Low voltage high frequency motors:

The easy answer is that one winding is failing.
You may have a high resistance fault in the supply conductors. (One phase low Amps, two phases high Amps.)
Less likely is that your generator has a serious problem.
Check the resistance of each winding, it will probably be less than 10 Amps however the three phase resistances should be equal..
Check the resistance to ground, it should be several meg-Ohms, even as low as several thousand Ohms to ground should not cause this issue.
A grounded winding will read close to zero Ohms to ground,

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Ohm's law
Not just a good idea;
It's the LAW!