Do you have a load reactor installed. These reactors can and do prevent unwanted harmonics at the motor. They also clean up the simulated sign wave produced by the VFD.
Do not confuse the load reactor with the line reactor. Both are relatively the same with the exception being that the load reactor is installed close to the drive just like the line reactor but is connected to the output leads of the drive.
However, 50 feet is not considered a great distance to any drive or motor manufacturer. Considerations are usually discussed at lengths exceeding 100 feet.
