Variable frequency drive and common mode voltage

[Kemme]

Hello all

Is there any way to measure the common mode voltage on a three phase motor driven by the variable frequency drive when the neutral point of the motor is not exposed?
Maybe creating a virtual neutral point with three resistors would help?
Or sum of all the output voltages ((U-V)+(V-W)+(W-U)) would do a proper measurement?

Any help will be appreciated.

[T3sl4co1l]

Yes, resistors can be used, as long as modest bandwidth is acceptable (good for line frequency and harmonics, okay for switching frequencies, unreliable for RFI frequencies).

You can also use three oscilloscope probes, and average them together (using the MATH / ADD function, assuming you can add two traces, then a MATH and a trace, together -- YMMV).

Tim