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.