May I ask a question about the single vs two phase stuff

[mivey]

I think that was me. I'll try to make the point with pictures instead of words. (Please pardon quick, crude editing of the Wikipedia image.)

If something momentarily disrupts the generator shaft rotation, the resulting voltage dips would be better represented by a graph like this...
View attachment 21015

...than by a graph like this.
View attachment 21016

The point is possibly even more more true when we're considering a 120/240 transformer with single phase primary.

Nice.:thumbsup:

 

[jaggedben]

Yeah, that's a lot of ifs. And will practically never be the case.

Actually, 'practically' speaking, there are plenty of cases where you don't have to account for power factor or multiple phases. For example, I would never have to understand the math in this article in order to calculate NEC solar inverter output requirements on a 120/240V service.

The "time delay" discussion has nothing to do with it. Time delay is something else altogether.

For the record, I think you are agreeing with me. [Edit: And that's corroborated by your last post. Typing at the same time. :thumbsup:]

 

[mivey]

I've been trying to make this point since post 241.

If the half-cycles are symmetric, then 180deg = -1.

If they are not symmetric, then -1 works better.

I went back and read your post 241. The problem is that for AC waveforms, -1 is OK only for a very specific set of circumstances. Almost everything we do with AC waveforms requires vector math of some sort and as such we are thus using degrees. I see no reason to use -1 when it is not applicable to the general case.

There is no mathematical difference in multiplying by -1 or multiplying by 1<180d. None. However, since the general solution to AC problems use vectors/degrees, I see no reason to say using -1 is preferred over using 1<180d.

If you want to be consistent, all of your numbers will be represented by a positive number in degrees and you will restrict the "-" to operations on those numbers. i.e., when you are subtracting two numbers.

Slightly different for rectangular coordinates of course but there we don't have degrees anyway.

 

[mivey]

That bit of the thread was in the context of discussing the broader meaning of 'phase' within the industry. IOW, if you have an ungrounded or corner grounded delta, you still have three phases even though there's no neutral. If you have a high-leg delta, in my opinion, you still have three phases even though there is a 'neutral' on one of them. I realize that the last point is controversial in the context of this this thread (but not in standard industry terminology). But I think the high-leg delta is illustrative of how it doesn't make sense to insist that the 'neutral' must be the reference, since it doesn't illuminate the three line-line phases.

With a high-leg delta transformer you can have it be the source for multiple systems of voltages. Use a 240/120 high-leg as an example.

You can have a 240 volt system with one voltage.

You can have a 240 volt system with two voltages.

You can have a 240 volt system with three voltages.

You can have a 208 volt system with one voltage.

You can have a 120 volt system with one voltages

You can have a 120 volt system with two voltages.

The name we use does not define every system. We can take away a single phase system but we do not call it a single-phase transformer. It is ALSO a source for single-phase.

That said, we meter it by using one line-line (240 volt) source and one line-neutral (208 volt) source. We do use the neutral as a reference.

 

[mivey]

I'm happy to allow that there's a mathematical definition for purposes larger than electrical engineering. But I think 'phase' means something different in the electrical industry, and there's a reason that the 120/240 service is call 'single phase' and the high-leg delta is called 'three phase.'

Because of the way it is normally used. 120/240 or 120/208 normally serve single-phase loads so that becomes the common name.

The name is just a name and not a physical description. It has a basis but not completely descriptive of the system.

Physically, the two-phase system is just a specific subset of the four-phase system and so the 5-wire two-phase is more correctly called a four-phase system. It was used to provide two phases so the name sticks. How about some history? There are too many for me to feel like putting them all here again but here are some quotes for you on names and about a 180d phase difference:


From the article "The Design and Action of the Rotary Converter" in Engineering Magazine vol 22, by David Rushmore, 1917

The names were given to the different systems before their relationships were thoroughly understood. Thus, the two-phase (so called) really comes between the three-phase and the six-phase. To be scientifically correct, the number of phases by which the machine or system is designated should be the number of alternating-current leads per pair of poles or 360 electrical degrees. we really have no single phase. what we call by that name is a two-phase. Then comes the three-phase, rightly named, after that the four-phase, misnamed two-phase or quarter-phase, and then the six-phase. As will be seen by comparing Figures 2 and 4, the six-phase consists of two interspersed three-phase systems and is taken from three transformers.

From: "Engineering Circuit Analysis", William Hayt, 1962, McGraw-Hill, pg 572:

The name single phase arises because the voltages Ean and Enb, being equal, must have the same phase angle. From another viewpoint however, the voltages between the outer wires and the central wire, which is usually referred to as the neutral, are exactly 180° out of phase. That is, Ean = -Ebn and Ean + Ebn = 0. In a following section we shall see that balanced polyphase systems are characterized by possessing a set of voltages of equal magnitude whose (phasor) sum is zero. From this viewpoint, then, the single-phase three-wire system is really a balanced two-phase system. "two-phase", however, is a term that is traditionally reserved for something quite different, as we shall see in the following section.

From "Alternating Current Machines", Sheldon, 1902:

If the zero ordinates of the two curves coincide, but the positive maximum of one coincides with the negative maximum of the other, as in Fig. 11, then Φ = 180° and the curves are in opposite phase.
...
The term polyphase applies to any system of two or more phases. An n-phase system has n circuits and n pressures with successive phase differences of 360/n degrees.
…

From: "Center-Tapped Transformer and 120-/240-V Secondary Models" William H. Kersting:

Distribution engineers have treated the standard “single-phase” distribution transformer connection as single phase because, from the primary side of the transformer, these connections are single phase and, in the case of standard rural distribution, single phase line to ground. However, with the advent of detailed circuit modeling, we are beginning to see distribution modeling and analysis being accomplished past the transformer to the secondary, which now brings into focus the reality that standard 120-/240-V secondary systems are not single-phase line-to-ground systems, but they are three-wire systems with two phases and one ground wire. Furthermore, the standard 120-/240-V secondary system is different from the two-phase primary system in that the secondary phases are separated by 180° instead of three phases separated by 120°.

From "Techniques of Circuit Analysis" by Carter/Richardson concerning forming polyphase sources from voltage sources separated by phase angle differences:

...two voltage phasors in opposition-that is, with a phase difference of 180 degrees; a single-phase transformer with a center-tapped secondary winding would be such a source.

From "Photovoltaic Power Systems and The National Electrical Code", Sandia National Laboratories:

In a utility connected system or with a 120/240-volt stacked pair of inverters, where the 120 /240-volt power consists of two 120-volt lines that are 180 degrees out of phase, the currents in the common neutral in the multiwire branch circuit are limited to the difference currents from any unbalanced load. If the loads on each of the separate branch circuits were equal, then the currents in the common neutral would be zero.

 

[mivey]

Actually, I've been one of those pointing out that there isn't a time shift with ideal sine waves. And I also keep pointing out that if you don't have ideal waves, then there's a practical difference between using math that reproduces a time shift and math that doesn't.

Yeah, I saw that later. I got lost in the thread and attributed it to you. Sorry. I got way behind.

 

[mivey]

Actually, 'practically' speaking, there are plenty of cases where you don't have to account for power factor or multiple phases. For example, I would never have to understand the math in this article in order to calculate NEC solar inverter output requirements on a 120/240V service.

Yes, there are plenty. As a percentage of the AC electrical world as a whole they are quite small in number IMO.

For the record, I think you are agreeing with me. [Edit: And that's corroborated by your last post. Typing at the same time. :thumbsup:]

I had you mixed up with some other dude.

 

[jumper]

The term phase-splitter is commonly used anyway.

Gads!

I ain’t using either myself, but yes-common terms.

“Phase splitter” sounds like something outta Star Trek.

 

[romex jockey]

good lord.....i need to go back to school! ~RJ~

 

[mivey]

But I think the high-leg delta is illustrative of how it doesn't make sense to insist that the 'neutral' must be the reference, since it doesn't illuminate the three line-line phases.

It doesn't make sense to say the neutral must be the reference. The reference is a choice.

Corner is valid. Neutral terminal is valid. There is no must.

For the 240/120 high-leg delta as an example, the neutral terminal is only a neutral for the 120/240 single-phase system we can take away. The neutral point for the three-phase system is floating inside the delta and does not have a terminal access.

 

[mivey]

good lord.....i need to go back to school! ~RJ~

I have been in school since I left school.

Life is a school. or is it a highway?

 

[mivey]

Gads!

I ain’t using either myself, but yes-common terms.

“Phase splitter” sounds like something outta Star Trek.

It is a weird term but makes sense enough I guess in context.

As long as one understands what is said, I guess you can let someone take a little liberty in how it is said.

 

[winnie]

If something momentarily disrupts the generator shaft rotation, the resulting voltage dips would be better represented by a graph like this...
View attachment 21015

...than by a graph like this.
View attachment 21016

In post 946, jaggedben raises a very important point.

I (and others) describe a phase shift as meaning a shift in time, but for many real physical systems things like transients hit all the phases at once, without a time shift.

This points to an error in how I am thinking about phase angle. Phase angle is _associated_ with a shift in time. It is a way of measuring the time offset between the zero crossings of a periodic signal. But phase angle does not mean that the underlying mechanism involves a time shift.

Clearly in a generator, the various phases are physically offset on the stator, and the phase angle is generated by the rotor poles passing at different times. But this is not the only way you could generate 3 phases; you could have an inverter where the different phases are produced by a table lookup by a microprocessor; everything happening at the same time but with an output that has a time offset in the zero crossings.

If, as jaggedben's diagram suggests, you wiggled the excitation control of a synchronous generator, you would modulate all of the phases at the same time. (Though I don't think that each phase would see the same modulation simply scaled, as suggested by jaggedben's first graph.)

I am sure that with another mechanism you could actually get a transient to 'roll' from one phase to the next.

IMHO this goes to the root of this whole discussion, distinguishing between the measurable phase angles in a system and the underlying mechanism that produces these phase angles.

Phase angle applies to single frequency sine waves and to periodic functions. Phase angle is _associated_ with a time shift, and _looks_ like a time shift, but could be created by an entirely different underlying mechanism. Phase angle must be understood simply as one of the parameters describing a sine wave, nothing more.

We can use phase angle to analyze transients and non-periodic functions, but only by first breaking the signal as a sum of single frequency sine waves (someone already mentioned Fourier analysis). So we can use phase angle to describe _each_ of the frequencies that make up a complex or transient signal, and _each_ of those frequencies will in general have a _different_ phase angle.

Back to our 'single phase center tapped transformer', if we use the neutral point as the reference, than the simple inversion is the equivalent of a 180 degree phase shift for each and every frequency going through that transformer.

If the mechanism of the transformer involved a time delay, then the phase shift would be _different_ for each different frequency going through the transformer. Such systems are possible, and have uses, but not for power distribution at the local level.

With regard to the question brought up of a pulse of 4 cycles of a sine wave, suggesting that inversion would give one result and a phase shift a different result. I believe that this is a terminology issue.

If you believe that a 180 degree phase shift necessarily requires a time delay, then there would be a 1/2 cycle delay difference between inversion and phase shift. If, as I now believe, that 'phase shift' simply describes sine waves, then you have to realize that a _pulse_ is not a single frequency sine wave, but is rather composed of lots of different frequencies. An inversion will result in a 180 degree phase shift for each of the component frequencies.

-Jon

 

[Adamjamma]

good lord.....i need to go back to school! ~RJ~

I say that but- I have learned more in the thirty five years since i Left high school, on a yearly basis, than I ever did in High school... I figured once that if I could afford the CLEP tests would probably be able to pass over eighty percent of them on what the real world has taught me... So, In class just about every single day

 

[ggunn]

Gads!

I ain’t using either myself, but yes-common terms.

“Phase splitter” sounds like something outta Star Trek.

Tube amplifiers that run in Class B or Class AB have a phase splitter between the preamp and power amp sections. It feeds the input signal (the "push") to one set of power tubes and the inverted signal (the "pull") to the other set.

 

[winnie]

mivey,

Thanks for your post 1008 with snips from the literature showing examples of usage.

I still prefer 'hemiphase' to express the special relation of phases related by inversion, but I'll leave that to the hpo motor folk.

-Jon

 

[jumper]

Tube amplifiers that run in Class B or Class AB have a phase splitter between the preamp and power amp sections. It feeds the input signal (the "push") to one set of power tubes and the inverted signal (the "pull") to the other set.

Sniff, sniff...smells like audio...no tube amplifiers in my standard trannies...

 

[jumper]

mivey,

Thanks for your post 1008 with snips from the literature showing examples of usage.

I still prefer 'hemiphase' to express the special relation of phases related by inversion, but I'll leave that to the hpo motor folk.

-Jon

If I gotta live with split phase and antiphase then I say you can go ahead and say hemiphase IMO.

 

[Besoeker]

If I gotta live with split phase and antiphase then I say you can go ahead and say hemiphase IMO.

Or anti-phase. Nothing is actually shifted.

 

[jumper]

Or anti-phase. Nothing is actually shifted.

Yeah, I had to throw in anti- phase.

I know exactly what you mean, but the term is not common here. Rarely see it.