Re: Another Possible Lower cost Servo Controller?

PTENGIN@... wrote:

> From: PTENGIN@...
>
> Gents,
> In looking at the specs of AC Servo motors, I realize that most are three
> phase motors with the required encoders/tachs and or resolvers added on to
> the back. Granted they are optimized for positioning systems (usually) and
> are not designed to be run at high speed for long periods of time. My
> question is, with many of the more advanced VFD's on the market, you can
> control velocity and direction with standard +-10 volt inputs. I again
> understand that VFD's are not optimized for positioning but what harm would
> there be in pressing a 3 phase motor / VFD / encoder set up into CNC service?
> The VFD's are significantly cheaper than AC Servo drives in most cases. If I
> am totally off the mark, please tell me why. TIA.

First, there are 2 kinds of AC servo motors (sort of). One has permanent magnets
in the rotor. It is usually called a brushless DC motor, but in fact is a synchronous
AC motor. Control of this is fairly easy, but you need an encoder to tell the drive
where the magnets are in relation to the coils.

The second kind is, indeed, very similar to an AC induction motor, although the
optimization of the rotor magnetics are a little different. The tricky part here is that
there is no fixed position of the rotor magnetic poles, so the drive has to figure out
where the poles are at any moment. This is commonly called a 'Flux-Vector'
drive. The trick for using it as a positioning drive is that the controller must always
keep currents flowing in the stator windings so that the rotor can remain magnetized
by induced current flow in the rotor bars. A Flux-Vector servo drive does this, even
when the motor is not moving. A VFD does not.

The performance difference is that a flux vector drive can hold motor position
indefinitely, and rock the motor in tiny increments to make small moves or hold
position against external forces. The VFD can't do any of this, and takes a while
to build up the rotor flux when starting the motor.

A VFD might be able to be used as a spindle drive, although things like hard
tapping on a mill (or lathe, for that matter) might be a problem, as they require
a pretty sudden reversal. But, the VFD would not be of any use in a positioning
servo, unless your tolerances were +/- 1 full inch or so.

Jon