5FP7 project starting with the focusing coil.

[ppppenguin]

Saturation depends on the frequency and applied voltage. Not on the VA rating. So if you want to apply 12.5Hz to a nominal 50Hz transformer you need to derate the winding voltages to a quarter of those stated. The voltages are RMS, whereas I think you're thinking of peak-peak. 50V p-p is 50/2.8 = 18V RMS. So you would need a winding specified for at least 72V RMS to work at 12.5Hz.