Has anyone had experience using dc injection from a VFD to motor as a anti-condensation method?

Customer has aprox 50 motors with VFD existing. Motors are in high moisture area. They can't / don't want to have heaters installed in motors.

Any experience / formulas or methods greatly appreciated.

Theoretically there is no problem doing it, but can the VFDs be configured to do this continuously? I don't know of any, but then again, I've never checked. As to formulae, I don't have much to offer. My company sells a Motor Winding Heater that does what you are planning, but is designed as a stand-alone product. The output is 1/2 wave AC (quasi DC). We found during testing that motor windings under 10HP seem to have a positive coefficient of temperature, while everything larger tends to have a negative coefficient. Therefore a 10A 1/2 wave output will work on any motor 10-100HP, but a 3HP motor may need 25A to get the same temp rise, effectively eliminating it as a solution because the wiring is usually not rated for that. Hope this helps.