A single phase to three phase rotary-type converter installed on a farm to drive a compressor has assymetric voltages to earth on the load side. I understand that this is a feature of such devices. The displacement between phases remains at 120degrees. Can any one tell me why an equal phase to earth voltage cannot be easily achieved?

*headscratch*
What kind of difference are we talking about? A few volts or something like the US corner grounded delta where some phases have 120V to ground an others 240?
The latter could only be accounted to the arrangement of the windings and remedied by rewiring the converter to run in a Y setup.