Single Phase Theroy

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nicholaaaas

Member
Location
Baltimore
Hello everyone btw. One day a co-worker and I were talking and he asked why do they call it single phase when there are two hot wires. I though about it for a minute and answered because it actually IS only one phase. It comes from your power supplier on only one line, It goes through a transformer the secondary coil is connected between the two outer taps, and the middle of the coil is center-tapped to get a mid point. Therefore it is single phase 240 with a grounded mid point.

When I asked my apprenticeship teacher he started drawing a picture of two sine waves 180? apart. And insisted it really was two distinct phases.

Who's right?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Hello everyone btw. One day a co-worker and I were talking and he asked why do they call it single phase when there are two hot wires. I though about it for a minute and answered because it actually IS only one phase. It comes from your power supplier on only one line, It goes through a transformer the secondary coil is connected between the two outer taps, and the middle of the coil is center-tapped to get a mid point. Therefore it is single phase 240 with a grounded mid point.

When I asked my apprenticeship teacher he started drawing a picture of two sine waves 180? apart. And insisted it really was two distinct phases.

Who's right?
Two phase power was common many years ago, but the phases were 90 degrees apart. Here's a Wiki page that talks about it:http://en.wikipedia.org/wiki/Two-phase_electric_power
It make be a semantic difference, but from that page:"Three-wire, 120/240 volt single phase power used in the USA and Canada is sometimes incorrectly called "two-phase". The proper term is split phase or 3-wire single-phase."
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Nick, welcome to the forum! :)

Yes, if you look at the phase (I prefer 'polarity') from only the center-tap's perspective, the two lines do resemble being out of phase.

But, if the center-tapped secondary was anything other than one phase, 120v + 120v would not = 240v, as with a 120/208v source.

Also, see if any of you guys get anything from this I posted a while back in response to a basic question about the grounded neutral:

http://forums.mikeholt.com/showpost.php?p=708650&postcount=4
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
single phase circuit is either:

1 current carrying conductor and the neutral (single-phase, 2-wire)
2 current carrying conductors, no neutral, (single-phase 2-wire)
2 current carrying conductors and a neutral (single-phase, 3 wire)

These descriptions are consistent with IEEE 100 (Dictionary) which is the authoritative resourch for all dicussions with legal implications.
 

Cold Fusion

Senior Member
Location
way north
Hello everyone btw. One day a co-worker and I were talking and he asked why do they call it single phase when there are two hot wires. I though about it for a minute and answered because it actually IS only one phase. It comes from your power supplier on only one line, It goes through a transformer the secondary coil is connected between the two outer taps, and the middle of the coil is center-tapped to get a mid point. Therefore it is single phase 240 with a grounded mid point.

When I asked my apprenticeship teacher he started drawing a picture of two sine waves 180? apart. And insisted it really was two distinct phases.

Who's right?
I think you understand clearly. The model you are using is the one I use. Both 120V sources point the same direction and are joined nose to tail at the neutral.

It's important to understand the concept of a "model". We can't see the electrons or even measure them directly. We can't see the electric field. So we develop a "model", a collection of equations, diagrams, formula, that describe how the system works. There are often several models that will work for any given system. You need to pick out the one that best fits what you are trying to do. In this case, the one you picked out works the best for me to describe a single phase 120/240V house services.

The 200+ thread Jim mentioned, is interesting, not because any of the participants really had anything to say about the truth of single phase, but rather because each was adamant their model was The Truth. I'm reading along thinking, "Who cares? Either will work - as long as one defines the model."

However, as you have noted with your instructor - some are rabid in their choice/selection of the model.

And, also as you have noticed, it is pretty hard for the apprentice to tell the instructor he is screwed up. Other than the mis-application of the idomatic term "2phase", he isn't too screw up.

cf
 

Cold Fusion

Senior Member
Location
way north
...http://en.wikipedia.org/wiki/Two-phase_electric_power
It make be a semantic difference, but from that page:"Three-wire, 120/240 volt single phase power used in the USA and Canada is sometimes incorrectly called "two-phase". The proper term is split phase or 3-wire single-phase."
gg -
I've never understood when and why the term "split phase" was ever associated with single phase 120/240.

The only place I see "split phase" in IEEE 100 is in conjunction with:

split-phase motor
A single-phase induction motor having a main winding and an auxiliary winding, designed to operate with no external impedance in either winding. The auxiliary winding is energized only during the starting operation of the auxiliary-winding circuits and is open-circuited during running.


There are no definitions connected with 120/240 single phase. Yes, I usually know what is meant by "split phase" from the conversation context, whether is is motors or house wiring. And generally it would not matter to me - I don't do houses (except mine) and rarely single phase motors.

I'm just curious - where does this come from?

cf
 
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nicholaaaas

Member
Location
Baltimore
As I'm laid off I had nothing better to do this afternoon so I think I can settle this once and for all.

First watch this video: http://www.youtube.com/watch?v=tUO3o5JTGhQ&NR=1

From this we can assume that there are two windings in series that add up to 240 volts the one end of each winding is connected on the center-tap. I assume it looks something similar to this:
pole%20transformer.jpg



Now at this point i'd like to retract my previous statement that it it one single winding. I believe it is two separate windings. The problem i have is everyone on the really long thread believes it; whether it to be one of two phases, to have one of the phases lead/lag the other phase by 180? like so:


incorrect%20sine%20wave.jpg



Being that there in only one expanding and collapsing electric field from the primary there is no way physically this could cause a lead or lag of a "phase." I do believe, however, if the two windings in the secondary are wound opposite of each other they could be "inversely in phase," that is, each "phase" is the direct opposite of the other at any point; as show here:


correct%20sine%20wave.jpg


It seems some want to argue that any hot wire constitutes a phase. And for all practical purposed that is correct. It seems others what to philosophical define what a phase is. That is useless. The question really is do you define phase order by the primary or the secondary? For all practical purposed and using standard names for systems it seems clearly that defining them by the primary is correct. And using the proper names; most anyone in the field should know what you are talking about.
 

hurk27

Senior Member
The way I understand it, is as a term that describes the connection to the actual generator.
In a 3-phase generator you have three windings or poles connected in a delta, any time you connect a load to just two of the three conductors you will be pulling power only from one pole, current will not flow from the other two poles into this load, thus it is only using one phase pole to supply the current?
 

nicholaaaas

Member
Location
Baltimore
The way I understand it, is as a term that describes the connection to the actual generator.
In a 3-phase generator you have three windings or poles connected in a delta, any time you connect a load to just two of the three conductors you will be pulling power only from one pole, current will not flow from the other two poles into this load, thus it is only using one phase pole to supply the current?

it has nothing to do with 3 phase power. i think we all agree "single phase" power for a home in the US is fed from only one primary line
 

hurk27

Senior Member
it has nothing to do with 3 phase power. i think we all agree "single phase" power for a home in the US is fed from only one primary line

Really?

I know of at least two forum members on here that have delta primary's which are always fed from line to line Gar is one of them

I know in most areas the primary service is a Y but ether way, whether a Y or a delta your still only loading one pole of the supplying generator.
 

Volta

Senior Member
Location
Columbus, Ohio
it has nothing to do with 3 phase power. i think we all agree "single phase" power for a home in the US is fed from only one primary line

If 'line' means wire, then two, usually.

If 'line' means ungrounded conductor, then maybe, as in Line to neutral, but rarely Line to ground.

If 'line' means phase winding, one is all that is needed
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Cold Fusion said:
And, also as you have noticed, it is pretty hard for the apprentice to tell the instructor he is screwed up.

I found that it was easy, often enjoyable and I did it many times. The instructors got used to it and eventually started to actually enjoy it as well.

I managed to get incorrect answers in our tests changed for the upcoming years.

I also finished #1 in my 40 student class.

A wise sage once said, "Never get upset with a student that argues with the teacher as he may be the only one in class paying attention."

It does help considerably if A) you are right and B) you bring documentation into class to support your allegations.
 

iwire

Moderator
Staff member
Location
Massachusetts
Question for Sparkys that insist 120/240 is not single phase:

If the nameplate requires 120/240 1PH 60Hz and you have it in your brain that 120/240 has two phases, what kind of phase converter are you going to use?

Hmmm???

That is pretty much my position, call the systems per their common names or you just add to confusion.

If I need a 240 volt motor for a home it will be called a single phase motor.

If I need a panel for that home it will be a single phase panel.

Also there is another, different system, although old and as far as I know now unused called "2 Phase" so lets not try to use "2 phase" for the common home service.

For me I think it just makes sense to call things by their common names and all this 'stuff' about it being single phase or 2 phase amounts to people just trying to prove they have the biggest brains.:roll:
 
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nicholaaaas

Member
Location
Baltimore
If 'line' means wire, then two, usually.

If 'line' means ungrounded conductor, then maybe, as in Line to neutral, but rarely Line to ground.

If 'line' means phase winding, one is all that is needed

I know out back of my parents house there is only one line. I'm guessing that is single-wire earth return set-up
 
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