Single Phase vs Three Phase flow of electrons with no neutral/grounded path?

[ggunn]

It is clear that the OP seems to think that there are two waveforms. I've been trying to dispel that notion in my responses to his questions. Simply saying "its just single phase" isn't going to clarify the situation for the OP. If the OP grasped the concept that "its just single phase," he wouldn't be asking the questions he's asking.

Like I said earlier, that was hard for me to wrap my head around at first, too. It's not intuitive just looking at the waveforms that the difference between two sine waves 120 degrees apart (which is what a load sees when you connect it across two legs of three phase power) is a sine wave as well. Or any phase difference apart, for that matter. Only the amplitude is affected by the amount of phase shift, not the shape of the wave.

It would have an effect if it were a three wire system with two of the three phases and a neutral, but I don't think anyone does that.

 

[david luchini]

It's not intuitive just looking at the waveforms that the difference between two sine waves 120 degrees apart (which is what a load sees when you connect it across two legs of three phase power) is a sine wave as well.

I think that's exactly what Smart$ graph in post #46 was intending to show.

 

[Besoeker]

That's great, but the system is question is 3 phase. How does a 3 phase system have only 1 phase?

Line to line is single phase. Actually, it doesn't matter how you get the 208V.
It could be a 480V primary 208V secondary single phase transformer. The result would be the same. That it might be derived line to line from a 208/120 three phase system doesn't change that.

I'm not introducing phase shifts that don't exist. The phase shifts exist because the system is a 208/120V, 3 phase wye.

The 208V line to line voltage is single phase.

If the system source was a generator or a transformer secondary, the voltage developed on each of the 3 windings of the source is 120V. The 120V voltage on each of the 3 source windings have 120 degree phase shifts.
Yes, the three 120V phases are mutually displaced by 120 degrees.
But if the the load is 208V and the supply is 208, the 120V doesn't come into it. It's a single phase circuit.

If I connect a load across two of the source terminals, L1 and L2, then KVL tells me that the voltage across the load will equal the voltage across winding 1 minus the voltage across winding 2.

Vl=V1-V2 = (120<0)-(120<-120) = 208<-30. The 120 degree phase shift between the two 120V source windings is precisely why the voltage between the terminals L1 and L2 is 208V.

It doesn't matter how the 208V is produced. It is still single phase.

 

[Besoeker]

It's not intuitive just looking at the waveforms that the difference between two sine waves 120 degrees apart (which is what a load sees when you connect it across two legs of three phase power)

It isn't two sinewaves 120 degrees apart. It is ONE sinewave. That's what seems to be getting missed here.
Sure if you take each of L1 and L2 with respect to neutral, there would be a displacement of 120 degrees. But if you have a 208V line to line load, neutral plays no part.

 

[david luchini]

It doesn't matter how the 208V is produced. It is still single phase.

Of course it matters how the 208V is produced. It is central to the question asked by the OP. It obviously matters to the OP and his understanding of electrical systems.

It isn't two sinewaves 120 degrees apart. It is ONE sinewave. That's what seems to be getting missed here.
Sure if you take each of L1 and L2 with respect to neutral, there would be a displacement of 120 degrees. But if you have a 208V line to line load, neutral plays no part.

You seem to be missing that what you are stating is exactly the problem that the OP is having. The title of his original post is "Single Phase vs. Three Phase flow of electrons with no neutral/grounded path." Instead of arguing semantics, why not try to provide an answer that will increase the OP's understanding of electrical systems?

 

[Besoeker]

Instead of arguing semantics, why not try to provide an answer that will increase the OP's understanding of electrical systems?

Explaining that 208V line to line is single phase instead of introducing non-existent phase shifts is a poor explanation?

 

[david luchini]

Explaining that 208V line to line is single phase instead of introducing non-existent phase shifts is a poor explanation?

Considering that that explanation was given early on, and the OP still had trouble understanding the point...I'd so just repeating the same thing would be a poor explanation.

And why are the phase shifts non-existent? You are ignoring what was asked in the question, and you are ignoring half of the circuit. The is no phase shift in the load portion of the circuit. There is a phase shift between the two windings of the 3 phase source that is supplying the 208V load. If there was not a 120 degree phase shift at the source, there wouldn't be 208V across the load. So how is this phase shift which is essential to the circuit, non-existent?

 

[Smart $]

Explaining that 208V line to line is single phase instead of introducing non-existent phase shifts is a poor explanation?

Yes

Additionally, in either of the systems mentioned, the neutral [point] does play a role, both in that voltages measured are referenced to it and in that current flows through it. In both systems, the current which flows through the load also flows through the neutral point.

 

[__dan]

Nice cut and paste job with the equations, BTW.

[/QUOTE]

315 is getting closer every day

Truly, I did not read the whole cut myself. I was only grateful someone appeared to calc and the number given was in the expected range. The body of the cut could have been the recipie for hard boiled eggs.

Two 115v secondary windings on one iron core, say the leads out are winding 1, X1 and X2, winding 2 is X3 and X4. Straight single phase. X2 and X3 are spliced and grounded to produce the neutral.

Now oscilliscope leads, black and red. Black is placed on X1, red on the neutral, X2+X3, reads 115v. Move red lead to X4 and it reads 230 v with same synch as the 115v sinewave, zero phase displacement. The windings add in voltage because the turn polarity is measured in the same direction, adding in series.

Place leads black and red to X1 and X2 then place leads black and red to X3 and X4 and the results are identical, 115v sinewaves with zero phase displacement, both in synch.

Place the black lead at the neutral, X2+X3, and move the red lead from X1 and to X4 and the result is 115v sinewaves with 180 deg phase displacement. The phase displacement is an artifact of the leads arrangement, effectively, the 115v is measuring what appears to be windings wound in opposite directions, which is not the actual. The lead polarity becomes reversed by the arbitrary, but conventional, act of making the neutral at the winding centerpoint. There is no phase displacement.

Of course with three phase there is phase displacement between phases, but the quantity is fixed and changing it is not available to the user. Changing the turns ratio from primary to seconday is the variable the user can change, adding or subtracting turns, windings and taps of windings, in series.

 

[__dan]

Someone gave the local volunteer firefighters a large commercial washer for their gear and my job was to wire it. I had to put in a rotating phase converter to make three phase.

I went with the recommended unit and upsized it for any future loads. Well, the red leg was higher by, I forget, 20 volts, and the washer made this bad singing sound in testing. I had to add a 3 phase fan motor to the load in testing to get it to run nice. The upsize unit did not like the lower load.

The importer who handled the phase converter wanted to sell me some kind of cap bank that would switch in and out with load to fix the problem, what they said. I said no thank you. No hesitation.

Found a small autotransfomer in my collection and trimmed the high leg by that 20 volts, wired on only one phase. No scope, passed the sound, voltage, and running load tests beautiful, not an open delta, an anomaly.

 

[ggunn]

It isn't two sinewaves 120 degrees apart. It is ONE sinewave. That's what seems to be getting missed here.
Sure if you take each of L1 and L2 with respect to neutral, there would be a displacement of 120 degrees. But if you have a 208V line to line load, neutral plays no part.

We are saying the same thing. Line to line is a single phase sine wave, no question, even though A phase to neutral and B phase to neutral are two sine waves 120 degrees apart. Whether it's one sine wave or two depends on your reference point.

 

[ggunn]

I think that's exactly what Smart$ graph in post #46 was intending to show.

I agree. It's hard (it was for me, anyway) to visualize just from thinking about the two line to neutral 120 degree displaced sine waves that the difference between them is just another sine wave, but that is the essence of how it all works.

 

[Besoeker]

We are saying the same thing. Line to line is a single phase sine wave, no question, even though A phase to neutral and B phase to neutral are two sine waves 120 degrees apart.

But no neutral is involved so why complicate matters by introducing it?

 

[Besoeker]

Yes

Additionally, in either of the systems mentioned, the neutral [point] does play a role, both in that voltages measured are referenced to it

If it is a load that is fed line to line there just two conductors neither of which is neutral. Why would you reference the two lines with respect to a neutral that isn't there?

 

[Besoeker]

Considering that that explanation was given early on, and the OP still had trouble understanding the point...I'd so just repeating the same thing would be a poor explanation.

And, with trouble understanding the point this

Vl=V1-V2 = (120<0)-(120<-120) = 208<-30. The 120 degree phase shift between the two 120V source windings is precisely why the voltage between the terminals L1 and L2 is 208V.

adds clarity?

 

[Smart $]

If it is a load that is fed line to line there just two conductors neither of which is neutral. Why would you reference the two lines with respect to a neutral that isn't there?

When you get in the panel, the neutral is there. And knowing that you measure 120VAC between N and L's, leaves an unknowledgeable person scratching their head wondering how in the h$%# is there only 208VAC between L's....

That is the gist of the discussion and explanation has ensued...

It takes no brainiac to know 208VAC loads get connected L-L in a 208/120 panel.

 

[david luchini]

And, with trouble understanding the point this...

adds clarity?

I would say yes. If the OP has in his mind two sources, two sine waves, then explaining how those two sources add together to form one sine wave across the load would add clarity.

If it is a load that is fed line to line there just two conductors neither of which is neutral. Why would you reference the two lines with respect to a neutral that isn't there?

I think you're still ignoring the question that the OP asked, and you're ignoring half the circuit which is integral to the question that the OP asked. You can't answer the question by ignoring the question.

 

[Besoeker]

I would say yes. If the OP has in his mind two sources, two sine waves, then explaining how those two sources add together to form one sine wave across the load would add clarity.

Yes, it's just one sine wave. That's the bottom line.

 

[Besoeker]

When you get in the panel, the neutral is there.

But not at the load.

 

[Smart $]

But not at the load.

It is only a load after you get in the panel... :blink: