Episode# 69: What is Load-to-Motor Inertia Ratio?

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The Motion Control Show

What is load-to-motor inertia ratio?  And what is its acceptable value?

What is inertia?
  • Inertia is that property of matter which causes a resistance to any CHANGE in the motion of a body.
  • Which wheel has the higher inertia if they have the same mass and diameter, but one is hollow?
  • If both wheels start down the incline at the same time, which one will hit the bottom first?
  • Examples of inertia equations

What is load-to-motor inertia ratio?
  • May also be called load-to-rotor inertia ratio.
  • Example calculation with typical values.

What is the ideal load-to-motor inertia ratio?
  • Graph of inertia ratio vs energy usage
  • What load-to-motor inertia ratio should I use for servos?
    • Different manufacturers will give different guidelines.
    • Some tuning algorithms are better than others.
    • How critical the application will also be a factor.
    • Typical ranges from 5:1 to 30:1.
  • What load-to-motor inertia ratio should I use for steppers?
    • Many manufacturers say 10:1. 
    • Other manufacturers don’t specify.
    • Steppers still have to overcome resonances and mass-spring system motions
  • What load-to-motor inertia ratio should I use for direct drives?
    • Direct drives allow for the loads to be bolted directly to them.
    • Can often handle loads 50:1 up to 200:1.

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.