The second vfd is running at a pwm freq. that your motor does no respond to.
Motor susceptibility to "singing" varies all over the place depending on
motor size, insulation system, enclosure, mass, air gap, etc. There are
many variables.

Virtually all small vfd's generate very similar waveforms. The main
variable is the pwm freq. This is user adjustable on most vfd's and is
provided by the manuf. to help eliminate or suppress "singing", as many
users think the noise is detrimental or annoying. The only other way is to
install a reactor between the vfd and the motor. However, this is
expensive, inefficient, and takes up space.

Randy


"Bob Engelhardt" wrote in message
...
R. O'Brian wrote:
The motor is the problem. ...

I'm certainly no expert, but I have a little direct experience with
this. I have a 3 ph motor and VFD on my drill press. It is the 2nd
VFD. The first one sang, this one does not. Same motor, same wiring.

Bob