GD2 is the Moment of Inertia of the rotor of any rotating equipment. You see, as per Newton's laws, any object will not change its position of rest unless an external force acts on it. Isn't it? To make a rotor to come up to its rated speed from rest also you need to apply a force. And, this force is called the torque. As the mass of the object increases, the torque required also increases. A measure of this mass is the GD2. From this information one can calculate the time required for a motor to accelerate from rest top rated speed under rated supply conditions.Flywheel effect. G is mass of motor rotor, D is the average (or effective) Diameter of the mass. Larger GD2 means more difficult to start up the motor. Smaller GD2 means easier to the start up. Each motor has different rotor GD2. ask motor vendor for a data-sheet. It is now more common to use inertia than GD2. Inertial is GR2, (R = D/2), so, Inertia is (GS2)/4.Inner radius = 0.15mOuter radius = 0.25 mWidth = 0.18m