Single Phase or Polyphase?

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jim dungar

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It doesn't just appear to be two currents. There are two different currents in the secondary winding. These are actually two different fluxes seen by the primary winding.

One single primary winding.
One single core.
One single magnetic path, through the core.
One single secondary winding.
Is the result, one single secondary current direction?

This should be a simple yes or no answer.

jim dungar

Moderator
Staff member

It is more helpful if you point out specific passages which you feel are important to the discussion such as from page 90 of your reference:
If the induced emf at an instant is from A1 to A2 on the HV winding it will rise from a1 to a2 on the LV winding.
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winnie

Senior Member
One single primary winding.
One single core.
One single magnetic path, through the core.
One single secondary winding.
Is the result, one single secondary current direction?

Nope.

As described, you have the same voltage induced in every turn of the secondary. This is the same throughout the transformer (neglecting minor flux leakages).

But you can have different _currents_ flowing via any of the available paths on the secondary.

Consider a standard center tapped single phase transformer. Depending upon the loads connected to the two legs, you can have different magnitude and phase of the current flowing on the two halves of the winding.

-Jon

jim dungar

Moderator
Staff member
Nope.

As described, you have the same voltage induced in every turn of the secondary. This is the same throughout the transformer (neglecting minor flux leakages).

But you can have different _currents_ flowing via any of the available paths on the secondary.

Consider a standard center tapped single phase transformer. Depending upon the loads connected to the two legs, you can have different magnitude and phase of the current flowing on the two halves of the winding.

-Jon

You only appear to have currents flowing in two directions because you choose to use the neutral as your reference point. If you use an end point of a the winding as your reference the currents flow in the same direction.

The number of phases should not depend on if a neutral is or is not used.

winnie

Senior Member
You only appear to have currents flowing in two directions because you choose to use the neutral as your reference point. If you use an end point of a the winding as your reference the currents flow in the same direction.

No, I do not mean currents that are in phase but measure 180 degrees out of phase because of the selection of reference point, I mean currents flowing at real different phase angles.

Consider a conventional 120/240V center tapped transformer. I connect a 120V 12A motor from leg A to neutral, and connect 12A of tungsten lighting from leg B to neutral. The current flowing on the transformer from terminal B to neutral will not be in phase with the current flowing from neutral to terminal A.

The number of phases should not depend on if a neutral is or is not used.

On this point I am pretty sure that I _disagree_ with Mivey and agree with you. However this is why I posted that there are numerous different uses of the term phase, and some of these usages are not quite consistent with each other.

The transformer that you've described is clearly a _single_ phase transformer.

Even with a center tap I agree that it is a single phase transformer. This is the common usage of the term phase to describe transformers.

But I'm not going to get my knickers in a knot if someone says that I tap phase A and phase B from the terminals of this single phase transformer. It is quite common and well understood to call each separate ungrounded leg of a system a 'phase'.

Furthermore, I am quite comfortable saying that the two legs of this single phase transformer are 180 degrees out of phase. This is yet another usage of the term, one that is entirely dependent upon the selection of reference point.

-Jon

jim dungar

Moderator
Staff member
... The current flowing on the transformer from terminal B to neutral will not be in phase with the current flowing from neutral to terminal A.

Yes, you have what appears to be current from B-> N and current from A->N.
But you can also see this as current flowing from N->B and from A->N.

But the current in the winding comes from the magnetic field created by the primary. The single magnetic field is moving in a 'fixed' direction relative to the secondary winding, therefore the 'different' currents in the winding must actually be in the same direction.

My point has been.
The number of phases should not change based simply on the presence or absence of a neutral.

I have little problem with people saying the currents in a 120/240V system appear to be 180? apart when viewed from the neutral. But that does not make it 2-phase. While it might simplify describing the operation of a 120/240 system it causes problems for similarly constructed ones that do not have or use a neutral, like the 240/480 and the 24/120 examples I have been using.

LarryFine

Master Electrician Electric Contractor Richmond VA
Consider a conventional 120/240V center tapped transformer. I connect a 120V 12A motor from leg A to neutral, and connect 12A of tungsten lighting from leg B to neutral. The current flowing on the transformer from terminal B to neutral will not be in phase with the current flowing from neutral to terminal A.
Yes, it will.

kwired

Electron manager
I still think all the OP wanted to know is what does polyphase mean? I think most of the posts are way over many peoples normal encounters.

winnie

Senior Member
Consider a conventional 120/240V center tapped transformer. I connect a 120V 12A motor from leg A to neutral, and connect 12A of tungsten lighting from leg B to neutral. The current flowing on the transformer from terminal B to neutral will not be in phase with the current flowing from neutral to terminal A.

Jim, Larry, please read the specific example again. You have _two_ circuits: Leg A terminal to a _motor_ to neutral, and Leg B terminal to tungsten lighting to neutral.

These two circuits are supplied by transformer coils on the same core, and therefore have the _same_ voltage induced in them.

The loads on these two circuits have different power factor (a motor on one, a resistor on the other). With applied voltage in phase, and different power factor, the currents must be out of phase.

The magnetic flux in the transformer doesn't create the current flowing in the secondary. In fact, the current flowing in the secondary will act to _oppose_ the flux. Instead the flux in the transformer creates the _voltage_ in the secondary.

Since this is a single phase transformer, then the _voltage_ induced in the two halves of the secondary has to be in phase (barring losses and leakage, but these are small order terms). But the _current_ flowing the two halves of the secondary does not need to be the same magnitude, and does not have to have the same phase.

kwired: threads in open forums are almost always like this Fasten your seatbelt and enjoy the ride!
-Jon

LarryFine

Master Electrician Electric Contractor Richmond VA
I have little problem with people saying the currents in a 120/240V system appear to be 180? apart when viewed from the neutral. But that does not make it 2-phase.
I agree. If one puts a pair of batteries in series, you couldn't get 3v if they weren't 'in phase', and likewise with 240v.

Moving the reference point is the only difference between a dual-voltage supply and a bi-polar one. 'Additive' requires 'in phase.'

jim dungar

Moderator
Staff member
Jon
Jim, Larry, please read the specific example again. You have _two_ circuits: Leg A terminal to a _motor_ to neutral, and Leg B terminal to tungsten lighting to neutral.

Does the motor current flow in the direction of A->N at an angle of -90? or is it N-A at an angle of 90?? Does the tungsten lighting current flow from N->B at an angle of 0? or might it be B->N at 180??

Regardless they are two different currents, at different phase angles but still flowing in the same relative direction as their voltages.

jim dungar

Moderator
Staff member
I still think all the OP wanted to know is what does polyphase mean? I think most of the posts are way over many peoples normal encounters.
Actually the OP did ask if (2) phase conductors + (1) neutral is single phase.

LarryFine

Master Electrician Electric Contractor Richmond VA
Actually the OP did ask if (2) phase conductors + (1) neutral is single phase.
The answer is yes: 120v 1ph and 208v 1ph.

LarryFine

Master Electrician Electric Contractor Richmond VA
Jim, Larry, please read the specific example again. You have _two_ circuits: Leg A terminal to a _motor_ to neutral, and Leg B terminal to tungsten lighting to neutral.
So, what would you call it if both of these loads were in parallel on a single 120v secondary?

winnie

Senior Member
So, what would you call it if both of these loads were in parallel on a single 120v secondary?

I would call this a single phase system. But as I stated before, I would call the center tapped transformer with the motor on one winding half and the resistor on the other half a single phase transformer.

If you have a center tapped transformer, and you have a motor in parallel with a resistive load on one half of the center tapped winding, and nothing on the other half, then you still have two _different_ currents flowing in the two winding halves.

Additionally, the two circuits (the one going to the motor and the one going to the resistive load) carry current with different phase angles.

This simply reinforces my point: we are using 'phase' in many different ways, and depending upon which _commonly accepted_ usage you are using, you will get different phase counts.

This is a _single_ phase transformer.

It develops a _single_ voltage phase in its windings.

It can supply current with different phase angles. Unless you have extremely odd loads (say a capacitor on one leg and an inductor on the other leg) the different phase angles of these currents will be more or less aligned.

It can carry different currents in the different halves of the secondary winding, with different phase angles (again, more or less aligned, but still different).

Remember that a 'network' service, which clearly has different phase angles available, and is derived from a set of transformer secondaries that are developing different voltage phase angles, is still _defined_ as a single phase service. IMHO this is simply a different use of the word phase.

-Jon

jumper

Senior Member
Actually the OP did ask if (2) phase conductors + (1) neutral is single phase.

Yep.

The answer is yes: 120v 1ph and 208v 1ph.

I said this 72 posts ago.

Why is it single phase?

Short answer: Because the teacher said so.

jumper

Senior Member
I think most of the posts are way over many peoples normal encounters.

Yeah, but they are fascinating and fun to read. I may not understand all of them, but I always learn a little in these type of threads. I like them a lot.

mivey

Senior Member
It isn't, but if any reference other than neutral is chosen, you have said, the 2-phase no longer exists. So why must the neutral point be the reference in order to determine the number of phases?
Because of the way the system is defined. Don't get the idea that I am saying that voltages 'disappear'. What I am saying is that the voltage will not fit in the particular system phase-counting bucket. You have to pick a reference in order to define a voltage. There is no universal designation for picking a reference point. Neither is there a universal designation to say which voltage has the angle from which all other angles will be measured.

There are several systems we can define using the transformer terminals. The transformer can supply any of these different systems. If the transformer is capable of supplying more than one order of system, I am saying the transformer is a supply of the highest order.

There are several type loads we can serve. Single-phase loads use only one pressure wave. Two-phase loads use two different pressure waves. Three-phase loads... you get the idea. Whether the voltages to these loads will all fit into a single phase-counting system bucket has to be determined.

Let's use the 240/480 volt transformer with the X1, X23, X4 terminals for example so we can put in some real numbers for some example systems:

System 1: A 240 volt single-phase system
System 2: A 480 volt single-phase system
System 3: A 240 volt two-phase system

Now pick a voltage reference and let's see what possible systems we can have:
X1 (or X4) Reference: We can have two single-phase systems, each with one voltage (one at 240 volts and one at 480 volts). Both voltages are present, but they are classified into two different systems.

X23 Reference: We can have two single-phase systems with one voltage each. One single-phase system is 240@0 and the other single-phase system is 240@180. This system can serve two independent loads from separate sides of the winding.

We can also have one two-phase system with two voltages. This type system serves loads that use both voltages and one voltage is 240@0 and the other voltage is 240@180.

Whether we have two systems with one voltage each or one system with two voltages, both voltages are used.

I was discussing the single current on the primary side of the transformer creating a corresponding current 'direction' in a single center tapped secondary winding.
Un-equal loads can cause the "neutral" to be different from the natural neutral point. With these loads, the neutral is just a grounded conductor. It is forcing the common connection point away from a point of natural equilibrium. Regardless of the direction you pick to be positive, there are times when the current in one side of the coil is positive and the current in the other side of the coil is negative. They are actually flowing in two different directions. Even when in the same direction, the flux can be different because of a magnitude difference.

Looking at the primary current is not representative of what is going on in the secondary because the net current in the primary is going to be based on the net flux of two different secondary currents. By looking at the primary current, the real secondary currents are hidden from you because you can't see the original individual fluxes.

Without the neutral, the currents would stabilize such that the voltage balance point was located inside the load away from the common connection point. In that case, there is no unbalanced current and the current in both sides of the secondary coil would be exactly the same. At that point, you would just have one single-phase load, one flux, etc.

I did not say that a grounded conductor must be counted, only that it may be. It is you who is putting requirements on which conductors must be used. I am looking for a definition that does not change based on which reference is used.
I have given you a method but you haven't been able to see it yet. It is a general definition that can apply to more than one type system. The formula to determine the available system types and which voltages you can put into each system bucket for counting phases does not change.

How you configure the source can restrict the number of available systems. Saying you don't want the neutral to make a change is like saying you are okay with saying we have a 480 volt source, but don't want to recognize that you can also get 240 volts if you use the center-tap. The neutral gives you an option you did not have without it.
There are two pairs of two L-L voltages. L1-L2 and L1'-L2'. L1-L1' and L2-L2' were never considered as valid connections (similar to the high leg in a 240/120 connection). This is why I refer to this as 2-phase 5-wire.
The high-leg is a valid connection but since its voltage is higher than the others, it is put in a system phase-counting bucket by itself. The only unique voltage in that bucket is one 208 volt high-leg so it becomes a single-phase 208 volt source. The four L-L voltages in the 5-wire are just as legitimate as any other voltage. They are real voltages. Whether or not we can find a practical use for them is immaterial.
With your logic, the presence or absence of a center tapped neutral changes the number of phases, but for some reason it doesn't affect a wye connection. With my logic it makes no difference.
Then I think you are misunderstanding something I have said. A 120/208 wye source with no possible connection to the neutral can only supply a single-phase 208 system. If we can use the neutral, the transformer can supply the following:
1) one single-phase system with one 208 voltage
2) two single-phase systems with one voltage each (one at 0 deg and one at 120 deg)
3) one two-phase system with two voltages (one at 0 deg and one at 120 deg)

FWIW, the utility industry considers the open-wye distribution system to be two-phase. The load of the system as a whole appears as a two-phase load. If an open-wye is labeled "single-phase" it would be because of the nature of the loads. As I have said before, the load types have been used as part of the labeling. If it is serving single-phase loads, it can get labeled as a single-phase supply. That does not mean it is no longer a valid source for two-phase loads.

If you consider two voltages and how you are going to classify systems for them to be counted in, there is a difference between two single-phase systems and one two-phase system.
Not all grounded connections use a neutral, not all 'non-end point' taps are neutrals. For example, there is a standard control power connection of 24/120V (x1-X23= 24V, and X1-X4=120V).
I know that. And not all "neutrals" are true neutral points.
I believe I have said there is a single current direction, created by a single magnetic field direction, such as X1->X23->X4. But through the magic of math, X1->X23 and X23->X1 can be interchanged, by following proper 'signing' rules, giving the appearance of two currents.
Pick any direction you want, but the currents will not always flow in the same direction or have the same magnitude. It is a physical reality, not math magic.
Do you say a high-leg 4-wire delta (one winding is center tapped) has these 3-phases: 2@180?, and 1@90? while ignoring the 3@120?? If you mention them all is this a 6-phase transformer?
No. To be included in a system phase count, the voltage must have the same magnitude and a different angle from another counted voltage. All of those phases are not classified in the same system as each system phase-counting bucket has its own voltage level. Here are examples of some of the systems a 120/240 high-leg transformer could supply:
1) Three 240 volt single-phase systems with one voltage each.
2) One 208 volt single-phase system with one voltage.
3) One 120 volt single-phase system with one voltage.
4) One 120 volt two-phase system with two voltages.
5) One 240 single-phase system with one voltage.
Each system has one voltage magnitude to use for counting voltages..

mivey

Senior Member
One single primary winding.
One single core.
One single magnetic path, through the core.
One single secondary winding.
Is the result, one single secondary current direction?

This should be a simple yes or no answer.
No.
It is impossible for you to define a direction (pick whichever one you want) such that the current in the first half will always have the same sign as the current in the second half at every point in time:

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