Transformer question

[ggunn]

The new neutral point, lets call it N' between the 240V terminations would need to be grounded per the NEC. However, as shown above, this neutral point is not electrically the same as the grounded neutral point of the 208Y/120V source supplying the buck-boost transformer. The 60V difference will cause problems, regardless if it is from a center tapped winding or the junction of two windings.

We use 208V delta to 480/277V wye with grounded neutral transformers all the time; what's the difference? Other than the obvious three phases, of course, it seems analogous to Wayne's top drawing to me.

 

[jim dungar]

We use 208V delta to 480/277V wye with grounded neutral transformers all the time; what's the difference? Other than the obvious three phases, of course, it seems analogous to Wayne's top drawing to me.

The difference is the isolation provided by a two wire.

Way back when Sorgel Transformers, eventually bought by Square D, published a Dry Type Transformer Study Course book which discussed Larry's connection method in detail and provided the vector diagrams (see part XXXII).
If the system neutral N or the boosted neutral N' were not bonded together via ground, the connection would work fine.
If you look at just the (2) 208V legs used, you see that there is 120V between them and N but 104V between them and N', therefore connecting both points together will cause a short circuit.

 

[LarryFine]

So, my single BB transformer idea will work for 240v loads, but not for 120/240v loads?

 

[jim dungar]

So, my single BB transformer idea will work for 240v loads, but not for 120/240v loads?

Yes.
The problem is bonding the boosted neutral to ground which connects it electrically to the system neutral.
Another suggestion would be to only boost to about 230V and then pull a system neutral to your load, your L-N voltage at the load of about 130V which might be tolerable to the end use equipment.

In either case, some day a troubleshooter will question their 240V L-G voltage readings.

 

[LarryFine]

The problem is bonding the boosted neutral to ground which connects it electrically to the system neutral.

But, as an auto-transformer, the output conductors would not be isolated from the supply, so it's not really floating.

And I would think you could use the secondary H2-H3 point as a load neutral, without tying it to the source neutral.

Sort of a hybrid not-quite-isolated, yet not-quite-bonded power source.

 

[jim dungar]

But, as an auto-transformer, the output conductors would not be isolated from the supply, so it's not really floating.

And I would think you could use the secondary H2-H3 point as a load neutral, without tying it to the source neutral.

You might, if you didn't have to bond it to ground per the NEC 250.20(B).

 

[wwhitney]

The new neutral point, lets call it N' between the 240V terminations would need to be grounded per the NEC. However, as shown above, this neutral point is not electrically the same as the grounded neutral point of the 208Y/120V source supplying the buck-boost transformer. The 60V difference will cause problems, regardless if it is from a center tapped winding or the junction of two windings.

Agreed for the first diagram I drew. But the question was if you take that first diagram, and add a connection from N' to N (the 208Y/120V neutral), does something bad happen? Or does it just switch the system to the second diagram I drew?

Cheers, Wayne

 

[jim dungar]

Agreed for the first diagram I drew. But the question was if you take that first diagram, and add a connection from N' to N (the 208Y/120V neutral), does something bad happen? Or does it just switch the system to the second diagram I drew?

Cheers, Wayne

It turns it into the second diagram. There is a voltage difference between your 208V and your 240V points, therefore the laws of physics won't allow both of these points to be 120V to any common reference point.

 

[wwhitney]

It turns it into the second diagram. There is a voltage difference between your 208V and your 240V points, therefore the laws of physics won't allow both of these points to be 120V to any common reference point.

Sure, I understand the last part. The question was whether there was some physics that required the coil to be a straight line on the voltage phase space diagram along its whole length, or just between connection points. There was a suggestion it had to be the former, but I was expecting the latter.

Cheers, Wayne

 

[gar]

220209-2018 EST

wwhitney:

The problem you have with your analysis is that you do not understand the operation of a transformer with a closely coupled primary and secondary.

A center tapped winding on a common high permeability core magnetic material has very tight magnetic linkage between the two parts of the winding. Doesn't matter whether this is two separate coils or a center tapped coil. If you excite 1/2 half of that coil. and look at the other half you will see an almost identical voltage waveform. If you put a fairly heavy load on the unexcited coil this will not much change the output voltage waveform. Now remove that load, and apply from a synchronized voltage source a voltage of equal magnitude, but shifted in phase by 120 degrees, and you will produce a very large current flow limited only by the leakage inductance and internal resistance of the transformer, and the voltage difference of the two phases. This might be many times the full load rating of the transformer. Burnout is likely.

See if you can visualize this problem. If not try an experiment.

.

 

[LarryFine]

The question was whether there was some physics that required the coil to be a straight line on the voltage phase space diagram along its whole length, or just between connection points. There was a suggestion it had to be the former, but I was expecting the latter.

Regardless of the quantity of windings on a single core, they all must be in phase.

You can not electromagnetically "bend" a single core to accommodate energization of separate windings form sources with differing timing. All inputs and outputs will be in phase with each other, so the vector is straight.

That's why a dual-voltage primary must be supplied by a single circuit, whether wired in series or parallel, and why a 3ph transformer has three cores, or core legs, with each primary/secondary pair sharing a separate leg.

 

[gar]

220209-2117 EST

whitney:

Do you know how to describe an equivalent circuit for a transformer?

One of the simplest for a single phase 1 to 1 transformer used at power line frequencies is simply an inductor ( leakage inductance ), and a resistance. Connect a series inductance, and resistance between two voltage sources ( V sin wt and V sin (wt - A) ) and calculate the current.

.

 

[kwired]

We use 208V delta to 480/277V wye with grounded neutral transformers all the time; what's the difference? Other than the obvious three phases, of course, it seems analogous to Wayne's top drawing to me.

Those are separately derived systems, only one common point, after bonding them together, normally that point will be the point that is grounded, especially when NEC comes into play. With only one common point there is no short circuit created. The autotransformer drawn to attempt to get 120/240 from 208/120 has more than one point of each "system" bonded to each other creating a current paths.

 

[synchro]

If there is access to a third phase "C" from the 208/120V transformer, below shows how you could get 236V L-L and 123V L-N from two 240V/32V buck boost transformers. As shown in green color, one buck-boost is fed by the 208V across A-C, and it adds its 28V output to the B-N voltage at a 90° relative phase shift . And as shown in red, the other buck-boost is fed by 208V from B-C, and it adds its 28V output to the A-N voltage at an opposite 90° phase shift.

 

[wwhitney]

The problem you have with your analysis is that you do not understand the operation of a transformer with a closely coupled primary and secondary.

Absolutely, I've been thinking only in terms of coils, without paying attention to how they are wound around a common core (or not).

Regardless of the quantity of windings on a single core, they all must be in phase.

OK, so to expand on that, what is the key detail that forces them to be in phase? Is it that the coils are interwound and the spatial extent of each coil (along the central axis) is the same? I.e. if you had a single iron core, but some coils are wound only around half its length, and other coils are wound around the other half of its length, would that allow the second diagram I drew?

Cheers, Wayne

 

[gar]

220209-2341 EST

wwhitney:

"OK, so to expand on that, what is the key detail that forces them to be in phase? Is it that the coils are interwound and the spatial extent of each coil (along the central axis) is the same? I.e. if you had a single iron core, but some coils are wound only around half its length, and other coils are wound around the other half of its length, would that allow the second diagram I drew?"

If you have multiple coils on a common high permeability core with negligible leakage flux, then all coils see essentially the same changing flux, and there is low leakage inductance.

If you take an EI transformer core structure, and put all the windings on one leg, then you get essentially all flux coupling all coils. But, if instead you put one coil on one outer core leg, another coil on the opposite leg, and nothing on the center leg, then magnetically the center core leg shunts magnetically some of the flux. Thus, incomplete coupling between the two coils, and effectively high leakage inductance between the two coils.

.

 

[LarryFine]

OK, so to expand on that, what is the key detail that forces them to be in phase?

That a single core can only have one varying magnetic 60 Hz flux going inside it.

If you had a standard 3ph transformer, there is no way you could parallel separate phases, because the sine waves, and thus the + and - peaks, are not in sync.

Is it that the coils are interwound and the spatial extent of each coil (along the central axis) is the same?

No, any coil anywhere on a given core (or leg leg) will be in sync with every other coil on that same core.

That's why you can wire two secondaries in series or in parallel, and you can treat the series connection like a center-tapped secondary.

 

[electrofelon]

If there is access to a third phase "C" from the 208/120V transformer, below shows how you could get 236V L-L and 123V L-N from two 240V/32V buck boost transformers. As shown in green color, one buck-boost is fed by the 208V across A-C, and it adds its 28V output to the B-N voltage at a 90° relative phase shift . And as shown in red, the other buck-boost is fed by 208V from B-C, and it adds its 28V output to the A-N voltage at an opposite 90° phase shift.

View attachment 2559371

I love this forum. Only oh MH do we start with "I have 208 but need 240" and end up with 2 pages on 127 degree phase shifts and unorthodox transformer connections

 

[kwired]

I love this forum. Only oh MH do we start with "I have 208 but need 240" and end up with 2 pages on 127 degree phase shifts and unorthodox transformer connections

Yet it isn't exactly off topic either.

 

[wwhitney]

That a single core can only have one varying magnetic 60 Hz flux going inside it.

If you had a standard 3ph transformer, there is no way you could parallel separate phases, because the sine waves, and thus the + and - peaks, are not in sync.

No, any coil anywhere on a given core (or leg leg) will be in sync with every other coil on that same core.

OK, what I don't follow is this:

I've seen diagrams of 3 phase transformers wound on a figure 8 (3 leg) core, presumably with each leg having a primary and secondary coil for one phase. That would suggest that the core does not force the different legs to all be in phase.

I've also seen a diagram of a single phase transformer wound on a loop (2 leg) core, where one leg has the primary coil and one the secondary coil. [I gather this is not done in practice, but was illustrated that way for educational purposes.] That would suggest that the core forces the two different legs to be in phase.

So what's the difference? Is it a matter of degree? Or does the 3 leg 3 phase transformer rely on a cancellation property that you get with a 3 phase input? I.e. if you took that transformer and instead energized it with 3 out of 4 phases of a 2 phase system, there'd be a problem?

Cheers, Wayne