Selecting and Connecting A Buck/Boost Transformer For 1Φ Phase Converter Feeder
SND,
Some things to consider when selecting a buck transformer to run a machine with a spindle drive.
I would not recommend selecting a unit that puts you at the top limit of the voltage range for the machine in question. Many Asian machines are built for 220 or 200V input, the 235V is the upper limit for reliable operation. Check the machine spec for the desired voltage, not the upper limit.
Utility voltages are not always stable and tend to shoot up when the HV line is lightly loaded, that is nights, weekends, and winter when the grid loads are lessened. Take some readings over time to find out what your swing typically is, before selection.
Sizing the unit, can be a little tedious at times because most tables and calculators provide a part number for the brand in question. Then you need to look up the part number in their catalog to find the actual KVA size and voltage range needed.
Here is the SqD online calculator, You will also need to get their catalog to look up the part number from the calculator result, to find the actual recommended unit specs. Once you know the specs you need, then you can use that info to do your price shopping with other vendors.
Buck and Boost Transformer Calculator - Schneider Electric United States
Digest 177 Sec. 14: Transformers | Schneider Electric
Next up is the connection diagram. All of them have two secondary voltages, 12/24, 16/32, 24/48. It matters which way they are connected to get the change you desire. Each unit is capable of two levels of transformation depending on the connections used.
Then just about all standard connection diagrams supplied by the manufacturers, supply unequal voltage outputs on the two legs, with respect to neutral and ground, from your split phase service. The transformation adjustment occurs only on one leg, the other leg passes straight through, with no voltage change. This imbalance can cause issues with some machinery, especially when being run through a phase converter, and then sent to a drive that normally wants a balanced Y system. You'll already have a high leg from your phase conversion manufactured output leg, when you combine that with one low leg and one high leg from your buck output, you end up with even more of an unbalanced output. Not the best setup for a drive that wants a symmetrical input legs.
There is a fix for this situation. And that is to modify the factory connection diagram, so that the adjustment occurs equally on both legs, feeding the converter. Then you only have the one hi leg (manufactured) from the converter.
Here's an example for one type of connection diagram, a standard OEM drawing, and a modified one for balance.
Running the numbers for your setup...
I selected your given input of 247V, Then I selected 225V (10%)buck output to stay well below your given maximum voltage. Then I used 60A for the capacity of your converter, and 100A for the capacity of your feeder.
Results were 1.5KVA 12/24 for the Converter capacity
and 3KVA 12/24 for the Feeder capacity
That would put the 2KVA unit right in the middle ground.
SAF Ω