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In classical mechanics, moment of inertia, also called mass moment of inertia, or rotational inertia, is the resistance of an object to rotational acceleration around an axis. This value is typically calculated as the ratio between the torque applied to the motor and the acceleration of the mass ridigly coupled with that motor.
There is a common misunderstanding that inertia is equivalent to load. Load usually presents as load inertia and load torque, where load inertia is the mass that will spin simultaneously with the motor rotor, while the load torque appears as an external torque applied on the motor rotor shaft. An easy way to differentiate the load inertia from load torque is to consider whether the load will spin together with the rotor shaft if the rotor shaft changes spinning direction. Direct couplers and belt pulleys with the mass rigidly mounted to the load shaft are examples of load inertia. Load inertia and motor rotor inertia contribute to the system inertia. Example of load torque include: gravity of a mass applied to one side of the motor rotor shaft, distributed clothes in a washing machine drum during the spin cycle, and the fluid viscosity of a pump. Load inertia of a system should be estimated with the load torque eliminated or minimized.
Figure 8-1 shows an example of a simple motion system. In this system, the Rotating Mass is rigidly coupled with the Motor. This means that the Rotating Mass rotates along with the motor and is considered as part of the inertia. The Non-Rotating Mass is not rigidly coupled with the motor and is considered as part of the load. During the inertia identification process, this Non-Rotating Mass should not be attached to the motor.