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

[Besoeker]

Ah, the difference between absolute and relative polarity. AC does not have absolute polarity but split phase waveforms display opposite relative polarity. Absolutely.

Which reverses every half cycle so it makes no sense to label one side positive and the other side negative.

:thumbsup:

 

[jumper]

By NEC standards this definition would make the old 2-Phase with the 90 degree phase displacement single phase.
Now to get Larry to agree.

Two Phase has it own motor chart in the NEC....

They are named differently. May not be logical, but that is the way it is.

 

[K8MHZ]

OK, crazy question for the thread.

Instead of using meters and scopes to determine a 'direction' of flow, why not use a compass? The compass won't need to pick a spot on the coil for a reference.

Of course, this would take a slow version of AC to see the compass needle move.

If we put a compass on each side of the 240 volt coil, one for each 120 volt section, would the compasses point in the same direction as they went from one cycle to the next, or would they be 'out of phase' and point at opposite directions?

 

[jumper]

The NEC quote does point out that there is some vaugery or inconsistency in industry nomenclature. In terms of physical properties, the 120/208 3-wire system has far more in common with the 90deg '2 phase' system than it does with the 120/240 'split phase' system. The only important difference between the former two is the degree of the phase shift. Both require two primary waveforms to be brought. So from the primary point of view, they are both '2-phase', while the 'split phase' is derived from a 'single phase' primary waveform.

Far be it from me to try to change industry standard nomenclature though.

It is what it is.

 

[jumper]

I believe that SG-1 was drawing waveforms for a '2-phase' (90 degree phase offset) 3 wire system.

If I understand his point, under the naming scheme used by the NEC where a two leg 120/208 service is called a single phase service, such a 2 leg 2 phase service would also be called single phase.

I disagree, not because the physics analogy is not correct, but because of how the two are used. Three wires, one grounded, with a phase offset between the ungrounded conductors of either 90 or 120 degrees; seems pretty similar to me...simply that they have different _uses_.

Two leg 120/208 is used for residential applications where ordinary 120/240 single phase might otherwise be used, but where you have lots of such users (apartment buildings and the like) and want to provide balanced loading to the three phase distribution system. You don't need a 'rotating field producing' phase offset.

Two leg '2 phase' is an old system used for industrial applications such as running motors, where you explicitly want a phase offset that can produce a rotating field.

-Jon

All I really care about is that everyone calls each set up the same thing. Our naming have screwed up for years.

 

[jumper]

This the pic you guys wanna discuss?

 

[jaggedben]

OK, crazy question for the thread.

Instead of using meters and scopes to determine a 'direction' of flow, why not use a compass? The compass won't need to pick a spot on the coil for a reference.

Of course, this would take a slow version of AC to see the compass needle move.

If we put a compass on each side of the 240 volt coil, one for each 120 volt section, would the compasses point in the same direction as they went from one cycle to the next, or would they be 'out of phase' and point at opposite directions?

I like it.

 

[GoldDigger]

No mathematics required. Hook up a two channel scope and you can visually observe the two difference phases.


The dot notation indicates the start and finish of the winding, not its polarity.

And what distinguishes the start from the finish? The direction (handedness) of the winding, and hence the polarity of the output voltage for a given input at the primary.

Sent from my XT1585 using Tapatalk

 

[jumper]

Uh, the dot is for keeping the keeping the orientation of the windings correct in respect to each other. Primary and secondary.

We really back to this, ugh!!!

 

[Besoeker]

And what distinguishes the start from the finish?

I think that's a question for the transformer designers on here.

For a one winding secondary it mostly doesn't matter. That includes centre tapped secondaries. There are exceptions. We designed and manufactured DC drives. The timing references for the firing pulses were from 400V to 50-0-50 transformers. The phase relationship between primary and secondary had to be right.

But does it matter on a 120-0-120 residential supply?

 

[jumper]

I think that's a question for the transformer designers on here.

For a one winding secondary it mostly doesn't matter. That includes centre tapped secondaries. There are exceptions. We designed and manufactured DC drives. The timing references for the firing pulses were from 400V to 50-0-50 transformers. The phase relationship between primary and secondary had to be right.

But does it matter on a 120-0-120 residential supply?

Are you referring to additive and subtractive polarity between primary and secondary windings or series and parallel of the secondary?

Or is this something else?

 

[jaggedben]

I think that's a question for the transformer designers on here.
...

It was a rhetorical question to which Golddigger provided the simple answer in the part of his post that you snipped.

But does it matter on a 120-0-120 residential supply?

Of course it does. Have you not been reading the last few pages? See post 521 for example.

 

[Besoeker]

Of course it does. Have you not been reading the last few pages? See post 521 for example.

OK. Explain why in simple terms for this ignorant Brit.
To clarify my question would it matter which you called the start and finish as long are you are consistent?

 

[Besoeker]

Are you referring to additive and subtractive polarity between primary and secondary windings or series and parallel of the secondary?

Or is this something else?

The phase relationship between primary and secondary.

 

[jumper]

Bes, please clarify, with pic, which diagram I posted that you have objections with.

Larry, ditto please.

 

[jumper]

The phase relationship between primary and secondary.

Of course that is important. But what does that do do with the way we analyze the secondary independently. Whether the neutral is a set reference or not?

 

[jaggedben]

OK. Explain why in simple terms for this ignorant Brit.
To clarify my question would it matter which you called the start and finish as long are you are consistent?

That isn't clarifying the question. It's changing it. The question was whether 'polarity' belongs in the discussion. And it certainly does.

 

[Besoeker]

Bes, please clarify, with pic, which diagram I posted that you have objections with.

Your diagram, the upper one in #530, shows the waveforms in anti-phase. That makes my point about there being two phases. The polarity signs, as I said, are an irrelevance. Misleading even.

 

[Besoeker]

That isn't clarifying the question. It's changing it. The question was whether 'polarity' belongs in the discussion. And it certainly does.

Totally disagree when it comes to alternating quantities. You simply can't label one positive since it is negative for half the time.

 

[jumper]

Your diagram, the upper one in #530, shows the waveforms in anti-phase. That makes my point about there being two phases. The polarity signs, as I said, are an irrelevance. Misleading even.

No it is not misleading at all. The windings are physically in series but the sine waves are wrt to the neutral. Opposite.The comments reflect that. Two different things going there.

Is the pic great, no. Accurate,yes, if you look at it correctly and in context.

Look at the current arrows, oops!