Re: What makes a motor a servo?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, bobmcknight@c... wrote:

>
> WHAT MAKES A MOTOR A SERVO ??????
>
> We used to have a computer parts surplus store.
> They had nice sized servo motors (probably from big
> multi disk drives). I never understood just how to classify
> these motors. They seemed to be just a big overly precision
> series motor. We were cautioned not to take them apart
> (same caution for steppers). I was familiar with steppers as they
> were used on radar antennas. I used to be able to go to Audels
> and get a little idea about the design of motors and the

characteristics

> of different motors. Some generators were easy to hook as motors,
> others not. We took the old Aircraft Generators and made
> welders out of them. Surplus Center still sells those and I have
> saved one for use on a VW Air Compressor (fire on two, compress
> on two. Volks Air and still have enough power when not compressing
> to power the Welder). And while on the subject, I still have trouble \
> understanding how a DC Brushless motor works.
>
> Bob McKnight
>

Hi Bob,

I assumed we answered that on the first few posts. the application is
the answer. if you take a motor and put an encoder on it and run it
from a servo driver, it is a servo motor.

What makes a GOOD servo motor is one that does not cog.

Jon Elson pointed out on post 80375
"magnetic cogging, the number of armature poles under the field poles
changes through a small rotation"

"constant low voltage is applied...you should get a constant rotation
rate...This is called velocity ripple."

"Finally, when a constant current is supplied (a little harder to do
with simple equipment) the motor should provide constant torque as it
is slowly rotated. If not, it has torque ripple."

These three things are bad.

In addition, some motors are manufactured with the brushes not in
exactly 180 degree alignment. This creates a unique output in one
direction with a lower power output if reversed.

A nameplate voltage that is lots higher than you will ever run them is
also not desireable. ie: a 66 volt power supply for a 230 volt motor.

These are some of what makes a DC motor a GOOD servo.

Dave