Buc Boost 208 to 120/240 with a Neutral

[ronaldrc]

I guess I have too much time on my hands again.


I have not tried this yet.

Had another idea about the Buc and Boost transformer.

I know there will be a grounding conflict with NEC grounding.

But that aside do you think this would work and if not why not, please explain why not?

 

[wasasparky]

I will assume the 208V is from a 208Y120V system.
It will work as long as you use the neutral fom the 208/120 system.
(Do not ground the centerpoint of the buck/boosts)

 

[mull982]

I will assume the 208V is from a 208Y120V system.
It will work as long as you use the neutral fom the 208/120 system.
(Do not ground the centerpoint of the buck/boosts)

I agree and I would think that grounding the neutral point shown would cause a fault?

 

[ronaldrc]

Agreeded

Agreeded

I agree with both of you.

I don't have a Threephase system at my resident.

But I am going to try this with my 240 volt single phase system.
I realize my resulting voltages will be higher but I am just interested in the conflicting voltage when I bond my Neutral conductors. Incoming neutral and my output neutral.

 

[winnie]

In the circuit drawn out in the original post, if the 208V comes from a 208/120V wye three phase source, then the 'derived neutral' (terminal H2, H3) would be at about 60V relative to the grounded neutral point of the wye.

-Jon

 

[mull982]

I agree and I would think that grounding the neutral point shown would cause a fault?

In thinking about my statement some more I wanted to clarify that the reason I think it would be a fault is due to the fact that we are not creating a seperately derived system and therfore grounding this poing would create a ground path back to the grounded wye point on the system supply?

 

[jim dungar]

I will assume the 208V is from a 208Y120V system.
It will work as long as you use the neutral fom the 208/120 system.
(Do not ground the centerpoint of the buck/boosts)

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

 

[Besoeker]

I agree with both of you.

I don't have a Threephase system at my resident.

But I am going to try this with my 240 volt single phase system.
I realize my resulting voltages will be higher but I am just interested in the conflicting voltage when I bond my Neutral conductors. Incoming neutral and my output neutral.

Your incoming neutral and output centre tap will be at different potentials. If you connect both to ground you will short out one of the windings.
Not a good idea.

 

[glene77is]

Winnie,

Could the OP test this 60 V difference by using a light bulb load,
between the (208) line side neutral the the boosted 'load' side neutral.
Then measure the voltage across the light bulb ?

I suggested the light bulb so that there is a minor load on the wierd system.

 

[LarryFine]

I suggested the light bulb so that there is a minor load on the wierd system.

That's why God invented wiggies.

 

[glene77is]

Larry,
Right again.

My wiggy thing has 3Kohm impedance, and pulls 40 mAmps.

What was I thinking?
My light bulb tester broke the day before I bought this wiggy thing.
The OP should Load the wierd transformer with a wiggy!

 

[rattus]

Worse than that:

Worse than that:

Your incoming neutral and output centre tap will be at different potentials. If you connect both to ground you will short out one of the windings.
Not a good idea.

You will fry both windings because you have connected 2 of the 3-ph legs through a 1:1 transformer.

 

[wasasparky]

NO, NO, NO.
Do not ever use the neutral of a 208Y/120 4wire system when creating a 3-wire buck-boost arrangement.

MAYBE, MAYBE, MAYBE

What I suggested is right out of the Square-D dry type transformer study course. Using the system neutral is recommended by a number of transformer manufacturers and has approval of most electrcial codes (once again - taken from the square-D course...)

 

[ronaldrc]

I should have made this clear when I first started redrawing these wierd and cluttered Transformer circuits.

I by no means ,mean for anybody to even consider using these configurations for regular
Electrical services by no means.

I myself use these in my shop and are small transformers usually smaller than a 5 KVA.

Thanks: Ronald

 

[jim dungar]

MAYBE, MAYBE, MAYBE

What I suggested is right out of the Square-D dry type transformer study course. Using the system neutral is recommended by a number of transformer manufacturers and has approval of most electrcial codes (once again - taken from the square-D course...)

Please provide the page number of your reference.
The on-line copy says that this is an improper connection, page 21 figure 26.

 

[ronaldrc]

Please see NEC art.210.9

Thanks

 

[Smart $]

Please provide the page number of your reference.
The on-line copy says that this is an improper connection, page 21 figure 26.

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.

Also note copyright on that course is 1993!

 

[wasasparky]

Please provide the page number of your reference.
The on-line copy says that this is an improper connection, page 21 figure 26.

Page 22 :smile:
"So much for the wrong way, what is the accepted solution? To run a neutral wire from the power transformer to the point of load and use it as a neutral..."

It goes on to say it is not a perfect solution, as their example gives you 129V L-N (I would imagine there is a buck/boost selection where you could make it closer to 120V...)

 

[iwire]

Also note copyright on that course is 1993!

Does electricity behave differently in 2009?:-?

 

[Smart $]

Does electricity behave differently in 2009?:-?

No

...but the course states, "...this connection... has the approval of most electrical codes."