- Location
- Wisconsin

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- Retired Electrical Engineer - Power Systems

Then why did you say you needed two phases, when the circuit works because of the two voltages?I couldn't have put it better.

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- Thread starter TimWA
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- Location
- Wisconsin

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- Retired Electrical Engineer - Power Systems

Then why did you say you needed two phases, when the circuit works because of the two voltages?I couldn't have put it better.

But of course. But as I know you know, if either the first subscript or the second subscript of the voltages are not the same, the subtraction result is going to be a voltage larger than either of the two voltages. We both know that we have 240v and 120v voltages, not 360v voltages so to set up a calculation to produce that result is misleading at best.Are you serious?

It is obfuscation to ask that a mathematical equation be able to be solved in other permutations?

Yes, I would expect any student taking a course to be able to solve a single equation for a for a single unknown.

For those following along:

Vxy - Vzy = Vxz (second subscripts match and go away and we are left with the remaining 1st-2nd subscript result)

Vxy - Vxz = Vzy (first subscripts match and go away and we are left with the remaining 2nd-1st subscript result)

The valid subtractions with the voltages we have are:

Vab - Van = Vnb (1st subscripts match)

or

Vab - Vnb = Van (2nd subscripts match)

or

Vba - Vbn = Vna (1st subscripts match)

or

Vba - Vna = Vbn (2nd subscripts match)

The obfuscation comes from comparing these differences, which is not what we are doing:

Vxz - Vxy (first subscripts match)

Vxz - Vzy (no subscript match)

In other words, we do not have these resultant voltages:

Vab - Vbn = 1.5*Vab = 3*Vnb

nor

Vba - Van = 1.5*Vba = 3*Vna

Ah.Then why did you say you needed two phases, when the circuit works because of thetwovoltages?

Progress again.:thumbsup:

What is the relationship between those

That is the waveform you would have for the voltages I have between the n & b terminals in my circuit.That is not a posting of the waveforms from the circuit where you had {Vbn@180? connected in parallel with Vnb@0?} which was in series with a parallel connection of {Van@0?and Van@0?}.

I am surprised you think it is not representing the voltages in my circuit. Can you please explain how the voltages I have labeled in my circuit are not represented by a graph of two sinusoidals displaced by 180?? I mean, that was the whole point of making my generator example in the first place. If you did not follow it, then I can understand why you failed to get my point.I am surprise you would not have noticed the difference.

120Vrms@PI L2---------->N---------->L1 120Vrms@0

Are V1n and V2n in phase? Of the same phase?

Well no. Because the phase angles on L1 and L2 did not change. Whattya know? The voltages are separated in time by half a period--PI radians. One is the inverse of the other.

- Location
- Ann Arbor, Michigan

An experiment to try.

Get two equal resistors. Connect one to phase A and the other to phase B from the center tapped secondary with the other end of the resistors going to neutral. The current in each resistor will be in-phase with its voltage.

Tie the two hot wires from the resistors together. Run this wire pair thru a current transformer and measure the current. What is the current? Are those two currents the same phase? Are the voltages the same phase?

.

- Location
- Wisconsin

- Occupation
- Retired Electrical Engineer - Power Systems

But of course. But as I know you know, if either the first subscript or the second subscript of the voltages are not the same, the subtraction result is going to be a voltage larger than either of the two voltages. We both know that we have 240v and 120v voltages, not 360v voltages so to set up a calculation to produce that result is misleading at best.

Misleading??

It must have been a tough assignment, I have no idea how you arrived at 360V.

If the voltages can be combined to produce a higher voltage, than by the virtue of mathematics then one of them should be able to be subtracted from the higher value without providing a misleading answer.

If you don't like your answer maybe the problem is your methodology and not the question?

Here let me do the simple phasor manipulation for you.

Van+Vnb=2Vab

2Vab = 240V@0?|-------------------->

Van = 120V@0?|--------->

2Vab-Van =120V@?? this requires the phasors to be connected tail to tail or head to head

So |--------------------> -|---------> = |--------->---------> = --------- I'll let you put in the direction.

Now

Van-Vbn=2Vab

2Vab = 240V@0?|-------------------->

2Van = 120V@0?|--------->

2Vab-Van =120V@?? this requires the phasors to be connected tail to tail or head to head

So |--------------------> -|---------> = |--------->---------> = --------- I'll let you put in the direction.

Do your your phasors differ?

Why?

- Location
- Wisconsin

- Occupation
- Retired Electrical Engineer - Power Systems

They are in phase. Just like I have repeated in the past.What is the relationship between thosetwovoltages?

It is the way your circuit is wired, not the phasing of the voltages.

I asked before, can the circuit be made to work by relocating and swapping one of your SCRs?

- Location
- Wisconsin

- Occupation
- Retired Electrical Engineer - Power Systems

No it is not.That is the waveform you would have for the voltages I have between the n & b terminals in my circuit.

I am waiting for someone to post an actual oscilloscope display of your paralleled Vbn and Vnb in series with your paralleled Van and Van outputs.

- Location
- Wisconsin

- Occupation
- Retired Electrical Engineer - Power Systems

120Vrms@PI L2---------->N---------->L1 120Vrms@0

Are V1n and V2n in phase? Of the same phase?

Well no. Because the phase angles on L1 and L2 did not change. Whattya know? The voltages are separated in time by half a period--PI radians. One is the inverse of the other.

Have you rewired the transformer?

Are you simply redrawing phasor directions while ignoring the subscripts? Because they no longer match.

You have phasors pointed in the same direction but you are saying they have different angles. This is not a proper representation of phasors, so I do not want to guess at what you are attempting to prove.

Then maybe you were not being misleading at all. The math may just be unfamiliar to you and you made an honest mistake.Misleading??

It must have been a tough assignment, I have no idea how you arrived at 360V.

Your first exercise was to take 240@0? and subtract 120@180?. How you do not know this yields 360@0? is beyond me.

Your next exercise was to take 240@0? and subtract 120@0?. This, of course, yields 120@0?

There is nothing wrong with my methodology and my post is correct.If the voltages can be combined to produce a higher voltage, than by the virtue of mathematics then one of them should be able to be subtracted from the higher value without providing a misleading answer.

If you don't like your answer maybe the problem is your methodology and not the question?

That, sir, is some of the worst labeling and notation I have ever seen. How can you possibly expect to do any phasor manipulations for me? I know you own a copy of Blackburn's "Protective Relaying" book. He has a good section on voltage labels and proper use of subscripts. You would do well to review that section.Here let me do the simple phasor manipulation for you.

Van+Vnb=2Vab

2Vab = 240V@0?|-------------------->

Van = 120V@0?|--------->

2Vab-Van =120V@?? this requires the phasors to be connected tail to tail or head to head

So |--------------------> -|---------> = |--------->---------> = --------- I'll let you put in the direction.

Now

Van-Vbn=2Vab

2Vab = 240V@0?|-------------------->

2Van = 120V@0?|--------->

2Vab-Van =120V@?? this requires the phasors to be connected tail to tail or head to head

So |--------------------> -|---------> = |--------->---------> = --------- I'll let you put in the direction.

I suspect they would but I am not even going to attempt to try to interpret the mess you just posted.Do your your phasors differ?

I'll take the high road on that question.Why?

Yes, it is.No it is not.

If you do not understand that the Vbn and the Vnb in my example are two sinusoidal waveforms with a 180? displacement, then I really don't know how posting an actual oscilloscope display of Vbn and Vnb that will display two sinusoidal waveforms with a 180? displacement is going to help you.I am waiting for someone to post an actual oscilloscope display of your paralleled Vbn and Vnb in series with your paralleled Van and Van outputs.

- Location
- Wisconsin

- Occupation
- Retired Electrical Engineer - Power Systems

Actually, the mistake is in assigning an angle of 180? to this voltage in the first place.Your first exercise was to take 240@0? and subtract 120@180?. How you do not know this yields 360@0? is beyond me.

The math does work when you acknowledge the double negative in your methodology.

Below are the waveforms for the voltages in my generator example (slightly offset so the plots don't lay right on top of each other).If you do not understand that the Vbn and the Vnb in my example are two sinusoidal waveforms with a 180? displacement, then I really don't know how posting an actual oscilloscope display of Vbn and Vnb that will display two sinusoidal waveforms with a 180? displacement is going to help you.

Here is the coupling of the voltages from my generators on the left side of my example (V@0? and V@180?) with the voltages from the center-tapped transformer on the right side of my example (V@0? and V@0?):

Here are the waveforms for the voltages from the generators:

Here are the waveforms for the voltages from the center-tap transformer:

Here they are on one plot:

You set up the exercise so you take ownership of any mistakes you allege are there.Actually, the mistake is in assigning an angle of 180? to this voltage in the first place.

The math does work when you acknowledge the double negative in your methodology.

Add:

You will note that my voltages from the left side of my generator that have a 180? displacement use the "industry standard blah blah blah" terminals for naming the voltages. One voltage Vx

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- Location
- Wisconsin

- Occupation
- Retired Electrical Engineer - Power Systems

I do not plan to take any phasor instruction from someone that cannot figure out that a result of 360V must be due to poor direction assignment, and instead blames the math.That, sir, is some of the worst labeling and notation I have ever seen. How can you possibly expect to do any phasor manipulations for me? I know you own a copy of Blackburn's "Protective Relaying" book. He has a good section on voltage labels and proper use of subscripts. You would do well to review that section.

Have you rewired the transformer?

Are you simply redrawing phasor directions while ignoring the subscripts? Because they no longer match.

You have phasors pointed in the same direction but you are saying they have different angles. This is not a proper representation of phasors, so I do not want to guess at what you are attempting to prove.

No transformer involved. They are simply voltages from a black box.

The voltages are the voltages seen at nodes 1 & 2. They are not necessarily the value of the phasors; that is left as an exercise to the student.

I'm quite sure your pride won't let you take instruction from me, that is why I suggested you learn from Blackburn. But I'm sure he would also agree that 240@0? - 120@180? = 360@0?I do not plan to take any phasor instruction from someone that cannot figure out that a result of 360V must be due to poor direction assignment, and instead blames the math.

I'm quite sure your pride won't let you take instruction from me, that is why I suggested you learn from Blackburn. But I'm sure he would also agree that 240@0? - 120@180? = 360@0?

Yep! That's the way phasors work!

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