Re: [CAD_CAM_EDM_DRO] Motor vs. Servo

To me the biggest distinction between a dc brush motor
being called a servo or just a motor is its armature
MOMENT OF INERTIA.

This is a measure of the "flywheel effect" of the spinning
armature. If the moment is high it will take a long time to
wind up or coast down for a given torque. In fact angular
acceleration equals torque divided by moment.

The rather amazing thing is that moment goes up as the
FOURTH power of radius for a homogenous cylinder. So
twice the diameter has 16 times the moment of inertia!

For this reason a servo will have either a very light rotor
or (more typically) a very long and skinny rotor. You can see
this very quickly looking at a typical servo. To make them
this way costs money.

As Ballendo said a "dc brush motor" can be used as a servo
if it has a tach, resolver, or encoder on it. It prob cannot
ramp up to speed very fast but that might be fine for some applications. For
example one of my bandsaws has a 2 hp
servo drive to run it, but it is just a shunt wound dc motor with
a tach. It does not need to change speed very fast. If it were used on a cnc
drive it would be much too sluggish to use.

All the requirements can be easily calculated.


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

----- Original Message -----
From: "jtfrimenko" <jtf@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, December 28, 2001 8:42 AM
Subject: [CAD_CAM_EDM_DRO] Motor vs. Servo


> Simple question: when does a brush-D.C. motor become a "servo" motor?
> Is it possible to use a non-servo brushed motor in a servo
> application? As soon as the name "servo" is attached, the price goes
> up. Is low speed controll the limiting factor? Are number of
> commutator segments the critical factor?
>
> john
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