Inertia and Motors

What is a simple definition for "moment of inertia," often described as inertia in a motor ?

Inertia describes the tendency of a body to resist changes in rotational speed for a given torque.

What is a typical ratio of the application load inertia to the motor inertia ?

It is not unusual that the application load is in a range of 1:1 to 5:1 to allow highest loop bandwidth and dynamic response. The “inertia mismatch” is a rule of thumb indicator for good controllability of the axis. For a ratio of 1:1 the necessary torque during acceleration is equally applied to the load and the motor.

Why do low inertia motors benefit an application?

  • Minimizes torque requirements and consequently can effect drive selection.
  • Motor selection is usually smaller in envelope and drive selection could require a lower continuous current rating.
  • Performance of machine can be improved (e.g., throughput)

Why is rotor inertia important to know?

A motor can not be selected for replacement by the power rating or the torque speed curve alone without knowing how the rotor inertia has effected the continuous torque required in the application.

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