2, how can you measure (2) voltages?
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Agreed. It's easy to forget that you're effectively "swapping the polarity" of your test leads when you keep one on N and move the other from L1 to L2, and should flip the sign (add a negative) of one of the readings.

If you were using a DC meter to test each of a pair of batteries in series, you'd move both leads from battery to battery. If you kept one lead on the 'center tap', of course one battery would appear to have a reversed polarity.
 
LarryFine said:
I disagree. If they weren't "in phase," they couldn't add to 240v. They are of opposite polarity, from the neutral's perspective.
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Larry, You speak with forked tongue. Two sinusoids of opposite polarity ARE 180 degrees apart. One and the same! Still single phase though!!

And, in general, to find the potential DIFFERENCE between two points, you SUBTRACT the potential at one point from the potential at the other. That is what the DIFFERENCE is all about. Of couse you must use a common reference--the CT--to do this. The math works.

It is also valid to simply add the magnitudes of V1n and V2n as you and Jim are doing. Although you cannot do this with a wye.
 
dbuckley said:
I would argue that the two hots are of opposite polarity but in phase - there is no time difference between the zero crossing points.
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dbuckley, to be in phase, the peaks must also coincide. With your definition, a wave would be in phase with its inverse.
 
rattus said:
It is also valid to simply add the magnitudes of V1n and V2n as you and Jim are doing. Although you cannot do this with a wye.
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But you do add them in a delta.

Not all systems have neutral points. We should not have to define or change the number of phases solely based on the presence of a neutral.
 
LarryFine said:
Okay, but we know the source is 3ph. :cool:


I'm getting picked on again! :mad: :)grin:)
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I'm not picking on you Larry, I'm just making the point that calling that 120/208 "single-phase" service single-phase is....... well....... just not quite right. I've argued in the past on this forum (and a lot of the posters have said I'm wrong) that this is not a single-phase service.
 
hardworkingstiff said:
I'm not picking on you Larry, I'm just making the point that calling that 120/208 "single-phase" service single-phase is....... well....... just not quite right. I've argued in the past on this forum (and a lot of the posters have said I'm wrong) that this is not a single-phase service.
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See your point Stiff, but the loads are all single phase, that is there are no 2-phase motors to be driven.

And, we can't call it 2-phase because that designation applies to the obsolete system that none of us have ever seen, or at least none under 80!
 
jim dungar said:
Are you saying the neutral connection changes how my (2) single phase voltages are connected, or just how your choice of neutral as a reference affects measuring them?
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Jim,

Of course I can't change the connection, even if I could climb poles which I tried only once.

Yes, the choice of reference only makes a difference in the phase angle of the measured voltage.

If you only have points 1 and 2, how can you measure (2) voltages?
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You swap leads if only in your mind, and if your scope is synched properly, you will see the phase shift.
 
jim dungar said:
But you do add them in a delta.

Not all systems have neutral points. We should not have to define or change the number of phases solely based on the presence of a neutral.
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Jim, I don't think anyone is suggesting that. The CT/neutral is always there in the 120/240V single phase system. If we choose that as a reference, it does not change the number of phases.
 
LarryFine said:
If you took two individual, but synchronized 120v 1ph sources, and connected them "in phase" in such a way that they'd form a 120/240v source, like we get from two 120v windings in series, I'd still call that single phase.
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I would too! But we can still swap leads and see a phase shift, but not another phase.
 
hardworkingstiff said:
I'm not picking on you Larry, I'm just making the point that calling that 120/208 "single-phase" service single-phase is....... well....... just not quite right. I've argued in the past on this forum (and a lot of the posters have said I'm wrong) that this is not a single-phase service.
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According to IEEE it is.......... 120/208Y is a modification of a three phase, 4 wire, system, where two legs are used to create a single-phase, 3-wire, open wye application.
 
LarryFine said:
Agreed. It's easy to forget that you're effectively "swapping the polarity" of your test leads when you keep one on N and move the other from L1 to L2, and should flip the sign (add a negative) of one of the readings.
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No Larry, if we choose "N" as the reference, we would record the measurements as is. We would NOT negate one of the readings! The whole idea of a common reference is to express the voltages relative to that common reference.

Let us connect batteries in such that we have a total of 240Vdc. If we add a center tap and call this "N" and call the positive and negative terminals L1 and L2, we would have,

V1n = +120Vdc
V2n = - 120Vdc

Just like many electronic circuits where,

Vcc = +5V
Vgg = 0V
Vee = -5V
 
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