2 Phases of a 3 Phase System

[Fitzdrew516]

I know there has to be a logical answer for this, but I can't seem to find it. I've been searching the internet, but the amount of misinformation that's out there is astounding. Here's the question -

Why is a feed that utilizes (2) phases off a 3ph system always called "single phase"? For example - You have a 208/120v 3ph, 4w panelboard that feeds a piece of equipment that is 208v single phase, but it uses (2) of the phases to do so. Why is this not called "2 phase"?

Thanks,
-Drew

 

[jumper]

Because 2 phase already exists and we have to call it something.

There is no official IEEE designation for it.

 

[jim dungar]

Don't equate the number of 'phase conductors' with the number of 'phase voltages'.
Phase voltage is between sets of 2 Line conductors. The neutral is not part of the description.

 

[winnie]

Don't look for a logical explanation, this is language!

With that said, voltage is always measured between two points, and current always flows in a closed path. This means that if you have two wires from a system (_any_ two wires) you will only have a single phase of voltage and a single phase of current flowing on those wires.

If you look at a normal 'wye' secondary, you have _six_ possible pairs of wires (A-B, B-C, C-A, A-N, B-N, C-N). Each one of these pairs of wires represents a different supply phase angle; but we _call_ a wye secondary three phase. We could connect a single phase load to any pair of wires; it will only see one of the six possible phases supplied by the system.

A related language/naming conundrum: if you take two of the 'phase' wires and the neutral from a three phase wye supply and use it to feed a panel, we call that a single phase panel. I believe that this is appropriate naming because such a panel is used in the same fashion as a 'true' single phase panel.

-Jon

 

[GoldDigger]

But it is possible (with transformers) to recreate the third phase from the two hots and the neutral. That is not possible with true single phase.

 

[K8MHZ]

IOW, why can we only get single and 3 phase from a three phase source, and not 2 phase?

Because three phase is 120 degrees apart.

In order to have two phase, we need two separate single phase sources (coils) 90 degrees apart.

 

[electrofelon]

Perhaps we should also note that single phase (from the utility) is always served from two legs of a three phase system (or it could be a line and neutral). Doesn't saying the two legs are "120 degrees apart" only make sense if it's a grounded system?

 

[K8MHZ]

Perhaps we should also note that single phase (from the utility) is always served from two legs of a three phase system (or it could be a line and neutral). Doesn't saying the two legs are "120 degrees apart" only make sense if it's a grounded system?

Ungrounded 3 phase deltas are also 120 degrees apart. Grounding has nothing to do with phase angles.

 

[GoldDigger]

Ungrounded 3 phase deltas are also 120 degrees apart. Grounding has nothing to do with phase angles.

But it does have to do with specifying a reference point relative to which to measure the voltage.
As stated, if you have available only two phase wires and no ground or neutral reference you cannot say that there is a 120 degree phase offset between them.

 

[electrofelon]

Ungrounded 3 phase deltas are also 120 degrees apart. Grounding has nothing to do with phase angles.

Right, but if you only have 2 of the three, are those 2 120 degrees apart? Then those two legs feed a single phase transformer. Is each line of the secondary 120 degrees apart? I'm trying to get at something and/or make a point....

 

[ggunn]

But it does have to do with specifying a reference point relative to which to measure the voltage.
As stated, if you have available only two phase wires and no ground or neutral reference you cannot say that there is a 120 degree phase offset between them.

Or if you have a three phase high leg service with a grounded center tap between the A and C phases, A and C will appear to be 180 degrees apart when referenced to the center tap.

 

[Ingenieur]

L-L is single phase
the phasor sum of 2 L-N phasors displaced by 120 deg result in a single phase

the 3 phases are
A-B A to B, not A and B EACH
B-C
C-A

but A, B and C are each a 'single' phase

 

[K8MHZ]

Right, but if you only have 2 of the three, are those 2 120 degrees apart? Then those two legs feed a single phase transformer. Is each line of the secondary 120 degrees apart? I'm trying to get at something and/or make a point....

If you only have two conductors, there is only one phase. It's the phases that are at different angles, not the conductors.

Phase A-B by itself will have no phase angle.

Phase A-B will have a phase angle of 120 degrees from phase A-C and also 120 degrees from phase B-C.

Think of it like a triangle. There will be an angle between any two legs of the triangle, but there must be two. There is no angle to a single side of a triangle in relationship to itself.

 

[GoldDigger]

One isolated wire without a voltage reference is zero phases. :angel:

 

[K8MHZ]

but A, B and C are each a 'single' phase

I disagree. There still has to be two conductors to establish a phase.

 

[electrofelon]

But it does have to do with specifying a reference point relative to which to measure the voltage.
As stated, if you have available only two phase wires and no ground or neutral reference you cannot say that there is a 120 degree phase offset between them.

That is what I was trying to say - GoldDiggers comment hadnt posted before my last post.

If you have two conductors from an ungrounded system, I dont think there is any way to tell if that system is from a single phase transformer or two lines of a three phase delta, other than using high impedance meters to measure the timing of the capactive coupling of each conductor.

 

[Ingenieur]

I disagree. There still has to be two conductors to establish a phase.

Not if you talking about the phase of a line current
it will have a ph ang vs time
I cos (wt + ph)
wt is the angle and ph the offset or shift
w = 2 Pi f

it is the construction/orientation of the poles of the gen source that establishes the ph relationships

a phase angle exists without reference

 

[GoldDigger]

Not if you talking about the phase of a line current
it will have a ph ang vs time
I cos (wt + ph)
wt is the angle and ph the offset or shift
w = 2 Pi f

it is the construction/orientation of the poles of the gen source that establishes the ph relationships

a phase angle exists without reference

Any phase angle without a reference is purely arbitrary.

 

[GoldDigger]

Not if you talking about the phase of a line current
it will have a ph ang vs time
I cos (wt + ph)
wt is the angle and ph the offset or shift
w = 2 Pi f

it is the construction/orientation of the poles of the gen source that establishes the ph relationships

a phase angle exists without reference

Any phase angle without a reference is purely arbitrary.
When it time=zero?

 

[Ingenieur]

Any phase angle without a reference is purely arbitrary.

Time is the reference
Doesn't matter what time only that all are measured at the same time