Re: [CAD_CAM_EDM_DRO] Re: 3 phase motor math

In a message dated 5/17/2004 10:06:42 AM Eastern Daylight Time,
jmkasunich@... writes:
Could a VFD be used to run an AC servo ?

Using a generic VFD to run an AC servo motor _might_ be able
to make it behave like a generic motor - but you certainly
won't get servo-like performance from it. And it's quite
possible it won't work at all (constantly tripping out the
drive, etc.) It would definitely take a lot of tweaking.
Not a project for a VFD newbie. (I have an old AC servo
motor sitting on the shelf, and one of these days I intend
to try it. I'm don't know if I'll succeed, and I design VFDs
for a living!)
My answer to the above question would be likely not. The drives used on true
AC servo motors are NOT your generic VFD. Generic VFDs are designed for use
with standard asynchronous 3 phase AC motors, preferably ones rated for VFD
use. If you are not looking for great accuracy you can use a standard 3 phase
AC motor with a VFD for positioning. OEMs do it all the time and I often
substitute VFDs and 3 phase AC motors for AC motors with electromechanical brakes
on them. However, the accuracy is nothing close to a true servo, be it AC or
DC.

Also, you MUST match the drive to the motor size. Trying to run a 5 Hp motor
with a 2Hp VFD will just destroy the VFD if you size the overloads for the
motor. If you size the overloads for the VFD then they will continually trip
and which makes the application useless as well. The applications where I use a
VFD for positioning are ones that have high ratios between the motor and the
part being driven. Typically 40:1 to 80:1 ratios, this makes the stopping
distance less important.

Also, in order to get any sort of accuracy, you will need to install a
braking resistor to decrease the braking time when stopping.

One other point regarding VFD use. Never put any contactors or overloads
between the VFD and the motor itself. If you do and these open while the VFD is
running, it will often destroy the VFD.

Randy Abernathy
4626 Old Stilesboro Road NW
Acworth, GA 30101-4066
Phone / Fax: 770-974-5295
Cell: 678-772-4113
E-mail: Cnc002@...

I furnish technical support, repair, and other related services for your
industrial woodworking machinery. My background as Senior Service Engineer for the
SCMI Group for nearly fifteen years with factory training, combines with my
extensive background in electronics, mechanics, pneumatics, electrical and CNC
machinery to offer you needed support for your machinery.


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