Re: [CAD_CAM_EDM_DRO] Re: servo Motors

>
> --- In CAD_CAM_EDM_DRO@egroups.com, "Mariss Freimanis"
> <geckohall@h...> wrote:
> > Hi,
> >
> > John, if you don't mind doing a little math and some careful
> > measurements, here's how you can characterize your motor.
> >
> > What you are looking for is your motor's stall torque.
> >
> > 1) Measure the armature resistance.
> > 2) Measure the no-load speed at rated voltage (30VDC)
> > 3) Use the following equation:
> >
> > T = (1352 times V squared)/(2R times S)
> >
> > Where:
> >
> > T=Stall torque in in-oz
> > V=Power supply voltage
> > R=Motor armature resistance in ohms
> > S=Motor's no-load speed in RPM
> >
> > The accuracy of the results depends on how accurately you measure
> the
> > armature resistance (most important) and how accurately the no-load
> > speed is measured.

Unfortunately, measuring the resistance on a brush motor is a brutal
experience in frustration! I think I've gotten close once, but the
brushes simply do NOT produce accurate or repeatable results with
a mA or so current from a DVM. I used an impedance bridge, but
that suffers from the same problem. You can twiddle the shaft a
hair, and get stable readings that vary from tens of Ohms to just
about an Ohm, on the type of servo motors we are talking about
for machine tools. Generally, the lowest reading you can possibly
obtain will be the most accurate.

It might be more accurate to use a precision ammeter and a DVM,
and read a locked-armature voltage drop at some modest current, like
a couple of amps.

Jon