Variable speed drive common mode voltage and EMI

How to measure or diagnose common mode voltage of three phase PWM variable speed drive? And how to solve EMI from motor Driver?

For your first question:
Could you let me know your definition of variable speed drive "common mode"? Is this the voltage between one phase IGBT leg and the neutral point of stator "Y" connected machine?

For your second question:
Again, I would like you to provide more detailed information regarding which EMI issue you have encountered in motor & drive system.

As far as I know, in order to reduce the common mode EMI issue, such as: ground current, radiated/conducted noise, motor shaft voltage and bearing current which are caused by high frequency IGBT switching (dv/dt), passive and active EMI filter can be built.

For passive EMI filter: it is widely used in industry, such as: component segregation, cable routing, connectors, grounding, shielding, common impedance coupling.

For active EMI filter: we can do voltage mode or current mode noise cancellation. Generally speaking, the active EMI filter is achieved by generate a voltage or current which has the same amplitude inversely series with the original noise. You can find more papers from IEEE Xplore.

There are two books I recommend you to read for EMC/EMI:
Noise Reduction Techniques in Electronic Systems by H.Ott
Introduction to Electromagnetic Compatibility by C.R.Paul