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

[K8MHZ]

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
  
  
  

But -120 + 120 = 0, so the above can't be representative of a split phase system.

 

[Besoeker]

I'm sorry, why can't you put the 'scope common terminal on L1, and put the two probes on N and L2? Assuming the inputs are properly isolated from the 'scope power source?

Cheers, Wayne

You could but you wouldn't be able to see the two 120V supplies. And how they relate to each other.
And then there is the minor issue of the common on the scope being connected to the supply ground.
You can get scopes with fully isolated inputs, we had one, but compromise on bandwidth.

 

[Ingenieur]

But -120 + 120 = 0, so the above can't be representative of a split phase system.

120 + (-) (-120)
polarity convention around the kvl loop must be observed

 

[Ingenieur]

You could but you wouldn't be able to see the two 120V supplies. And how they relate to each other.
And then there is the minor issue of the common on the scope being connected to the supply ground.
You can get scopes with fully isolated inputs, we had one, but compromise on bandwidth.

a proper meaningful measurement

both neg probes on the common point
each positive on each supply line

makes no sense to put a neg and pos on the same common neg/neut/gnd point

 

[jaggedben]

no, it is not a choice
the neut is the 'common' or reference or 'negative' and polarity should be observed

Most two channel 'scopes have a common ground terminal for each. If you want to measure the two aides of the 120-00-120 simultaneously to compare them. using the neutral as the common is the only choice.

The limitations of typically available measurement equipment shouldn't determine the ontology of what is being measured in this context. (We're not doing particle physics here.) This is not what anyone means by 'choice' in the context of this discussion.

If one had to, we could set up two separate, ungrounded scopes, and send the signals through some optical isolators to some type of display that was set up to overlap the outputs simultaneously. That of course is not worth the money because all of us in this discussion understand the system and the math well enough to derive multiple waveform graphs from the information that any real or theoretical scope setup provides. That's what we mean by choice.

 

[Besoeker]

But -120 + 120 = 0, so the above can't be representative of a split phase system.

Polarity signs like + and - don't sit well with alternating voltages.

 

[wwhitney]

if you have a scope, observe polarity and use 2 probes, L1-n and L2-n, you WILL get 2 waveforms 180 deg out of phase

Only if the the voltage source is a perfect sine wave with no noise, distortion, or harmonics, i.e. basically never in the real world. But you will get two waveforms that are negatives of each other (the ratio may be slightly off from -1 if the secondary coil tap isn't exactly in the center).

Cheers, Wayne

 

[wwhitney]

Polarity signs like + and - don't sit well with alternating voltages.

e^i*pi = -1.

Cheers, Wayne

 

[Besoeker]

Only if the the voltage source is a perfect sine wave with no noise, distortion, or harmonics, i.e. basically never in the real world. But you will get two waveforms that are negatives of each other (the ratio may be slightly off from -1 if the secondary coil tap isn't exactly in the center).

Cheers, Wayne

If you want to be pedantic. The fundamentals, and mostly that's what matters, are 180° apart.

 

[Besoeker]

e^i*pi = -1.

Cheers, Wayne

Or.......

e^iπ + 1 = 0

But rather off topic.

 

[wwhitney]

If you want to be pedantic.

Any thread with 230 posts is already pedantic.

Cheers, Wayne

 

[Besoeker]

And now 232................sorry mods.

 

[wwhitney]

But rather off topic.

No, it means that for a pure sine wave f(t) = Re (e^i*w*t), a phase difference of 180 degrees is the same as multiplication by -1.

But more broadly, I am disagreeing with your repeated assertion that for AC waveforms, +/- 1 doesn't matter.

Cheers, Wayne

 

[Besoeker]

But more broadly, I am disagreeing with your repeated assertion that for AC waveforms, +/- 1 doesn't matter.

Cheers, Wayne

OK.

 

[Ingenieur]

The limitations of typically available measurement equipment shouldn't determine the ontology of what is being measured in this context. (We're not doing particle physics here.) This is not what anyone means by 'choice' in the context of this discussion.

If one had to, we could set up two separate, ungrounded scopes, and send the signals through some optical isolators to some type of display that was set up to overlap the outputs simultaneously. That of course is not worth the money because all of us in this discussion understand the system and the math well enough to derive multiple waveform graphs from the information that any real or theoretical scope setup provides. That's what we mean by choice.

when you have a common point
the measurements should be made with the same polarity reference from the point, ie, both neg or both pos, but since grounded neg is logical and convention
and the same polarity used for all measurements
common sense
sound scientific practice

with you logic I could adjust scales, time ref, etc. and make 3 phase look like 1 phase

 

[Ingenieur]

No, it means that for a pure sine wave f(t) = Re (e^i*w*t), a phase difference of 180 degrees is the same as multiplication by -1.

But more broadly, I am disagreeing with your repeated assertion that for AC waveforms, +/- 1 doesn't matter.

Cheers, Wayne

OR
w1 = 2 Pi f
w2 = 2 Pi f + Pi = 2 Pi(f + 1/2)

and f DOES NOT equal (f + 1/2)

 

[wwhitney]

w1 = 2 Pi f
w2 = 2 Pi f + Pi = 2 Pi(f + 1/2)

and f DOES NOT equal (f + 1/2)

????

Wayne

 

[ggunn]

when you have a common point
the measurements should be made with the same polarity reference from the point, ie, both neg or both pos, but since grounded neg is logical and convention
and the same polarity used for all measurements
common sense
sound scientific practice

with you logic I could adjust scales, time ref, etc. and make 3 phase look like 1 phase

It matters not. L1 to N, L2 to N, L1 to L2. Measure any two and you can derive the third.

 

[ggunn]

The thing is, all of us (most of us, anyway) are in total agreement about what it is. All of the wailing and gnashing of teeth is over what to call it.

https://www.youtube.com/watch?v=nb9XE5Dlf7g

 

[jaggedben]

...

with you logic I could adjust scales, time ref, etc. and make 3 phase look like 1 phase

That's an unfair misrepresentation of what I said. Nothing I said implied shifting aspects of reality such as time, space, or electromagnetism. It was all about how one could take measurements.