Drive Size: Ratio of Inertia's

[jharjula]

I am trying to size an AC or DC servo motor. Some manufacturers recommend the reflected load inertia not be 10x greater than the rotor inertia, while others do not mention it at all. Is it necessary to have this restriction? Continuous torque is ~10 in.lb and starting torque is ~20 in.lb. Drive reverses every 5-10 seconds.

 

[gcaudill]

My thought is that the restriction is to prevent the motor from being overhauled if you wish to brake the inertia to a stop vs. coasting. I think most servo motors can deliver high momentary torque which is useful for different purposes. If you are simply reversing directions you should consider the decel/accel times that will be achievable based on the reflected inertia vs. available motor torque.

 

[jbartos]

Suggestion: The application may end up with an oversize motor if the reversals are very abrupt in time.

 

[jharjula]

Thanks guys. Theoretically the motor has enough torque to stop the load in 0.5 seconds. The manufacturer (Baldor) recommends checking the running torque against the motor's stall torque and the starting/stopping torque against the peak torque of the motor. In addition they recommend the ratio of the load/motor inertias be less than 10.

I can meet the 1st two requirements but the 3rd is making the motor too large dimensionally. If I meet the 3rd requirement, I have way too much torque.

I'm wondering if I have enough torque to accel/decel the load, why do I need to worry about the ratio of inertias?

 

[wired1]

if the inertia of the motor is used as a baseline, then the ratio serves as a convenient way to gauge the ability of the system to respond to changes (ie. ability to accelerate within a reasonable time). i have seen this ratio advocated by other manufacturers but i believe to be a rule-of-thumb rather than a hard design standard. if you want your system to be more responsive, stay within the 10:1 ratio. if you decide go outside of that, then your system may get unduly stressed while accelerating.

 

[jbartos]

Suggestion: The higher load inertia, the higher torque is needed for the fast or abrupt start. E.g. axial fans with the high inertia may be starting for 18 seconds or more depending on their sizes.