Combine 2 separate 120V iso-transformers, tie secondaries in series,isolate primaries

[mmontoya]

I want to connect two separate 120V (7.5KVA) isolation transformers, tie the secondaries, but not the primaries. The input is 120V for each primary, with the goal to have 240/120V on the output. I guess you can call the primaries of the two transformers as floating and/or isolated primaries. I tried searching all of the world wide web for this type of application, but it seems very uncommon. Typically, the primaries are connected right at the transformers. The logic I had was that the transformers are technically connected at the neutrals upstream at a terminal block, but that distance is about 50 feet. To clarify my intention, I drew what the circuit would like below (link is below as well). I want to know if this is possible, what should I do to make this works, NEC concerns such as following 450.7 Parallel Operation, and what issues should I look out for. Also, how would the output look if the 2nd transformer below had Line 2 and the Neutral switched on the primary?

I did see issues online about non-matching polarity, insufficent insulation in the transformers, ease of mis-wiring, equal impedance's and ratios, and others. All help is appreciated.


https://i.stack.imgur.com/dnxiv.jpg

 

[GoldDigger]

As drawn, there will be zero volts where your drawing shows 240V.
Either keep the direction of the primary windings the same and connect to opposite Ls or connect to a single L and reverse the sense of one primary winding.

You do not show OCPD for the circuit. Code and common sense would require that the two breaker poles be either common trip or handle tied. This is a very unconventional application for an MWCB.

More common and more justifiable would be to use two transformers, both connected L1 to L2, to create a center tapped secondary.

 

[gar]

171005-1314 EDT

Your drawings are fine except you have not shown "DOTS" on your windings to provide phasing information.

I will define primary as the side of the transformer where power is input. And secondary as the side from which power is obtained.

So long as insulation and coil voltage ratings are not exceeded I do not care where the primaries are connected. However, to do what I expect you want to do the primaries must be connected to the same power source, and of a phase shift of either 0, or 180 relative to each other.

Whether you get 240 V or 0 V from line to line of the summed secondaries is only a function of the phasing of any one of the windings.

Any NEC issues are a function of your application.

.

 

[steve66]

Why connect the primary windings to the neutral? Normally, they would just be connected together.

 

[gar]

171005-1456 EDT

steve66:

What does your statement mean?

You certainly would not connect the primaries in series. Therefore, with 120 V primaries and some sort of 120 V source with a neutral, why would you not connect to the neutral?

Really you could work from any 120 V sources so long as they had the correct phase relationship.

.

 

[GoldDigger]

Why connect the primary windings to the neutral? Normally, they would just be connected together.

If connected in parallel they would have to be a different turn ratio than that used in the circuit shown.
If connected in series, without a common core, they would be suitable only for driving a line-to-line load. Using the created center tap on the secondary side would unbalance the voltages.

 

[mmontoya]

The power supply is 240V/120V, 180 degree US split phase. There are currently Single Pole, 30A, 120V, GFCI circuit breakers on the primary side of each transformer, sorry for not showing that. There will also be a 2 pole, 240/120V, GFCI circuit breaker on the tied Secondary of the transformer.

 

[steve66]

171005-1456 EDT

steve66:

What does your statement mean?

You certainly would not connect the primaries in series. Therefore, with 120 V primaries and some sort of 120 V source with a neutral, why would you not connect to the neutral?

Really you could work from any 120 V sources so long as they had the correct phase relationship.

.

Why not? Is any different than a standard 240v to 240v center tapped transformer?

Certainly if the loads on both transformers are balanced, the primary currents will be equal, and the 50' run back to the neutral will only create more voltage drop.

But I haven't thought much about what happens if the loads are unbalanced. With two separate transformers, there are two separate cores. Does having a common core on a single transformer make any difference if the loads are unbalanced?

I guess my question is really why not just use a single 240v to 240v center tapped transformer? Then there is no neutral connection, and you save 100' of neutral wire, and the voltage drop is less. The inputs and outputs are no different.

 

[Jraef]

Let's start here. What is your intended GOAL in doing this? As of now, I see absolutely zero purpose in this exercise.

 

[GoldDigger]

But I haven't thought much about what happens if the loads are unbalanced. With two separate transformers, there are two separate cores. Does having a common core on a single transformer make any difference if the loads are unbalanced?

A common core, either in isolation or autotransformer configuration, allows drawing current from the single L-to-L primary to supply magnetic flux to allow any balanced or unbalanced loads on the secondaries.

 

[victor.cherkashi]

power is taken from different phases. I recall from school when you sum two vectors 120V0°+120V120° you will never get 240Volt

Sent from my Nexus 5X using Tapatalk

 

[gar]

171005-2017 EDT


mmontoya:

Has your question been answered in your mind?

As I said before your drawings are fine. You just have to get the phasing correct to get 240 V out. With the secondaries in series incorrect phasing will not produce any problem except zero output voltage line to line on the output.

You can also put the two primaries in parallel and supply from a single 120 V source, and that source does not need to be grounded or have a neutral.

With two separate transformers what you can not do is put the two primaries in series and feed line to line from a source that does not tie that mid point of the primaries to the mid point of the voltage between the two line source.


steve66:

A closely coupled transformer reflects the secondary load impedance to the primary by approximately the square of the turns ratio.

You figure out what happens with an unbalanced load. What happens to the primary voltages when the primaries are connected in series? When primary voltage is well above its rating what happens?

.

 

[steve66]

A common core, either in isolation or autotransformer configuration, allows drawing current from the single L-to-L primary to supply magnetic flux to allow any balanced or unbalanced loads on the secondaries.

That's right. So, so why not use a single transformer with a 240V primary, and a 240/120V secondary, like this:

https://www.grainger.com/product/1T...17540&ef_id=WQCadgAAAReL8g7Z:20171006005047:s

I assume the OP is worried about the GFCI breakers. I'm assuming he can supply the primary with a 30A/2 pole GFCI breaker, but maybe he has another reason he cant do that?

power is taken from different phases. I recall from school when you sum two vectors 120V0°+120V120° you will never get 240Volt

Sent from my Nexus 5X using Tapatalk

Since there wasn't a L3 shown on the original diagram, I assumed we were talking about a 240/120V single phase input where L1 and L2 are 180 degrees apart.