Sorta of a brainer teaser

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jim dungar said:
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.
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Thank you.
 
Larry

We see transformers a little differently. All the dry type transformers I have hooked or worked on up to a 112.5 KVA. have a single piece lamination in three tiers primary and secondary on same tier, phase A on one phase B on one and phase C on one.

The magnetic fields do not effect each other that much or enough to do what you said.The magnetic flux just embodies the coil of wire on that tier.

Since each phase is being driven from different sources or alternator pole at the power plant at a different time 120 electrical degrees apart they can't be synchronized or thats the way I see it.

Ronald :)
 
LarryFine said:
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.
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Current in each 480V winding is 1/4 the current in the corresponding 120V winding.

There cannot be any current in the 120V winding that is not connected, therefore there cannot be any current in the corresponding 480V winding.

The 480V windings are connected in a delta with nothing connected to the terminals, therefore the current in all windings is identical - any current just circulates around the delta.

Since there cannot be any current in the 480V winding corresponding to the unconnected 120V winding, there cannot be any current in any of the 480V windings.

Since there is no current in any of the 480V windings, there can be no current in any of the 120V windings.
 
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!
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Ok rattus

If you leave XO open I would have 208 on the two secondaries because thats what the source voltage feeding it is.

I think you would have 416 on one primary and am not sure about the other two.

I don't understand where I would get 832 on the third primary.

And you've talked to me long enough to know I know all there is to know about letting the smoke out of anything electrical. :D
 
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ronaldrc said:
The magnetic fields do not effect each other that much or enough to do what you said.The magnetic flux just embodies the coil of wire on that tier.
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That is what I was talking about. A single 3-phase transformer behaves as three 1-phase units, and not as a single core with three primary and three secondary windings.

Since each phase is being driven from different sources or alternator pole at the power plant at a different time 120 electrical degrees apart they can't be synchronized or thats the way I see it.
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Agreed again. You can't couple different phases into a single transformer.
 
ronaldrc said:
Ok rattus

If you leave XO open I would have 208 on the two secondaries because thats what the source voltage feeding it is.

I think you would have 416 on one primary and am not sure about the other two.

I don't understand where I would get 832 on the third primary.

And you've talked to me long enough to know I know all there is to know about letting the smoke out of anything electrical. :D
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Ronald, I thought I had corrected this already, but maybe not.

In effect you are dealing with three individual transformers, and you are backfeeding two of the transformers, therefore, 416 volts are induced in each of the two primaries.

But the phasing of these primary voltages is such that they cancel each other, and the third primary and the open secondary see zero voltage. Draw in the polarity dots to see what I mean.

You do not have the autotransformer effect as in a CT single phase secondary.
 
Rattus

When I thought about those voltages last night I wasn't thinking in terms of the 3 phase.
After I posted I thought about where you might be getting that 832 volts on the normally primary or high side. Your thinking it would work like an auto transformer? But with it tied together in a delta, I'm not really sure what the voltage would be but I don't think it would be that high. And you might be right it might let the smoke out of it. :D

Is that what you were refering to? :)
 
ronaldrc said:
Rattus

When I thought about those voltages last night I wasn't thinking in terms of the 3 phase.
After I posted I thought about where you might be getting that 832 volts on the normally primary or high side. Your thinking it would work like an auto transformer? But with it tied together in a delta, I'm not really sure what the voltage would be but I don't think it would be that high. And you might be right it might let the smoke out of it. :D

Is that what you were refering to? :)
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Ronald, with X0 unconnected, you have a single phase problem, but you cannot have an autotransformer because the transformers behave as three separate transformers. There is no common iron.

The 208V sees two magnetizing inductances in series which causes a 104V drop across each of the two secondaries. Then, there must be 416V on each corresponding primary.

However, and this is key, the phasor sum of these two induced voltages is zero because they are 180 degrees out of phase so to speak.

Therefore, the third primary sees zero volts. No smoke. My first comment about the smoke is wrong!
 
rattus said:
The 208V sees two magnetizing inductances in series which causes a 104V drop across each of the two secondaries. Then, there must be 416V on each corresponding primary.
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So what we are saying is that with X0 not grounded we will see 104V across each coil. However is X0 is grounded we will see 102V across each winding because were are reading the system voltage of 208V to ground?
 
mull982 said:
However is X0 is grounded we will see 102V across each winding because were are reading the system voltage of 208V to ground?
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If you meant 120v, then yes and no. Yes to 120v, but no to 208v. It's 208v line-to-line, 120v line-to-neutral, just like the source.
 
Furthermore:

Furthermore:

Larry must have been there and done that.

If we connect X0 to the neutral, we see 120V to neutral on all three secondaries. We also see 480V on all three primaries, and the phasing is that of a normal delta.
 
rattus said:
If we connect X0 to the neutral, we see 120V to neutral on all three secondaries. We also see 480V on all three primaries, and the phasing is that of a normal delta.
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Not if we're talking about using only two lines and the neutral. Nothing would be energizing the third transformer section (small induction aside).
 
Aw contrary, Larry:

Aw contrary, Larry:

LarryFine said:
Not if we're talking about using only two lines and the neutral. Nothing would be energizing the third transformer section (small induction aside).
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Larry, think open delta, and let the third xfrmr be a load on the open side. The third primary is energized by the 480V from the other two. Then 120 volts are induced in its open secondary.
 
rattus said:
Larry, think open delta, and let the third xfrmr be a load on the open side. The third primary is energized by the 480V from the other two. Then 120 volts are induced in its open secondary.
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I was thinking 120/208, since that's where we started. The source was two lines and the neutral, and we were discussing the effect of connecting the neutral to the source.

I maintain that a 3ph xfmr, which is not a single-core device, fed from such a source would have no real power induced in the third, unconnected phase.
 
LarryFine said:
I was thinking 120/208, since that's where we started. The source was two lines and the neutral, and we were discussing the effect of connecting the neutral to the source.

I maintain that a 3ph xfmr, which is not a single-core device, fed from such a source would have no real power induced in the third, unconnected phase.
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Ignore the unconnected winding for a minute. The 208Y/120V secondary is connected open wye, two phases and a neutral. The 480V primary, without the winding associated with the unconnected secondary, is an open delta connection.

Now, consider the third delta winding to be connected across the open delta - 480V. This 480V induces 120V in the unconnected 120V winding.
 
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