Power Distribution At Residential Level Understanding

[jaggedben]

Sorry, no, that is backwards.

They are of opposite polarity, which in this case is indistinguishable from a 180 degree phase shift.

The blue part is helpful. The red part is semantics.

L1-n and L2-n are 180 degrees apart.

L1-n and n-L2 are 0 degrees apart.

You can define the polarities either way you want, so it's a matter of stating which way you are defining it.

 

[ggunn]

No offense, but you are an electrical engineer and you have to ask about the phase relationships of a center tapped winding of a transformer?

-Hal

Electrical engineering is a wide and varied field. My concentration in engineering school was in device physics and I spent 23 years in semiconductors before moving to solar about ten years ago. I didn't know crap about three phase power, transformers, or the NEC at first. I've learned a few things since then.

 

[charlie b]

No offense, but you are an electrical engineer and you have to ask about the phase relationships of a center tapped winding of a transformer?

Sorry, Hal, but I have to say that I find that offensive. We don't question each other's qualifications here. Otherwise someone would be saying similar things to electricians (e.g., you are an electrician and you don't know how to bend conduit?). An EE degree normally will not include any course work related to the NEC, and most will not touch power systems distribution or analyses. My own introduction to 3-phase design, from the academic world that is, was a senior-level course that I took during my masters degree program, 12 years after I received my BSEE degree. I learned 3-phase systems from my work environment, not from school.

 

[ggunn]

Click here at your own risk

Roger

I'll just say this: If you put a glitch on a waveform at its maximum, shift it by 180 degrees, replot it and superimpose it on the original, the two glitches are at the same voltage but not at the same time. If you invert the glitched waveform, replot it, and superimpose it, the glitches are at the same time but not at the same voltage. It's a distinction without a difference for a single frequency sine wave.

 

[Ingenieur]

Electrical engineering is a wide and varied field. My concentration in engineering school was in device physics and I spent 23 years in semiconductors before moving to solar about ten years ago. I didn't know crap about three phase power, transformers, or the NEC at first. I've learned a few things since then.

doesn't everyone take ckts 1 and 2?
kvl, kcl, thevenin, etc, applied to ac/dc
at most abet schools regardless of concentration the first 4-5 courses are the same
ckts 1 and 2
basic machines/power
basic ss devices (lol mine included tubes)
fields

 

[drcampbell]

... you are an electrician and you don't know how to bend conduit? ...

Could happen, if all your work was residential/light commercial (NM & MC) or industrial. (rigid conduit assembled with straight segments and pre-fab elbows)

 

[ggunn]

doesn't everyone take ckts 1 and 2?
kvl, kcl, thevenin, etc, applied to ac/dc
at most abet schools regardless of concentration the first 4-5 courses are the same
ckts 1 and 2
basic machines/power
basic ss devices (lol mine included tubes)
fields

Of course, but when I switched fields it had been over 20 years since those classes and I had lived in low voltage DC land since then. I remembered that ideally VI on one side equals VI on the other, but if I would have been asked how split phase was made I wouldn't have known the answer. At one time I could have solved Fourier and convolution problems and plotted root loci but certainly not now. Use it or lose it.

 

[JFletcher]

Could happen, if all your work was residential/light commercial (NM & MC) or industrial. (rigid conduit assembled with straight segments and pre-fab elbows)

Yep, and I remember one threadabout a lifetime industrial electrician that had never handled Romex.

I bet I put less than a stick of EMT a year in my pipe benders, but have run more Smurf pipe (ENT) than I care to admit.

It takes all kinds... that's one thing that makes this forum great, is the wide variety of backgrounds and experience of the posters and members.

 

[mivey]

What I don't understand is how do we take a single phase with 60Hz frequency and break that into the two 120V phases that are 180° out of phase.

Just to be clear, I understand how we can step down the higher single phase run (before transformer) from higher voltage primary to 240V and run a neutral to divide into 120V "pieces" on the secondary side, but what I can't find is some clarification as to why these are out of phase with each other. Is this something that just inherently happens when we tap a transformer with the neutral to get two 120V phases?

It goes back to the basics of how we define a voltage. For any voltage we must pick a reference point. No reference point is pre-defined. It really is just that simple.

If we pick the center-tap as the reference point, then the signals (L1-N and L2-N) are 180 degrees out of phase. If we pick L1 as a reference for one signal and N as the reference for the other signal (N-L1 and L2-N) then they are in phase. Fundamental physics and EE stuff that you can refresh yourself with.

The defined set of voltages with a phase difference is not just a paper definition but is aligned with the physical world. Kind of the whole point in making definitions to follow what we discover in the physical world. We use these to harness the world around us and they really would not be useful if they did not help us interact with the physical world.

For example, we can use this relationship to produce a third phase from only two phases using an open wye-wye transformer bank. The math says we should get the same set of three-phase voltages we would get if we had separate transformers with a 180 degree difference. With the open-wye bank, we use half of the winding in one polarity and half in the other polarity and the physics produce exactly what the math predicts.

 

[mivey]

doesn't everyone take ckts 1 and 2?
kvl, kcl, thevenin, etc, applied to ac/dc
at most abet schools regardless of concentration the first 4-5 courses are the same
ckts 1 and 2
basic machines/power
basic ss devices (lol mine included tubes)
fields

For EE yes. Doesn't mean they will remember it later if they haven't been playing with it since then. But they should be able to refresh that knowledge.

 

[ggunn]

For EE yes. Doesn't mean they will remember it later if they haven't been playing with it since then. But they should be able to refresh that knowledge.

Agreed. An EE degree doesn't teach you everything about all things electrical, it just gives you a grasp of the fundamentals that will enable you to understand an explanation when you get it.

 

[K8MHZ]

I like to make the analogy of two 1.5 volt batteries connected in series. (- +)(- +)

From one end to the other, you will get 3.0 volts. There are still two 1.5 volt sources if you 'center tap' the cells.

To me, this is kind of like a center tapped transformer, and it shows how multiple voltages can be had without multiple phases, and how the center can be both positive and negative at the same time.

I don't know how great the above is for understanding alternating current, but it works for me.

 

[ggunn]

I like to make the analogy of two 1.5 volt batteries connected in series. (- +)(- +)

From one end to the other, you will get 3.0 volts. There are still two 1.5 volt sources if you 'center tap' the cells.

To me, this is kind of like a center tapped transformer, and it shows how multiple voltages can be had without multiple phases, and how the center can be both positive and negative at the same time.

I don't know how great the above is for understanding alternating current, but it works for me.

It's valid, only it switches polarity 120 times per second.

 

[Ingenieur]

Agreed. An EE degree doesn't teach you everything about all things electrical, it just gives you a grasp of the fundamentals that will enable you to understand an explanation when you get it.

or develop your own understanding by building on the basics
it teaches you how to learn, how to teach yourself

 

[mivey]

I like to make the analogy of two 1.5 volt batteries connected in series. (- +)(- +)

From one end to the other, you will get 3.0 volts. There are still two 1.5 volt sources if you 'center tap' the cells.

To me, this is kind of like a center tapped transformer, and it shows how multiple voltages can be had without multiple phases, and how the center can be both positive and negative at the same time.

I don't know how great the above is for understanding alternating current, but it works for me.

Kinda sorta except you don't get the phase change happening so you need to:

Remove the endcaps and labels so there is no set +/- end designation. This removes the pre-selected reference point for the positive direction.

Set the batteries to discharge and recharge in the opposite direction (so a cap would work better) every 1/120th of a second. This removes the fixed polarity.

Now you are closer to being able to simulate a sinusoidal signal with positive voltage terminals defined on terminals away from the midpoint. The positive peak for the first waveform occurs 1/120th a second before the positive peak of the second waveform.

 

[mbrooke]

Agreed. An EE degree doesn't teach you everything about all things electrical, it just gives you a grasp of the fundamentals that will enable you to understand an explanation when you get it.

Ideally theory, code and hands on make the best EEs/electricians. While they are related, often they are worlds apart in what you learn.

 

[Ranes]

Really, I'm not blaming you. I'm just trying to understand today's educational system that hands out what they like to call an EE degree without the recipient knowing fundamental concepts. Maybe it's because I'm an old grey haired guy who has worked with many EEs over the years. None would ever say "I don't understand analog because I spent most of my degree studying digital and computers".

Guess it's just another symptom of our pathetic educational system.

But I digress...

-Hal

As an old grey haired guy, you must not know how the educational system has changed since the time period in which you could have gone to college.

I think its safe to say that science/engineering is advancing rapidly and there is a lot more information for younger generations going to college to catch up on then that of previous generations. BSEE, at least in my experience, is fairly specialized so that one can catch up faster since its such a diverse field after college.

Do you think EEs that design micrometer level circuits for computer processors need to know specifics on procedure with cellular towers sending/receiving data from smartphones? But to anyone in those fields, there are concepts that are likely basic knowledge to them.

I believe your comment stems from some resentment for the EE degree, rather than actually trying to comment on how the educational system works.

That being said, I did take the basic power courses in my degree, and this might have been a line in a lecture that I missed, but my memory of the class is that there was a lot of emphasis put on higher up in the electrical distribution, not at low voltage, possibly because a decent amount of EEs don't get into low voltage design?

Regardless, I had a :slaphead: moment right after I posed this question. Its fairly simple to conceptualize if you think about the transformer with a neutral tap and polarity. The further in depth comments were helpful, and I am glad that there are those willing on this forum, so Thanks!

 

[RumRunner]

Really, I'm not blaming you. I'm just trying to understand today's educational system that hands out what they like to call an EE degree without the recipient knowing fundamental concepts. Maybe it's because I'm an old grey haired guy who has worked with many EEs over the years. None would ever say "I don't understand analog because I spent most of my degree studying digital and computers".

Guess it's just another symptom of our pathetic educational system.

But I digress...

-Hal

I don't know what kind of education you had achieved but your double entendre: I'm not blaming you, pathetic educational system, no offense, but you are an electrical engineer blah blah blah. . . . but those are simply insults flying in the face of narcissistic selfishness.

So what if an EE asks something that you felt he should have known. He doesn't deserve the treatment let alone being a new-ish member. OP did not come here to be humiliated.

Do you know everything there is to know about electricity? Someone asking a question is a sign of humility that deserves recognition. . . a not being “know- it- all guy”.
That is commendable.

No one is perfect . For someone who thinks that he is, is simply being a charlatan. (a quack)

You are not an engineer who was nurtured to adhere to some code of ethics consistent with professional conduct.

Engineers along with various disciplines like doctors, economists, teachers etc would never say things in the context (yet uncivilized behavior) you have demonstrated.

To become a credentialed PE or other calling, they had to take an oath to uphold the profession and not denigrate others who belong to the profession. It is a part of the PE exam.

Are you any better than those professionals who had gone through what you call “pathetic educational system”?

I certainly wouldn't mind listening to any proof that you are and can do a lot better.

You have indicated that you are a mature person but it's not too late to make some attitude adjustment so you can be more humbling to your readers and thereby not offend anyone that has happened here right now.

I have been accused of being thin-skinned and you might even accuse me being so. . . however, other posters that are not amused of what you've said makes me think that I am far from being thin-skinned.


And to OP, I appreciate your unabashed and honest attempt to inquire and come up with the answer you seek in the midst of humiliation.

 

[Ingenieur]

As an old grey haired guy, you must not know how the educational system has changed since the time period in which you could have gone to college.

I think its safe to say that science/engineering is advancing rapidly and there is a lot more information for younger generations going to college to catch up on then that of previous generations. BSEE, at least in my experience, is fairly specialized so that one can catch up faster since its such a diverse field after college.

Do you think EEs that design micrometer level circuits for computer processors need to know specifics on procedure with cellular towers sending/receiving data from smartphones? But to anyone in those fields, there are concepts that are likely basic knowledge to them.

I believe your comment stems from some resentment for the EE degree, rather than actually trying to comment on how the educational system works.

That being said, I did take the basic power courses in my degree, and this might have been a line in a lecture that I missed, but my memory of the class is that there was a lot of emphasis put on higher up in the electrical distribution, not at low voltage, possibly because a decent amount of EEs don't get into low voltage design?

Regardless, I had a :slaphead: moment right after I posed this question. Its fairly simple to conceptualize if you think about the transformer with a neutral tap and polarity. The further in depth comments were helpful, and I am glad that there are those willing on this forum, so Thanks!

even back in the good old days all ee's had basic courses in all disciplines
my concentration was power, but I had courses in digital comm and signals

you can't really specialize until you know the basics, which usually means grad school
I completed my msee 30 years after my bs, so I have seen the old and new systems

some observations
grading is more generous now
workload/homework is lighter
students expect to be force fed, not have to 'do the work'

I know I'll get the grumpy old grouch &@$@$ spiel but so be it
I've hired, mentored, etc alot of young engineers, and their institutions are doing them a disservice by easing up the standards
the students only bear a portion of the blame
it seems students pay for a degree now a days rather than earn it
it's more a business than a public service to better society

 

[Ingenieur]

I don't know what kind of education you had achieved but your double entendre: I'm not blaming you, pathetic educational system, no offense, but you are an electrical engineer blah blah blah. . . . but those are simply insults flying in the face of narcissistic selfishness.

So what if an EE asks something that you felt he should have known. He doesn't deserve the treatment let alone being a new-ish member. OP did not come here to be humiliated.

Do you know everything there is to know about electricity? Someone asking a question is a sign of humility that deserves recognition. . . a not being “know- it- all guy”.
That is commendable.

No one is perfect . For someone who thinks that he is, is simply being a charlatan. (a quack)

You are not an engineer who was nurtured to adhere to some code of ethics consistent with professional conduct.

Engineers along with various disciplines like doctors, economists, teachers etc would never say things in the context (yet uncivilized behavior) you have demonstrated.

To become a credentialed PE or other calling, they had to take an oath to uphold the profession and not denigrate others who belong to the profession. It is a part of the PE exam.

Are you any better than those professionals who had gone through what you call “pathetic educational system”?

I certainly wouldn't mind listening to any proof that you are and can do a lot better.

You have indicated that you are a mature person but it's not too late to make some attitude adjustment so you can be more humbling to your readers and thereby not offend anyone that has happened here right now.

I have been accused of being thin-skinned and you might even accuse me being so. . . however, other posters that are not amused of what you've said makes me think that I am far from being thin-skinned.


And to OP, I appreciate your unabashed and honest attempt to inquire and come up with the answer you seek in the midst of humiliation.

he is not the only one questioning why a bsee does not have the knowledge base to analyse how 120/240 is derived

this is not personal against the op, but more commentary on the system

the op's query can be answered (I did) AND questions raised why new bsee's do not have the basic tools they need