One of the problems with measuring the output of VFDs is to find an appropriate reference. Some drives use true Bi-Polar power to drive the H-Bridges and in that case, the common line between the two supplies is ground referenced and you could simply probe any of the output lines referenced to ground.
Some other drives simply rectify the incoming AC line and send that rectified output to the power rails of the H-Bridges. In that case, the only way I can think of to safely look at the output would be to use 2 channel differential. (Chan A on U, Invert Chan B and tie to V or W. That would give a phase to phase waveform on you scope.