Buck and boost transformer

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Gsxrnum1

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Location
California
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Electrician
I have a machinery that requires 220vac 4 wire (3phases with neutral) . Onsite at the electrical panel I have 208 wye 4wire. I’m planing to use a buck and boost transformer to boost my voltage from 208vac to 220vac . Since the machinery needs a neutral can I just pull the existing neutral from the 208vac wye panel to the machinery ?
 
If you plan to use an autotransformer and you need the neutral at the machine, then you _must_ bring the supply neutral to the machine, and you _must_ use a transformer arrangement that maintains this neutral.

The common 'open delta' buck/boost arrangement that uses two transformers to raise the voltage won't work. You would need three L-N connected boost transformers.

Also note that 220V is not a common US voltage. You will want to confirm that you really need 220/127 wye, and not something else like 240/120V high leg delta, or 380/220V wye.

Jon
 
Winnie,

Thanks for the quick response. I’m trying to understand why is it that I cannot use the neutral conductor that is existing in the 208v electrical panel ? Without having it go through a transformer (it’s for my knowledge) ?

Checking with the manufacture right now . Thanks
 
Draw a schematic of your proposed transformer connection, and calculate the resulting line-neutral voltages. Is the source neutral still neutral relative to the transformer output voltage.

The most common 3 phase buck/boost arrangement does not preserve the neutral.

The 3 phase wye connection maintains the symmetry around the neutral point, but requires connection to that neutral.

Pondering further, I could imagine a delta connected buck/boost transformer which would maintain symmetric line-neutral voltage, and the neutral would not need to be connected to the transformer windings.

Jon
 
Gsxrnum1 said:
Thanks for the quick response. I’m trying to understand why is it that I cannot use the neutral conductor that is existing in the 208v electrical panel ? Without having it go through a transformer (it’s for my knowledge) ?
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The supply neutral will indeed pass through to the load.

In a wye system, each line's voltage is raised (or lowered) individually, as if it was a single 1ph circuit, so the neutral must be used by each one, as well as the load if needed.

The neutral itself does not "go through" a transformer, but each transformer will be connected to it, and it will remain at its 0v level on the way to the load.
 
winnie said:
Pondering further, I could imagine a delta connected buck/boost transformer which would maintain symmetric line-neutral voltage, and the neutral would not need to be connected to the transformer windings.
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However, that voltage symmetry would be unstable.
 
I'd verify what the machine actually was designed to work with.

Many 208/120 systems tend to run maybe 212-215 volts. I doubt I would even try to boost that unless it is something really sensitive to minor input differences.

Being rated 220 might mean it can be used directly on 208 or 240 volt systems?
 
winnie said:
Draw a schematic of your proposed transformer connection, and calculate the resulting line-neutral voltages. Is the source neutral still neutral relative to the transformer output voltage.

The most common 3 phase buck/boost arrangement does not preserve the neutral.

The 3 phase wye connection maintains the symmetry around the neutral point, but requires connection to that neutral.

Pondering further, I could imagine a delta connected buck/boost transformer which would maintain symmetric line-neutral voltage, and the neutral would not need to be connected to the transformer windings.

Jon
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A boost transformer in an open delta connection will not have a symmetrical L-N voltage on its output. Your machine may not like this in symmetry. If your machine needs a 3-phase 4-wire feed, purchase a third transformer.

Pardon my looseness with some terminology. Looking at a typical connection two legs are transformed from 208V to 220V in relationship to the third leg which is not touched. The result will be two legs at 127V to neutral and ground and the third leg at 120V. My guess is the cost for 3 transformers will still be less than the profits the equipment will produce over its lifetime.
 
winnie said:
delta connected buck/boost transformer
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Maybe I'm misunderstanding these that have come up in some recent threads, but how is a delta connected b/b transformer fed from a 208 wye not going to have a 139 volt L-N potential and remain symmetrical?
 
Joethemechanic said:
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Maybe I'm misunderstanding these that have come up in some recent threads, but how is a delta connected b/b transformer fed from a 208 wye not going to have a 139 volt L-N potential and remain symmetrical?
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If you boost the 208V L-L in a wye source, you _must_ similarly boost the L-N. So if you were to boost the 208V to 240V, and maintain symmetry, then you _must_ get a 139V L-N. The OP was asking about boosting to 220V L-L, when means a 127V L-N. Thus my caution that they carefully confirm exactly what they really need.

If it is necessary to maintain symmetric L-N voltage with a buck/boost system, the best way to do this is is with a wye autotransformer configuration.

-Jon
 
winnie said:
If you boost the 208V L-L in a wye source, you _must_ similarly boost the L-N. So if you were to boost the 208V to 240V, and maintain symmetry, then you _must_ get a 139V L-N. The OP was asking about boosting to 220V L-L, when means a 127V L-N. Thus my caution that they carefully confirm exactly what they really need.
If it is necessary to maintain symmetric L-N voltage with a buck/boost system, the best way to do this is is with a wye autotransformer configuration.

-Jon
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Yeah I must be misunderstanding the posts here and on the other thread about the single phase B/B transformer from a WYE source. There are ways to end up with 120 on one leg in relation to neutral, but that is not symmetrical and has elevated N-G potential. And most likely require specially wound transformers.

So for the OP requirements of 120/240, the only simple transformer option is a high leg delta, provided the machine will operate on a high leg
 
Joethemechanic said:
Yeah I must be misunderstanding the posts here and on the other thread about the single phase B/B transformer from a WYE source. There are ways to end up with 120 on one leg in relation to neutral, but that is not symmetrical and has elevated N-G potential. And most likely require specially wound transformers.

So for the OP requirements of 120/240, the only simple transformer option is a high leg delta, provided the machine will operate on a high leg
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The OP has not said what they need. Just that they want to use a buck boost yet have a neutral. We don't know if they need a 120V L-N in a 240V high leg system (where only some of the legs are 120V to neutral), or a 127V balanced L-N in a 220V wye system, or if they could make due with the completely unbalanced system where you have a single leg at 120V L-N and the other legs at elevated voltage.

I admit I probably made the wrong assumption in answering the question.

If you _need_ 220V three phase (and don't care about L-N balance), and also _need_ a single 120V L-N, and can be absolutely certain when supply leg goes to the 120V load, then you just need a very solid assurance that the 120V loads will be connected to the 120V leg, and then use an open delta autotransformer to get the required L-L voltage.

-Jon
 
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