Battle of the Phases
- Thread starter rattus
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quogueelectric
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- new york
This is crazy town.
coulter said:
Had to go back and look. Yes that is correct, but since all along we have been using the neutral as a reference, we should reverse the left hand arrow. That is, all arrows should point away from the neutral--just like a wye. That is correct also.
The choice of a reference is the crux of this discussion.
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mivey
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crossman said:
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.
quogueelectric
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Reference is not relevant to the basic understanding that there is only 1 winding and it therefore must be in phase with itself. Current flows in the same direction in both halves of the winding in the same time frame so why would you draw voltage vectors in the opposite direction. All arrows should point in the same direction period.rattus said:
coulter said:
rattus said:
Go back and read your post. It looks to me like you reversed your self in the same breath. If I'm translating correctly, you want the transformer connection as shown in the second sketch to go with the vectors shown in the first sketch. Is that right?
carl
coulter said:
I took another look, and your first diagram is wrong. You show phasors of 120V @ 0 and 120V @ 180. But these voltages are equal. These phasors should be identical--say 120V @ 0. Subtract and the result is 0V.
In the second, you chose 120V @ 0 for both phasors. Connect them head to tail and the sum is 240V @ 0.
Or you could define the neutral as a common reference. Then we have 120V @ 180 and 120V @ 0, then you must take the difference,
120V @ 0 - 120V @ 180 = 120V @ 0 + 120V @ 0 = 240V @ 0
Now bear in mind, the phasor arrows are not intimately tied to the polarity marks.
I will try to come up with a diagram and some better prose after I satisfy my craving for Tex-Mex.
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wirenut1980
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Just my 2 cents, I agree with Rattus in that in a single phase 120/240 V system, leg 1 to neutral voltage is 180 degrees out of phase with leg 2 to neutral even though we are only dealing with one phase out of a 3 phase system (I say a three phase system because that is the "real" source, i.e the utility generator. On a phasor diagram, 2 phasors that are 180 degrees out of phase point away from each other, and if you compare the waveforms, you will get what you see on the diagram posted earlier in the thread, post 38 first diagram by mivey.
Whatever the crux is with this conversation, it only seems to be about semantics. And rattus, if you thought this thread was not meant to go to 200 posts, you are dreaming.
Whatever the crux is with this conversation, it only seems to be about semantics. And rattus, if you thought this thread was not meant to go to 200 posts, you are dreaming.
quogueelectric said:
Au contraire Messr. Quogue. You are missing the point that it is all about the reference which I have been emphasizing all along. It is moreorless standard engineering practice to establish a reference point, preferably a ground or neutral bus. That is exactly what we do with a 3-phase wye, so why not with 120/240 single phase? Note that I said single phase, not two phase!
Now with due respect to all, few people, engineers included, have a good handle on phasors. It is not an easy subject, and for that matter, I believe many engineering schools don't teach the subject anymore.
So it is that electricians and many engineers work in magnitudes and only a few work with phasors.
Follow the exchange between coulter and me. Perhaps you will see what I am talking about.
Here are your diagrams:
Here are your diagrams:
Carl, please see the attached diagram. First we have the miswired transformer where identical voltages appear on L1 and L2. Since the voltages are equal in magnitude and phase, their phasors must be identical. Let V1n = 120 @ 0, then V2n = 120 @ 0. That is the arrows point in the same direction, and that direction is 0 degrees. This is not a matter of polarity marks.
Now consider the second diagram with a properly wired transformer. If V1n = 120 @ 180, then V2n = 120 @ 0. To find V12 or V21 we merely subtract one phasor from the other as indicated in the sketch.
Oops, had it backwards, but only one person noticed.
Here are your diagrams:
Carl, please see the attached diagram. First we have the miswired transformer where identical voltages appear on L1 and L2. Since the voltages are equal in magnitude and phase, their phasors must be identical. Let V1n = 120 @ 0, then V2n = 120 @ 0. That is the arrows point in the same direction, and that direction is 0 degrees. This is not a matter of polarity marks.
Now consider the second diagram with a properly wired transformer. If V1n = 120 @ 180, then V2n = 120 @ 0. To find V12 or V21 we merely subtract one phasor from the other as indicated in the sketch.
Oops, had it backwards, but only one person noticed.
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winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
IMHO you'all have been arguing '6' versus 'half dozen'.
In a single frequency system, an inversion is _indistinguishable_ from a 180 degree phase shift. You can't call one correct and the other incorrect, because they both represent exactly the same condition. You can pick one or the other depending upon which better suits the problem being considered.
Consider a 'black box' with 4 terminals, called H, N, L1, L2. You don't know what happens inside the box. You only know that you can connect a 7.2KV 60Hz source between terminals H and N, and get '120/240V single phase' from L1, L2 and N. What is inside the box might be a 'transformer' with a primary coil and a secondary coil, with a center tapped secondary. Or you might have a 7.2KV motor spinning a 240V generator with a center tapped output coil. Or there might be a motor with 2 separate 120V generators, or some other high weirdness.
If the output is a pure sinusoid, then you could not tell if it was derived by inversion or by a time delay. I absolutely agree that in a center tapped secondary, leg B is the inverse of leg A; there is no time delay involved. But the inverted output could not be distinguished from the output of a system with a real time delay of 180+N*360 degrees. IMHO even though there is no time delay involved in a single phase transformer secondary, it is entirely correct to describe the two output legs as being 180 degrees apart, if such a description suits the problem at hand.
Rattus, for your next pot stirring post, I suggest that you discuss the equality/inequality of 0.999999... and 1.0
-Jon
In a single frequency system, an inversion is _indistinguishable_ from a 180 degree phase shift. You can't call one correct and the other incorrect, because they both represent exactly the same condition. You can pick one or the other depending upon which better suits the problem being considered.
Consider a 'black box' with 4 terminals, called H, N, L1, L2. You don't know what happens inside the box. You only know that you can connect a 7.2KV 60Hz source between terminals H and N, and get '120/240V single phase' from L1, L2 and N. What is inside the box might be a 'transformer' with a primary coil and a secondary coil, with a center tapped secondary. Or you might have a 7.2KV motor spinning a 240V generator with a center tapped output coil. Or there might be a motor with 2 separate 120V generators, or some other high weirdness.
If the output is a pure sinusoid, then you could not tell if it was derived by inversion or by a time delay. I absolutely agree that in a center tapped secondary, leg B is the inverse of leg A; there is no time delay involved. But the inverted output could not be distinguished from the output of a system with a real time delay of 180+N*360 degrees. IMHO even though there is no time delay involved in a single phase transformer secondary, it is entirely correct to describe the two output legs as being 180 degrees apart, if such a description suits the problem at hand.
Rattus, for your next pot stirring post, I suggest that you discuss the equality/inequality of 0.999999... and 1.0
-Jon
Winnie - Maybe you are right, but I let me finish. Hang on just a little longer guys.
Okay, you like the second set on your picture. I'm good with that now that I understand your definitions. I agree the polarity marks don't matter too much, they aren't much use with multi-phase. They just seemed like good way to show why I picked my preferred arrow direction.
Just so I am clear: For single phase, you like the polarity marks with one at the centertap and one at the out side edge, and the arrows pointed away from each other?
carl
Rattus -rattus said:
Okay, you like the second set on your picture. I'm good with that now that I understand your definitions. I agree the polarity marks don't matter too much, they aren't much use with multi-phase. They just seemed like good way to show why I picked my preferred arrow direction.
Just so I am clear: For single phase, you like the polarity marks with one at the centertap and one at the out side edge, and the arrows pointed away from each other?
carl
Rick Christopherson
Senior Member
Oh come on now...Rattus does not stir the pot! He is a model citizen of this forum, and everything he says is golden. That is so unfair of you to be this judgmental of him.winnie said:Rattus, for your next pot stirring post, I suggest that you discuss the equality/inequality of 0.999999... and 1.0
Oops, I think I bit my tongue while it was stuck in my cheek there for a moment. Tastes salty.
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