I've seen the light (I think...)

dangercraft wrote:

If this is the case, could one drive the the screw with a VFD and an AC motor and get the same results as using a servo and drive in velocity mode?


Frank

The typical VFD has a bandwidth of a few Hz. The default LinuxCNC servo loop
has a 1 KHz update rate, giving it a theoretical bandwidth of 500 Hz. So, right there,
you have a serious delay between command and response. This would make the
servo loop seriously unstable.

Flux Vector VFDs maintain the induced rotor flux, allowing the motor to respond
more quickly even at zero speed, and generally have higher bandwidth as well.
Still, they need to be designed for positioning control to be suitable. With a 1 KHz
servo update rate, delays in response of more than a few ms will make servo
tuning difficult to impossible.

Finally, typical AC induction motors have MASSIVE rotor inertia, perhaps
20 times greater than a servo motor of the same ratings. Induction
servo motors do exist, but they are specially designed to minimize rotor
inertia.

Jon

Hey John,
That makes perfect sense, thanks!

Frank