May I ask a question about the single vs two phase stuff

[jaggedben]

Usually when I engage in debates (or perhaps, 'debates'?) on this forum I tend to moderate my position over time. But in this case I'm starting to feel the opposite about it.

Ingenieur, Besoeker, (mivey?):

It's been pointed out that in the physical world there are multiple ways to derive a 120/240V AC source with a waveform(s) resembling a sign wave.

Can you not agree that, for most such cases, multiplying the instantaneous voltage L1-N by -1 is a more reliable way to predict the simultaneous, instantaneous voltage L2-N than employing any vector function?

For a transformer source derived from a single primary waveform, that's true if there's any distortion or interruption of the primary.
As gar pointed out, for two generator windings attached to the same shaft at 180deg, if something disrupts the shaft motion, that's true.
The only common exception I'm unsure about would be two inverters that are synced electronically. That depends on the syncing algorithm. However, I suspect that employing a vector function would still not be more reliable than multiplying by -1.

 

[buffalonymann]

they are not in phase, they are 180 deg out
the motor argument is a strawman

split-phase
ˈsplitˌfāz/
adjective
adjective: split-phase

• denoting or relating to an induction motor or other device utilizing two or more voltages at different phases produced from a single-phase supply.
  
  different implies more than 1
  
  
  

Notice the middle picture, he is showing 240 volts across two positive points :blink:
Two points at the same potential = 0 volts and other people have tried to tell him this, but he just can't grasp it.

For those who missed my statement - L1 is positive and L2 is negative therefore potential is 240 volts. It has to be this way or no current will flow, it has to be this way or Michael Faraday and Joseph Henry are frauds. Ok now that is settled; L1 is positive and N is negative at the very same time that N is positive with respect to L2. Very simple to understand, not sure why this guy can't grasp and really don't care.

 

[Besoeker]

Can you not agree that, for most such cases, multiplying the instantaneous voltage L1-N by -1 is a more reliable way to predict the simultaneous, instantaneous voltage L2-N than employing any vector function?.

I don't know what you think needs to be predicted. We simply know.

 

[mivey]

I've been ignoring this guy because he just cannot understand electrical theory - Notice his middle picture, he is showing 240 volts across two positive points :blink:
Two points at the same potential = 0 volts and other people have tried to tell him this, but he just can't grasp it.

For those who missed my statement - L1 is positive and L2 is negative therefore potential is 240 volts. It has to be this way or no current will flow, it has to be this way or Michael Faraday and Joseph Henry are frauds. Ok now that is settled; L1 is positive and N is negative at the very same time that N is positive with respect to L2. Very simple to understand, not sure why this guy can't grasp and really don't care.

Don't talk about "this guy". Talk about the info posted. Talking about individuals is not helpful and not acceptable. Discussing the information posted is what we are here for.

 

[wwhitney]

I don't know what you think needs to be predicted. We simply know.

Jaggedben obviously means predict as in determine. I.e. what procedure should one apply to the L1-N voltage waveform to determine the L2-N voltage waveform?

Cheers, Wayne

 

[mivey]

Usually when I engage in debates (or perhaps, 'debates'?) on this forum I tend to moderate my position over time. But in this case I'm starting to feel the opposite about it.

Ingenieur, Besoeker, (mivey?):

It's been pointed out that in the physical world there are multiple ways to derive a 120/240V AC source with a waveform(s) resembling a sign wave.

Can you not agree that, for most such cases, multiplying the instantaneous voltage L1-N by -1 is a more reliable way to predict the simultaneous, instantaneous voltage L2-N than employing any vector function?

For a transformer source derived from a single primary waveform, that's true if there's any distortion or interruption of the primary.
As gar pointed out, for two generator windings attached to the same shaft at 180deg, if something disrupts the shaft motion, that's true.
The only common exception I'm unsure about would be two inverters that are synced electronically. That depends on the syncing algorithm. However, I suspect that employing a vector function would still not be more reliable than multiplying by -1.

Why the L2-N prediction? They are separate waveforms and are rarely mirrors of each other and practically never exact matches. Or is it more of an academic question?

The -1 multiplier is okay, but it doesn't really show that we are taking the start of the 1st waveform to be 8.333 ms before the 2nd waveform.

We have defined the positive direction to be away from the center. The first waveform begins a positive rise from zero away from center then 8.333 ms later the second waveform begins a positive rise away from center.

 

[mivey]

Jaggedben obviously means predict as in determine. I.e. what procedure should one apply to the L1-N voltage waveform to determine the L2-N voltage waveform?

Cheers, Wayne

An 8.333 ms delay.

 

[Carultch]

Jaggedben obviously means predict as in determine. I.e. what procedure should one apply to the L1-N voltage waveform to determine the L2-N voltage waveform?

Cheers, Wayne

The point Wayne is trying to make, is that it is more correct to negate, than it is to phase shift, when calculating instantaneous L2n from known instantaneous L1n. It is a coincidence that it is also correct to phase shift in the special case of the sine waveform, but in the case of a generalized waveform, it isn't. That is, based on the nature of how a split phase source is produced. A sawtooth wave is an example where phase shifting by 180 degrees alone, will get the wrong answer.

 

[jaggedben]

I don't know what you think needs to be predicted. We simply know.

Suppose there's a distortion of the waveform coming from the primary. You are scoping L1-N with a single channel scope. What mathematical operation best predicts the timing of the expected distortion in the L2-N waveform? Does it occur at the same time or 8.333ms later?

 

[wwhitney]

An 8.333 ms delay.

To my knowledge that procedure will often give you an incorrect answer.

For example, say I'm monitoring the L1-N waveform, and then I turn on a large L1-L2 load that causes an observed voltage dip L1-N. Would you expect the voltage dip to occur in L2-N 8.33 ms later?

Cheers, Wayne

 

[mivey]

To my knowledge that procedure will often give you an incorrect answer.

For example, say I'm monitoring the L1-N waveform, and then I turn on a large L1-L2 load that causes an observed voltage dip L1-N. Would you expect the voltage dip to occur in L2-N 8.33 ms later?

Cheers, Wayne

That is not what you asked but okay.

If I have a system dip, it occurs right then on both. The same happens with a genset providing two 180d waveforms from a common shaft with two 180d oriented windings.

Take the two 180d genset waveforms and synch them and you have the same scenario of two smaller waveforms or one larger waveform.

I don't see an issue.

 

[Besoeker]

Suppose there's a distortion of the waveform coming from the primary. You are scoping L1-N with a single channel scope. What mathematical operation best predicts the timing of the expected distortion in the L2-N waveform? Does it occur at the same time or 8.333ms later?

I wouldn't use a single channel scope to determine that.

 

[mivey]

I wouldn't use a single channel scope to determine that.

When measuring, I like to move both my leads all over the place so the frame of reference can remain a mystery!

 

[mivey]

That is not what you asked but okay.

Maybe you did ask that. I lost track of who said what.

 

[Besoeker]

When measuring, I like to move both my leads all over the place so the frame of reference can remain a mystery!

Indeterminate even.

 

[Ingenieur]

I've been ignoring this guy because he just cannot understand electrical theory - Notice his middle picture, he is showing 240 volts across two positive points :blink:
Two points at the same potential = 0 volts and other people have tried to tell him this, but he just can't grasp it.

For those who missed my statement - L1 is positive and L2 is negative therefore potential is 240 volts. It has to be this way or no current will flow, it has to be this way or Michael Faraday and Joseph Henry are frauds. Ok now that is settled; L1 is positive and N is negative at the very same time that N is positive with respect to L2. Very simple to understand, not sure why this guy can't grasp and really don't care.

you do not understand kvl/kcl, you have 3 loops and 2 nodes
one loop can't be considered because it contains the other 2
the polarity is reflected in the phasor angle of 180 deg (not star trek phaser)
also since common/gnd that side of the source/supply MUST be neg, hence the oppositevor line side must be pos
I have a MSEE conentration power, PE and 30 years experience

not settled
both L1 and L2 are 'positive' since i by convention flows - to + or n to load
there is a difference between ac and dc lol

pick up a textbook and read it
you lack a very basic grasp of the material

 

[Ingenieur]

An 8.333 ms delay.

yep
1/60 x 1/2 = 0.0083333... sec
or (Pi/2Pi)/60

 

[ggunn]

yep
1/60 x 1/2 = 0.0083333... sec
or (Pi/2Pi)/60

But if the input to the transformer glitches, the glitch appears on L1 and L2 simultaneously, not separated by 1/60 sec.

 

[Ingenieur]

But if the input to the transformer glitches, the glitch appears on L1 and L2 simultaneously, not separated by 1/60 sec.

same for 3 phase
although it occurs simutaneously it is seperated by Pi or 8.33 mS timescale NOT time

 

[Ingenieur]

note each 3 ph line is labelled '+'
but you have v across them