Re: [CAD_CAM_EDM_DRO] 90v DC motor as servo?

Chris,

The biggest difference I see is in polar moment of inertia.
The dc motors you mention often accelerate slowly compared
to a servo. They are designed for steady state operation.
For example a typical 1/2 hp pmdc scr rated motor has
.2 - .4 oz-in/sec^2 while a dc servo of the same torque
might be .05 or less.

The biggest give away with conventional rotors is simply diameter. Moment of
inertia grows rapidly with diameter,
so servos are often long and low diameter. Special rotors need not be.

Motor moment of inertia needs to be a specific value
for a given load for optimum performance. If it is too high
very large currents ($ for servo amps) will be required to
start and stop it rapidly.

I have used pmdc motors for servos in some apps and never had too much
problem with cogging. I test them by seeing if they will run smoothly at
some tens of rpm at a low voltage
with no load. It just depends on the application.

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html

----- Original Message -----
From: "Chris Stratton" <stratton@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, May 18, 2001 11:14 PM
Subject: [CAD_CAM_EDM_DRO] 90v DC motor as servo?


> I'm curious if someone could comment on how well one of the 90v DC
> brush motors (designed presumably for SCR speed control) would operate
> as a servo motor?
>
> Alternatively, what are the basic properties a motor needs for
> positioning servo use? Is 'cogging' bad?
>
> Thanks,
>
> Chris
>
> --
> Christopher C. Stratton, stratton@...
> Instrument Maker, Horn Player & Engineer
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