Single Phase

[electricalist]

I love when a customer asks this very question. After reading all the answers I feel much more comfortable saying I have no clue.

 

[kwired]

I love when a customer asks this very question. After reading all the answers I feel much more comfortable saying I have no clue.

I often know the answer to customer questions but never know how to explain in a manner that they will understand, or if I do - they will forget it in a very short time anyway. GFCI and AFCI are about the worst thing to explain why we need them to a non electrician and worse yet to someone that does have a little understanding of electricity as they think they are just a simple circuit breaker of some sort, and they always ask why we can't just depend on the breaker back at the panel?

 

[Jraef]

And now, just to be the muckraker...

In countries OUTSIDE of North America, two-out-of-three phases (so the "2 pole" described in the OP) is referred to as "2 phase".

Here in North America (actually just Canada and the US), we actually still have some legacy true "two phase" power distribution, where there are two district phases separated by 180 degrees. So that terminology causes all kinds of confusion for people here when reading literature from IEC equipment mfrs. They will give a "2 phase" rating of a device, which is what WE would call "single phase", but people reading it here sometimes then look up what "2 phase" means and Google will find the references to the legacy true two phase systems in an around Philadelphia, NY and Niagara. That's why I am mentioning it in this conversation, so that if you see something describing a "2 phase" rating and it is an IEC device, don't get excited; that's just how they describe 2 pole.

 

[GoldDigger]

We do not usually consider a 180 degree phase shift as a second phase since it can be derived by a transformer. Legacy two phase in the US has a 90 degree separation.
It often (always?) consists of four wires plus a neutral so that the phase angles are 0, 90, 180, and 270. But we do not call it four phase for the same reason that we do not call 120/240 three wire "two phase".

 

[Jraef]

We do not usually consider a 180 degree phase shift as a second phase since it can be derived by a transformer. Legacy two phase in the US has a 90 degree separation.
It often (always?) consists of four wires plus a neutral so that the phase angles are 0, 90, 180, and 270. But we do not call it four phase for the same reason that we do not call 120/240 three wire "two phase".

Yep, sorry. You are right. I keep forgetting that. I've never actually WORKED on a 2 phase system, I've only dealt with it from afar and had to learn about it after the fact and that first encounter was described to me as having a 180 degree phase shift, so my brain keeps getting stuck there. Someone corrected me like you just did, but it's so rare that i forget again.

Years later I came across this issue with the Germans I worked constantly describing "2 phase" this and that and I knew it was rare even here in the US, so I thought that odd and investigated what they meant. They thought I was nuts, I had to prove to THEM that we had a true two phase system over here. I think at the time I told them it was 180 degrees out as well, so they are likely still messing that up in their minds too...

 

[mivey]

It often (always?) consists of four wires plus a neutral so that the phase angles are 0, 90, 180, and 270. But we do not call it four phase

They originally were called quarter-phase systems and were noted as being correctly called a four-phase system. It was only the usage that made people accustomed to calling it a two-phase system. A 3-wire 90 degree two-phase system is still recognized as part of a four-phase system. Six-phase systems are recognized in the same manner. Nothing new, this has been the case since the turn of the 20th century.

 

[Tony S]

Sorry it?s UK voltages.

If you put an oscilloscope on the two phases you will get a single sine wave, a dual trace oscilloscope connected between Ph-N-Ph will give opposing sine waves. An awful lot of confusion comes from this. Some will say the O/P is 180? opposed but the windings are at 0? if they were at 180? you can see what happens. Not a great deal of use.

 

[Tony S]

They originally were called quarter-phase systems and were noted as being correctly called a four-phase system. It was only the usage that made people accustomed to calling it a two-phase system. A 3-wire 90 degree two-phase system is still recognized as part of a four-phase system. Six-phase systems are recognized in the same manner. Nothing new, this has been the case since the turn of the 20th century.

A Scott transformer.

 

[GoldDigger]

A Scott transformer.

With the vector diagram included, that is one of the best explanations of how a Scott T configuration works that i have seen.
I would, however, tend to call it a bank or set of transformers rather than one Scott transformer.

 

[Tony S]

If you really want confusing here?s a LeBlanc transformer, the O/P is exactly the same as the Scott.

I gave up trying to do a 4Ph 5 wire, I confused myself.

 

[wwhitney]

A Scott transformer.

Nice diagram, I was wondering about one thing. That section of the coil to the left of where the HV red line attaches, the (1 - sqrt(3)/2) length of it, does it have to physically be there for the transformer to work? Or is it just included in the diagram to make the physics clearer?

Thanks,
Wayne

 

[Tony S]

Normally what they do is make two sets of identical windings with tappings at:
100%
86.5% (Sqr3/2)
50%
0%

OK it wastes a bit of copper but it saves on setting up time. Our old friend economics rules I?m sorry to say.












Why can?t an engineering site accept basic mathematical characters?

 

[mivey]

Sorry it?s UK voltages.

If you put an oscilloscope on the two phases you will get a single sine wave, a dual trace oscilloscope connected between Ph-N-Ph will give opposing sine waves. An awful lot of confusion comes from this. Some will say the O/P is 180? opposed but the windings are at 0? if they were at 180? you can see what happens. Not a great deal of use.

I guess they might get confused but then one would have to question their proficiency with an oscilloscope.

What also confuses people is when they try to make a naming convention more than what it is. It is not meant to be a canonical definition/description of the system, it is simply a naming convention.

It is this attempt to stretch the function of the naming convention that forces people to try to fit things in only a particular way. For example, your drawing shows how one might assume that taking phase-opposed voltages means the voltage across the series combination is zero. That occurred because of trying to force a reference point for the two voltages across the two windings.

The windings aren't at a degree anything. It is the voltages that have a phase relationship and they can be taken either way. The choice of reference defines how the voltage is taken. Just because the windings are in series does not mean the voltages from them have to be taken with only one possible reference convention. In fact, we use this flexibility of voltage references in windings to develop other voltages in some interesting and useful transformer banking applications.



Since you seem to have some knowledge of vectors let me give an example why the reference point is not some universal constant:

Take the series windings for a 3-wire 120/208 single-phase supply derived from a 3-phase source. We can have two L-N voltages: 120<0d and 120<120d. The L-L voltage can be found as 120<120d ? 120<0d = 207.8<150d.

Now let the source have a slight phase shift so we then have two L-N voltages given by 120<0d and 120<122d. The L-L voltage can be found as 120<122d ? 120<0d = 209.9<151d.

Now let the source have more of a phase shift so we then have two L-N voltages given by 120<0d and 120<150d. The L-L voltage can be found as 120<150d ? 120<0d = 231.8<165d.

Continue with the source phase shift progression so we then have two L-N voltages given by 120<0d and 120<179.9999d. The L-L voltage can be found as 120<179.9999d ? 120<0d = 240<179.99995d.

As we approach a limit where the second voltage reaches 180d, we have a physical set of phase opposed voltages. You might ask: How is that possible when we have the same system we can get with in-phase voltages? The difference becomes our choice of a voltage reference point. These phase-opposed voltages also map to the same physical space as two voltages that are in phase but with one taken in the other direction across the winding. This is not just a matter of a single core physical system but can also be the result of a physical phase-displacement of 180d. We can get this set of voltages by using two windings in series on a common core or even by two generators physically rotated 180d to each other. Either way, the fact remains that the in-phase and phase-opposed voltages can be made to map to the same physical space.



Some added notes about the two windings might also be interesting:

You might note that the voltages across the two windings are not the self-same same voltage. With one two-lead winding I will have the exact same current leaving one lead as I will have entering the other. There is only one current in one winding.

With two windings and 3 leads, the current in one winding does not have to be the same as the current in the other winding. In fact, you will find that due to winding impedance differences, the voltages from the two windings are not exactly in phase nor do they make exact polarity pairs. While this difference might be hard to see physically in most cases, it should be clear that they are separate voltages that can be out of phase. The point is not to focus on the minute impedance-caused phase difference but to emphasize that they are not the exact same voltage and can react differently to loads. The fact that the winding currents can differ in phase should be obvious.



I?m not suggesting we change names but rather recognize that naming conventions are simply naming conventions, nothing more. It is also great when people remember the fundamental concept that a voltage reference point is a choice and not pre-determined.

 

[robertmorkel]

There are many international ciruit breakers that look like a 2 pole device but are really 1 OCPD and 1 switch. This is similar to our "switched neutral" breakers except the grounded conductor plugs on to the busbar.

On single phase 220vac, you have 2 "hot" wires that each test at 120v to ground but 240v from one to another. Each side can act independently as a 110-120 leg but the two of them are 180 degrees out of phase with one another, allowing them to essentially act as a team.

 

[Tony S]

On the subject of phase shifting, I inherited this little beauty when I changed companies.

 

[kwired]

There are many international ciruit breakers that look like a 2 pole device but are really 1 OCPD and 1 switch. This is similar to our "switched neutral" breakers except the grounded conductor plugs on to the busbar.

On single phase 220vac, you have 2 "hot" wires that each test at 120v to ground but 240v from one to another. Each side can act independently as a 110-120 leg but the two of them are 180 degrees out of phase with one another, allowing them to essentially act as a team.

Square D has taken a step in that direction with the dual function AFCI - GFCI circuit breakers, I imagine others will eventually have something similar. I don't see the simple single pole breaker disappearing from the market either though- until codes make it unneeded anyway.

 

[mivey]

On the subject of phase shifting, I inherited this little beauty when I changed companies.

Good spot for an ungrounded_Y-Y with a delta tertiary.

 

[Tony S]

Good spot for an ungrounded_Y-Y with a delta tertiary.

For the LV yes but it would cost money to have the transformer made. I?d had two 550/433V 150KVA units made for a previous company and knew what the price would be like.
I?d about as much chance as a snowball in hell. We just battled on with the situation.

The 550/433V were able to be used either way by swapping the E & N links around.

 

[mbrooke]

On the subject of phase shifting, I inherited this little beauty when I changed companies.

Wouldn't the transformer on the left be different from the right?

 

[mivey]

Wouldn't the transformer on the left be different from the right?

It should be. That was the issue.