Re: VFD Positioning?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "C.S. Mo" <cs@v...> wrote:

> Assuming you put an encoder w/Index on your spindle, what kind of
> positional accuracy can you get controlling the spindle with

a 'typical'

> VFD (not one of the super accurate ones - but a standard FM50 or

FM100 as

> example)?
>
> As in, is it possible to have the spindle turn x degrees? Or can

you only

> control RPM?
>
> --C.S.

One of my lathe controllers supports spindle indexing. This is
accomplished with a flux-vector VFD. The spindle drive motor is
coupled to the VFD with an encoder. The lathe spindle also has an
encoder tied into the controller.

The flux vector drive can lock the motor armature similar to a
stepper motor. Plus the motor can be rotated at near zero rpm with
high torque. Because of the closed loop connection between the
motor and the VFD precise control of the motor is possible. The
motor is essentially the same as a regular 3 phase motor.

With this setup the lathe can index the spindle with high accuracy
and hold it in position. Plus the spindle can rotate slow enough to
do milling on the rotating part using an auxiliary milling spindle.

The 7.5hp Baldor flux drive can be picked up for under $1000 (Baldor
list is around $1800). Then you need an encoder on the spindle
motor and some means to cool the motor at low rpm running.


I know of another application of flux drives/motors where they
replaced large servos on a huge aircraft spar mill when it was
retrofitted with a modern control. In this case the flux drive
setup acts like servo motors for axis positioning. I don't know why
the system designer chose to use flux vector drives rather than
servos, possibly a hp issue.

Doug