VFDs and Unscreened Output Cables

Provided that the impedance of the decoupling circuit is very low to the transient currents, and the impedance between the motor frame and the VFD frame is very low, the majority of the transient current will return to the DC bus and not cause any issues. Current that does not return directly to the DC bus will flow through the structure of the building finding paths back to the input of the VFD and return that way.
If we connect the VFD to the motor using unscreened cable, we will have an equal impedance between the VFD output and the motor, and between the motor frame and the VFD frame.
The motor capacitance appear close to a short circuit to the transients and so we have a potential divider where the transient voltage on the motor can be as high as 50%. As the motor is mounted on the structure, there are multitudes of earth paths and the noise current flows in all directions.
To eliminate the transient current spread, you either have to totally isolate the motor from ALL other paths, conductive and capacitive, or you reduce the impedance between the motor frame and VFD frame considerably so that the transient voltage on the motor frame is small. As the transient voltage on the motor frame is reduced, the stray currents are also reduced.
Using a screened cable correctly bonded between the VFD frame and the motor frame is an easy way to reduce the earth return impedance dramatically with the screen impedance being 1/20 - 1/50 of the impedance of the phase conductor. This reduces the stray noise voltage from the VFD by a factor of 20 to 50.

Voltage between Motor Frame and VFD frame with unscreened cable.

Voltage between motor frame and VFD frame with screened cable.

For more information, see VFDs cause EMC