Single Phase/Two Phase Discussion

[Smart $]

Only 1? loads can be powered from the subpanel.

I disagree.

And the isolation transformers you mention are... 1? loads.

Can one connect a standard three-phase load to this service directly? (No)

Configuring electrical apparatus to derive a third phase from two is no more common to the service than utilizing a VFD or rotary converter on a typical 120/240 service.

Keep in mind this type of service is delivered, as I understand it, for residential application. Any premises in the area of the POCO transformer banks supplying these services that qualifies for three-phase service is going to get three-phase service rather than using some apparatus to derive the third phase.

 

[winnie]

Smart$,

Good point.

In addition to using single phase service equipment, it will be _used_ in essentially the same fashion as a single phase service.

IMHO it is still a polyphase service, in that you could derive a rotating field, or derive the third phase, but in terms of the loads served, the equipment used, etc. it fills the role of a common residential single phase service.

The only real difference will be in how to calculate loads and current carrying conductors.

As far as the ratings of service equipment used: single phase
As far as the homeowner is concerned: single phase
As far as the phase angles: three phase
As far as loads on the neutral: three phase

-Jon

 

[Smart $]

In addition to using single phase service equipment, it will be _used_ in essentially the same fashion as a single phase service.

Correct.

IMHO it is still a polyphase service, in that you could derive a rotating field, or derive the third phase, but in terms of the loads served, the equipment used, etc. it fills the role of a common residential single phase service.

It's not a matter of opinion. It is a polyphase service... just not a complete polyphase service. This type of service is actually closer to a Two Phase service than the one I used as an example in the "previous discussion". It could quite literally be called a "120? two-phase service", whereas the original Two Phase was a 90? two-phase service.

The only real difference will be in how to calculate loads and current carrying conductors.

As far as the ratings of service equipment used: single phase
As far as the homeowner is concerned: single phase
As far as the phase angles: three phase
As far as loads on the neutral: three phase

I believe your last two points will be quite confusing for DIY'ers...

 

[jim dungar]

120/208Y is not poly-phase. It only has a single line-line voltage therefore it is single phase.

You cannot change the rules for which rationale or formula to use depending on if a neutral or ground conductor is available. If you do then you need to always specify that your system contains a neutral even if a circuit does not.

If you only use two lines from any ungrounded transformer how do you determine the number of phases in your circuit if you measure 240V line to line? Remember the laws of physics and the rules of circuit analysis do not require a ground reference even though the NEC does.

Some possible standard transformers are:
240V 2-wire, 1 winding
120/240V 3-wire, 2 winding
240V 3-wire, 3 winding
240V 3-wire, 2 winding
240/120 4-wire, 3winding
240Y/138 3-wire, 3 winding
138/240Y 2 wire, 2 winding

 

[Smart $]

Some food for thought...

120/208Y is not poly-phase. It only has a single line-line voltage therefore it is single phase.

By that defintion they errantly named a Two Phase 3-wire system, as it only had one line-to-line voltage...

The simple and basic truth is: A 120/208Y service is not single phase, and the logic for categorizing it as such is flawed. Only from perspective of utilizing common single phase loads can it be called such. And that is to ignore a major portion of the physics involved.

You cannot change the rules for which rationale or formula to use depending on if a neutral or ground conductor is available. If you do then you need to always specify that your system contains a neutral even if a circuit does not.

I'm not exactly sure what you are inferring here...

Nonetheless, such a 120/208Y service has no neutral. It has a grounded conductor and two ungrounded line (or phase, out-of-phase, not-in-phase, etc.) conductors. If the grounded conductor were ungrounded, the voltage and phase relationships would remain the same with respect to each other, barring any malfunction in the system.

Rationale is often biased to the premise of the developing entity.

If you only use two lines from any ungrounded transformer how do you determine the number of phases in your circuit if you measure 240V line to line? Remember the laws of physics and the rules of circuit analysis do not require a ground reference even though the NEC does.

That is not the case here. You have structured the question around the rationale you referred to earlier. Let's put it on track with the issue at hand...

You have a transformer arrangement that is uncertain but know it is an AC 60Hz system. You are provided with three output leads from the arrangement. [Using safe practices, of course...] How do you determine if the output is single-phase or polyphase with a voltmeter?

 

[winnie]

120/208Y is not poly-phase. It only has a single line-line voltage therefore it is single phase.

You cannot change the rules for which rationale or formula to use depending on if a neutral or ground conductor is available. If you do then you need to always specify that your system contains a neutral even if a circuit does not.

In 120Y/208V you have _three_ conductors; two ungrounded conductors and the grounded conductor. With three conductors I can define _three_ voltages, the line-line voltage and the two line-grounded voltages. These three voltages will be out of phase. The zero crossings of these AC voltages will happen at different times.

In 120/240V you have _three_ conductors; two ungrounded conductors and the grounded conductor. There are again three defined voltages; line to line and two line-ground. But in this case all these voltages are in phase or exactly inverted. The zero crossings happen at the same time.

I totally agree that a two terminal load is _always_ a single phase load, no matter what the phase count is in the supply system. The equations will only be different if the load is a three terminal load. But with a three terminal load, some systems will respond differently to the different sources.

For example, consider two 120V 10A heaters on a MWBC. Both heaters are on. On a 120Y/208V system, the grounded conductor carries 10A. On a 120V/240 system, the grounded conductor carries no current.

-Jon

 

[jim dungar]

You have a transformer arrangement that is uncertain but know it is an AC 60Hz system. You are provided with three output leads from the arrangement. [Using safe practices, of course...] How do you determine if the output is single-phase or polyphase with a voltmeter?

You are changing my question, I asked about two conductors that measured 240V between them.

Jon,
Are you sure about your 120/208Y neutral carrying 10A? In your perfectly balanced circuit is your neutral even needed?

 

[rattus]

I will stick my neck out:

I will stick my neck out:

Jon,
Are you sure about your 120/208Y neutral carrying 10A? In your perfectly balanced circuit is your neutral even needed?

Firstly, the third wire in this system is not a neutral; it is a grounded conductor, and it is needed for single phase loads.

Secondly, the grounded conductor does indeed carry 10A at a phase angle halfway between the two load currents. Do the math.

Thirdly, I don't believe you can balance this system with only two phases.

 

[Smart $]

You are changing my question, I asked about two conductors that measured 240V between them.

You know darn well it's a staged question. Phasing cannot be determined without a third reference.

 

[LarryFine]

If you derive a 208/120v, 3-wire system (2 hots & 1 neutral) from a 208/120v, 4-wire system, you will measure 120v line-to-neutral 1 ph, and 208v line-to-line 1 ph.

The 208v will appear as a normal, symmetrical sine wave. The voltage difference (208 vs 240) is what accounts for the different (120 deg. vs 180 deg.) phase angle.

Added: To see the 120 deg., you would need something like a dual-input oscilloscope and you'd need to reference the neutral. The hots alone would produce a single wave.

 

[winnie]

You are changing my question, I asked about two conductors that measured 240V between them.

If you have only _two_ conductors, as far as those two conductors are concerned, you have a single phase. Doesn't matter what you get these two conductors from; it is single phase. The question of single phase or more only becomes relevant if you have more than two conductors.

Jon,
Are you sure about your 120/208Y neutral carrying 10A? In your perfectly balanced circuit is your neutral even needed?

If you have a 120/208Y system, and connect two perfectly balanced loads between the two lines, without a connection to the grounded conductor, then the loads will each see 104V. Now connect the mid point to the grounded conductor, and you get 120V on each load. The only thing that could change is phase angle.

-Jon

 

[jim dungar]

Firstly, the third wire in this system is not a neutral; it is a grounded conductor, and it is needed for single phase loads.

Secondly, the grounded conductor does indeed carry 10A at a phase angle halfway between the two load currents. Do the math.

Thirdly, I don't believe you can balance this system with only two phases.

Rattuss,
If there are no line to neutral (point) loads then why is it required?
And yes I wish there was a better description for the word neutral, because all center taps are neutrals but not all neutrals are center taps.

If there is no neutral (point) conductor the voltage drop across the resistive loads connected in series, will be equal and the 2-wire circuit will still work, although the heaters' output will be drastically reduced. So yes the addition of a neutral (point) conductor does affect the circuit.

I agree that the supply "system" may not be able to be balanced.


Smart $,
Yes it is a staged question. My definition of a phase = line-line voltage works for any circuit.

I prefer to fight the battle to use the ANSI voltage standards. I always try to list the voltage, then the number of phases and finally the number of wires. So using my method, a 240/120 3PH 4W system can supply branch circuits of:
240/120 3PH 4W
240V 3PH 3W
240V 1PH 2W
120/240V 1PH 3W
120V 1PH 2W
208V 1PH 2W

 

[engy]

Seems we are over-complicating things?
120/208, dare I say single phase, is simply a subsystem using two phases of a three phase system.
We did not create a new animal here.

I dare say single phase, by imagining a two or three phase motor connected to two legs of a 208Y120 system...think it would spin?

 

[winnie]

I dare say single phase, by imagining a two or three phase motor connected to two legs of a 208Y120 system...think it would spin?

A 'normal' three phase motor, designed for operation at 208V, connected to only two of the three legs: you only have two leads connected to the motor, and therefore only single phase. The motor will not develop a rotating magnetic field, and will thus not self start. If it is already spinning, it will probably continue to spin.

However if the motor were wound with three 120V phases, designed for connection to each of the supply legs and the neutral, then it would start and spin just fine, with reduced capacity. Motors are generally _not_ wound nor connected in this fashion, but with two phases plus the grounded conductor of a 120/208 system, you have everything needed to create a rotating field.

-Jon

 

[jim dungar]

Seems we are over-complicating things?
120/208, dare I say single phase, is simply a subsystem using two phases of a three phase system.

No, it uses 2 line/hot conductors not phases. Because there is only one line-line voltage it is a single phase circuit.

 

[Smart $]

Smart $,
Yes it is a staged question. My definition of a phase = line-line voltage works for any circuit.

But what constitutes a LINE?

Consider, just for the sake of discussion, a corner-grounded open-delta system, where the mutual corner is grounded. Do you or not have three LINE conductors? Do we not consider the grounded conductor as a LINE conductor just because it is grounded and common to both windings?

How does the described system differ from a 120/208Y service (i.e. two of three phases of a grounded-neutral wye system) in terms of "what is a LINE"? You have the ends of two windings connected together and grounded in both. You have voltage across each winding. You have voltage across the unconnected ends of the windings. Except for the magnitude of the voltages and degrees they are out-of-phase, both perform in extremely similar capacity.

As we have said, the 120/208Y service has no neutral. If it is not a neutral, it is a LINE, albeit a grounded one.

I prefer to fight the battle to use the ANSI voltage standards. I always try to list the voltage, then the number of phases and finally the number of wires. So using my method, a 240/120 3PH 4W system can supply branch circuits of:
240/120 3PH 4W
240V 3PH 3W
240V 1PH 2W
120/240V 1PH 3W
120V 1PH 2W
208V 1PH 2W

I have no qualms with this as I use the same method.

 

[rattus]

Rattuss,
If there are no line to neutral (point) loads then why is it required?
And yes I wish there was a better description for the word neutral, because all center taps are neutrals but not all neutrals are center taps.

If there is no neutral (point) conductor the voltage drop across the resistive loads connected in series, will be equal and the 2-wire circuit will still work, although the heaters' output will be drastically reduced. So yes the addition of a neutral (point) conductor does affect the circuit.

I agree that the supply "system" may not be able to be balanced.

First, in a poly-phase system, I understand "phase voltages" to be the voltages across the secondary windings. Likewise, "phase currents" are the currents through the secondary windings. "Line to line voltage" would be the voltage between any two lines. "Line current" would be the current through any of the lines.

Second, I understand Smart's problem to be that of two 120V loads connected across the secondaries with a common return. If we drive 120V loads, we must have this common return. Yes, if only a 208V load is present, the common return carries no current.

Third, I think of a neutral as being a point with a potential such that the line to neutral voltages are equal in magnitude.

 

[dnem]

A 120/240-volt single-phase transformer has the grounded conductor hooked up so there is no influence from any other power source. The 'neutral' is only connected (in the center) of the windings.

A 208Y/120-volt has the neutral connected to the end of three seperate windings. Even though only two (legs, phases, whatever) are brought to a house served with 208/120-volt single phase, doesn't the winding (that was not brought to the house) still influence the voltage at the house?

Let's say legs A and B along with the neutral are brought to your house. The winding for leg C goes out, A and B are still OK. What happens to the voltage in your house? Do you still have 208/120? Something tells me you don't, you have 208/???. Without the C winding, won't you lose some of the 120-volts?

edit: In my simple mind, it seems you would have 208/104 since you then have the neutral tapped in the center of two windings (with no additional influence) just like the single-phase transformer.

If you ignore the primary windings you would have a point. . But obviously you can’t ignore the primary windings. . Whether or not the 3rd phase coil is present, the voltages induced in the secondary coils hit maximum voltage value 120? apart. . 208/104 could only be achieved if they were 180? apart.

Using the simplest example possible, take the residential 120/240 3wire as the example. . Single coil of wire in the transformer secondary. . The grounded point is always the flat line base line that all compares to in the sine wave.

If you ground the center tap wire you get 2 sine wave values of identical voltage levels, but 180? apart. . It’s the result of looking from the center point of the coil in one direction for one value and then in the other direction for the other value.
Now if you ground one end of the coil, you’ll still get 2 sine wave values, but this time one with be double the voltage of the other one and both values with be in phase or 0? apart. . It’s the result of looking from one end of the coil to the center point for one value and then looking farther in the same direction to the farthest opposite end of the coil for the other higher voltage value.

You have to first totally understand what your looking at in the basic 120/240 single coil before you can move on and understand other things. . If you can understand how you can have one coil, one induced voltage, and yet 2 separate sine wave values, then you can move on.

But in your example you would still have 2 separate induced voltages 120? apart because of the primary coils.

A 208Y/120-volt has the neutral connected to the end of three seperate windings. Even though only two (legs, phases, whatever) are brought to a house served with 208/120-volt single phase, doesn't the winding (that was not brought to the house) still influence the voltage at the house?

No, the ratio between primary and secondary determines the voltage.

Let's say legs A and B along with the neutral are brought to your house. The winding for leg C goes out, A and B are still OK. What happens to the voltage in your house? Do you still have 208/120? Something tells me you don't, you have 208/???. Without the C winding, won't you lose some of the 120-volts?

“winding for leg C goes out, A and B are still OK. What happens to the voltage in your house? Do you still have 208/120?”
Yes, still 208/120. . The amperage levels will be affected, not the voltage levels. . Currents from one coil circulate in the other coils so losing one coil will effect the current in the remaining coils.

David

 

[dnem]

Reading this entire discussion up to this point has lead me to the conclusion that the word “phase” has more than one meaning in our electrical industry and in our discussions and can not have an agreed upon usage until it is defined more specifically.

I’m seeing 3 different definitions being used:
1) phase coil, which can number 1, 2, or 3 (and theoretically higher but not found in reality)
2) phase load, which can number 1 or 3
3) phase conductor, which can number 1, 2, or 3

A single phase coil is connected to a single phase load.
3phase coils are connected to a 3phase load.
But 2phase coils are connected to a single phase load. . There is no 2phase load. . And that’s where most of the dispute is coming from.

As far as the subject of phase conductors go, I think there is still a question about the number of phase conductors in a 208Y/120 3wire system. . The 3rd phase might not be present at the service or it might not even exist. . If it doesn’t exist, is the neutral still a neutral or is it a 3rd phase conductor, as smart has suggested ?

But what constitutes a LINE?

Consider, just for the sake of discussion, a corner-grounded open-delta system, where the mutual corner is grounded. Do you or not have three LINE conductors? Do we not consider the grounded conductor as a LINE conductor just because it is grounded and common to both windings?

How does the described system differ from a 120/208Y service (i.e. two of three phases of a grounded-neutral wye system) in terms of "what is a LINE"? You have the ends of two windings connected together and grounded in both. You have voltage across each winding. You have voltage across the unconnected ends of the windings. Except for the magnitude of the voltages and degrees they are out-of-phase, both perform in extremely similar capacity.

As we have said, the 120/208Y service has no neutral. If it is not a neutral, it is a LINE, albeit a grounded one.

 

[engy]

No, it uses 2 line/hot conductors not phases. Because there is only one line-line voltage it is a single phase circuit.

I disagree. A 480-208Y120 has (basically) three 480-120V transformers (phases)

A 120/208 "single" phase feeder has two of the three 120V phases.
Still not two-phase or three-phase, just a portion of a 3-phase system where ya gan get 120 and 208V.

You can only connect 1-phase loads to this animal, which is why I call it single phase.

It ain't two phase - doesn't have two phases 90degrees out of phase...
It ain't three phase - cause we only brought two of the three phases...

Maybe it's not single phase either