Dear Rannam,
The generator DOF results in a rigid-body mode, which in MBC3 shows up as a pair of zero-valued (or near-zero-valued) frequencies with +/- inf damping.
See the following post for more guidance on how to interpret the full-system mode shapes resulting from application of a FAST linearization and MBC3: Eigenanalysis FAST.
I hope that helps.
Best regards,