Mr Mike Mayhem (Electrical)(OP)12 May 23 16:37
"....How can 2X line frequency be at 60hz if a motor is operated by a vfd. .... as this does not seem logical. If my intuition is correct; line frequency is relative to the vfd output frequency not the supply to the inverter".
1. I am of the opinion that the word "line" usually refers to the utility power source conductors. Not the conductors from the VFD output to the motor.
2. If the power source "line" is 60Hz, the frequency does NOT change, irrespective whether it supplies a VFD or the motor directly.
3. The VFD output frequency is adjustable. It is possible to adjust the VFD output frequency to 2x the input "line" frequency. The VFD output conductor can be subjected to 120Hz and actually at much higher carrier frequencies....etc
Che Kuan Yau (Singapore)

op - why don't you rephrase your question in such a way that we don't have to parse "line frequency".

Are you seeing 120hz vibration on a motor fed from vfd?

OP: The term "line frequency" is generally used to refer to the frequency of the INCOMING waveform. If the drive is connected to a local distribution bus that sees the typical "grid" frequency of 50 or 60 Hz, then the DRIVE's line frequency is the same value - 50 or 60 Hz. The motor, which is downstream of the drive, sees the drive output waveform as its input. Thus, the MOTOR's line frequency is 2x whatever the drive is putting out - which can range anywhere from a very low value (even 0 Hz) to a very high value (some drives output in the thousands of Hz).

Converting energy to motion for more than half a century

Quote (che12345)

1. I am of the opinion that the word "line" usually refers to the utility power source conductors.

Quote (gr8blu)

Thus, the MOTOR's line frequency is 2x whatever the drive is putting out

Case in point, the meaning of this particular phrase (line frequency) is not clear in this context. Each responder has their own idea of what it should mean.

I don't have any vote what line frequency should mean in this context. Less ambiguous terms might be utility frequency or vfd output working frequency, depending on what is meant.

My request to op: please rephrase your question in such a way that we don't have to parse "line frequency".