Is split-phase AC the same as "regular" AC?

[Randy222]

This is completely confusing. What are you even trying to say here?

In both of Analog Kid's diagrams, there is only one phase feeding the primary of the transformer, and only one phase on the secondary. So there are no phase angles, no 120° anywhere.

Confusing? 

Reply #95. Am I not seeing 3 phases with two of them hooked to the primary?
It's actual how it works when there is phase shift between the feed lines. The red line is your volt meter or scope across the primary coil of the transformer when you hook the primary across two phases that are shifted 120deg, or 180deg, or 0deg.

[IanB]

This is completely confusing. What are you even trying to say here?

In both of Analog Kid's diagrams, there is only one phase feeding the primary of the transformer, and only one phase on the secondary. So there are no phase angles, no 120° anywhere.

Confusing? 

Reply #95. Am I not seeing 3 phases with two of them hooked to the primary?
It's actual how it works when there is phase shift between the feed lines. The red line is your volt meter or scope across the primary coil of the transformer when you hook the primary across two phases that are shifted 120deg, or 180deg, or 0deg.

To have a measurable phase angle, you need more than two wires. Since there are only two wires going to the primary, it is a single phase arrangement, and there are no phase angles involved. There is a single phase feeding the primary, and a single (split) phase drawn from the secondary.

[IanB]

To be clear about this, a phase angle is a difference between two AC voltages, just like a voltage is a difference between two wires. And since a voltage is a difference between two wires, the only way to have a difference between two AC voltages is to have at least three wires so that you can have two voltages to compare.

It doesn't make sense to talk about a phase angle as an absolute quantity.

[Randy222]

This is completely confusing. What are you even trying to say here?

In both of Analog Kid's diagrams, there is only one phase feeding the primary of the transformer, and only one phase on the secondary. So there are no phase angles, no 120° anywhere.

Confusing? 

Reply #95. Am I not seeing 3 phases with two of them hooked to the primary?
It's actual how it works when there is phase shift between the feed lines. The red line is your volt meter or scope across the primary coil of the transformer when you hook the primary across two phases that are shifted 120deg, or 180deg, or 0deg.

To have a measurable phase angle, you need more than two wires. Since there are only two wires going to the primary, it is a single phase arrangement, and there are no phase angles involved. There is a single phase feeding the primary, and a single (split) phase drawn from the secondary.

Last I looked, a 3ph feeder has three wires.
If you want to call Ph1 + Ph2 = one phase, so be it. What you name it does not remove fact the the diagram in #95 is using two phases that are shifted from each other by 120deg.  

[Andy Chee]

So just for my own benefit, is it safe to say that this is how split phase is derived from 2 phases of 3-phase power?

(Attachment Link)

(I only drew 3 lines for the 3-phase part. There's no ground conductor there, is there?)

Basically, yes, but the primary (MV) side is an uncommon arrangement, as it requires two line conductors to feed the transformer.

It is more common to ground the low side of the MV primary winding and common it with the LV ground/neutral. (The three MV phases will commonly come from a grounded wye secondary of the supply transformer at the substation, so ground/neutral can be the return from the pole pig back to the substation.)

Something like this?

Most of our streets have 4-wires (3 phase Wye configuration hot-hot-hot-neutral). 

Your diagram appears to show 3-wire delta configuration, your connection between "hot" and ground/neutral will not work on delta configuration....because there's no neutral!!

As an aside, some of our streets have transformers to convert Delta to Wye, as seen in this photo;

[IanB]

Last I looked, a 3ph feeder has three wires.
If you want to call Ph1 + Ph2 = one phase, so be it. What you name it does not remove fact the the diagram in #95 is using two phases that are shifted from each other by 120deg.  

Once again, no.

If you take any pair of wires from a three phase circuit, that gives you one phase. Two wires, one phase.

They are not "shifted from each other" because they carry a single phase AC voltage.

To have a phase angle or phase shift, you need at least a third wire in the circuit.

But two wires alone are just one phase.

[Randy222]

To be clear about this, a phase angle is a difference between two AC voltages, just like a voltage is a difference between two wires. And since a voltage is a difference between two wires, the only way to have a difference between two AC voltages is to have at least three wires so that you can have two voltages to compare.

It doesn't make sense to talk about a phase angle as an absolute quantity.

That sounds a bit confusing.

Is "120deg" not an absolute number in std 3ph generation? 

Delta or Y generators, 120deg is it, physically constructed that way.

[Andy Chee]

To be clear about this, a phase angle is a difference between two AC voltages, just like a voltage is a difference between two wires. And since a voltage is a difference between two wires, the only way to have a difference between two AC voltages is to have at least three wires so that you can have two voltages to compare.

It doesn't make sense to talk about a phase angle as an absolute quantity.

That sounds a bit confusing.

Is "120deg" not an absolute number in std 3ph generation? 

Delta or Y generators, 120deg is it, physically constructed that way.

"120deg" is relative, not absolute.

If you can't get your head around the concept, think of GMT time offsets as relative time, and GMT itself as absolute time.

[IanB]

Your diagram appears to show 3-wire delta configuration, your connection between "hot" and ground/neutral will not work on delta configuration....because there's no neutral!!

Ah, but those electrical engineers are clever fellows! They have thought of that. They only install a transformer with a single wire MV feed when the MV circuit has a wye configuration with a neutral/ground return path back to the substation 

[Randy222]

Last I looked, a 3ph feeder has three wires.
If you want to call Ph1 + Ph2 = one phase, so be it. What you name it does not remove fact the the diagram in #95 is using two phases that are shifted from each other by 120deg.  

Once again, no.

If you take any pair of wires from a three phase circuit, that gives you one phase. Two wires, one phase.

They are not "shifted from each other" because they carry a single phase AC voltage.

To have a phase angle or phase shift, you need at least a third wire in the circuit.

But two wires alone are just one phase.

Respectfully do not agree with that.

Two independent voltages derived from two separate coils at different times will combine. This does not remove the fact that the two lines are literally phase shifted 120deg from each other.

Work the problem the other way. How could you possibly get higher voltage in the "1ph" than the peak voltage on any of the 2ph lines? Simply not possible with 1ph, the source lines must be phase shifted. 

[Andy Chee]

Your diagram appears to show 3-wire delta configuration, your connection between "hot" and ground/neutral will not work on delta configuration....because there's no neutral!!

Ah, but those electrical engineers are clever fellows! They have thought of that. They only install a transformer with a single wire MV feed when the MV circuit has a wye configuration with a neutral/ground return path back to the substation 

Is this common practice? Or is it a state by state affair?  Or power grid owner by power grid owner affair?

[IanB]

Your diagram appears to show 3-wire delta configuration, your connection between "hot" and ground/neutral will not work on delta configuration....because there's no neutral!!

Ah, but those electrical engineers are clever fellows! They have thought of that. They only install a transformer with a single wire MV feed when the MV circuit has a wye configuration with a neutral/ground return path back to the substation 

Is this common practice? Or is it a state by state affair?  Or power grid owner by power grid owner affair?

I'm not in the industry, I just describe what I see.

I can only suggest you watch this video that I linked above, and see what you see.

Obviously, there will be regional variations, but North America in general has a lot of common practice in power distribution.

[Randy222]

To be clear about this, a phase angle is a difference between two AC voltages, just like a voltage is a difference between two wires. And since a voltage is a difference between two wires, the only way to have a difference between two AC voltages is to have at least three wires so that you can have two voltages to compare.

It doesn't make sense to talk about a phase angle as an absolute quantity.

That sounds a bit confusing.

Is "120deg" not an absolute number in std 3ph generation? 

Delta or Y generators, 120deg is it, physically constructed that way.

"120deg" is relative, not absolute.

If you can't get your head around the concept, think of GMT time offsets as relative time, and GMT itself as absolute time.

I dunno Andy, last I looked the generator is physically built with output coils that are exactly 120deg apart from each other. I guess you could install a 2nd (but same) generator next to the 1st and rotate the whole thing by 10deg, then I guess we could say the 1st one has phases that are 130deg (or 10deg) relative to the 2nd generator ??

[Andy Chee]

To be clear about this, a phase angle is a difference between two AC voltages, just like a voltage is a difference between two wires. And since a voltage is a difference between two wires, the only way to have a difference between two AC voltages is to have at least three wires so that you can have two voltages to compare.

It doesn't make sense to talk about a phase angle as an absolute quantity.

That sounds a bit confusing.

Is "120deg" not an absolute number in std 3ph generation? 

Delta or Y generators, 120deg is it, physically constructed that way.

"120deg" is relative, not absolute.

If you can't get your head around the concept, think of GMT time offsets as relative time, and GMT itself as absolute time.

I dunno Andy, last I looked the generator is physically built with output coils that are exactly 120deg apart from each other. I guess you could install a 2nd (but same) generator next to the 1st and rotate the whole thing by 10deg, then I guess we could say the 1st one has phases that are 130deg (or 10deg) relative to the 2nd generator ??

Wrong interpretation.

A better interpretation would be wiring up a new coal/nuclear generator plant to the grid.  In this case, you need to wire phase A to phase A, phase B to phase B, phase C to phase C, otherwise sparks and fireworks.

But if you have a 3-phase motor at home, you can wire phase A to phase B, phase B to phase C, phase C to phase A.  Your motor will run just fine without any problem.

[Randy222]

Wrong interpretation.

A better interpretation would be wiring up a new coal/nuclear generator plant to the grid.  In this case, you need to wire phase A to phase A, phase B to phase B, phase C to phase C, otherwise sparks and fireworks.

But if you have a 3-phase motor at home, you can wire phase A to phase B, phase B to phase C, phase C to phase A.  Your motor will run just fine without any problem.

Generator to grid does not remove fact that the 3ph of the generator are fixed (not relative) 120deg shifted from each other. How you sync that onto the grid has nothing to do with fact that the generator output has 3 lines, all 120deg shifted from each other.

So I still not following your relative comment.

[Andy Chee]

Wrong interpretation.

A better interpretation would be wiring up a new coal/nuclear generator plant to the grid.  In this case, you need to wire phase A to phase A, phase B to phase B, phase C to phase C, otherwise sparks and fireworks.

But if you have a 3-phase motor at home, you can wire phase A to phase B, phase B to phase C, phase C to phase A.  Your motor will run just fine without any problem.

Generator to grid does not remove fact that the 3ph of the generator are fixed (not relative) 120deg shifted from each other. How you sync that onto the grid has nothing to do with fact that the generator output has 3 lines, all 120deg shifted from each other.

So I still not following your relative comment.

The relative comment applies to the household motor. 

Wiring A to B, B to C, C to A, is relative.
Wiring A to C, B to A, C to B, is relative.

Wiring A to A, B to B, C to C, is absolute.

How about a car cylinder firing order? Is 1-3-4-2 the same as 4-2-1-3?  Relatively speaking it is exactly the same, but if you have a newer car with a crankshaft position sensor, then there is an absolute zero degrees of crank rotation, making the two firing orders absolutely different.

[IanB]

Generator to grid does not remove fact that the 3ph of the generator are fixed (not relative) 120deg shifted from each other. How you sync that onto the grid has nothing to do with fact that the generator output has 3 lines, all 120deg shifted from each other.

So I still not following your relative comment.

The phase rotation is "fixed" in some academic sense, but nobody in the field has an oscilloscope with a long enough probe to reach back to the generator to establish an absolute reference. So all you can do is look at the conductors locally at your disposal, and then all you can do is compare them to find differences between them. If you have two wires to look at, then all you can see is an AC voltage at a given frequency like 60 Hz. That is a single phase. Your oscilloscope will draw a sine wave with a voltage and a frequency, and will set 0° on the trace according to however you set up the triggering.

If you have a third wire to look at, then you can compare it to one of the first two, and then you might be able to see a (relative) phase angle between two voltages. Having three wires will allow you to draw two traces on the oscilloscope.

[Randy222]

How about a car cylinder firing order? Is 1-3-4-2 the same as 4-2-1-3?  Relatively speaking it is exactly the same, but if you have a newer car with a crankshaft position sensor, then there is an absolute zero degrees of crank rotation, making the two firing orders absolutely different.

Yet the phase shifting for the cylinders are hard set 180deg, absolute, not relative?

[Randy222]

If you have a third wire to look at, then you can compare it to one of the first two, and then you might be able to see a (relative) phase angle between two voltages. Having three wires will allow you to draw two traces on the oscilloscope.

Having 3 wires I can get all 3 lines on my 4ch scope, each showing 120deg phase shift.

L1------------L2------------L3
Probe1+    Probe1-
                Probe2+         Probe2-
Probe3-                          Probe3+

[paul cotter]

In Ireland on long rural runs we have a three wire system with no neutral connection whatsoever and on the 10kV line the three phases sit nominally ~5.7kV to ground(earth) but not exactly. The advantage of this system is that if a tree falls against one of the wires that wire will go to zero volts wrt ground(earth) and the other two will go to 10kV but importantly rotation is maintained and customers on downstream transformers will not notice any change. Obviously having a tree in contact is not good and to detect this problem we use a transformer with star connected primary and open delta winding on the secondary to measure residual voltage. This fault condition is called a ground(earth) fault.
I hope this digression has not confused anyone.

[IanB]

Having 3 wires I can get all 3 lines on my 4ch scope, each showing 120deg phase shift.

Relative to each other. Which is the point at hand.

[langwadt]

How about a car cylinder firing order? Is 1-3-4-2 the same as 4-2-1-3?  Relatively speaking it is exactly the same, but if you have a newer car with a crankshaft position sensor, then there is an absolute zero degrees of crank rotation, making the two firing orders absolutely different.

a crankshaft sensor cannot tell the difference between the two

[SteveThackery]

How about a car cylinder firing order? Is 1-3-4-2 the same as 4-2-1-3?  Relatively speaking it is exactly the same, but if you have a newer car with a crankshaft position sensor, then there is an absolute zero degrees of crank rotation, making the two firing orders absolutely different.

I don't think the two firing orders are "absolutely different".

The 4-stroke cycle takes up two revolutions of the crankshaft - 720 degrees. A crankshaft position sensor cannot distinguish between the two rotations, which is why cars also have a camshaft position sensor - the camshaft taking 720 crank degrees to make one rotation. In your example, in one crankshaft rotation cylinders 1 then 3 are fired; in the other rotation cylinders 4 then 2 are fired. But the crankshaft starts in exactly the same position for both rotations. So a crankshaft position sensor alone cannot distinguish between the two rotations, meaning there is no "absolute zero" in that sense, and the two firing orders are therefore not "absolutely different".

4-stroke engines have always needed a camshaft position sensor in order to fire the right spark plug at the right time. In pre-electronic times, the distributor acted as a virtual camshaft position sensor, rotating at camshaft speed and being adjusted to open the points at just the right time and sending the spark to the right cylinder.

[Randy222]

Looks like the engine knowledge has surfaced. 
And w/ or w/o any sensors, the crank is machined in absolute phasing, two at 0 and two at 180, never gonna change that. If one wished to use a sharpie and put a mark +10deg from 0, and then call the phasing -10 and +170, "relative to the mark", have at it.

There's nothing relative in 3ph phase shift, no matter when or how you look at the 3 lines, 120deg shifting is always there.

[IanB]

The firing order of the cylinders in an engine is also completely relative.

Basically, you would have ...342134213421342134213... going on forever.

4 comes after 3, 2 comes after 4, 1 comes after 2, 3 comes after 1. These are all relative positions in the firing order.

There is no absolute here, because the crankshaft is circular, and a circle has no beginning and no end. The only thing regulated and determined are relative positions (or angles) such as between valve opening and closing, or when the spark fires, relative to the crankshaft position.

If you identify a crankshaft position and label it 0 degrees, or top dead center, this is just a convenience to help with maintenance and adjustment. It has no intrinsic meaning beyond that. It is exactly the same as choosing a node on a circuit diagram and arbitrarily labeling it as "zero volts" to use as a reference for other voltages. All voltages are relative, but it is convenient to establish a base reference to measure against.