Is it Single or Two Phase?

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jim dungar said:
If there two 120V transformer windings connected in series (winding X1-X2 and X3-X4) the only reason you would say that the voltages are subtracted from each other is to justify your 180 degree logic. Because when the discussion is 3-phase wye circuits the common statement becomes "add the voltages vectorially". How about some consistency?
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No Jim,

That is the way it is. If we reference both V1 and V2 to neutral and assign zero phase angle to V1, then:

V1n = 120V @ 0
V2n = 120V @ 180

Now to obtain V12, we must measure the DIFFERENCE in potential between L1 and L2. This is fundamental in any voltage measurement.

Then V12 = V1n - V2n

V2 <----------n---------->V1

V2 ----------------------->V12

It would be more precise to say subtract vectorially.
 
zbang,

Please note that the reference you are using is about DC derived from AC 3 phase by using an anode at the end of each coil. This is referred to as "six phase" because for the DC output. It is still actually 3 phase.
 
Phase and polarity

Phase and polarity

When we have a single phase transformer, there is NO degrees of "in phase" or "out of phase". There is only ONE phase. You can not center tap a phase coil and create a second "phase". All you are doing is taking a potential difference measurement from another point in the coil. There is no 180 degrees difference in a single phase transformer.

There is however a polarity difference between each end of the coil. There is a polarity difference between a center tap and each end of the coil. There is NO phase or angle difference of a single phase coil or transformer.

ctap.jpg
 
Phase Angle/Polarity in a 120/240V Service:

Phase Angle/Polarity in a 120/240V Service:

Dave,

You have just said, I think, that the voltages on L1 and L2 relative to the neutral are inverses of each other. This is true.

It is also true that this is equivalent to a 180 degree phase difference between the two voltages! This is fundamental.

I have illustrated this with a rudimentary phasor diagram in a previous post.

Example:

Let

V1n = 170V*cos(wt)

and let V2n be the inverse of V1n,

V2n = -170V*cos(wt) = 170V*cos(wt + 180)

The phase difference between L1 & L2 is indeed 180 degrees. Check it out with a scope sometime, but be sure to trigger the scope on only one voltage.

One must understand trig and phasors to appreciate this argument.

Even so, this is NOT a two-phase system, and I have never claimed it was!
 
rattus said:
It is also true that this is equivalent to a 180 degree phase difference between the two voltages! This is fundamental.
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This can be said about a single, non-tapped secondary: the end polarities are always 180 degrees opposite one another at any instant in time. That's why I say that the presence of the tap changes nothing.
 
LarryFine said:
This can be said about a single, non-tapped secondary: the end polarities are always 180 degrees opposite one another at any instant in time. That's why I say that the presence of the tap changes nothing.
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Larry,

Yes indeed, swapping the test leads will result in a 180 degree phase shift, or in other words, reversing the polarity of a sinusoid results in a 180 degree phase difference.

You can look at V1 and V2 any way you wish, but if you use a common reference such as the neutral, you will see the 180 degree phase difference. That is why we must subtract the two phasors to obtain V12. We do the same thing in a three phase wye. We subtract (vectorially) any two phase voltages to obtain the line voltage between them.

Agreed?

But, the 120/240V system is still single-phase!
 
Rattus,

Why do you want to talk about subtracting voltages on a 1-phase 120/240V 3-wire circuit but you talk about adding voltages on a 3-phase 208Y/120 4-wire system?

Do you also say the currents in a 3-wire balanced neutral "subtract to zero"?

I am making it a personal goal to be more consistent in terminology and methodology (but then again, I also want to loose weight).
 
jim dungar said:
I am making it a personal goal to be more consistent in terminology and methodology (but then again, I also want to loose weight).
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LOL :D

I resemble that remark! :D
 
Add or Subtract?

Add or Subtract?

Jim, read my previous post again. I said you SUBTRACT phase voltages in a wye to obtain line voltages. You also subtract V2n from V1n to obtain V12 in the single phase system or vice-versa.

In general, you subtract one phasor voltage from the other (common reference point) to obtain the voltage between them.

For currents, it depends on the assumed directions of the currents. If they are in the same direction in the neutral, one ADDS the phasor currents. If they are assumed to be in opposite directions, one SUBTRACTS the phasor currents.

This gets sticky, and engineers who know better are sometimes confused by it.
 
rattus said:
That is why we must subtract the two phasors to obtain V12. We do the same thing in a three phase wye. We subtract (vectorially) any two phase voltages to obtain the line voltage between them.
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jim dungar said:
Why do you want to talk about subtracting voltages on a 1-phase 120/240V 3-wire circuit but you talk about adding voltages on a 3-phase 208Y/120 4-wire system? Do you also say the currents in a 3-wire balanced neutral "subtract to zero"?
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rattus said:
I said you SUBTRACT phase voltages in a wye to obtain line voltages. You also subtract V2n from V1n to obtain V12 in the single phase system or vice-versa. In general, you subtract one phasor voltage from the other (common reference point) to obtain the voltage between them.

For currents, it depends on the assumed directions of the currents. If they are in the same direction in the neutral, one ADDS the phasor currents. If they are assumed to be in opposite directions, one SUBTRACTS the phasor
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Keep in mind that adding a negative number is the same as subtracting a positive number. It is much more accurate to say that we add voltages or currents when doing this kind of work. This is where the phase angle comes in.

For example, when calculating neutral current, if we add the two or three line currents, keeping the relative phases in mind, we end up with the neutral (aka 'difference') current. In the case of a 240/120 1ph system, we add L1 at 0 deg. and L2 at 180 deg. and wind up with the difference.

This does not mean that there are two phases in the electrical sense, merely that the vectors of the two currents are in opposite directions, mathematically speaking. If we add 10 amps at 0 deg. to 12 amps at 180 deg., the result is 2 amps.

If we add 10 amps and 12 amps at the same vector angle (at 0, 120, or 180 degrees relative to the outside world), we end up with 22 amps, which explains why we must never share a neutral for two lines of the same electrical phase. Math is so intertwined with electricity; that's why we need to understand math.
 
Larry,

It is no more accurate to add than it is to subtract. You just have to know which operation to perform.

You must agree that to obtain a voltage difference one must subtract one phasor from another. If you add two wye phasors, your result will be grossly wrong. If you add two delta phasors, your result will be the negative of the third phasor.

Your example of neutral currents is correct. But you have said that these currents are 180 degrees apart. That means that the voltages are 180 degrees apart. Why can't anyone admit that? It does not mean there is a second phase as in the case of a true two-phase system.

Yes, an understanding of math is crucial especially vector math or phasor math which is based on trig.
 
To me, Phasors have magnitude and direction and are emanating from the origin point in the center of a 2D plane.

Two phasors describe the adjacent sides of a parallelogram.

The diagonal of the parallelogram from the origin to the corner opposite the origin is both the magnitude and direction of the "sum" of the two phasors.

I "see" this instead of the text string of trig. I derive the trig from the XY coordinate system of the 2D plane.
 
al hildenbrand said:
To me, Phasors have magnitude and direction and are emanating from the origin point in the center of a 2D plane.

Two phasors describe the adjacent sides of a parallelogram.

The diagonal of the parallelogram from the origin to the corner opposite the origin is both the magnitude and direction of the "sum" of the two phasors.

I "see" this instead of the text string of trig. I derive the trig from the XY coordinate system of the 2D plane.
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Al,

This works if you are ADDING phasors, but not for subtracting. For example consider the 120/208V wye. Let Van = 120V @ -30 and let Vbn = 120V @ -150.

Van + Vbn = 120V @ -90 which has no meaning.

Vab = Van - Vbn = 208V @ 0 which is the line to line voltage.

You could have rotated Vbn 180 degrees on your sketch, then you would get the correct result.

The difference of these two phasors
 
But when the "origin" of the phasor diagram is seen at the "virtual neutral" in the center of the Delta, the phasor at the 4 wire 120/240 volt delta center tap "neutral" has both magnitude and direction.
 
I will not agree that the L1-L2 voltage is made up of two single phase voltages seperated by 180 degrees.

I agree that the L1-N current and the L2-N current appear to be 180 degrees apart, but this is nothing more than a slight of hand.

Given a transformer with a single primary current and a single primary line-line voltage. This single primary current creates a single magnetic field in the transformer. This single magnetic field creates a current in a secondary winding X1-X2 and an identical current in another secondary winding X3-X4. The transformer is wound so that the secondary currents are in the same direction (i.e. X2->X1 and X4->X3). When the windings are connected in series per the nameplate the winding current path is effectively X4->X3&X2->X1. This arrangement provides 3 possible load current paths of X1->X4, X1->X2&X3, and X2&X3->X4. The slight of hand comes in when we apply a principle of double negatives where A=-1*(-A) or in our case, path X1->X2 = the opposite of path X4->X3

There is no Thevinan network that I have been able to create that will allow unbalanced L-N loads coincident with L-L loads while supporting the concept of the load currents flowing in opposite directions.
 
"I will not agree that the L1-L2 voltage is made up of two single phase voltages seperated by 180 degrees."

But mathematically, it can be represented that way.

I can define "up" as pointing to the ground, and "down" as pointing to the sky, and always know which way to look as long as others use the same convention.

"I agree that the L1-N current and the L2-N current appear to be 180 degrees apart, but this is nothing more than a slight of hand."

When one is at it's positive peak, the other is at its negative peak.

We can define the space around us with rectangular, cylindrical, or polar coordinates too, more mathematical slight of hand, but sometimes that slight of hand makes the math easier.



But yes, no matter how you slice it, it is single phase.
 
engy said:
When one is at it's positive peak, the other is at its negative peak.
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This is only if you insist on using the neutral as your voltage reference, and then change the relative polarity of the end points. This is like relabeling the ends of a series of batteries.

Why should the explanation and math have to change if the reference becomes one of the end points instead of the neutral?
 
rattus said:
It is no more accurate to add than it is to subtract. You just have to know which operation to perform.
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I meant grammatically more accurate.

You must agree that to obtain a voltage difference one must subtract one phasor from another. If you add two wye phasors, your result will be grossly wrong. If you add two delta phasors, your result will be the negative of the third phasor.
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If vector angle is taken into account, adding is accurate. 120 + (-120) = 120 - 120.

120 (0 deg) + 120 (120 deg) + 120 (240 deg) = 0. No subtracting, but still a zero net.

Your example of neutral currents is correct. But you have said that these currents are 180 degrees apart. That means that the voltages are 180 degrees apart. Why can't anyone admit that? It does not mean there is a second phase as in the case of a true two-phase system.
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Then we have no disagreement here. Again, even with no neutral, the ends of a single-phase AC source are 180 degrees apart.

Yes, an understanding of math is crucial especially vector math or phasor math which is based on trig.
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Then you should understand that adding two voltages of differing polarities is the same as subtracting one from another of the same polarity.
 
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