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

I wonder if he is spending to much time worrying about the voltages instead of thinking about what the transformer is actually doing. As previously stated, make it simple, a 3 phase transformer is nothing more than 3 single phase transformers configured in either wye or delta. You can have either scenerio with a neutral or not. It all depends on what you need. All of our Motor control centers are 480 3 wire. No neutral. We have a 480/277 lighting panel, There is a neutral. But essentially it is the same setup. We just need a neutral for the 277 lighting load. Keith and Jim have given wonderful explanations.

thanks all


the problem is that the basics of this form are not clear to me , so my statements are not stable !!

if i study the basics well as keithkyll said , may be i have the answer

thanks keithkyll , thanks leitmotif , and thanks Jim Dungar

I'm not a power engineer, but here is my explanation of transformers, it may or may not A transformer is two coils, around a core, transferring energy to the other proportional to the number of turns. Three Phase transformers are just 3 coils, tied together, keep in mind, that your pictures above are only one side of the transformer. Most likely the secondary. Now, without a neutral, both sides of the secondary can be hot or have voltage potential with respect to ground. We can reference (force), one line of a circuit to ground, making it neutral. Thus limiting the potential shock hazard, to the grounded operator/ technician. Like on a 480/120 transformer, one line is grounded, to ensure that it is 0 volts, and the other line is 120V referenced to ground. Likewise, if we have a center tapped transformer, we can ground the center tap, making it neutral. We will then have half the voltage from the center tap, to either end of that coil with the center tap 0V to ground. Just like a potentiometer, if we measure half way across a coil, or resistor, we will have half the total voltage.
-Mur

You can't have more than one phase on an iron core. A 3 phase transformer is 3 single phase transformers in one box.
The main reason for a earth ground (neutral) is safety. For instance, you have a 440 primary, and 24 secondary. What happens if you get a primary-secondary short? 440V on you 24V (safety!) circuit. The second thing is capacitive and resistive coupling. Even if you don't have a primary-secondary short, the still is some leakage. The floating 24VAC winding will measure 440 to ground. The third is scientific. We are pushing electrons around. They need to come from somewhere. The Earth is a source of electrons.In actual practice, there's enough electrons in the copper.

magdyfayad said:

thanks all


the problem is that the basics of this form are not clear to me , so my statements are not stable !!

if i study the basics well as keithkyll said , may be i have the answer

thanks keithkyll , thanks leitmotif , and thanks Jim Dungar

Click to expand...

Maddy
Stop thinking neutral instead think "common".
PROJECT take any two batteries of equal voltage call em A and B
Wire negative of A to positive of B. Strip i cm insulation from this conductor. This is a common. Now measure
Positive of A to negative of B
Positive of A to common
Negative of B to common.

NOw take two litebulbs of equal voltage and wattage. Connect one side of a litebulb to A Pos. Connect one side of other lite to B Neg. Connect the two unconnected sides of the lites together and connect that to the common.

Turn both on
How much current
does lite A draw
does lite B draw
what is current in neutral OOPS now you have me doing it -- COMMON.

Turn one off
What is current in lite that is lit
What is current in common ---- WHY?

Does current in common differ with two lites than with only one
WHY?

ASSIGNMENT
Draw the current path for only one lite lit.
Draw teh current path for only the other lite lit.
Draw the current paths for lites A and B when both are on. Assume that the current for both goes thru the common. Use different colors.

Understanding this will go a long ways to understanding centertapped transformer windings. To rationalize AC it is easier to use intantaneous voltage and current. That is why I have you simulate using batteries.

Now then for further study
GIVEN 4 wire 3 phase Wye grounded neutral
What is worse getting shocked by one phase to ground
OR across 277.

GIVEN
Edison 3 wire 240 line to line and 120 either line to neut.

What is worse one leg of 240 to ground
OR acrosss 120?

Dan Bentler

thanks very much leitmotif


as i told before i am not electerical power engineer , so the basics are not claer to me , so i must understand the basics well

thanks all

i read more and more what keithkyll and jim dungar said

i think the problem for me i donot differ between the phase conductor and the phase voltage

so i can say - and i hope there is the right - any voltage source must be single phase regardless the two wires of it the both are hot i,e : the two wires are two phase conductors or the two wires one of them is hot and the another is a neutral

so if i take R ( phase conductor )and S ( phase conductor )- either from star connection or delta connection - and applied R and S to a load , i must say it is a single phase voltage source - not say two phases voltage source - but with two phase conductors or with two hot wires

also if i take R ( phase conductor ), S ( phase conductor ) and T ( phase conductor ), - either from star connection or delta connection - and applied R , S , and T to a three phase load , i must say it is three single phase voltage sources - not say three phases voltage source - with three phase conductors or with three hot wires

i hope this result is correct !!

thanks all

CORRECT! (almost)

"i must say it is three single phase voltage sources - not say three phases voltage source"

You can say three phase voltage source. Three single phase voltage source doesn't sound right to me.

You are correct. Also, you get 100% for spelling. Your English grammar is almost perfect! I am impressed.

I believe you got it maddy.

Yes, you now have the basic concepts. Leave the exact details to the electrical engineers.

thanks all

but i want to know why did sliver say : " You can say three phase voltage source. Three single phase voltage source doesn't sound right to me." ???

i am an electerical engineer , but not power so i have not the basics of the generators and the tranformers , so i learned from all some basics for the three phase

as i am an electronnic enginner , but i have not worked at electronics ,
as i have worked at automatic control and PLCs .

thanks

In reality you have 3 single phase voltage sources, but in the states we call it 3 phase. No big deal, just a name. Don't let that distract you. You have the concept.

The only issue is English grammar. You can't say the words 'single' and 'three' in the same sentence. It's either 'three phase' or 'single phase'.

and also , at Egypt we call it three phases voltage source !!!


but as keithkyll and Jim Dungar said there is no two phases voltage source and there is no three phases voltage source , as any voltage source must be single phase because it consists of two wires and the current passes through the first wire is equal to - in value and in angle - the current passes through the second wire

i know it is as you said " just a name " and as i said to you we also said that at Egypt !!

but which i learnt for this disccusion it is wrong to say two phases voltage source or three phases voltage source ,
but the correct to say one single phase voltage source or two single phase voltage sources or three single phase voltage sources

thanks all