single vs. 3 phase

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Don Randall

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Long Thread

Long Thread

I see that some people dislike this thread. Because it is so long? Because of no consensus on much of it? I have been reading it for several days, not through yet. I have found it very interesting, amusing, informative, different and unusual viewpoints, and just a good read overall. Don
 
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Don Randall

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Yes Gar. A lot of algebra and trig was involved when we had to do all those ac problems, using a slide rule, pencil and paper, as I recall, we converted polar coordinates to rectangular, did the addition, then converted back to polar. Or if making a chart just polar to rectangular. Also figured, capacitive and inductive reactance, impedances, resonance of tuned circuits, instantaneous ac voltages, etc. A very slow, tedious procedure. I couldn't do it now without a review and I sure wouldn't use a slide rule. It's been 46 years and I've forgotten most of it. (But I still have my Pickett slide rule.) And I think I still know the basics of ac circuits, although I'm sure you guys can prove me wrong on a lot of stuff. (Frequencies were in cycles per second instead of hertz when I was learning electronics.) Don
 
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Don Randall

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winnie said:
Essentially, each 'phase' (transformer core and associated coils) has _two_ phase angles associated with it.

-Jon
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Jon, I thought it would have 360 different whole number phase angles associated with it. I thought the term phase as in phase difference is used as a comparative term. In other words, it is meaningless when relating to a single sine wave, but used to compare more than one sine wave. Maybe you can explain this to me. I need all the help I can get. Don
 
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winnie

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Electric motor research
Don,

You are correct that phase relation is meaningless without a reference. I was being sloppy.

I was trying to express that an output and its inverse are both present from a single transformer core, and that the two have _different_ phase angles, associated by a definite difference (180 degrees), but that these two phase angles (which might be 0 and 180, or 17.3 and 197.3, or any other pair separated by 180) are present in a _single_ 'phase'.

-Jon
 
engy

engy

Senior Member
Location
Minnesota
Don Randall said:
L1 and L2 are never out of phase with each other in a standard center tapped, single phase, transformer.. It's not possible.
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So L1-N and L2-N are not out of phase?
Then how do you explain L1-N = 120V while L2-N = -120V?

Don Randall said:
Just an illusion You get by incorrectly using the neutral as a reference point for your observations.
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Seems to me the industry commonly uses neutral/ground as a reference point:-?
 
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Don Randall

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winnie said:
Don,

You are correct that phase relation is meaningless without a reference. I was being sloppy.

I was trying to express that an output and its inverse are both present from a single transformer core, and that the two have _different_ phase angles, associated by a definite difference (180 degrees), but that these two phase angles (which might be 0 and 180, or 17.3 and 197.3, or any other pair separated by 180) are present in a _single_ 'phase'.

-Jon
Click to expand...

Also, if you had two single phase transformers with a single output on each, and you wanted to connect the outputs together, in series, in phase with each other, how would you go about determining the proper connection to get the desired results? Just curious. Don
 
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rattus

Senior Member
Don Randall said:
engy said:
So L1-N and L2-N are not out of phase?
Then how do you explain L1-N = 120V while L2-N = -120V?

I would explain it by saying that 120v and -120v do not add up to 240v, the voltage between L1 and L2. Don
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Since the two voltages are measured relative to a common reference, we subtract to find the algebraic difference.
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rattus

Senior Member
Now we are getting somewhere:

Now we are getting somewhere:

Don Randall said:
engy said:
Really? Can you show me an illustration? I believe one vector would point to the right 120 divisions and the negative would point to the left 120 divisions. It seems like they would cancel each other. What do you think? Don
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If a phasor points opposite the direction of summation, subtract. If a phasor points in the direction of summation, add.

That is, - 120V @ 180 = 120V @ 0
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gar

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EE
080429-1659 EST

Two equal but opposing vectors sum to zero. Also the sum of all voltages around a closed loop equal 0. The sum of all currents at a node equal zero. Fundamentals of circuit analysis. This is like in the study of statics where the sum of all forces at a point are 0 and the sum of all moments are zero.

A few posts back the question was asked about connecting two separate secondaries in series. Simply add their vectors. If their magnitudes are the same, then opposing the sum is zero, and not opposing the output is 2 times one of the outputs. Use a voltmeter.

A center tapped transformer has two outputs relative to the center tap that have a 180 deg phase difference. This can be easily seen in a full wave center tapped rectifier circuit.

.

.
 
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Don Randall

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Location
Bethany, OK USA
My argument is not that N-L1 and N-L2 cannot be considered as circuits that are 180? out of phase with each other by people who want to take that view. (However L1-N circuit is in phase with N-L2 circuit.) My argument is that L1 and L2 conductors are not out of phase with each other. If they have a potential difference between them, then they have to be of opposite polarity but since they are common to one circuit, they are not out of phase with each other. They are conductors, not separate circuits. Don
 
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Don Randall

Member
Location
Bethany, OK USA
gar said:
080429-1659 EST


A few posts back the question was asked about connecting two separate secondaries in series. Simply add their vectors. If their magnitudes are the same, then opposing the sum is zero, and not opposing the output is 2 times one of the outputs. Use a voltmeter.

The reason I asked, someone earlier stated that you have to have a common reference to make voltage and phase measurements. I've never know this to be a necessity, just wanted to find what common reference point would be required to connect the two transformers. I also would just use a voltmeter. Don
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