Buck Boost Transformers

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steve66

Senior Member
Location
Illinois
Occupation
Engineer
I'm trying to boost a 3 phase 208V/120Y to 240 V, 3 phase. The load needs a source capacity of 30 amps.

Square D has the following site which shows transformer model numbers, and wiring diagrams:

http://www.buckboostcalculator.com/index.cfm

They also have this catalog:

http://download.schneider-electric....eference=39000-323-01&p_EnDocType=Instruction sheet&p_File_Id=681213257&p_File_Name=39000-323-01.pdf

If that link doesn't work, its the first document here:

http://www.schneider-electric.us/si...&Country=us&lancer=oui&autnmQueryBis=500SV46B

Anyhow, the two don't agree at all. I'm getting everything from needing 0.5KVA transformers, to 1.5 KVA transformers. Some show needing a 16/32V secondary winding, some show a 24/48V secondary.

Some show the two main windings in parallel, and some in series.

Does anyone know the equations that give the relationships between input voltage, output voltage, and KVA capacity? Or can anyone help me figure out the best configuration to use?

As an example, in the configuration, I select 3 phase, 208V input, and 13.3% boost to 236 volts. It gives me a wiring diagram #7, with a 500SV46B transformer. But there isn't anything in the other document that even remotely matches that.
 

Smart $

Esteemed Member
Location
Ohio
I'd go with a 32V unit secondary.

The transformer must be rated for the current through the unit secondary times the voltage across the unit secondary.

32V ? 30A = 960VA

3 ? 1kVA, 120V PRI - 32V SEC in wye configuration should suffice.

The other configs are probably for open delta connection with 208V PRI. Some of the units may have a 240V PRI. Primary supplied with 208V will only gives 86.7% the secondary rated voltage, which may be why the 48V secondary version is recommended. You'd get 249.6V 3? open delta.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Smart$ gave you the _key_ equation: for buck-boost arrangements, the _transformer_ rating is the output current * secondary voltage. Basically most of the load power is supplied by the feeder; the transformer just needs to supply the voltage _change_. So if you have a 16V secondary carrying 30A, it is only a 480VA transformer...even if the load is using far more power.

Going from 208V, you will often use transformers with 240V rated primaries. The rating is at 240V, so at 208V you are only using 87% of the rating...or in other words if you calculate that you need 100VA in the transformer, because it is really a 240V device it will need to be rated as 115VA.

When going from 208 to 240, you have several options. _If_ you have the neutral available, then the most 'symmetric' option is to use 3 transformers, with 120V primaries and 18V secondaries (if you can get them, 16V secondaries are more common and usually close enough). You use the transformers to create a 239/138V wye supply.

Another common approach is to use only 2 transformers in an 'open delta' configuration. You connect the transformers from (say) phase AtoB and AtoC to get your A to B' and A to C' boosted voltage. The B' to C' voltage will boost to match the other two. This requires a pair 208V to 32V transformers, which will likely really be 240:36V transformers.

Finally (and not very commonly) you can use three transformers with 208V primaries and 20V secondaries (240V to 24V) to derive 240V in a closed delta configuration.

The biggest benefit of the second approach (using 2 transformers open delta) is that one of your supplies is still 120V to neutral, if you need that.

-Jon
 

Smart $

Esteemed Member
Location
Ohio
I'd go with a 32V unit secondary.

The transformer must be rated for the current through the unit secondary times the voltage across the unit secondary.

32V ? 30A = 960VA

3 ? 1kVA, 120V PRI - 32V SEC in wye configuration should suffice.

The other configs are probably for open delta connection with 208V PRI. Some of the units may have a 240V PRI. Primary supplied with 208V will only gives 86.7% the secondary rated voltage, which may be why the 48V secondary version is recommended. You'd get 249.6V 3? open delta.
As pointed out to me through PM by Jon, that should have been 16V unit secondary. I was originally thinking 208V primary and got my 'wires' crossed. So let's try that again....

16V ? 30A = 480VA

3 ? 0.5kVA 120V PRI - 16V SEC units wired in wye configuration should suffice.

That'll get you 235.6 phase voltage.

If you go with 24V unit secondary...

24V ? 30A = 720kVA

Likely looking at 1kVA units for wye config' here, and will get you 249.4 phase voltage.
 
Last edited:

steve66

Senior Member
Location
Illinois
Occupation
Engineer
The transformer must be rated for the current through the unit secondary times the voltage across the unit secondary.

Sounds simple enough, and you all seem to agree. But these xformers always have a dual voltage rating on the small windings - 16/32 or 24/48 volts.

So wouldn't the power calculation depend on if the smaller windings are placed in series or parallel?

In parallel means only half as much current through each winding.

I understand that the windings in series give you the higher voltage (32 or 48), and in parallel you get the lower voltage (16 or 24). But the tables always seem to be a little off - 208 volts with a 32 volt winding might get you to 236V instead of 240V.

And for applications in the 208V range, it seems like the main windings should always be in series if the main windings are 120/240V rated. But some of the tables and diagrams show these in parallel.

I thought it would be easy to find an answer on the internet, but so far I've drawn a blank on finding anything that lists the basic formulas.
 

Smart $

Esteemed Member
Location
Ohio
Sounds simple enough, and you all seem to agree. But these xformers always have a dual voltage rating on the small windings - 16/32 or 24/48 volts.

So wouldn't the power calculation depend on if the smaller windings are placed in series or parallel?

In parallel means only half as much current through each winding.
You're using both windings either way with the same voltage* across each winding

For example, 2 ? 16V ? 15A = 16V ? 30A = 32V ? 15A = 480VA


I understand that the windings in series give you the higher voltage (32 or 48), and in parallel you get the lower voltage (16 or 24). But the tables always seem to be a little off - 208 volts with a 32 volt winding might get you to 236V instead of 240V.
*The tables don't explain that you may be using a unit with a primary rated 240V. You're powering it with 208V while the unit primary to secondary voltage ratio will remain the same.

For example, 240:24 oc 208:20.8 (oc substituted for directly proportional symbol --> ?)

And for applications in the 208V range, it seems like the main windings should always be in series if the main windings are 120/240V rated. But some of the tables and diagrams show these in parallel.
You may be converting 208 to 240, but the unit configuration may utilize the line to neutral voltage (120V), and thus wired in parallel.

I thought it would be easy to find an answer on the internet, but so far I've drawn a blank on finding anything that lists the basic formulas.
In order to understand the basic formulas, you have to understand the principles and realities of configuring. When you understand the latter, you realize the formulas are uber basic and change with the transformer and configuration utilized.
 
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