2-Phase, No, not single phase!

[davedottcom]

Yes I said 2-Phase! Most of you will laugh and say it doesn't exist, But you might want to hold those replies until someone that has seen it replies first! Some of you "older" guys may have seen it. I know it was popular in very old parts of Philadelphia Pa. used in factories and commercial buildings. I've only "heard" about it myself, from elder fellow electricians and also from one of my Penn. State Electrical Engineering professors, however he never explained it in any detail. And THAT is EXACTLY what I was hoping someone could do for me! I would really appreciate it if someone with knowledge of or experience with 2-phase systems please share it with me. I know from past visits to this site that there are some extremely knowledgeable people here so I figured this is a good place to ask!
Thanks in advance,
Dave

 

[bennie]

Re: 2-Phase, No, not single phase!

Two single phase generators were installed on a common shaft. One generator was 90 electrical degrees from the other.

The purpose was for starting single phase motors.

Phase A to B was at 90? displacement. Phase B to A was at 270? displacement.

[ September 25, 2003, 12:34 AM: Message edited by: bennie ]

 

[Guest]

Re: 2-Phase, No, not single phase!

Bennie,
You are a treasure!

If you did not exist we would have to invent you!

I cherish your every post.

Most respectfully,

../Wayne C.

 

[davedottcom]

Re: 2-Phase, No, not single phase!

Thanks Bennie, that's very interesting to say the least!
Are the voltages from A phase to B phase and from each of the phases to ground the same as a single phase system?....or is voltage from the B phase to ground higher then A phase to ground?

It sounds like it would be 120V from A to ground, and 240V from B to ground...is that right?

Thanks again Bennie, you are one of the guys I knew would know about this!

Dave

 

[frenchelectrican]

Re: 2-Phase, No, not single phase!

yeah i heard about duex (2) phase system but there is insering twist about 2 phase they did have 3 wire 2 phase system and the other verison have 4 wire and last one is 5 wire 2 phase system it is very compated how to hook up the 2 phase motors i read it some time back. but right now i know it is almost nonexisting now most run ether 1 or 3 phase nowdays but i wish i can find the link for wiring digraph how that system ran but i remember it clear it say phase 1-AB and phase 2 AB but it was roated 120 or 240 electrical degrees but the 5 wires are rotated 90 if i am right.

yeah bennie you are on bat with it but dang i dont have link here

 

[big john]

Re: 2-Phase, No, not single phase!

I thought two-phase systems were still in existance.

Here's the scenerio I don't understand: A lot of panels are listed for 120/208 on a "single-phase," three-wire service. My understanding of that setup is that you're taking two phases of a three-phase system into the panel along with a neutral.

I suppose this system is used in applications were you only expect single-phase loads, so the fact that the two panel buses are only 120 degrees out of phase isn't a big deal. But it seems to me that despite calling it "single phase", that is still a two-phase system.

Or am I simply grossly misinformed about how this setup actually works?

-John

[ September 25, 2003, 01:22 AM: Message edited by: big john ]

 

[big john]

Re: 2-Phase, No, not single phase!

Sorry for the double post, I went to edit my first one and hit the "Quote" button instead.
Didn't notice until too late.

[ September 25, 2003, 01:22 AM: Message edited by: big john ]

 

[iwire]

Re: 2-Phase, No, not single phase!

John if I got you right, what you are describing is say using a 2 pole breaker from a 3 phase panel it might feed a heater on another panel.

This would be "single phase" even though you have 2 of the 3 phase conductors.(clear huh?)

Two phase like davedottcom is asking about is entirely different and I can not provide anymore input, but read Bennie's post it seems to say all we need to know.

You could have 4 phase 6 phase etc. it just happens 3 phase has worked out to the most efficient way to distribute power.

It is the best trade off between conductor size and conductor quantity to run a given load.

 

[bennie]

Re: 2-Phase, No, not single phase!

Please note I edited my first post. The phase difference is 90? and 270? respectfully.

This system allowed reversal of the motor by switching A and B, to B and A sequence.

There was 3,4, and 5 wire systems. I don't know if any are in use today, I would tend to think not. This system would not be compatible with the power grid.

 

[jbucshot]

Re: 2-Phase, No, not single phase!

on navy ships they use 2 of 3 phases to provide power, they are electrically out of phase with each other and add to 120 but they are like 60 volts to ships ground? :roll: all power comes from 3 phase systems transformed down and using 2 hot wires from one of the 3 transformers talk about balaning problems.

 

[charlie b]

Re: 2-Phase, No, not single phase!

Originally posted by big john:I suppose this system is used in applications were you only expect single-phase loads, so the fact that the two panel buses are only 120 degrees out of phase isn't a big deal. But it seems to me that despite calling it "single phase", that is still a two-phase system.

Two or more sine waves that cross ?Zero? at the same points in time, and that reach their peak values at the same points in time, are ?in phase.? If any sine wave reaches its peak before or after another sine wave, the two are ?out of phase? with each other. When we speak of ?single phase,? or ?2-phase,? or ?3-phase,? we mean (in order) ?a single sine wave, so there is no question of being in phase or out of phase with any other,? and then ?a pair of sine waves that are not in phase with each other,? and finally ?a set of three sine waves, no one of which is in phase with any other.? The way real systems are built, the two sine waves of a 2-phase system are 90 degrees out of phase from each other, and the three sine waves of a 3-phase system are 120 degrees out of phase from each other.

Consider a panel with two hots and a neutral, with the two hots coming from two of the three phases of a 120/208V system. If you measure the ?left-hand hot? to ground and measure the ?right-hand hot? to ground, and compare the two, you should find that their magnitudes are about the same (about 120 volts), and that the two are 120 degrees out of phase with each other. If you take a measurement across the two hots, you only have a single sine wave - it is ?single phase.? Its magnitude will be about 208 volts, and it is meaningless to talk about its being out of phase with anything else.

Does this help clear up the mystery?

 

[charlie b]

Re: 2-Phase, No, not single phase!

Originally posted by jbucshot:On navy ships they use 2 of 3 phases to provide power, they are electrically out of phase with each other and add to 120 but they are like 60 volts to ships ground?

My recollection of the Navy's power distribution systems is that the voltage to ground is meaningless. They are ungrounded delta systems, so the voltage from any point to ground can vary all over the map. The intent is to limit the impact of battle damage. The system will continue to operate with no detectable changes, if anything causes a single short circuit to ground. They have ground detection systems that they test on (at least) an hourly basis - to discover the ?first ground? before there is a chance for a ?second ground? to occur.

 

[bennie]

Re: 2-Phase, No, not single phase!

Only in electric work does 1 plus 1 equals 3, and 3 minus 1 equals 1.

Two phase is not 2 of 3 phases.

Think of a motor with a run and start winding. Apply power to the run winding it will only hum. Apply power an instant later to the start winding the rotor will turn in the direction of the start winding.

The sine wave of the run phase will be at maximum when the start phase begins. The run phase will then be at zero when the start phase reaches maximum. The resultant magnetic field will cause the rotor to turn.

 

[brian john]

Re: 2-Phase, No, not single phase!

In downtown Philly PA. I ran into a 2 phase 5 wire system, this was in the early 90's, I was not there to look into the AC power system. I was there to inspect a 48VDC system installed by a local firm. They (the contractor) was less that happy to see me and did not want me poking my Local 26 nose into their local jurisdiction work much less their panels. Therefore I did not (could not) investigate but I sure wanted to.

 

[big john]

Re: 2-Phase, No, not single phase!

I suppose I'll just have to accept 120/208 1P with the same faith that I accept that 120/240 is single phase.

What gets me, though, is that with 120/240 it is easy to see that you only need single-phase to the primary, and as such you'd get single phase out of the secondary. I can't think of any way to derive a 120/208 1P service without using a three-phase transformer and simply dropping one phase.

And with that in my mind, I'm always thinking of it as two phases of a three-phase system. Hence my notion that it is two-phase.

-John

 

[iwire]

Re: 2-Phase, No, not single phase!

Originally posted by big john:
with that in my mind, I'm always thinking of it as two phases of a three-phase system. Hence my notion that it is two-phase.

And you would be right, you have two phases of a three phase system, you just do not have two phases of a "Two phase system"

You just got to love our trade, conductors that are inside a building can still be "outside" a building, 4" raceways do not measure 4" and two phase conductors are not necessarily two phase.

 

[bennie]

Re: 2-Phase, No, not single phase!

The reason two phases of a 3 phase system is only single phase is due to the events occurring in the same time domain. The ends of the transformer windings are equal and opposite, in the same time domain.

 

[jhines]

Re: 2-Phase, No, not single phase!

Scott-T transformation can be used for 3 phase conversion to 2 phase with phase shift of 90 degrees.

 

[frenchelectrican]

Re: 2-Phase, No, not single phase!

Bennie; indeed i did look up my old electrcal book and i ran into the transformer connetcions and i will admit my mistake on my first post about phase shifting. it is a 90 degrees shifted but most common methold to connet is T format using open delta to hook up the " 2 phase system" and i will try to use the goggle sherch for old 2 phase system [ yeah i know it is about history now]

merci , marc

 

[rrrusty]

Re: 2-Phase, No, not single phase!

If you look at the "Ugly'S" It gives a brief illustration of the 2 phase motor hook-up for 3 wire and 4 wire and would this also be considered a delta with a grounded leg?