Sorta of a brainer teaser

Status
Not open for further replies.
But Jim , remember if you measure across any two lines of a three phase 208/120 system you will get 208 not 240 we are not connecting the neutrals. :)
 
I would think that for the diagram in figure #29 the Line to X0 point would be 104V. I see this as basically being a voltage divider with 208V being the input voltage.

Do we need to take any sort of phase shift (30deg) into account due to the offset of these two windings?
 
ronaldrc said:
But Jim , remember if you measure across any two lines of a three phase 208/120 system you will get 208 not 240 we are not connecting the neutrals. :)
Click to expand...

But if there is no input neutral, then as far as the circuit is concerned, the two transformer windings are in series. Two equal impedances in series will see 50% of the applied voltage across each of them. 208V applied will yield 104V across each winding which means the output will be 104/208 single phase 3-wire.
 
Jim my apology

I read your response wrong , I thought you implied there would be 120 volts developed across each 120 volt winding. You know how I half way read by now surely? :)
 
winnie said:
I think that a harder question is 'what happens with a 120V supply', such as in the diagram in post #1.

-Jon
Click to expand...

Why would it make a difference if the voltage was 120-volts from a single-phase source or 120-volts from a 3-phase source. The coil only see 120-volts and there is no other reference to the source.
 
I agree, with the new #29 post diagram and no connection of the neutrals, the voltage would be 104 V on each energized winding.

Now, if you shorted one energized winding, what would the current be? I say zero (in an ideal transformer). The voltage would be zero on the shorted winding and 208 V on the unshorted winding. Any current in the secondary winding would have to have current in the corresponding primary (delta connected) winding. Since there is one secondary winding that is open, it can have no current in it. Thus the corresponding primary winding can have no current. The only current that can flow in the primary would circulate in all three windings, so no current can flow in any winding.
 
jghrist said:
Now, if you shorted one energized winding, what would the current be? I say zero (in an ideal transformer).
Click to expand...
If you were to short any energized winding, the resultant current would be high enough to trip the OCPD. A low load impedance is reflected back to the source.
 
Hello I appreciate all the response on this.

Lets discuss the first diagram I posted.

When we test the voltage on a 208/120 volt single lamination three phase transformer from any ungrounded terminal leg Line #1,2 or 3
we get 120 volts to XO. This tells me the only different in this transformer and a single phase is that it has two more windings and cores
although all on the same lamination.

So this to me says that the winding turns are set at a ratio of a 120 volts.And the reason we get 208 instead of 240 is because of the
interactions of the three phase source voltages being offset at 120 degrees. This in reality would mean that that voltage developed in the Wye three phase would be subtractive by 16 volts else the voltage would be 240 volts.

That said I think this three phase transformer would act exactly like a single phase transformer on a single phase or on two legs of a three phase system. 240/120 or 208/104 respectively.

Thanks :)
 
ronaldrc said:
Hello I appreciate all the response on this.

Lets discuss the first diagram I posted.

When we test the voltage on a 208/120 volt single lamination three phase transformer from any ungrounded terminal leg Line #1,2 or 3
we get 120 volts to XO. This tells me the only different in this transformer and a single phase is that it has two more windings and cores
although all on the same lamination.

So this to me says that the winding turns are set at a ratio of a 120 volts.And the reason we get 208 instead of 240 is because of the
interactions of the three phase source voltages being offset at 120 degrees. This in reality would mean that that voltage developed in the Wye three phase would be subtractive by 16 volts else the voltage would be 240 volts.

That said I think this three phase transformer would act exactly like a single phase transformer on a single phase or on two legs of a three phase system. 240/120 or 208/104 respectively.

Thanks :)
Click to expand...

For the single-phase, yes 240/120. From 2/3 of a wye I believe it depends on what you do with XO. Ground it and you have 208/120, leave it ungrounded and you have 208/104. That's the way I understand it.
 
More teasing:

More teasing:

ronaldrc said:
Here is the circuit instead of a single phase 120/240 volt transformer. :)



three_phase_example.jpg
Click to expand...

Ronald, let this tease your brain:

If you connect X0, you will have 120V on all three secondaries and 480V on all three primaries.

If you leave X0 open, you will have 104V on two of the secondaries, then you will have 416V on two of the primaries, but you will have 832V on the third primary. Then you will have 208V on the open primary--in an ideal transformer. With a real transformer you cannot tell because the smoke would be too thick!
 
Quick side-note: A 3-ph transformer should really be represented as three individual units, and not three sets of windings on a single core.
 
Correction:

Correction:

rattus said:
Ronald, let this tease your brain:

If you connect X0, you will have 120V on all three secondaries and 480V on all three primaries.

If you leave X0 open, you will have 104V on two of the secondaries, then you will have 416V on two of the primaries, but you will have 832V on the third primary. Then you will have 208V on the open primary--in an ideal transformer. With a real transformer you cannot tell because the smoke would be too thick!
Click to expand...

Not quite! The third primary will see zero volts because the induced voltages on the other two primaries cancel each other.
 
Rattus

When you say leave XO open do you mean take the three connections of the 120 windings loose at XO?


Larry

Aren't most the dry type Transformers use to convert 480/277 to 208/120 of the single lamination type?


Not being an expert in transformers I don't know how much magnetic isolation there is between the phases
this could have an effect on the difference between a single and a three phase being used the way I show them?


Ronald :)
 
hardworkingstiff said:
For the single-phase, yes 240/120. From 2/3 of a wye I believe it depends on what you do with XO. Ground it and you have 208/120, leave it ungrounded and you have 208/104. That's the way I understand it.
Click to expand...

Now, just to be picky. The ANSI/IEEE convention for naming system voltages is:

Multi-wire grounded (bonded) single phase - list the L-G voltage first, then a forward slash and then the L-L voltage: 120/240 or in this case 104/208.

Multi-wire grounded (bonded) three phase - list the L-L voltage first, then the letter Y if applicable, then a forward slash, and finally the L-G voltage: 208Y/120, 480Y/277, and 240/120.

I have spent 30yrs trying to unlearn the bad habit of saying 120/208V. Now I try (but often fail) to include the number of phases and wires, as I find it does reduce confusion, for example: 120/240V 1PH3W and 240/120 3PH 4W
 
Rattus and Larry

Would the third unhooked phase even matter since it is a open circuit?

Now I'm wondering how much difference there would be between using a single and three phase transformer, on the single phase the primary and secondary are on the same core itself, on the three phase it is all one piece of laminated steel but still a different core so to speak. :confused:
 
Jim

You mean this ain"t right 240\120, 208\120, 208\104 or 208\480?

It would be confusing I guess but us old timers know what we mean. :)
 
ronaldrc said:
Larry

Aren't most the dry type Transformers use to convert 480/277 to 208/120 of the single lamination type?

Not being an expert in transformers I don't know how much magnetic isolation there is between the phases
this could have an effect on the difference between a single and a three phase being used the way I show them?
Click to expand...
If the three primary-secondary pairs were genuinely wound on a single core, there would be no wat to have three separate phase timing waveforms; they'd be locked into a single sine wave.

A 3-phase transformer has three separate cores bolted in to a single frame, but the three winding pairs are not electromagnetically coupled to each other. They are effectively three individual units.

Here's why it matters (and please remember this is from my grasp of how things work, and not formal education):

Two 120v windings in series will function fine as a voltage divider if on a single core, because the electromagnetic function locks them together in the voltage-per-turn ratio sense. That's how center-tapped secondaries survive load imbalances. If fed 240v, the output would be 120/240v; if fed 208v, the output would be 104/208v, as expected.

Two 120v windings on separate cores in series would suffer the same issues as an open-neutral condition in a service: the voltages on each side of the 'neutral' would vary with load shifts. The volts:turn ratio would not be locked between them. With only line-to-line loads, the center tap would measure as expected.

Now, if we connect the transformers' 'neutral' point to the source's neutral, we'd better make sure the single-core/separate-core situation matches the source. If we connected two windings on a single core two a 208/120v source, all kinds of evil will happen. Two separate transformers (e.g., 2/3 of a 3ph unit) are required.

If the source is single phase, then the connection to the source neutral is "optional" with a single-core-sharing pair of windings, but mandatory if there are separate cores and you want the center-tap's voltage to be stable, and not floating as I described above.

So, using the 2/3 of a 3ph unit, you'd measure 104v with the neutral floating, and 120v with the neutral connected. The former would be useless as a source, but the latter would basically be in parallel with the 2/3 of the source power supply. The circuit impedance would be all that keeps transformer differences from creating more evil.


So, how'd I do? :smile:
 
ronaldrc said:
Rattus and Larry

Would the third unhooked phase even matter since it is a open circuit?
Click to expand...
To answer this, I'd say that, in theory, no useful voltage would be developed on the third secondary at all, and the same for the corresponding unconnected primary. The other two primaries should develop their design voltage (minus a bit of VD-compensation, perhaps.)
 
Status
Not open for further replies.
Top