single vs. 3 phase

[mivey]

...Perhaps you should should state it a few more times if you haven't quite reached that plateau yet...

Can I get in on some of that? I had a long day and am in need of a boost of self-righteousness. Smart $, you were just sooooo wrong. Thanks, I needed that. (P.S., give me a little while up here on my perch before pointing out my errors please!)

Back to topic:
Carl & Jim, I was at Ga Tech today and was perusing some of the reference material at the bookstore. One of the books mentioned that single-phase was one voltage source supplied on two wires. The author also called two sources at 180 degrees two-phase (as well as the 90 degree stuff we already know about). I did not buy the books ($160+ for the set...I miss being a student...NOT), but it made me think about something.

You have mentioned in the past that the center-tapped transformer with a neutral reference really was just one source and should have the phasors drawn in one direction.

You may have already mentioned this in the past but do you see that it would make any philosophical difference (in your opinion of course) if the voltages were supplied by two different generators? Suppose they were mechanically tied or synched so that the waveforms looked exactly the same as the waveforms obtained by the center-tap reference. We could still get a 240 volt waveform like with the single winding.

In the center-tapped case, we start with one voltage and "split" it into two voltages. In the 2-generator case, we start with two voltages but combine them in series so we can mathematically sum them to one. Would you still take issue with phasors with a 180 degree difference? Would it matter if it were feeding the most complex load you could imagine instead of the original simple resistor circuit?

Just trying to clarify your opinion in my mind. Thanks

[edit: 3-wire supply, of course]

 

[mivey]

Smart $,
Generally speaking, I don't think engineers are necessarily better than non-engineers. I make mistakes all the time, forget stuff, mis-learn things, change my mind, etc. As I have said in the past:

A degree is just an acknowledgement by an established group that you devoted your time in a particular endeavor and was successful. That endeavor may provide you with a different set of tools or knowledge than someone else.

This does not diminish the effort that is put forth learning through other means that are not recognized with a degree. No one can say one is more valuable than the other, only that they are different.

There are non-engineers who have a much better understanding of some "engineering" concepts than a lot of engineers. You do not have to be an engineer to engage in "poor engineering" or "excellent engineering" as it can just mean the method or practice of applying scientific knowledge or principles to the problem or task at hand.

I can appreciate the fact that you were trying to line up your arrows with a diagram or trying to talk to a lay-person. Be a rebel if you want, but what is so bad about trying to follow standards? If you are going to teach the lay-person, why not teach them a method that follows standard practice? How many sub-cultures in our field do we need? We have a big enough mess as it is (with plenty of blame for the engineers and non-engineers alike).

Of course when all is said and done, I do like the quote someone posted a while back: "that is the nice thing about standards, there are so many to choose from":smile:

 

[jim dungar]

You have mentioned in the past that the center-tapped transformer with a neutral reference really was just one source and should have the phasors drawn in one direction.

You may have already mentioned this in the past but do you see that it would make any philosophical difference (in your opinion of course) if the voltages were supplied by two different generators? Suppose they were mechanically tied or synched so that the waveforms looked exactly the same as the waveforms obtained by the center-tap reference. We could still get a 240 volt waveform like with the single winding.

Lenz's Law/Right Hand [screw] Rule says that the direction of the current in a coil is related to the direction in which the magnetic flux cuts across the conductors. In a center-tapped transformer there is only one flux direction so the current in each half of the windings must be in phase.

I definitely do not agree with calling two individual transformers in series as a 2-phase system if they are fed from a single source/L-L voltage. I have often stated my preference of naming poly-phase systems by the number of different usable L-L not L-N voltages.

edit: corrected reference to Lenz's law/right hand screw rule

 

[rattus]

Not quite Jim:

Not quite Jim:

Lenz's Law(?)/Right Hand Rule says that the direction of the current in a coil is related to the direction in which the magnetic flux cuts across the conductors. In a center-tapped transformer there is only one flux direction so the current in each half of the windings must be in phase.

The flux is a function of the primary voltage, not load current. Now if the neutral current is zero, the same current flows in both halves of the secondary. However, one can define opposite currents in the two halves, and this is perfectly valid. That is one of the precepts of steady state analysis. Some like it this way; you do not.

I definitely do not agree with calling two individual transformers in series as a 2-phase system if they are fed from a single source/L-L voltage. I have often stated my preference of naming poly-phase systems by the number of different usable L-L not L-N voltages.

Agreed! Any voltage obtained by inversion does not constitute another phase.

 

[quogueelectric]

This thing still got legs??? Just stopping back to see what is going on.

 

[Smart $]

Can I get in on some of that? I had a long day and am in need of a boost of self-righteousness. Smart $, you were just sooooo wrong. Thanks, I needed that. (P.S., give me a little while up here on my perch before pointing out my errors please!)

ok... TIMES UP :grin:

You have mentioned in the past that the center-tapped transformer with a neutral reference really was just one source and should have the phasors drawn in one direction.

I did no such thing! (IIRC)

As far as I'm concerned, phasors are just a tool. Use the concept to the fullest extent, but do not substitute such for a better tool.

Here's an analogy which I believe fits pretty good... A common-day hammer generally has two sides (I would say peens, but some do not have two peens). Some are designed where the each side performs a different task better. However, it is up to the user of the hammer to decide or learn which side works better. Yet, if either side performs the function desired, is using one side any more wrong than using the other? What if the hammer was a typical engineer's hammer?

You may have already mentioned this in the past but do you see that it would make any philosophical difference (in your opinion of course) if the voltages were supplied by two different generators? Suppose they were mechanically tied or synched so that the waveforms looked exactly the same as the waveforms obtained by the center-tap reference. We could still get a 240 volt waveform like with the single winding.

If the criteria is output only, all that matters is no difference in functionality.

In the center-tapped case, we start with one voltage and "split" it into two voltages. In the 2-generator case, we start with two voltages but combine them in series so we can mathematically sum them to one. Would you still take issue with phasors with a 180 degree difference? Would it matter if it were feeding the most complex load you could imagine instead of the original simple resistor circuit?

As I said above, phasors are just a tool. Use it as such.

Just trying to clarify your opinion in my mind. Thanks

Does the above help?

 

[Smart $]

Smart $,
Generally speaking, I don't think engineers are necessarily better than non-engineers. I make mistakes all the time, forget stuff, mis-learn things, change my mind, etc. As I have said in the past:

[self-quote]

There are non-engineers who have a much better understanding of some "engineering" concepts than a lot of engineers. You do not have to be an engineer to engage in "poor engineering" or "excellent engineering" as it can just mean the method or practice of applying scientific knowledge or principles to the problem or task at hand.

I can appreciate the fact that you were trying to line up your arrows with a diagram or trying to talk to a lay-person. Be a rebel if you want, but what is so bad about trying to follow standards?

If you are going to teach the lay-person, why not teach them a method that follows standard practice? How many sub-cultures in our field do we need? We have a big enough mess as it is (with plenty of blame for the engineers and non-engineers alike).

I did my best not to infer "all" engineers. I apologize if I implied otherwise. The hard part about it is, even though not all engineers are self-righteous, if I somehow shorten the field, the ones that are, but have convinced themselves otherwise, take the escape clause?"not me" :grin:

How does one relate a discontinuity in standards. I say one must find common ground then elaborate. Standards and following them means nothing if persons the standard affects do not understand it.

If one's purpose is to teach calculus to someone that only knows basic arithmetic, does he just teach calculus and expect the student to learn everything in between on their own? If so, what are the odds the student will succeed at learning calculus?

I changed phasor orientation in a diagram to match transformer symbology... something seen more commonly by electricians not adept in engineering-level phasor practices or standards. If someone unknowledgeable asked for elaboration so as to better understand it, I would have provided such. I would have taken it upon myself as being duty bound. But when someone else starts saying it is wrong, how many of the people seeking a better understanding are going to ask for clarification?

 

[mivey]

Does the above help?

Thank you for explaining your position. Sorry about the confusion but the stuff following "Back to topic" was directed to Carl & Jim based on some of their past statements.

I did not make that clear, but now we won't have to beat your opinion out of you. Ve half veys uf may-king u tock.

 

[Lxnxjxhx]

"poor engineering"

"poor engineering"

The worst engineers I ran into were "political appointees" of a sort. If someone else did something right, these golden boys got the credit. If they did something wrong, someone else got blamed.
A lot of us were on the factory floor trying to fix the excrement that they designed.

All in all they were "doin' a heckuva job", but I always wondered why the tips of their noses were brown.
The company that hired these guys went from 1800 to 400 in a few short years. I left at 1100.

 

[jim dungar]

The flux is a function of the primary voltage, not load current. Now if the neutral current is zero, the same current flows in both halves of the secondary. However, one can define opposite currents in the two halves, and this is perfectly valid. That is one of the precepts of steady state analysis. Some like it this way; you do not.

I never said that flux is a function of load current.

The "right hand screw rule" lets you determine the direction of current in a conductor based on the direction the conductor cuts across a magnetic field (wrap the fingers of your right hand in the direction of the field and your extended thumb points in the direction of the current). If you only have one primary flux you will only have one secondary flux; if you only have one secondary flux you can only have one current direction.

Farady's Law and Lenz's Law are fundamental to describing the way transformers work. I don't think they should be ignored.

 

[rattus]

I never said that flux is a function of load current.

The "right hand screw rule" lets you determine the direction of current in a conductor based on the direction the conductor cuts across a magnetic field (wrap the fingers of your right hand in the direction of the field and your extended thumb points in the direction of the current). If you only have one primary flux you will only have one secondary flux; if you only have one secondary flux you can only have one current direction.

Jim, with all due respect, I think you are confusing the RH screw rule with the RH rule. The RH screw rule describes the direction of the magnetic field created by a current in a wire. The RH rule involves the motion of a conductor through a magnetic field.

You must agree that there can be only one flux in the core, but we often see different currents with different phase angles in the halves of a split-phase secondary. That is, it is not necessarily true that the currents in the two halves are in phase even if there is a single flux.

How can that be, one may ask? The flux is created by the magnetizing current through the primary inductance. Any flux created by the primary load current is canceled by the flux created by the secondary load currents.

The floor is yours.

 

[coulter]

...Carl & Jim, ...Just trying to clarify your opinion in my mind. ...

I appreciate the request. However, I don't think I have much to add that hasn't already been said. I'm not much of a believer in the concept of: "Saying things louder and more often makes them more true". And I'm certainly not implying you are either.:smile: (Truth - not sarcasm)

... I was at Ga Tech today and was perusing some of the reference material at the bookstore. One of the books mentioned that single-phase was one voltage source supplied on two wires. The author also called two sources at 180 degrees two-phase (as well as the 90 degree stuff we already know about). ...

Interesting. I would have hoped for better from an educator.

"...called two sources at 180 degrees two-phase..."
Well, It probably is. But I don't know what one would do with that system, the two phases would sum to zero. Strange concept for a textbook.

...You have mentioned in the past that the center-tapped transformer with a neutral reference really was just one source and should have the phasors drawn in one direction. ...[/quote
Yes, that would be the orientation that is the most intuitive for me to set up solutions. However, as I also mentioned, I don't recall ever working impedance or SCC calculations for 1ph systems. Other than a mathematical curiousity 1ph systems are pretty much, well, trivial. The times I have been concerned with VD on 1ph, I ignored any pf - perhaps not sufficiently rigirous, but appeared suitable for the task. Perhaps I am not qualified to judge the correct orientation to teach in a class (also truth - not sarcasm)

...do you see that it would make any philosophical difference (in your opinion of course) if the voltages were supplied by two different generators? ...

Not really. In one of the 1000 odd posts in this and the other threads, I posted a sketch that showed two series sources, +ends oriented up, labeled 120V<0. (Where "<" is "phase angle") As I recall, that sketch started with one source, +end up, labeled 240V<0, the next sketch showed the two sources in series adding to 240V, infering the two were equivalent. This would appear to be the same as what you are suggesting.

However, thanks for asking:smile:

carl

 

[mivey]

...Interesting. I would have hoped for better from an educator....Strange concept for a textbook...

I did not check the author's pedigree, (I'm pretty sure he was not a Ga Tech professor) but I'll see if I can find the book on-line. This was not a textbook, but was in the reference section of the student bookstore. The textbook shelves were almost empty as final exams start next week. That being said, I have listed other well-known & well-respected textbook/reference materials that say the same thing about "180 degrees two-phase".

...the next sketch showed the two sources in series adding to 240V, infering the two were equivalent. This would appear to be the same as what you are suggesting.

I think so. I was trying to distinguish between a source from a single winding vs two independent generator outputs with no transformer (I guess 2 separate windings will do).

Thanks for your & Jim's response

 

[winnie]

If you only have one primary flux you will only have one secondary flux; if you only have one secondary flux you can only have one current direction.

Have to disagree with this one, big time.

The core flux induces secondary _voltage_. With a center tapped secondary, the same _voltage_ is induced on both halves. But entirely different _current_ may flow on the two halves, both different magnitude and different phase.

I know that I keep bringing up 'T' connected transformers. Consider a 'T' connected secondary with a suitable 3 phase 'neutral' tap on the teaser coil. This transformer effectively has a _wye_ output secondary. Now connect this secondary to a balanced wye resistive load. You will find a clear phase difference between the currents in the two halves of the main coil.

-Jon

 

[jim dungar]

Jim, with all due respect, I think you are confusing the RH screw rule with the RH rule. The RH screw rule describes the direction of the magnetic field created by a current in a wire. The RH rule involves the motion of a conductor through a magnetic field.

Memory issues strike again.

The [Maxwell's] right hand screw [/grip] rule is used to determine the direction of a magnetic field generated by a current

The [Flemings] right hand [palm] rule is used when working with the direction of the force associated with an induced emf and a field. Commonly used with generators

The left hand rule is often used with motors as it can be applied to determine direction caused by a current.

But regardless of the "rule", Faraday's, Lenz's, and Maxwell's law are what apply.
A single center tapped winding on a single transformer core only has one magnetic flux therefore it can only have one direction for its secondary current and one direction for its secondary induced emf.

 

[rattus]

A single center tapped winding on a single transformer core only has one magnetic flux therefore it can only have one direction for its secondary current and one direction for its secondary induced emf.

Jim, you are right on the induced emf, however the phase angles of the currents are affected by the loads and can be different and often are.

 

[jim dungar]

Jim, you are right on the induced emf, however the phase angles of the currents are affected by the loads and can be different and often are.

This sound like you agree that the voltages in the two halves of a center tapped transformer coil must be in phase with each other.

And yes, loads can affect the phase angle of the secondary current. My mind was focused on my previous discussions of flux, voltages, and currents as they relate to a center tapped winding with purely resistive loads.

 

[weressl]

This sound like you agree that the voltages in the two halves of a center tapped transformer coil must be in phase with each other.

And yes, loads can affect the phase angle of the secondary current. My mind was focused on my previous discussions of flux, voltages, and currents as they relate to a center tapped winding with purely resistive loads.

They are not in phase they are 180* out of phase to each other.

One half is a lefthanded turn winding and the other is a righthanded turn winding when you look at from the tap point and you have to since that is the common reference point for the voltage measurements, it is the same potential for both circuits. If you would have two windings one top on the other going in the same direction, then you would have no voltage difference betwen the coil ends on the same extreemes and if you connect one end and ground it, you would have 115V to each opposite end and NO voltage difference between the two open ends insted of 230V in the case of a single winding, center tap.

 

[crossman]

They are not in phase they are 180* out of phase to each other. One half is a lefthanded turn winding and the other is a righthanded turn winding when you look at from the tap point and you have to since that is the common reference point

Oh! Laszlo knocks one out of the park! Good point!

 

[crossman]

The [Maxwell's] right hand screw
The [Flemings] right hand [palm]
The left hand rule is often

Are we talking conventional flow or electron flow? Is right left or right right?