2 Phases of a 3 Phase System

[just the cowboy]

As said before

As said before

There is a Two phase system where the windings are 90 degrees out not 120 or 180, we had it in school. Philadelphia still has some I think.

 

[GoldDigger]

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

If you only have one measurement, it is always measured at the same time. But my time may be different from yours
So my phase angle and yours will rarely match.

 

[Ingenieur]

If you only have one measurement, it is always measured at the same time. But my time may be different from yours
So my phase angle and yours will rarely match.

Doesn't matter
as long as your and my measurements measure all signals under observation at the same time, or more accurately, simultaneously (even though your time reference may be different than mine) the relative phase delta between them will be the same as will the fundemental if sampled

 

[K8MHZ]

Doesn't matter
as long as your and my measurements measure all signals under observation at the same time, or more accurately, simultaneously (even though your time reference may be different than mine) the relative phase delta between them will be the same as will the fundemental if sampled

Still, in order to have an angle, there must be more than one phase. In this case it's 'your' and 'my'.

If the angle between your phase and Golddigger's phase is 120 degrees, what is the angle of just your phase after Golddigger turns off the power and no longer has any phases?

 

[GoldDigger]

Doesn't matter
as long as your and my measurements measure all signals under observation at the same time, or more accurately, simultaneously (even though your time reference may be different than mine) the relative phase delta between them will be the same as will the fundemental if sampled

That only works if I have a reference to which to measure each voltage independently. Without the third wire (or ground) I do not have that reference. And if I have a third wire we can all agree we have three phases.

 

[Ingenieur]

Definitions may be the issue
the definition I use for 'phase'
the ang between the img and real components of a SINGLE signal

1 + 1j
phase is 45

1 - 1j
phase is -45

each signal has a phase independent of the other

the difference is the phase 'offset' between the 2 signals (each with an independent phase angle)
(1 + 1j) - (1 - 1j) = 2j or 90 deg

when measuring 2 phase lines that is what you are measuring
the absolute phase of 1 new signal, created as a result of the difference of 2 signals
that is why L-L is single phase, it only has 1
but a-b, b-c and c-a each have 1 for a total of 3

Mag/ang
277/0 = 277 + 0j
277/120 = -(-138.6 + 240j)
difference = 415.7 - 240j = 480/-30
each signal has it's own phase as does the difference

a lot of it is nomenclature and terminology

 

[Sahib]

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

I offer a simple answer which may be as refreshing as a cool drink on a hot summer day.

It is this:

A true 2 phase system is capable of creating a rotating magnetic field by itself whereas a single phase system such as mentioned by you is not unless aided by auxiliary devices such as capacitors to create the second phase.

 

[topgone]

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.

This! A requirement for anyone to see/measure the phase differences of two conductors/lines is to have a reference point!

 

[iwire]

This! A requirement for anyone to see/measure the phase differences of two conductors/lines is to have a reference point!

It seems so basic it is amazing it seems to escape some.

 

[kwired]

I offer a simple answer which may be as refreshing as a cool drink on a hot summer day.

It is this:

A true 2 phase system is capable of creating a rotating magnetic field by itself whereas a single phase system such as mentioned by you is not unless aided by auxiliary devices such as capacitors to create the second phase.

Absolutely. That may not really be how to define what is single and poly phase, but is a good test in determining which you have.

 

[Ingenieur]

It seems so basic it is amazing it seems to escape some.

Single line of a 3 phase system
Apply amp clamp
measures phase angle between V and I
cos of that phase is pf
you have 3 phases in a single non-referenced conductor
the abs phase of V, I and the difference or pf

the definition of ph qty is how many sinusoilal signal peaks of the same fundemental freq occur in Pi radians

 

[K8MHZ]

Single line of a 3 phase system
Apply amp clamp
measures phase angle between V and I
cos of that phase is pf
you have 3 phases in a single non-referenced conductor
the abs phase of V, I and the difference or pf

the definition of ph qty is how many sinusoilal signal peaks of the same fundemental freq occur in Pi radians

In order to have the above, don't you have to have a reactive load on the circuit? I am pretty sure this thread pertains to un-loaded sources or supplies.

 

[kwired]

In order to have the above, don't you have to have a reactive load on the circuit? I am pretty sure this thread pertains to un-loaded sources or supplies.

I agree we are likely talking about unloaded source/supply, no load means no current and with no current there is no power factor, a voltage wave is still present. It otherwise doesn't need to be a reactive load though, a true resistance load has 100% power factor and still fits in what was mentioned.

 

[iceworm]

Single line of a 3 phase system
Apply amp clamp
measures phase angle between V and I
cos of that phase is pf
you have 3 phases in a single non-referenced conductor
the abs phase of V, I and the difference or pf

the definition of ph qty is how many sinusoilal signal peaks of the same fundemental freq occur in Pi radians

That only works if I have a reference to which to measure each voltage independently. Without the third wire (or ground) I do not have that reference. And if I have a third wire we can all agree we have three phases.

This! A requirement for anyone to see/measure the phase differences of two conductors/lines is to have a reference point!

Real slow - I haven't had my second cup yet: How are you measuring this V? GD tg and others are correct, one needs a reference to measure a voltage.

 

[K8MHZ]

I agree we are likely talking about unloaded source/supply, no load means no current and with no current there is no power factor, a voltage wave is still present. It otherwise doesn't need to be a reactive load though, a true resistance load has 100% power factor and still fits in what was mentioned.

Correct.

I was thinking a perfect power factor meant no phase angle. I guess 0 degrees is still an angle.

 

[Ingenieur]

In order to have the above, don't you have to have a reactive load on the circuit? I am pretty sure this thread pertains to un-loaded sources or supplies.

the OP did not reference load
only why is a 3 phase LL not 2 phase

even unloaded your service has a pf as delivered from the utility
and seldom is it 1.00
they provide reactive power we consume it

even though your system is 'off' the grid is drug down by others

my point is the term 'phase' needs defined before we make blanket statements

in the op's case it is not so simple
do we base it on the machine poles and speed?
in a wye each sec xfmr coil is a ph-n
adding any 2 is ph-ph but is a 'single' phase (add the other 2 combinations for the other 2 phases)
and has nothing to do with how it is measured nor is it dependent on another signal/reference

 

[Ingenieur]

2 phase
a 1 + 0j or 1 ang 0
b 0 + 1j or 1 ang 90
each has a phase angle
independent of the other
does not need measurement to 'exist'

a-b
1 - 1j or 1.414 ang -45
a single phase

2 times (1.414^2) the power with a wire only 40% larger (40% more current)

 

[jumper]

.... why is a 3 phase LL not 2 phase

in the op's case it is not so simple

Sure it is Iggy.

See post #2.

 

[Ingenieur]

Sure it is Iggy.

See post #2.

So line to line is a true 2 phase signal?
but we don't call it that because the ieee has no definition and there is a 'real' 2 phase )90 deg offset)?
and to exist it must be measuredbto some reference?

clear as mud
:lol:

 

[jumper]

So line to line is a true 2 phase signal?
but we don't call it that because the ieee has no definition and there is a 'real' 2 phase )90 deg offset)?
and to exist it must be measuredbto some reference?

clear as mud
:lol:

Y'all are overthinking this. Not uncommon here, threads like this usually have about 300+ posts with no agreement.

OP merely asked:

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"?

And the answer is.....?