Buc Boost 208 to 120/240 with a Neutral

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jim dungar

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See page 22, figure 26A and the text immediately above the figure.

It says to essentially connect the supply neutral to the load neutral, bypassing connection to the midpoint connection of the BBXFMR.

Okay, I see your point.

But to split hairs.

I said simply to never use buck boost to create a 3-wire 120/240 system. I inferred but did not state the problem was the derived neutral point.

The Square D literature, you correctly quoted, says:
If you need a 3rd wire get it from the original system not from the buck-boost. Square D definitely says do not make a connection to the derived neutral.

Square D does say that if you use a 120/240-12/24 buck boost the L-N voltage will be about 129V. The OP was using a 120/240-16/32 transformer so the L-N would exceed the +10% tolerance of a nominal 120V device.
 

mull982

Senior Member
NO, NO, NO.

Do not ever use the neutral of a 208Y/120 4wire system when creating a 3-wire buck-boost arrangement. There will be about a 60V difference between grounded neutral of the wye system and the the neutral point of the derived system. This also means that the derived system voltages will be about:

L-L = 236V
L-N = 118V
L-G = 133V (?)
N-G = 60V

I am not seeing where this 60V difference is coming from. Can someone point it out real quick?
 

Smart $

Esteemed Member
Location
Ohio
Okay, I see your point.

But to split hairs.

I said simply to never use buck boost to create a 3-wire 120/240 system. I inferred but did not state the problem was the derived neutral point.

The Square D literature, you correctly quoted, says:
If you need a 3rd wire get it from the original system not from the buck-boost. Square D definitely says do not make a connection to the derived neutral.

Square D does say that if you use a 120/240-12/24 buck boost the L-N voltage will be about 129V. The OP was using a 120/240-16/32 transformer so the L-N would exceed the +10% tolerance of a nominal 120V device.

I agree with your assessment... I was just pointing out something I believed you missed.

I am not seeing where this 60V difference is coming from. Can someone point it out real quick?

It involves vector math.

Go to the Square D Study guide we have been discussing and look at page 21 figure 26.

http://ecatalog.squared.com/techlib/docdetail.cfm?oid=090089268007917c
Your link leads to the wrong publication :confused:


Here's a direct link:

http://ecatalog.squared.com/pubs/El...ltage Transformers/Mini Power-Zone/I1(R1).pdf
 

mull982

Senior Member

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I agree with your assessment... I was just pointing out something I believed you missed.

Here's a direct link:

http://ecatalog.squared.com/pubs/Electrical%20Distribution/Low%20Voltage%20Transformers/Mini%20Power-Zone/I1(R1).pdf
One thing I've not seen mentioned is that tying the transformer "neutral" to the supply neutral also creates a timing issue, because the two halves of the B/B transformer share a single core.

If two separate B/B transformers were used instead, it could work, except the primaries would then be receiving 120v, eliminating the need for the B/B in the first place. Can't win 'em all. :roll:
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If I understand correctly the only difference between an Buck-Boost and an autotransformer is that a Buck-Boost is for single phase, and an Autotransformer is for 3-phase?
No, it's interconnecting the secondary with the primary that makes a buck-boost into an auto-transformer.

An old-fashioned automotive ignition coil is actually an auto-transformer (no pun intended.) The points terminal (the -)is common to both the primary and the secondary.
 

ronaldrc

Senior Member
Location
Tennessee
I1R1.jpg
 

ronaldrc

Senior Member
Location
Tennessee
Larry

I need to think about that, but seems to me that wouldn't be an issue, or the issue was resolved when the wye changed the 240 to 208?

And theres that dow gon weird diagram again, Glene:D
 
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Smart $

Esteemed Member
Location
Ohio
One thing I've not seen mentioned is that tying the transformer "neutral" to the supply neutral also creates a timing issue, because the two halves of the B/B transformer share a single core.

The "timing" issue is what causes the 60V disparity to begin with, and it doesn't magically disappear if the connection is made.

If two separate B/B transformers were used instead, it could work, except the primaries would then be receiving 120v, eliminating the need for the B/B in the first place. Can't win 'em all. :roll:

Actually it wouldn't eliminate the need... because the two 120V sources are still 120? out-of-phase... but it would work (as in not let smoke out... there would still be voltage issues).
 
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ronaldrc

Senior Member
Location
Tennessee
Larry

I don't think there would be a timing issue. Why would this be any different from
taking two legs of the standard wye system.

Smart

The voltage different is because of the buck boost low voltage windings.

If you don't connect the grounds together there is no problem.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Actually it wouldn't eliminate the need... because the two 120V sources are still 120? out-of-phase... but it would work (as in not let smoke out... there would still be voltage issues).
My point was that connecting the source neutral to the load renders it with 120-0-120v, the desired product, but with the same 208v and 120 deg. between the lines.

You cannot feed two lines and the neutral from a 208/120v supply to a single transformer, because the common core will attempt to lock them together, phase-wise.

That's why I mentioned that using two separate transformers would work, but only in the sense that the smoke stays inside; it won't deliver a center-tapped 120/240v.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
We have been down this road before, I will agree to disagree.
Okay, no biggie, really. Maybe I've been over-complicating it. (Is that the same as under-simplifying it?)

Picture taking a 3-wire MWBC from a 208Y/120 panel, two hots and the neutral. Feed it to a 1-ph transformer with a 120/240v primary, connecting all three wires: L1 to H1, N to H2&H3, and L2 to H4.

What would happen?
 

ronaldrc

Senior Member
Location
Tennessee
I agree no biggy

I don't know what I am missing here, but if the output and the input voltages match it should just power the transformer up. And I don't mean let the smoke out of it either.
One core or three cores shouldn't have anything to do with this.



Line #1 and Line #2 are just one phase not two. If your taking your power from those two legs only the other two phases play no part. The neutral of the three phase is just like the neutral of a single phase when just using two legs. Although it won't balance out without the third leg.

The only effect the three phase wye system has is the interaction of the three phase mixing and beating it down from 240 volts to 208 volts.
 
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rattus

Senior Member
I agree no biggy

I don't know what I am missing here, but if the output and the input voltages match it should just power the transformer up. And I don't mean let the smoke out of it either.
One core or three cores shouldn't have anything to do with this.



Line #1 and Line #2 are just one phase not two. If your taking your power from those two legs only the other two phases play no part. The neutral of the three phase is just like the neutral of a single phase when just using two legs.

The only effect the three phase wye system has is the interaction of the three phase mixing and beating it down from 240 volts to 208 volts.

Ronald, by connecting the neutral to H2 and H3, you are indeed applying two phases to a single phase transformer.

Let

V1n = 120 @ 60, and let
V2n = 120 @ -60

Now connect everything except H4. The voltage at H4 is

120 @ -120, a 120V difference exists between L2n and H4!

Connecting V2 to H4 will result in New Year's fireworks.
 
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