Gd^2 of the motor

Moment of inertia, I (in "scientific speak"), is calculated from the mass of an object and a measure of how far the mass is from the axis of rotation.

For a cylinder, I = ½mr², where m is the total mass and r is the radius.

For a "point mass" at radius r, I = mr².

For a motor rotor, it gets complicated! It's generally a lot easier to measure it than to calculate it^[1].

[1] ack. to Redfred.

Try starting here...

http://www.springerlink.com/content/g17072gw37131826/

First google hit I found and I'm sure there are many more.

I have similar equations in my Electrodynamic Machines "textbooks". Those are bound sheets of paper which were frequently used last century to convey information

Go to WolframAlpha for any math problem.

You are looking for a rotational or torsional moment of inertia. As one of the contributers has said it is not easy to calculate it - it is not very easy to measure it either unless the armature is available on its own. It is possible to measure it in place but bearings and their friction may modify the results somewhat.

It needs the armature to be suspended from a wire and measure the period of oscillations and then work back to the torsional moment of inertia.

I would suggest a book called Roark's Formulas for Stress and strain by Warren C Young ISBN 0-07-100373-8. It may be better to get it from the library first and if you find it useful it will help you in Mechanical engineering throughout your life.