Why can't they use a variable resistor to control speed like fan control, provided the motor is a very low capacity one?
Converting to three phase and then using VFD seems to be an overkill.
As per Indian Standard 2026 Part 1, typical value of %Impedance for 12.5 to 25 MVA transformers is 10.0%
IEC might also be the same, because the clause 0.6 of the above standard states that "This standard has been based on IEC Pub 76-1 Power Transformers Part 1 - General"!!