RE: [CAD_CAM_EDM_DRO] Rotor inertia.

Hi Jason

I'll give a stab at an answer for you.

Rotor inertia is simply a measure of the "flywheel
effect" of a spinning mass. In a cnc this is usually
transformed into linear motion and then becomes the
effect of a moving mass tending to stay in motion
unless acted on by an external force.

With rotating things inertia is actually measured
as a moment of inertia so you will see that term a lot.

Now classically it was desirable to match rotor moment
of inertia with the load equivalent moment of inertia.
This came from transmission line theory.
You will find it often called impedance matching,
but the term refers to mechanical impedance- not electrical.
Inertia is a portion of that term.


When impedances are matched maximum power can be delivered.
A certain length and weight of baseball bat can transfer the maximum
energy to a ball... but the regulation range bats have a mismatched higher
mechanical impedance than the ball. That is one reason some would like to
hollow out and "cork" the bat.

Nowdays with modern servos and amps it is a little less critical though.

I will give an example of my latest gantry machine. It happens to
be inertia matched. The rotor inertia of the NEMA 56 size servo
is about the same as the load- about 500 kg of mass and some ballscrews.
The equivalent moment of inertia of that is transformed considering
things like ballscrew pitch etc.

But precise matching is not so important as I mentioned. If the rotor
inertia predominates the motor will determine how fast the system
will accelerate. If the load predominates _it_ will... by loading down the
motor.

Effective inertia can be changed by gear ratios and the like.

High torque to moment of inertia ratio motors generally cost
more for a given rpm... it's kind of like 0 to 60 mph time in a car. Light
but powerful.

In rotating things torque = moment of inertia x angular acceleration

In sraight line moving things force = mass x acceleration

A good basic engineering text will have a wealth of information about this
as will countless internet sites.

Did this help some?

Les

Leslie M.Watts
L M Watts Furniture
Tiger Georgia USA
(706) 212-0242
http://www.lmwatts.com
Engineering page:
http://www.lmwatts.com/shop.html
CNC surplus for sale:
http://www.lmwatts.com/forsale.html
CNC carved signs:
http://www.lmwatts.com/signwp.html




-----Original Message-----
From: Jason Cox [mailto:sinergy@...]
Sent: Thursday, October 30, 2003 7:26 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Rotor inertia.


Hello All,

Just wondering when selecting servo motors what impact does rotor inertia
have on the final function of a cnc machine?

From my research motors with a far lower inertia are much cheaper, but does
this really matter. Surely the redution in inertia of the rotor would be
minimal say compared to the inertia of the machine table.

Thanks for any thoughts.

Regards,

Jason Cox.