OK, that's interesting; thanks. I'd never read that explanation. (I'd sooner trust the BPA article than that online forum that seems to have a lot of clueless, unscientific types posting to it.)
So what they're saying is that, basically, even though electricity moves at [about] the speed of light, electrons actually move very slowly. Verrrrrry slowly.
But that doesn't change the fact that in both DC and AC, *current* flows, almost instantaneously in both cases. It's just that current doesn't necessarily coincide with electron motion.
(I have no idea now how electron and current flow actually works, but again, that's at the quantum physics level. Us boneheaded electricians can still go on thinking that AC currents flow, not "jiggle".)
For a 3-phase transformer, you use 2 transformers in a Scott connection for the primary, and with a Scott connection on the secondaries. The primary is 3-phase. The secondary is also 3-phase. The voltage in the transformers is at 90 degrees - 2-phase. A disadvantage is the transformer currents are not in phase with the voltage so the transformers can't be used at their full rating. It is practical for small 3-phase transformers.
I am too lazy to look up prices (which also requires matching quality). My notes say over a 1/2 HP motor is cheaper in 3-phase. You don't need a winding that is only used to start the motor. And you don't need the start switch paraphernalia and often a capacitor. Motor control is likely more expensive.
Fans of "2-phase" could ask for a 120/240 2-phase service from their utility.
Simpler, I agree. Whether or not the volume (of single-phase) motors exceeds the difference in complexity is the question. Also, I suppose, it depends on who's buying (in what quantity - inventory costs as well as manufacturing).
Bud Would you please guide me out of this morass these folks have constructed for themselves. You know how the dry transformers that we install all the time have output voltage selection taps to compensate for the variations in utility input voltage so we still end up with
120 volts for the general purpose receptacles? What do you say we wire one with a conductor off of each voltage selection tap. Do we know have six phase or eight phase. Dam I cant keep my tongue that far out in my cheek without it starting to hurt.
I just thought of another question for the two phase crowd. How many phases on the output of a high leg center tapped phase delta transformer? By there logic it must be four. Dam my cheek is starting to hurt again!
-- Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison
Ok everyone. Without any explanations included. Weigh in on the final answer. In phase or 180 out? You are allowed only yes or no. Is the sentence in question correct?
Well, all I know is I can buy 2HP 3 phase motors for considerably lower cost than 2HP single phase here in Waterloo and they are generally smaller as well. When you get to 5HP and higher, the difference REALLY becomes obvious. Not sure how 1/2 HP compares.
Also, lots of decent used 3 phase motors are available CHEAP, while good used single phase are less common (because 3 phase only burn out or need bearings, while single phase can also have starter problems, bad caps, etc - and are also more prone to burning out when starter problems occur.)
Fractional-HP motors are what I was really thinking about. Certainly above a couple of HP the numbers go the other way. A single-phase 100HP motor would be a rare thing indeed. ;-)
A lot of those things that burn out on a fractional-HP motor are pretty easy to fix, making the motors a lot cheaper (scrap yard cheap).
When a 3 phase motor ends up in the scrapyard because it outlasted the machine it was on, you only need, at worst, a pair of bearings. When a single phase motor ends up in the scrapyard you likely need a $16 starting cap as well as the bearings - and may also need to cleen/repair the starting switch.
The big problem is you need a 3 phase supply to run the 3 phase motor
- and outside of industrial plants 3 phase is "relatively" rare.
It's been awhile since I asked any silly questions. Probably at least two minutes so I'm behind on my quota. Isn't the distinction also about timing? There is no time with three phase power when the voltage is zero on all three lines. There is in single phase. Did that happen with the old two phase systems?
My understanding is that the ancient two phase had a phase difference of 90 deg, so there would also not be any time when there is no voltage difference between the conductors.
I have to join dpb, David, and Jeff in saying that krw is totally wrong.
To address some of the specifics:
krw: "No they,(2 hots in 240V service) in fact, aren't. One is the negative of the other. "
One is the negative of the other and it's also 180deg out of phase with the other. Take a sine wave centered around zero, shift it by 180deg and it becomes the negative of the other.
From dpb: "There are two meanings of "phase" here which is the difficulty in common usage. The generation is indeed a single electrical phase; the two derived currents are out of phase (in time) with each other. "
I said about the same thing many posts ago. Is the 240V service in use today generally referred to as two phase? No. Is it the same as the ancient two phase system that was referred to that way? No, because there the phases differed by 90deg. What is the phase relationship between the two hots in today's 240V service? They have a phase difference of 180deg.
">> No they, in fact, aren't. One is the negative of the other.
Matlab is wrong. "
Explain then how that is. If I have 3 signals that are 120 deg out of phase, krw agrees that consists of 3 phases. Apparently he agrees that the ancient 2 phase system, where the phase difference was 90deg, consists of 2 phases. So, why exactly if we have a system where there are two conductors, one of which is 180deg out of phase with the other, does it no longer qualify as consisting of 2 phases?
It's merely a special case of phase difference, where they happen to be 180deg out of phase and one is the direct opposite of the other. Try this mental experiment. Take two identical signals that are in phase, ie the phase difference is zero. Start adding phase shift, 1 deg at a time. Do you not now have a system with two phases? Continue until you have 180 deg of phase shift. Why now are there not still two phases?
Answer: In fact there are, it's just a special case where one can now be called the opposite or negative of the other.
">Of course, because one "leads" the other by pi radians...
krw: Wrong, obviously. "
Actually it's absolutely correct. I also don't understand the comment:
"Math engineering"
A core component of electrical engineering is math. Most of the courses are heavily math oriented. And the math engineers use is the same as the math everyone else uses. And the phase relationship of two signals would be described the same by anyone using math.
krw: "Nope. Define CT as zero. The signals at each end are the same but opposite sign"
Geez, opposite sign is exactly how you get 180deg phase difference.
">Not "source", simply a demonstration of how the phase shift leads to the
Complete bullshit. "
I'm a degreed Electrical Engineer, and it isn't BS to me. If you take a signal centered around zero volts, and another one in the same system that is also centered around zero volts, but is 180 deg out of phase and they share that common zero volt referrence, then:
One leads the other by 180deg One lags the other by 180deg One is the opposite of the other One is the negative of the other. You have two phases
That is exactly what you have with a 240V service. Is it generally referred to as a two phase system? No. Probably because it can be generated off of ONE phase coming from the power plant via a center tap transformer. But how I generate it matters not a wit. KRW, ask yourself this. You say you're an engineer. Here's a simple test:
1 - I have a graph of what we all have been describing, what I outlined above. Two sine waves on a graph, both centered around the zero voltage axis, both from the same system. One is shifted by 90 deg from the other. How many phases are there in this system?
2 - Same graph, but now one is shifted by 180 deg from the other. How many phases in this system?
3- Same graph, but now I have 3 sine waves, one shifted by 90, one by
180. How many phases in this system?
My answer, and I think the answer from all other 3 who disagree with you would be:
Unbelievable. People are actually replying and answering this stupid question without apparently even realizing that the question already answered itself. Did it not state: ""The two insulated wires each carry
120 volts, but they are 180 degrees out of phase "? Beyond that, it wants a yes or no answer to something that is mutually exclusive, ie "In phase or out?" How the hell do you guys answer that with a yes or no?
The problem some of you are having is understanding the difference between what something may be commonly called by those in the trade and the engineering concepts and definitions of systems, phase, etc.
And in the case of 240V service in your house, the answers you seek are:
Q What is the phase relationship of the two hots? A They are 180deg out of phase with each other
Q Does it matter how that phase difference was achieved? A No, from an electrical engineering perspective, we just need to look at the voltage waveforms and current flow on the service cable. Plot the waveform on a graph paper and you have your answer.
Q Is this commony called a two phase system? A No. Probably because it's created by a transformer that uses one of the three phases generated by the power plant.
It was defined to me some 30 years ago, but what I came away with was that it is "in phase" if the voltage and current reach peak and 0 at the same instant. That is what I understand a single phase transformer does. (Ignore capacitance and inductance)
Nobody I know would call 120/240 2-phase. You wouldn't buy a single core transformer and specify whether it is in-phase or 180 degrees out of phase. You don't get multiple phases out of a single transformer. If you ask for a 2-phase transformer you will completely confuse the transformer rep.
Analysis of real multiphase electricity commonly uses phasor analysis, using SQR(-1).
A simple 120/240V system is single phase with the math handled with
*trivial* plus and minus signs. "2-phases" confuses trivial math. Calling it 2-phase confuses communication with anyone who understands multiphase power systems.
If 120/240 is 2-phase, then single phase has no particularly useful meaning.
If you tell a utility you want a 120/240V 2-phase service what will they say? That is after they stop laughing.
-------------------- I never heard 120/240 called split phase either. A wiki article says it is. But the article also says 120/240 "it is sometimes incorrectly referred to as 'two phase'"
Whether 2 phases confuses anyone or not has no bearing on the fact that there are two phases. I could describe many physical processes by either very simple terms or varying degrees of complexity. When looking at electrical waveforms, that trivial plus and minus sign can equate to being described as 180deg out of phase.
I noticed you didn't specifically refute any of the statements:
I never said to call it 2-phase power, nor do I recall anyone else here really doing so. A couple of us have said consistently that while you can view a 240V service as having two phases that are 180 deg out of phase with each other, that terminology is not commonly used to refer to the actual service.
Let's get back to basics. The definition of phase and it's use in electrical engineering goes to the very roots of the discipline. IT doesn't depend on what terms people commonly call something. It doesn't depend on how the phases are generated. IT's not limited to only AC power systems. I can take any linear system that has a sine wave going into it and ask how many shifted sine waves of voltage are present in that system and what are their relationship to each other. I can ask a student to plot the voltages at various points in the system. Let's say I have a box with 3 wires coming out. Between A and C, there is one sine wave. Between B and C, there is an identical sine wave, but it's shifted by 90deg. I ask, what is the relationship between them and how many different phases are present.
What would your answer be?
Now the same experiment, but with the sine waves shifted by 180deg instead of 90. Is the correct answer that it is now simply a plus and minus, trivial issue and there is only one phase present?
Again, neither I nor anyone else I believe, has called it "2 phase service". What common terms are used and what things really are, are two different things. If I told probably 1/4 of Americans that my son is a homosapien, I'd get the same reaction because they don't even know what it means. It doesn't make it untrue or not technically correct.
Unfortunately, I'm heading out for a few days. I'll have to pick back up on this later on.
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