Single Phase/Two Phase Discussion

[dnem]

Please see post #17:

120/208Y is single phase, three wire , open wye service, where 208Y/120v is a three phase, 4 wire service.

I was going to post the whole table, w/ notes but it seems we have lost the capability to upload pictures. Maybe George can get the webmaster to bring it back.

The full table is also reproduced in the IEEE Std 141 Red Book. If you can't get a hold of it, send me a PM and I will see what I can do.

If I might deviate from the subject for a post or 2.

"120/208Y is single phase"
That doesn't sound possible

I can understand 120/240 is a single phase with a center tap, 120v from either end of the single phase to the grounded center tap, 240v from one end of the single phase to the other end.

But 120/208 is not a center tapped single phase. . It's 2 completely separate phases and the grounded point isn't a center tap. . The only way you can get 208v is by using the square root of 3 multiplied by the voltage to ground. . Doesn't that fact prove that it isn't single phase ?

David

 

[kingpb]

In some areas of the country you may get 2 legs of a larger 208Y/120V system instead of the usual 120/240V split phase service. Why this is done, for example; say a large three phase bank will serve perhaps a whole block of a city with 208Y/120V three phase 4 wire service. But there is a mix of smaller business, residential, etc. The single phase services required will get two legs of this larger service, hence the 120/208Y designation. The utility obviously has to deal with unbalance.

 

[dnem]

Can somebody post a link to a thread discussing 2 phase vs single phase ?

With residential 120/240v, I can understand how people screwup and call it 2 phase. . But I'd like to read comments about the use of 2phases of a 3phase wye with a neutral.

David

 

[dnem]

In some areas of the country you may get 2 legs of a larger 208Y/120V system instead of the usual 120/240V split phase service. Why this is done, for example; say a large three phase bank will serve perhaps a whole block of a city with 208Y/120V three phase 4 wire service. But there is a mix of smaller business, residential, etc. The single phase services required will get two legs of this larger service, hence the 120/208Y designation. The utility obviously has to deal with unbalance.

"The single phase services required will get two legs of this larger service"
I would have said,
The residential services required will get two legs of this larger service

120/240 is a single phase service (center tapped single phase)
120/208 is a 2 phase service

And I would point to the sine wave as proof. . The grounded point is the flat line reference line. . Each of the hots have a sine wave that peaks at 120v from the straight line base line. . And the two sine waves are not in phase. . You're going to have one sine wave peak, 120 degrees later the other sine wave peak, and 120 degrees later a gap because the third phase is missing. . 120 degrees after the gap is the low peak from the first wave.

David

 

[kingpb]

I might suggest someone post a new topic under engineering/calculations to discuss the 2 phases of a 120/208Y vs. single phase designation.

 

[jim dungar]

Remember a single conductor is not a phase. A true phase is the voltage across two conductors.

The standard convention is to refer to a Line-Line voltage as a phase voltage. This convention allows us to have consistency whether we are talking about wye or delta three phase connections and 2-wire or 3-wire single phase connections.

The majority of the posts in this discussion illustrate the problems of how our industry has taken too many short cuts in describing electrical systems. There are so many times that the context of the discussion dictates the meaning of the word "phase". Transformers are not built with "phase" markings on their terminals, motors do not have "phase" markings on their windings so why does the rest of our industry insist on calling every Hot/Line conductor a phase?

 

[kingpb]

The majority of the posts in this discussion illustrate the problems of how our industry has taken too many short cuts in describing electrical systems.

You are correct, sir!

 

[dnem]

Remember a single conductor is not a phase. A true phase is the voltage across two conductors.

I agree. . A single conductor is not a phase. . A true phase voltage is the voltage across two conductors connected to the same phase coil because the phase is actually a coil of wire inside the transformer across which a voltage is induced.

The majority of the posts in this discussion illustrate the problems of how our industry has taken too many short cuts in describing electrical systems. There are so many times that the context of the discussion dictates the meaning of the word "phase". Transformers are not built with "phase" markings on their terminals, motors do not have "phase" markings on their windings so why does the rest of our industry insist on calling every Hot/Line conductor a phase?

Yes ! . I also think it creates more of a problem in the delta configuration.

In the delta configuration every conductor is connected to the end of two different coils so if the phase coils are designated A, B ,& C, then every conductor should have a dual designation. . Conductor AB, conductor BC, & conductor AC.

In the wye configuration, it's obvious that the wire commonly called the A phase conductor is not the only wire connected to the A phase coil. . But it is the only ungrounded one connected to the A phase coil. . The other wire connected to the A phase coil does triple duty for all of the phase coils and is grounded. . So calling the "hot" conductors A, B, & C is better than in delta altho still not completely accurate.

Getting back to the 120/240 and 120/208

120/240 three wire is a single phase coil with maximum voltage values being reached uniformily across the whole coil at the same moment. . When the center is tapped, grounded, and displayed as a flat line base line, this creates a sine wave display that has 2 values 180 degrees apart that result from looking from the center point in one direction for one value and then in the other direction for the other value. . But it's still one phase coil. . Therefore single phase.

120/208 three wire is two separate phase coils with maximum voltage values being reached at two different moments. . The common neutral is shared, grounded, and displayed as a flat line base line. . The phase coils all hit max voltage 120 degrees apart. . They are independent coils with voltages induced at different points in time. . Even if you eliminate one of the 3 coils, the other two coils remain two separate coils. . Therefore 2 phase.

David

 

[jim dungar]

A true phase voltage is the voltage across two conductors connected to the same phase coil because the phase is actually a coil of wire inside the transformer across which a voltage is induced.
David

Not really. You are creating a different set of rules for when you have a grounded conductor and when you don't.

Using your analysis how do you describe the 3 line conductors in an ungrounded wye transformer or motor?

A consistent description must exist even if there is no "center tap" or "neutral" or "ground reference".

In a 120/240 3 wire circuit there is a single line-line voltage. In a 120/208Y 3 wire circuit there is also a single line-line voltage. These can not be a different number of phases than is a 240V 2 wire or a 208V 2 wire circuit.

 

[George Stolz]

Off topic posted moved - hey, it's better than 'deleted', right?

Off topic posted moved - hey, it's better than 'deleted', right?

I've moved posts from another thread to a new thread, this one. The conversation had gotten far enough off topic to not serve and probably utterly confuse the original poster, and so I took King PB up on the idea to move them. Sorry for any confusion that may have resulted.

Can somebody post a link to a thread discussing 2 phase vs single phase ?

For a previous discussion on the topic, click here.

 

[Smart $]

120/208 three wire is two separate phase coils with maximum voltage values being reached at two different moments. . The common neutral is shared, grounded, and displayed as a flat line base line. . The phase coils all hit max voltage 120 degrees apart. . They are independent coils with voltages induced at different points in time. . Even if you eliminate one of the 3 coils, the other two coils remain two separate coils. . Therefore 2 phase.

As I pointed out in the "previous discussion", there are two perspectives from which to discuss the matter: utilization vs. generation. Unless participants in the discussion are talking from the same perspective, all sorts of disagreements will bound!

From the utilization perspective, a 120/208Y system is a single phase supply in the sense only 1? loads can be powered by this portion of the complete system. The same is true if you were to connect a 120/240 subpanel to a 240Δ/120 service. Only 1? loads can be powered from the subpanel.

 

[infinity]

In my unscientific way I have have always viewed the 208Y/120 system as having three phases, AB, BC, AC. Forgetting about the phase relationship between their physical coils and just thinking that a connection to one of them is a single phase connection. There are three, choosing one would be a single one of the three possible combinations.

 

[hardworkingstiff]

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.

 

[Smart $]

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.

I don't believe so. I can't say from experience. Losing a winding shouldn't affect the other two?provided they remain in working condition. The out of phase relationship and volatges on the delivered two of three Lines and Neutral are not dependent on the third. However, if it is a Δ-Y service, the delivered two of three Lines are dependent on all three Lines feeding the primary.

 

[hardworkingstiff]

I don't believe so. I can't say from experience.

I can't say from experience either.

I keep thinking back to the Oregan Fudge Factor. The example that was used was a 10-amp single-phase load connected at 208-volts as compared to 2 10-amp 120-volt loads connected from (one each) the same two phases to neutral.

Assuming unity power factor (also disregarding any transformer losses, etc):
The 208 load consumes 2080 watts.
The 2 120-volt loads consume 2400 watts.
Each of the two windings under either condition sees the 10-amp load.

Where is the additional 320 watts that the 2 120-volt loads are using come from? It must be the 3rd winding. When connected at 120-volts, some of the current is going through the 3rd winding.

I may be way off base here. I hope some of you smarter people can straighten me out.

 

[jpweygandt]

240 Volt 3 Phase Delta Corner Ground

240 Volt 3 Phase Delta Corner Ground

This message is for David, Inspector Medina County Ohio, and all who have been on this discourse. I have learned a lot by reading here. I had an electrical contracting firm in Medina County at one time. 240 Volt 3 Phase Delta Corner Ground systems are not overly abundant in our area. At one time both voltages of Delta services were common. In Ohio most of them have been replaced with Y services. In the early days they were not grounded. The state required utilities to ground those services that were already installed and any new installations. New installations were usually meant to make existing systems larger or increases in amperage size.

Some existing ungrounded systems were corner grounded. The state required a grounded conductor for all services. Thus, the installations of Y services. I know of no services in Medina County, Ohio using Delta services. However, I know of at least several in Summit County, Ohio. Dry type transformers were used to create lighting services.

During World War I and II when copper was really scarce many three phase motors were fed with just two wires and the third was obtained at the location by attaching the third wire to the building steel. Thus the early Delta systems were less expensive and easier to install.

Jerry

 

[hardworkingstiff]

During World War I and II when copper was really scarce many three phase motors were fed with just two wires and the third was obtained at the location by attaching the third wire to the building steel. Thus the early Delta systems were less expensive and easier to install.

Jerry

WOW, they used the building steel as a conductor?

 

[Smart $]

I can't say from experience either.

I keep thinking back to the Oregan Fudge Factor. The example that was used was a 10-amp single-phase load connected at 208-volts as compared to 2 10-amp 120-volt loads connected from (one each) the same two phases to neutral.

Assuming unity power factor (also disregarding any transformer losses, etc):
The 208 load consumes 2080 watts.
The 2 120-volt loads consume 2400 watts.
Each of the two windings under either condition sees the 10-amp load.

Where is the additional 320 watts that the 2 120-volt loads are using come from? It must be the 3rd winding. When connected at 120-volts, some of the current is going through the 3rd winding.

I may be way off base here. I hope some of you smarter people can straighten me out.

The 208V load's curent, 10A, is out of phase by 30? with that of the system voltage applied at each end. This has the effect of a 30? power factor: cos 30? = .867

P = E ? I ? pf = 120 ? 10 ? .867 = 1040W​

The above result is per connected winding.

Whereas with a 120V load of 10A, the current is in phase with the system voltage applied at each end. Hence, a 0? power factor: cos 0? = 1

P = E ? I ? pf = 120 ? 10 ? 1 = 1200W​

Again, the above is per winding.

 

[hardworkingstiff]

The 208V load's curent, 10A, is out of phase by 30? with that of the system voltage applied at each end. This has the effect of a 30? power factor: cos 30? = .867

P = E ? I ? pf = 120 ? 10 ? .867 = 1040W​

The above result is per connected winding.


Whereas with a 120V load of 10A, the current is in phase with the system voltage applied at each end. Hence, a 0? power factor: cos 0? = 1

P = E ? I ? pf = 120 ? 10 ? 1 = 1200W​

Again, the above is per winding.

I wish I was a little more educated. I don't understand why you would calculate power for a 208 load using 120-volts.

Let me try to articulate my thinking with a different example. Let's say there is a 3-phase 208Y/120-volt transformer feeding a few different residences having single-phase services.

I think we can agree that if there is a load of 2080 watts at 208 (just from one residence), there would be 10-amps on one leg, 10-amps on the other leg, and 0-amps on the neutral going back to the transformer. If the load was 2400 watts @120-volts split evenly between the 2 legs, then there would be 10-amps on one leg, 10-amps on the other leg, and 10-amps on the neutral. It just seems to me that that 10-amps on the neutral has to go somewhere, it just doesn't dissapear in thin air (or does it?).

Back to the scenario of losing one winding. You would be left with 2 windings connected in series with the center tap being grounded, just like the secondary of a single phase transformer. What's the difference?

 

[winnie]

IMHO, calling two phase legs plus the neutral of a 120/208 system 'single phase' is quite a confusing misnomer.

The _only_ sense that it is 'single phase' is that you have two 'hot' legs plus a grounded conductor, and can thus use common single phase service equipment. By this meaning, 'single phase' service equipment with suitable voltage ratings could be used for a corner grounded delta system.

With a polyphase system, power can always be delivered to the load by some combination of the supply phases. This cannot be true in a single phase system; the available power always drops to zero at two points on the supply cycle. With a polyphase system of _any_ phase count, you can use transformers to derive a polyphase system of any other phase count. But with a single phase system you need some form of energy storage to derive a polyphase output, eg. an VSD with its capacitor bank, or a rotary phase converter with its spinning rotor.

From the utilization perspective, a 120/208Y system is a single phase supply in the sense only 1? loads can be powered by this portion of the complete system. The same is true if you were to connect a 120/240 subpanel to a 240?/120 service. Only 1? loads can be powered from the subpanel.

I disagree. As you note:

The 208V load's curent, 10A, is out of phase by 30? with that of the system voltage applied at each end. This has the effect of a 30? power factor:

A unity power factor load from leg A to neutral will have a different phase angle from a unity power factor load between leg A and leg B. If we call leg A to neutral our reference phase 0?, then leg B to neutral becomes 120?. The inverse of leg A added to the inverse of leg B, derived, for example, using suitable 120V to 120V isolation transformers, would be a derived leg C, at 240?.

I posit that with any three conductors of a three phase system, eg. two phases plus neutral, you can re-derive the entire three phase system with suitable transformers. I believe that 'open-wye to open-delta' transformers are sometimes used for distribution.

I wish I was a little more educated. I don't understand why you would calculate power for a 208 load using 120-volts.

Smart$ was giving you the basic principal behind your question. With the 120Y/208 'single phase' system, you have a real phase angle difference between your two supply legs, as referenced to the neutral. Even though your load is unity power factor, the current flowing through it must be in phase with the voltage connected to it. The current is in phase with the 208V. It turns out that the 208V is 30? out of phase with the 120V legs.

I think we can agree that if there is a load of 2080 watts at 208 (just from one residence), there would be 10-amps on one leg, 10-amps on the other leg, and 0-amps on the neutral going back to the transformer. If the load was 2400 watts @120-volts split evenly between the 2 legs, then there would be 10-amps on one leg, 10-amps on the other leg, and 10-amps on the neutral. It just seems to me that that 10-amps on the neutral has to go somewhere, it just doesn't dissapear in thin air (or does it?).

Think about the currents flowing into the 'neutral point'. The rule is that the sum of all current flowing into or out of a point must be zero; electrons cannot 'pile up'.

In the 208V situation, you have a circuit of 2 coils and the load. The _voltages_ developed by the two coils are not in phase; that is why two 120V coils in series will only sum to 208V. But the _current_ flowing through the entire circuit _must_ be the same, and thus must be in phase. It happens to be in phase with the _sum_ voltage of the two coils.

In the dual 120V situation, you have _two_ circuits that happen to share the neutral point. The currents in these two circuits are in phase with the individual coil voltages, thus the currents of these two circuits are no longer in phase. Coming into the neutral point, you have three 10A currents, one from each of the coils, and the net current of the two circuits. These three currents happen to phase balance such that the net is zero.

Back to the scenario of losing one winding. You would be left with 2 windings connected in series with the center tap being grounded, just like the secondary of a single phase transformer. What's the difference?

The difference is that in the 240V center tapped situation, the two coil voltages are in phase, but in the 'two legs of a wye' situation, the two coil voltages are not in phase.

-Jon