Two Phase Systems

[Joethemechanic]

Hi, my name is Joe and I am new here. I am from Philly and have been around for a few years. Philly at one time had a Two Phase generating and distribution system. I'm old enough to remember when it was common to work in two phase buildings. I've set up Scott connections, and I even know a few tricks with a delta secondary that I've used over the years to get emergency power to some of the two phase loads.

But anyway, I am very familiar with the "how" of the system, my question has more to do with the "why?"

What was the reasoning behind going with a two phase system in the first place? To me it just seems like a waste of copper. Most places went with three phase. Was there something different about the customer base and their types of loads here that made Philly decide against three and go in favor of two? Was it just that all the factors were not properly understood? Was it just that Philly was an early system, and they just copied what worked at Niagara Falls?

The only other thing that keeps popping into my head is, I think, years ago, an instructor told me that there was some concern with a harmonic on a three phase system that the engineers at the time believed would not occur if they went with the two phase.

I just can't remember. Maybe something to do with supplying single phase 25 cycle for rail traction? But they were M-G frequency converters and I can't see how it should matter.

Damn, where is that 130 year old engineer when you need him? lol

 

[jim dungar]

Two phase was easy to generate and distribute and was suited very well for large motor loads (like subway trains).
It took several years and some scientific break throughs (thank you Tesla) before three phase was economically feasible.

 

[SG-1]

Here is a web site with a short history of 2-Phase Power.

http://www.3phasepower.org/2phasesystems.htm

 

[Joethemechanic]

Ok so here is where I was going with this 2 phase thing,,,,,

Triplen Harmonics, and Non-Linear Load Stuff

---

Ok, just at the brain storming stage but,,,,,


What if we were to use Scott connected transformers to convert our 3 phase service to a 2 phase 5 wire 120/240 source.

And we use this newly created 2 phase source to feed panel/panels from which to run MWBCs to serve all our non-linear loads.

Now hopefully we will have all our non-linear loads firing 180 degrees from each other and not really loading our neutrals much provided they are reasonably balanced.

A neutral isn't even necessary on the 3 phase side. And we should end up with fairly decent load balancing on all 3 phases on the primary side.

Question one should be,,,,, Will we save any copper?

Question two,,,,,, will we save enough copper to make the transformer losses negligible?

Question three ,,,,, If the whole idea sucks, do you think we could form a barbershop quartet and call them "The Triplen Harmonics?

 

[T.M.Haja Sahib]

Ok so here is where I was going with this 2 phase thing,,,,,

Triplen Harmonics, and Non-Linear Load Stuff

---

Ok, just at the brain storming stage but,,,,,


What if we were to use Scott connected transformers to convert our 3 phase service to a 2 phase 5 wire 120/240 source.

And we use this newly created 2 phase source to feed panel/panels from which to run MWBCs to serve all our non-linear loads.

Now hopefully we will have all our non-linear loads firing 180 degrees from each other and not really loading our neutrals much provided they are reasonably balanced.

A neutral isn't even necessary on the 3 phase side. And we should end up with fairly decent load balancing on all 3 phases on the primary side.

Question one should be,,,,, Will we save any copper?

Question two,,,,,, will we save enough copper to make the transformer losses negligible?

Question three ,,,,, If the whole idea sucks, do you think we could form a barbershop quartet and call them "The Triplen Harmonics?

While very appreciative of your beautiful circuit diagrams,I want to ask,Are they necessary?Because ordinary 120v/240v service lines have the same property of eliminating third order harmonics in the neutral, dispensing with above Scot-connected transformer.

 

[kwired]

If panel 1 and panel 2 in the diagram have identical loads isn't the three phase supply unbalanced? Therefore you have inefficiencies that would require larger supply than if it were balanced.

If the source were actually 2 phase this would not be an issue.

 

[T.M.Haja Sahib]

If panel 1 and panel 2 in the diagram have identical loads isn't the three phase supply unbalanced?

It is not unbalanced.

 

[Joethemechanic]

While I am not much of a believer wild harmonic theories, I am not sure if they should be given credibility. Many engineers are specifying oversize and separate neutrals for circuits that are designed to serve non-linear electronic loads. Personally I do not think it is anywhere as large of a problem as it is being made out to be. But there must have been a few cases of overloaded neutrals to support the hype.

How would an equal load across a 2 phase Scott T secondary be anything other than balanced in the 3 phase primary side? I am thinking you are not understanding the phase shift between the primary and secondary in a Scott T connection. A Scott T is not simply like just dumping one phase

 

[kwired]

While I am not much of a believer wild harmonic theories, I am not sure if they should be given credibility. Many engineers are specifying oversize and separate neutrals for circuits that are designed to serve non-linear electronic loads. Personally I do not think it is anywhere as large of a problem as it is being made out to be. But there must have been a few cases of overloaded neutrals to support the hype.

How would an equal load across a 2 phase Scott T secondary be anything other than balanced in the 3 phase primary side? I am thinking you are not understanding the phase shift between the primary and secondary in a Scott T connection. A Scott T is not simply like just dumping one phase

It is obvious to me that we are not dumping a phase. I am just curious if the output were same load on both phases if the input current would be same amps on all three phases, that is not obvious without possible use of higher level math skills than average Joe uses daily.

What is primary current in your example if the two phase has 10 amps each phase of load and the input voltage is 240 volts?

I see it as 4800 VA which should be about 11.5 amps if balanced across three phases.

 

[Joethemechanic]

It is obvious to me that we are not dumping a phase. I am just curious if the output were same load on both phases if the input current would be same amps on all three phases, that is not obvious without possible use of higher level math skills than average Joe uses daily.

What is primary current in your example if the two phase has 10 amps each phase of load and the input voltage is 240 volts?

I see it as 4800 VA which should be about 11.5 amps if balanced across three phases.

Well as a guy who grew up in philly, where we still have large buildings running on 2 phase (Youth Study Center , just to name one) I can tell you that I know empirically that all 3 phases, will be balanced on the primary, if both phases on the secondary are equally loaded. And of course vise-versa.

I never applied the math because I never doubted it for a minute. I was working on this stuff when I was in my early teens. I amp clamped all the phases, and never doubted what the old timers told me for a minute.

 

[kwired]

Well as a guy who grew up in philly, where we still have large buildings running on 2 phase (Youth Study Center , just to name one) I can tell you that I know empirically that all 3 phases, will be balanced on the primary, if both phases on the secondary are equally loaded. And of course vise-versa.

I never applied the math because I never doubted it for a minute. I was working on this stuff when I was in my early teens. I amp clamped all the phases, and never doubted what the old timers told me for a minute.

View attachment 6204

I am not doubting you just asking information. Probably will never need to apply any of this but do find it somewhat important to know a few basics. I have more than once heard people ask what is 2 phase or is this 2 phase. The only real two phase that I run into is the phase shift in a single phase motor that essentially makes a single phase motor a two phase motor.

I figured the fact that the 86.6% tap on the one coil probably has something to do with balancing things. I am not a math genious but am not totally ignorant to some advanced math either.

 

[jim dungar]

I figured the fact that the 86.6% tap on the one coil probably has something to do with balancing things. I am not a math genious but am not totally ignorant to some advanced math either.

The tap has to do with providing the correct voltage.

This is a reasonable tech resource for interconnecting single-phase transformers. It uses some 'dated' utility terminology as opposed to the IEEE terminology which is sees to be more popular these days.
http://www.cooperindustries.com/con...urces/library/201_1phTransformers/R201902.PDF

 

[T.M.Haja Sahib]

I figured the fact that the 86.6% tap on the one coil probably has something to do with balancing things...........

It may not be that 86.6% tap............It is the equal secondary ratios of the two transformers that make the three phase system balanced.......

 

[Joethemechanic]

Yes, the 86.6 does. And yes single phase motors are pretty much 2 phase motors. The capacitor shifts the phase 90 degrees and creates a second phase.

You can also use a 4 wire 240/120 delta high leg to supply 2 phase motor loads. Although it will be a bit unbalanced as you will see in the diagram below. I can tell you I have seen them run for years like this though. Most of the 2 phase motors were designed for 220 V so you can see that the 208 V isn't really that low and the 240 V isn't really all that high.