Two phases or not?

"David Nebenzahl"

** It's clearly two phase power, no doubt about that.

There are two AC waves that differ only in phase and if both "phases" are loaded equally, the neutral current is zero.

So it is completely analogous to three phase power.

The 90 degree system is just an obsolete odd ball only a moronic pedant would even mention.

.... Phil

I do industrial EE work and terminology can really get screwed up at times.

You have a CT (center Tap) this gives you 2 phases, using the CT as the common, which of course are 180 degrees apart. How ever, if you need 230 volts, you would need to use the 2 outer legs which will only give you a single phase for that circuit. Hence, only one winding over all. CT means nothing here.

To qualify for a phase, it must has a common or an opposing leg.

Lets look at a floating Delta 3 phase system, even though it has no common to qualify it as 3 circuits 120 degree's out of phase, it does have 3 separate windings, each 120 degree's out and can supply a circuit on each winding if it wished.

Looking at a STAR (WYE), one end of each phase (winding) is connected to a common point.. This common point does not need to be used but is in some cases.

The point is, there are 3 individual windings that are out of phase with each other. Makes it 3 phase...

In the case of residential, the CT will give you 2 phases of 180 degree's when you only need 115 volt circuits how ever, because the transformer here is really only a single winding with a CT, when you need full voltage for 230 appliances, that circuit ends up being a single phase.. How ever, most appliances like that also have the CT/Neutrual so it can use half of that to operate the control electronics.

Some people call it split phase, only because you are taking a single phase and splitting it in the middle. But that does give you a 2 phase source if you to use the CT as the common for both.. It would be the same as 2 transformers with their secondaries joined on one side only to form a CT.

Remember "Split Phase" only means a single winding with a CT it, hence, you have split the phase in half!

Now here's a tricky one.. Just think of the 3 phase system with a CT in each winding. Now we're talking ! :)

Jamie

Now here's a tricky one.. Just think of the 3 phase system with a CT in each winding. Now we're talking ! :)

Jamie

--------------------------------------------------------------------------------

-------------------------------------------------------------------------------- Never seen one like that, but have seen a center tap on one winding to add

240/120 split phase to 3 phase 240 Delta. Tap I've seen on Wye (star) garners 120 from any phase to neutral and 208 from phase to phase. (all this if my memory isn't completely FUBAR)

CT

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d

his

The system you mention was somewhat common in the '30s for large apartments/ medium hotels/ small commercial and the like where they would have a centre tapped 115/230V say 50kVA transformer across one phase and a pair of say 5kVA 230V transformers across the other two phases of a 3 phase supply [typically 2300V back then] and connected in delta with the 50kVA. The lighting and appliances were powered off the 50kVA while the elevators and laundry motors were powered from the

3 phase. It seems that most of those systems were replaced with 3 phase 4 wire wye or one large single phase transformer and a three transformer delta system when the primary distribution was upgraded or when the customer load increased after WWII. I still see the occasional 3 phase open delta [two transformers] where the 3 phase load is fairly light such as rural irrigation pumps that are a bit too large HP for single phase.

Neil S.

If there were two electrically isolated windings, with opposite polarity, you would clearly have two phases. (I think.)

But the center tap gives them a common neutral. So are they still separate phases? I'm inclined to say yes, because in a conventional three-phase system, loads can be attached to individual phases, or across two phases for a higher voltage. And these are considered multi-phase systems.

Well, if you want to be correct, the house power is split phase, and not two phase. I suppose there is an arguement that it is two phase, but say that to a power engineer and you'll get the old raised eyebrows response!

Ding! We have a winner.

Thank you peter.

Jeff

Reminds me of the story of two carpenters. One says, "Half of my nails have the head on the wrong end." His partner responds, "You dummy, those are for the other side of the house!"

And the other carpenters doing the other side of the house: "Damn, I just cut this board too short." His partner replies, "Simple, just turn it around and cut somemore off the other end..."

On 1/22/2011 5:27 AM PeterD spake thus:

Welllll ... that's pretty much what I wrote. So I take it you agree with me that it is, in fact, 2-phase power, correct?

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   To me, the *plonk...* reminds me of the old man at the public hearing
   who stands to make his point, then removes his hearing aid as a sign
   that he is not going to hear any rebuttals.

In a word, no.

Power distribution being what it is, the 240/120 transformer on the pole is sourced with _ONE_ phase of the 3-phase feed for the area. That there is a center tap allowing for 120/120 or 240 loads is immaterial. It's STILL single phase.

Jeff

"Jeffrey Angus"

** Totally irrelevant how the 2-phase is derived. ** Pedantic nonsense.

There are two AC waves that differ only in phase and if both "phases" are loaded equally, the neutral current is zero.

So it is completely analogous to 3-phase power.

The 90 degree system is just an obsolete odd ball only a moronic pedant would even mention.

.... Phil

On 1/22/2011 4:49 PM Phil Allison spake thus:

I have to say I totally agree with Phil here. Especially the reference to "true" 2-phase power brought up in this argument (the one over on a.h.r), meaning that totally antiquated and obsolete system.

Those with which I disagree seem to think that just because the two phases aren't somehow manufactured separately, by two different windings of a generator or some such, that the center-tapped xfmr can't possibly supply two separate phases, when it so clearly does just that.

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On 1/22/2011 4:45 PM Jeffrey Angus spake thus:

Keep in mind the premise of my original question. Remember I pointed out that 120-0-120 current supplied by a center-tapped transformer is *not* called "2-phase" by those in the electrical power industry, but that it is, in fact, 2-phase power. Do you disagree that there are actually two separate phases of power present at the secondary of the transformer?

Regardless of what they call it ...

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   To me, the *plonk...* reminds me of the old man at the public hearing
   who stands to make his point, then removes his hearing aid as a sign
   that he is not going to hear any rebuttals.

Nope, single phase.

The definition of "more than one" phase is a difference of something other than 0 or 180 degrees.

With either 2-Phase (the old 90 degree system) or 3-phase (current 120 degree system) the phases can NOT be generated with a simple transformer. They are generated by multiple alternators mounted on a common shaft.

With a single transformer, and a center tap, you have two voltages, in phase, that add together.

It's single phase.

Jeff

having a CT in a winding gives you 2 phases, 180 apart..

Calling it split phase is just a method of doing it.

Lets look at it this way..

Take a control xfomer..

If I was to wire the secondary as

X1, X2+X3, X4;

X2 and X3 being the CT, I now have a source that has 2 secondaries (2 windings) that can give me 180 degree shift via the CT. This gives me 2 phases..Why? because they are not in phase with each other.. It does not matter if they are only 1 degree off from each other.. They would be two difference phases, because we are using the CT as the common point. Same as, if you were to use the STAR Center of a WYE transformer as the common point, this would give you 3 phases which we all know are 120 degrees different from each one. The analogy isn't any different if you had the pole pig which is just a single winding with a CT in it on the secondary side for your common. Other wise known as a split phase because can treat that as a single phase to get the full voltage or split phase to get half voltage with 180 shifts.(2 phases)

Now, take that same xformer I have above there and....

X2, X1+X3, X4;

What do you get? You get two different power points sharing a CT but in phase with each other. And yes, I've seen this done before to avoid over voltage through grounds if the neutral was ever lifted for some reason. This basically is only one phase and does not allow you to use them for double voltage. In fact, you'll get no voltage between X2 and X4.

Oh well.

Jamie

Oh, so now that we have a CT as a neutral (common), how do you explain the two different phase angles we now have ?

Last time I went to school, phase count was the number of phase angles you have that are unique in degrees.

does not matter how you derived it..

If I plug in an inverter that operates from a single phase circuit and it generates 3 phases that are 120 degree's apart, does that mean it much be single phase because it started from a single phase source?

Jamie

On 1/22/2011 6:49 PM Jamie spake thus:

Excellent question. I await answers.

So far, we have two objections to a center-tapped transformer giving two phases, both very doubtful:

1. Not two phase because the "source" (i.e., the other side of the xfmr) is single phase.
2. Not two phase because two phases that are 180° are not actually two phases (?!?!?!).

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   who stands to make his point, then removes his hearing aid as a sign
   that he is not going to hear any rebuttals.

"Jeffrey Angus"

** Who's " definition " is that ????

I wonder ........

..... Phil