J

#### John Boland

<< English units, the moment of inertia has the dimension, ft-lb sec^2, and

the dimension for torque is ft-lb, then what is the dimension for

"stiffness?" Or "compliance?" >>

A good physics or mechanical engineering text should cover this.

May I suggest working with linear motion in MKS units first? The English "system" is filled with derivative, not fundamental, units (pounds, for

example, versus slugs) and rotary motion has those nasty radians floating around.

Dimensional analysis falls apart in our (electrical) world, too, if we treat amps and ohms as fundamental units instead of using:

E (volt)

I (coulomb / sec)

R (volt - sec / coulomb)

Watt (volt - coulomb / sec)

This "Inertia Ratio" issue may result from an over-simplification of the mathematics of the natural frequency of the bare motor and its stable

control, versus the natural frequency of the load. Not torque, speed, or acceleration, ...

We know that a bare motor's internal magnetic / mechanical frequency response can be poor - for example, with a "normal" induction motor and a VFD - or spectacular (to me) - with a DC servo. Rigidly couple equivalent HP / RPM / inertia rated motors of these two types to an equivalent - inertia flywheel and test the frequency responses. There will be a tremendous difference,

although the "Inertia Ratio" is 1:1.

Couple the same DC servo motor to the same equivalent - inertia flywheel with a low "K" torsion spring (where "K" is the torque per resulting radial deflection - in units of your choosing). Test the frequence response. You can

make it as bad as you want by reducing the ratio of spring stiffness to load inertia (softer springs).

In electrical terms, the electro-mechanical system behaves like a lumped transmission line.

Have a good weekend, folks. I got my asbestos undies on.

"Lurker John" G. Boland, president

VisiBit Corporation

One Parker Square Suite 408

2525 Kell Boulevard

Wichita Falls, Texas 76308

940.322.9922

940.723.1478 fax