Sine wave filter or DV/DT filter on VFD

Instead of using brushes, I have been using either a sine wave filter or a dv/dt filter at the output of the variable speed drive. A sine wave filter eliminated all the high frequency from the VFD output and I have very good luck with that implementation -- have not seen bearing burn out. When using dv/dt filter, I like to keep the dv/dt below 200 volt / micro-second. The dv/dt filter design must have the common mode capability. That seems to work too. However, some people think that the filter is an expensive solution - like a 100 amp sine wave filter can cost US$1,500 to 2,000.

I am using non-insulated bearing. The shaft diameter of my motors are quite large - 100mm to 130mm and insulated bearing will cost a lot even if you can find them. I am using standard bearings from SKF and FAG.

A good sine wave filter removed all the high frequency from the PWM voltage and seems to works fine. Yaskawa has also built a filter for me for testing and it even works better. With a high frequency current probe (15Mhz) band width, I didn't see any common mode current going to the motor (I guess the level is below the noise floor of my measuring equipment). We typically pull out our motors after 18 months of continuous service in the oil field and found only normal ware on the bearing.

Using Dv/DT filter should also work if the filter is designed to remove both differential mode and common mode. I have less experience with dv/dt filter due to my application are mostly oil field related, and I am able to convince my customer to paid the extra for the Sine wave filter.

For the question regarding to the bearings of the driven equipment, it has not come to my attention. I can just assume if the common mode current does not arc through the bearing in the motor, it should not arc through the bearing of the driven equipment.