single vs. 3 phase

[hardworkingstiff]

While not perfectly descriptive of the physics, I am content to accept naming such a service 'single phase'.

-Jon

IMHO, calling two phase legs plus the neutral of a 120/208 system 'single phase' is quite a confusing misnomer.

The last one came from
http://forums.mikeholt.com/showthread.php?t=82963&page=2
message 20

I'm not content to call it (2 legs of a 208Y/120) single-phase because it indeeds act differently than a traditional (120/240) single-phase service. I won't argue a lot, I'm just not content to conceed. :smile:

 

[mivey]

I'm not content to call it (2 legs of a 208Y/120) single-phase because it indeeds act differently than a traditional (120/240) single-phase service. I won't argue a lot, I'm just not content to conceed. :smile:

This whole thing can be a confusing mess :smile:

I like Jim's comment from post #69 in the other thread:

Part of the problem is that our industry has taken short cuts in terminology for a long long time.

 

[winnie]

The last one came from
http://forums.mikeholt.com/showthread.php?t=82963&page=2
message 20

I'm not content to call it (2 legs of a 208Y/120) single-phase because it indeeds act differently than a traditional (120/240) single-phase service. I won't argue a lot, I'm just not content to concede. :smile:

I believe that I spent quite a few posts in the thread above arguing for that exact point. It finally dawned on me that the 'nominal value' assigned by ANSI/IEEE was simply a _name_, and while not good physics, it does describe how the service is used.

You are welcome to not concede, and you won't get an argument from me; the _physics_ is different. But you will need to take your argument to a higher authority.

-Jon

 

[hardworkingstiff]

This whole thing can be a confusing mess :smile:

I like Jim's comment from post #69 in the other thread:

Yea, I think Jim's on to something there.

 

[K8MHZ]

The best way to adequately illustrate the difference between single and multi phase systems is with an oscilloscope. That way we can actually 'see' the phases. A O scope will also prove the existence of a single phase regardless of the method of transformer tapping.

It takes a special transformer to get a two phase supply. If I remember correctly the secondary would have to be an open delta. Two phase systems never really caught on as there is essentially no benefit for their use, especially when compared to a three phase system.

 

[rattus]

Seems to me:

Seems to me:

Since all available load devices are single-phase, it makes sense to call this a single-phase service.

 

[peter52]

Psychojohn post #8 makes sense. The load would see the difference of the two signals as 1 phase. If you use a rotating vector to analyse the difference between the two signals you would have one signal, or phase.( some how this makes sense in my feeble mind)

 

[hardworkingstiff]

Since all available load devices are single-phase, it makes sense to call this a single-phase service.

If you put 93 octane gas in a vehicle that recommends 87 octane, does it become 87 octane?

 

[quogueelectric]

The best way to adequately illustrate the difference between single and multi phase systems is with an oscilloscope. That way we can actually 'see' the phases. A O scope will also prove the existence of a single phase regardless of the method of transformer tapping.

It takes a special transformer to get a two phase supply. If I remember correctly the secondary would have to be an open delta. Two phase systems never really caught on as there is essentially no benefit for their use, especially when compared to a three phase system.

Yes I wish you would put it on an O scope to see that if you go A toB phase you willread the combined vectors of two windings 120* out of phase with each other in a sinusoidalpattern. You should clearly see that the resulting waveform will nit be sinusoidal but a resultant waveform of the two combined waves 120* out of phase with each other. Once you introduce the neutral you will be able to show thw phase difference between A to N and B to N which will clearly demonstrate that you have 2 windings with a 120* phase shift Therefor a two phase system. No different than the old two phase except for the 30* extra degrees of phase shift. Trace both waveforms and you will clearly see it.

 

[rattus]

Yes and no:

Yes and no:

Yes I wish you would put it on an O scope to see that if you go A toB phase you willread the combined vectors of two windings 120* out of phase with each other in a sinusoidalpattern. You should clearly see that the resulting waveform will nit be sinusoidal but a resultant waveform of the two combined waves 120* out of phase with each other. Once you introduce the neutral you will be able to show thw phase difference between A to N and B to N which will clearly demonstrate that you have 2 windings with a 120* phase shift Therefor a two phase system. No different than the old two phase except for the 30* extra degrees of phase shift. Trace both waveforms and you will clearly see it.

Yes, technically there are two 120V voltages 120 degrees apart, but the resulting 208V waveform, Vab, is sinusoidal.

It is called single-phase for two reasons:

1. To avoid confusion with the obsolete two-phase system.

2. Because there are no load devices available which would take advantage of the two-phases.

 

[coulter]

Yes I wish you would put it on an O scope to see that if you go A toB phase you willread the combined vectors of two windings 120* out of phase with each other in a sinusoidalpattern. You should clearly see that the resulting waveform will nit be sinusoidal but a resultant waveform of the two combined waves 120* out of phase with each other. ...

(red letter emphasis mine)

quogu-
I must be mis-understanding you. Are you saying the waveform one would see on an o-scope with the leads on ph a and ph b is not sinusoidal? Have to be an isolated channel scope, but that's okay. I think the wave form one sees is a perfect sine, well, okay, maybe a perfect cosine:-?

edit: I didn't see rattus' post. I agree that the resultant is sinusiodal. But I don't agree with rattus' answer as to why two legs of 208 is called single phase - cause I really don't have a clue. I wasn't there when it was named - have never read anyone's commentary that was there.

carl

 

[quogueelectric]

(red letter emphasis mine)

quogu-
I must be mis-understanding you. Are you saying the waveform one would see on an o-scope with the leads on ph a and ph b is not sinusoidal? Have to be an isolated channel scope, but that's okay. I think the wave form one sees is a perfect sine, well, okay, maybe a perfect cosine:-?

edit: I didn't see rattus' post. I agree that the resultant is sinusiodal. But I don't agree with rattus' answer as to why two legs of 208 is called single phase - cause I really don't have a clue. I wasn't there when it was named - have never read anyone's commentary that was there.

carl

YES BIG FAT YES and this is the evidence needed to prove once and for all that when you observe a waveform from two sinusoidal waveforms that are 120* out of phase that the resultant wave is not sinusoidal but the product of the additive waves. Can I get an amen.

 

[Rick Christopherson]

Yes, technically there are two 120V voltages 120 degrees apart, but the resulting 208V waveform, Vab, is sinusoidal.

It is called single-phase for two reasons:

1. To avoid confusion with the obsolete two-phase system.

2. Because there are no load devices available which would take advantage of the two-phases.

A little consistency would be nice. Why is it that you want to call a 120/208 system single-phase, but then you want to call a 120/240 system two-phase?

 

[quogueelectric]

Yes, technically there are two 120V voltages 120 degrees apart, but the resulting 208V waveform, Vab, is sinusoidal.

It is called single-phase for two reasons:

1. To avoid confusion with the obsolete two-phase system.

2. Because there are no load devices available which would take advantage of the two-phases.

You are so wrong it isnt even funny.

 

[rattus]

Not from this corner:

Not from this corner:

YES BIG FAT YES and this is the evidence needed to prove once and for all that when you observe a waveform from two sinusoidal waveforms that are 120* out of phase that the resultant wave is not sinusoidal but the product of the additive waves. Can I get an amen.

No amens from this corner because,

It is common knowledge that the 208V between any two phases of a 120V wye is sinusoidal.

The sinusoidal issue is easily proven.

The 208V is the phasorial difference, not the sum. The product would be 1440V squared.

References have been posted which defines this service as single phase.

Whatever you call it, only single-phase loads can be driven.

 

[rattus]

A little consistency would be nice. Why is it that you want to call a 120/208 system single-phase, but then you want to call a 120/240 system two-phase?

Stay on subject Rick.

 

[quogueelectric]

No amens from this corner because,

It is common knowledge that the 208V between any two phases of a 120V wye is sinusoidal.

The sinusoidal issue is easily proven.

The 208V is the phasorial difference, not the sum. The product would be 1440V squared.

References have been posted which defines this service as single phase.

Whatever you call it, only single-phase loads can be driven.

Common Knowledge?? Prove it!

 

[Smart $]

YES BIG FAT YES and this is the evidence needed to prove once and for all that when you observe a waveform from two sinusoidal waveforms that are 120* out of phase that the resultant wave is not sinusoidal but the product of the additive waves. Can I get an amen.

Sorry, Q. No AMEN from this corner.

The resultant waveform is sinusoidal. Additionally, the resultant waveform is the difference, not the product.

 

[rattus]

Common Knowledge?? Prove it!

OK. Rather than go into a mathematical proof, just think of an open delta. The voltage on the open side is sinusoidal is it not? Same set up here.

If what you are saying is true, then we could not drive 208V three phase motors from a wye system.

Listen also to what Smart and Carl have to say about the matter.

 

[quogueelectric]

Ok Uncle!!>>>>