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

magdyfayad said:

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 connection at USA homes not as figure c

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What makes you think that? You are wrong. The voltage doesn't dictate the type of connection.

magdyfayad said:

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

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There is no such thing as a three phase transformer. The connection in figure C IS the way US homes are connected. It is single phase. Why are you concerned about the other 2 phases (A-B, C-A)? Why do you care that it's 3 phase? You are only using one of the 3 phases.
If I have 3 12 volt batteries in series, Someone could connect and get 36 volts. I can connect to just one of the batteries, and get 12 volts. I don't care what anybody else does with the other batteries. I'm only using 1 of the 3 phases.

Voltage doesn't dictate the connection. If you would study how a generator works, you would understand this. I guess you won't study, so I will continue to say the same thing over and over again in your 2 posts until you get it.

There is 2 types of 3 phase in the US that are close. 208/120 is 208 between phases. If I use any 2 wires, I will have single phase 208.
The other type is 120/240. It has 240 vac between phases. If I use any 2 wires, I will have single phase 240 VAC.

Magdyfayed: I want you to find a drawing of a DC generator. It will have one coil. It will have a magnet with a north and south pole turning on a shaft. I want you to explain the position of the magnet in relation to the coil at the 2 peaks of the sine wave and the zero crossing.

In a US house measuring line to line is 240, line to neutral is 120. Neutral is a grounded current carrying conductor. See attached.


I'm not a power distribution guy, but I believe the 2300 volts shown is usually from line to ground on a Y three phase connection, hence it is only single phase. Those who know better please correct me.

yes , i am also not power engineer and the question which keithkyll is difficult to me as i do not understand the question , then how can i answer ??


also i think as Tom Jenkins said and as skipfast drew the 2300 volts shown is usually from line to ground on a Y three phase connection , ans also may be i am wrong !!

magdyfayad said:

yes , i am also not power engineer and the question which keithkyll is difficult to me as i do not understand the question , then how can i answer ??

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How can you understand the answer if you don't understand the question? I am not being difficult. I have answered the question several different ways, yet you still don't understand. That is because you don't know how a generator works. It is very simple. I will not tell you. You can find the information on the internet.

magdyfayad said:

also i think as Tom Jenkins said and as skipfast drew the 2300 volts shown is usually from line to ground on a Y three phase connection , and also maybe i am wrong !!

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There is no ground on the primary. It is shown as theoretical (dashed line marked A). The ground on the secondary is real. It is primarily for safety. Main reason is to protect you from 2300 volts if a primary to secondary short develops.

thanks

i know you try to help me , but i must do something and study what do you say , so i will study the basics of the generator ,

may be it is the all answers for my all questions !!

thanks

I have always thought that the reason three-phase systems became so ever widespread and popular was a very simple one: this is the easiest and the cheapest way to achieve smooth rotating motion. A few things are simpler that a three-phase AC induction asynchronous motor: a "squirrel-cage" rotor and three stator windings at 120 degrees from each other; no brushes, no slip rings. It works.

Could it have been four, five or six phases? Sure it could - and the rotation would be much smoother. But would it be worth the added costs? I don't think so. Could it be two phases? Hmmm... I don't think one could achieve smooth rotation with two magnetic fluxes generated by currents at 180 degrees apart... in essence, in couneterphase.

Yes, the voltages in two line wires of the much-discussed split-winding circuit (aka "Edison circuit") are shifted 180 degrees apart so technically one may call it a "two-phase" system. However, nobody does because there are no loads that need both the lines and the neutral at the same time: it is always either two line wires (240V) or a line and a neutral (120V).

Maddy

Dont worry how the alternator is wound - it makes absolutely no difference with respect to your question (IF I understand it)
on how do you get single phase from three phase.
Just concentrate on 3 phase no neutral no ground just three wires.

Phase to phase voltage on the line is 240 volt
ie A to B B to C and A to C.
Now then to derive single phase pick any two let us say A to B --
you will have 240.

When you have this understood then we will get a little more confusing and do star and delta. After that we will get into neutrals.

Dan Bentler

thanks all

after discuusions , i want to write the results which i reached to it , and i want any one to comment to each one if my results need comments :

the most generator stations at USA produce three phase voltage delta connection A , B , and , C , but little generator stations at USA produce three phase voltage star connection A , B , , C , and N .


first for most generator stations at USA produce three phase voltage delta connection A , B , and C , :

1- for some regions , it takes two hot wires come from A, B as the voltage between them 240 VAC

2- for another regions , it takes two hot wires come from A, C as the voltage between them 240 VAC


3- for another regions , it takes two hot wires come from B, C as the voltage between them 240 VAC


4- the two hot wires come from A, B are in phase i.e : single phase

5-the two hot wires come from A, C are in phase i.e : another single phase

6- the two hot wires come from B, C are in phase i.e : another single phase

7- the angle betwen any two phases is 120 as for between the phase consistos of pole A , pole B and the phase consistos of pole A , pole C , is 120 and so on

8 - if we applied the two hot wires come from A, B which are in phase i.e : single phase - as the voltage between them 240 VAC - to the primary of a transformer , and the secondary consists of three wires ( two hot wires and a nuteral ) , then the voltage between the two hot wires at the secondary is also 240 VAC and also in phase i.e also single phase , but there is a difference of the voltage ampliude by 180 for the two hot wires at the secondary as when any hot wire is 120 vac with respect to nuteral , then the another hot wire is - 120 vac with respect to nuteral and vice versa , as at any instance the voltage between the two hot wires must be in phase and fixed all the time at 240 VAC


second for little generator stations at USA produce three phase voltage star connection A , B , C and N ( nuteral ) :

9- then we did not need any transformer to obtain the N ( nuteral ) as the N ( nuteral ) is already obtained from the generator station

10- the two hot wires come from A, B are not in phase as they are two phases and the voltage between A, B is : 208 VAC


11- the two hot wires come from A, C are not in phase as they are two phases and the voltage between A, C is : 208 VAC

12- the two hot wires come from B, C are not in phase as they are two phases and the voltage between B, C is : 208 VAC

13- any hot wire and the nuteral are in phase - single phase - and the voltage between them is 120 VAC

14 - so the staement which keithkyll said it which is " There is 2 types of 3 phase in the US that are close. 208/120 is 208 between phases. If I use any 2 wires, I will have single phase 208.The other type is 120/240. It has 240 vac between phases. If I use any 2 wires, I will have single phase 240 VAC."

i think it must must be modified said as keithkyll said " I will have single phase 208 " as i think the correct to be " I will have two phase 208 " , my modifaction is right or not ??



i hope these final results which i reached all are correct !!

wait for any comment to obtain the good understand

thanks all

NOPE

Utilities do not use a neutral. Just 3 phases each 120 apart. all ungrounded. Utility distribution can be anywhere from 2300 V (old systems) 12.5 KV (current) to 230 KV.

Industrial distribution may or may not have a neutral.
Do not worry about a neutral for now. Get a good understanding of just 3 phase. Join the US Navy if you have to - they use nothing but ungrounded 3 phase.
Go back to the message I gave you that showed a math calculation. DO that and see if that makes sense.

Dan Bentler

I don't know if this is any use but its my attempt to help.

Here are two sine waves one for three phase and the second is single phase using the same B and C phases of earlier drawings.

If you draw lines between the purple dots that is when the voltages are at there peaks
On 3 phase you get 6 lines at 60 degree spacings
On single phase you get 2 lines at 180 degree spacing

Neutral occurs as the middle of both drawings at 0 .

thanks leitmotif , Gil47 , and keithkyll

i must return again to message which leitmotif said

but are all the 14 statements which i wrote at my last post right ?? , or something right and something wrong , and why ??


and also a question why at figure c obtain the nuetral by splitting the phase which still connected to delta connection ??, as i think to obtain the nuetral it must take the phase from delta connection ( two hot wires in phase ) without any splitting and applied the two hot wires to the primary of a transformer , and the output of this transformer then contain the nuetral , so i asked what is the meaning of take the nuetral by this method ??


thanks all

Everything in electronics is dealt with 1 thing at a time. 3 phase is explained 1 phase at a time.
A power source has 2 wires - supply and return. It can only be 1 single phase. It is impossible for it to be 2 phases.
A generator has a coil of wire. There is 2 wires - The beginning of the winding, and the end. Start and Finish. That's 2 wires - a single phase.
A 3 phase source is 3 of these coils. Each of the 3 generator coils generates a sine wave. The 3 generators are mechanical ganged together one third of the rotation apart. This creates 3 sine waves, each 120 degrees out of phase with each other.
I have 3 windings. That means 6 wires. I connect them in a Wye connection. The 3 Starts (or Finish) are connected together for the Neutral. I still have 6 wires, but you don't see them. I still have 3 separate circuits, each with 2 wires - supply and return.
If I need some power, I take 2 wires. It doesn't matter where these 2 wires come from. One is supply, the other return. They can be from 3 phase, single phase, or a hot and neutral. I don't care. I can only have ONE SINGLE PHASE between 2 wires.

Now for the confusing part. From any phase to neutral, I have 120. If I use 2 hots, they are 120 degrees out of phase with each other. If the phases were 180 degrees apart, I would get 240 - 120 from each coil. Because I am using 2 hots 120 degrees apart (no neutral), I must use a formula to calculate the voltage. We use the square root of 3. 120 * SQRT(3) = 208.
Don't let that confuse you. We still only have 2 wires, therefore we only have one single phase. If I look at it on a scope, I will see one sine wave.
Study the posts from others. They are accurate. Study how a magnet generates a sine wave, i.e.; study how a generator works (for the third time!). Then you will understand.
Statements 1-7 are correct. Statements 8-14 must be rewritten. The modification is not correct, because there is no such thing as '2 phase 208'.
(Some may argue that we can have 2 phase AC. I agree, but not in this discussion. One example of 2 phase is a syncronous motor. 2 coils, 3 wires, reversable. Second phase from a cap).

magdyfayad said:

...why at figure c obtain the neutral by splitting the phase which still connected to delta connection ??

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The phase on the secondary is not connected to the delta connection. The delta is on the primary. The secondary generates it's own phase from the magnetism in the iron of the transformer.

magdyfayad said:

, as i think to obtain the neutral it must take the phase from delta connection ( two hot wires in phase ) without any splitting and applied the two hot wires to the primary of a transformer , and the output of this transformer then contain the neutral , so i asked what is the meaning of take the neutral by this method ??

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The neutral is a reference - the Earth. Neutrals do not pass through transformers. Hots don't either.
You must first understand how to make a magnet with electricity, how a generator uses a magnet to make electricity, and how a transformer uses magnetism to do it's job. Without that foundation, you will never understand.

If you are asking about household and office voltage in the US you should take the diagrams in your post #23 and burn them. They are three phase diagrams and are only pertinent to power generation, distribution, and large industrial uses.

Once the power leaves the utility's pole transformer the three phase generation and distribution has absolutely nothing to do with the power feed to the home or office. Only one phase is used at the utility pole transformer for each power feed, and only one phase is used in the home.

In the US most households use single phase three wire service. Line to line is 240 VAC, line to the grounded neutral wire is 120 VAC.

magdyfayad said:

14 - so the staement which keithkyll said it which is " There is 2 types of 3 phase in the US that are close. 208/120 is 208 between phases. If I use any 2 wires, I will have single phase 208.The other type is 120/240. It has 240 vac between phases. If I use any 2 wires, I will have single phase 240 VAC."

i think it must must be modified said as keithkyll said " I will have single phase 208 " as i think the correct to be " I will have two phase 208 " , my modifaction is right or not ??

Click to expand...

For the most part you are correct. Stop thinking of what the generators produce, it is not important. Most utilities in the US generate at 3-phase voltages between 2400V and 13.8kV. These generator voltages are then transformed up to 69kV-230kV (or higher) and then transmitted around the country. The transmission voltages are then transformed down to distribution levels of 5kV to 42kV. During all of this it does not matter if the transformers are delta or wye connected.

The distribution voltages are then stepped down to the typical utilization voltages of 120/240 1-phase 3-wire, 240/120 3-phase 4-wire, 208Y/120 3-phase 4-wire, 480V 3-phase 3-wire, or 480Y/277 3-phase 4-wire. It is only the secondary connection of the final transformer that makes a difference if it is delta or wye(star) connected.

A typical residence is fed using a single transformer with a single primary winding therefore its output is always single phase even if the secondary has a center tap (after all there is only one magnetic circuit).

Now the confusing part, the circuit that is used can be different from the system that feeds it. This is why it is important to remember that a phase voltage (measured from a hot line to hot line) is not the same thing as a phase conductor (a wire connected to a reference point). If your circuit has one phase voltage then it is a single phase circuit.

So, if your circuit has conductors from Phase A and from Phase C it is single phase regardless if the circuit contains a neutral. This is why we can take two phase conductors from a 208Y/120 system and still call it a 208/120 single phase circuit (although some people call this a network circuit instead).