why the two out of phases is called single phase ??

Yes, the 2 hot wires are in phase with respect to EACH OTHER.
Hot with respect to what? Every electrical circuit has 2 wires for power.
A floating circuit doesn't care if the leads are hot/hot or hot/neutral, because there is no ground reference. At any given point in time, one lead is positive and the other lead is negative.

On standard, machine generated AC, no matter what your reference, each single cycle of the waveform (to pick an arbitrary point) starts at 0 degrees (0 potential), and passes in turn through 90 deg (max potential), 180 deg (0 potential), 270 deg (minimum potential) and back to 0 (0 potential). Period.

'Three Phase' supplies simply have 3 'overlapping' sine waves, offset by 120 degrees to each other. Single phase, has two wires, not three, but the complete cycle is still 360 degrees.

skipfast said:

I have attached a drawing of a single phase, residential distribution transformer. I hope this helps.

Click to expand...

Here in Seattle USA (from looking up at the poles) the residential primary is supplied by two conductors and NOT grounded. Back at the "residential substation 3 phase comes in is split up and single (or if you want to say it that way 3 singles) taken out to the homes

Commercial of course is 3.

The ground is obtained at the service point in the house or business.

Why it is called singe phase instead of two phase who the heck knows. Some of the terms in electrical have a source that sometimes makes no sense. My favorite is the BLDC ie brushless DC motor which is actually a 3 phase motor - now who in the heck cooked up that term?

Dan Bentler

The ground on the primary is only for brevity, all that is needed is potential difference. Ground is always 0V so the other leg will always swing to a different potential. The point is that the 2 secondary legs have the same phase, thus called single phase and not 2 phase which would be 90 degrees out.

please , i want a certain answer to understand


if the generator station produces three phase delat connection , can we take two phases of these delta connection and applied these two hot wires - delta connection which are two phases - to the primary of the transformer and obtain at the secondary also two hot wires but in phase i.e : acts as single phase or not ??

i.e : i think if the primary of the transformer is two phases , the secondary must be also two phases , and if the primary of the transformer is single phase , the secondary must be also single phase , but to connect at the primary of the transformer two phases and obtain at the secondary two hot wires as a single phase i think it is impossibble !!!

so as skipfast drew , i think it is the only method to obtain at the homes at USA two hot wires in phase as they are in phase , am I correct or not ??

thanks

magdyfayad said:

please , i want a certain answer to understand


if the generator station produces three phase delta connection , can we take two phases of these delta connection and apply these two hot wires - delta connection which are two phases - to the primary of the transformer and obtain at the secondary also two hot wires but in phase i.e : acts as single phase or not ??

Click to expand...

Correct. Single phase.

magdyfayad said:

i.e : i think if the primary of the transformer is two phases , the secondary must be also two phases , and if the primary of the transformer is single phase , the secondary must be also single phase , but to connect at the primary of the transformer two phases and obtain at the secondary two hot wires as a single phase i think it is impossible !!!

Click to expand...

The primary of the transformer cannot be 2 phases. Each winding can only be single phase. A 3 phase transformer is 3 single phase transformers in one box.

magdyfayad said:

so as skipfast drew , i think it is the only method to obtain at the homes at USA two hot wires in phase as they are in phase , am I correct or not ??

Click to expand...

Correct.

I'm wondering if your not confusing the term phase in some instances. Earlier someone said look at a generator. So from line one to line two is considered one phase. Line2 to line3 one phase. Line3 to line1-one phase. Together they equal 3 phase. 120 degrees apart. On a typical usa home service, you have two secondary power wires and a neutral and a ground. From one power wire to neutral is 120v. From the other power to neutral is 120v. From line to line is 240v. Now due to where the neutral is, from one line to neutral to the other line to neutral is 180 degrees apart. That is why the neutral current from one side cancels out the neutral current from the other side.

then the only method to obtain two hot wires - in phase - at home as it the secondary of a transformer is to make the primary also in phase which means we have two methods to make the primary in phase :

1- the generator station must be star connection , and take one hot wire and the nuteral and applied it to the primary and then obtain at the secondary two hot wires in phase

2- the generator station may be delta connection , but put after that transformer 1 : 1 which its primary is three phase delta connection and the secondary is three phase star connection with ratio 1 : 1 , and for the secondry of this transformer i take one hot wire and the nuteral and applied it to the primary of another transformer and then obtain at the secondary of this another transformer two hot wires in phase

do these notes are right or not ??

if these two notes are right , what is the meaning of the figure c as shown below as it is wrong wiring !!!! am i correct or not ??




wait for your answer

thanks all

With AC, measure any two points you get a sine wave. If you have a device that is using these two points (L1-N, L1-L2, A-B, B-C, A-C ...) you call that single phase.

If you are using 3 points (3 sine waves) that are 120 degrees out of phase with each other (A-B,B-C,A-C) that is refered to as three phase.

If you lose a winding or blow a fuse on a three phase motor it can be called single phasing. (not a good thing)

magdyfayad said:

please , i want a certain answer to understand


if the generator station produces three phase delat connection , can we take two phases of these delta connection and applied these two hot wires

Click to expand...

magdyfayad,

See Jim Dungar's post

Jim Dungar said:

Most times when I see confusion about single phase and three phase it is because people forget that the word phase is usually context sensitive. For example, to measure a phase voltage you need two points, but many people call a single conductor a "phase" when they mean a "phase conductor"

Click to expand...

i repeat again


then the only method to obtain two hot wires - in phase - at home as at USA- which is the secondary of a transformer - is to make the primary also in phase which means we have two methods to make the primary in phase :

1- the generator station must be star connection , and take one hot wire and the nuteral and applied it to the primary and then obtain at the secondary two hot wires in phase

2- the generator station may be delta connection , but put after that transformer 1 : 1 which its primary is three phase delta connection and the secondary is three phase star connection with ratio 1 : 1 , and for the secondry of this transformer i take one hot wire and the nuteral and applied it to the primary of another transformer and then obtain at the secondary of this another transformer two hot wires in phase

3- i.e : i think if the primary of the transformer is single phase , the secondary must be also single phase , and if the primary of the transformer is three phases ( 3 single phase transformers in one box ) , the secondary must be also three phases ( 3 single phase transformers in one box ) ,

as it is wrong to say the primary of the transformer is three phases delta connection and obtain at the secondary two hot wires as a single phase , i think it is impossible !!!

do these above notes are right or not ??

if the above notes are right , what is the meaning of the figure c as shown below as i think it is wrong wiring !!!! am i correct or not ?? or what is the explansion ??





wait for your answer

thanks all

On figure c. If lineA to line B is 230v, line b to line c is 230v, line a to line c is 230v. Then, from line b to neutral you will have 115v. From line c to neutral you will have 115v. Although our company runs 480v system a machine shop up the street from us uses this exact 230v/115v system. I believe it's very common in the USA.

the figure c explains the delta connection which are two phases between two hot wires B , C - as the delta connection contain three phases - and split by the nuetral ,
but we assumed the two hot wires which come to USA homes are not two phases as we considered it a single phase with 180 degree for the voltage amlitude

so if my assume is correct , what is the explansion of figure c and when we use it ??

or my assume is wrong , but then what is the coorect assume ??
i.e : if the figure c explain the actuall connection at USA homes , how they are delta connection and called B , C are not two phases but single phase ??

i think my question is clear now !!

thanks

In the US, If I measure from A-B I will read 208VAC. If I measure from B-C, I will measure 208VAC. They are 2 different phases, but I only care if I am using all 3 phases. If I am only using 2, then it is 1 single phase.
Phase 1 = A-B
Phase 2 = B-C
Phase 3 = C-A
Three single phases. There isn't six wires, only 3. Each phase shares a wire with the next phase.
If you would please study a generator, then it would become clear. Otherwise, it will take 3 more pages of posts to explain it. Go find out how a generator works!

yes , all you said i agree with you ,

but if we measured the voltage between the two hot wires at USA homes , it will be 240 VAC not 208 !! so the connetion at USA homes not as figure c

as i think the connetion at USA homes is a secondry for single phase transformer , but if the connection at USA homes is at figure c , then the secondry is for three phase tranformer not single phase transformer !!


thanks