# Inertia Matching with TWO or more motors

by hp1
Tags: inertia, matching, motors
 P: 1 Been trying to find an answer to this all day. Would really appreciate any help! Assuming the electric motors are perfectly synced-up, would using multiple motors effectively split the inertia loads evenly among the 2/3/4 motors? Correct me if I'm wrong, but with the equation 1/2Mr^2 for inertia loads, each motor is essentially responsible for moving half the mass (or 1/3, 1/4 and so on and so forth), it seems the inertia would logically also be cut by the same ratio. Thanks in advanced for any help!
P: 88
 Quote by hp1 ... Assuming the electric motors are perfectly synced-up, would using multiple motors effectively split the inertia loads evenly among the 2/3/4 motors? Correct me if I'm wrong, but with the equation 1/2Mr^2 for inertia loads, each motor is essentially responsible for moving half the mass (or 1/3, 1/4 and so on and so forth), it seems the inertia would logically also be cut by the same ratio. ...
Assuming that you are talking about a servo or other dynamic system, the main reason for inertia matching is to minimize control issues due to backlash and lack of rigidity in the system between load and motor.

If as you suggest, the motors are perfectly synchronized, and mechanical connections were identical in terms of total rigidity and backlash, I think that you are theoretically correct that only total rotor inertia matters (whether from 1, 2, 3 or 100 motors).

Practically, this isn't possible; there is always a compromise. Additional motors would likely result in an increase in effective backlash and loss of rigidity. Although the total motor inertia and theoretical mismatch may be the same, dynamic performance would be compromised if only to a small and acceptable degree (depending on application).

I have found that going to the next larger frame size motor offers several advantage over increasing the number of motors (torque and rotor inertia more than double, no parts count increase, cost typically doesn't double). Also, 2 small shaft connections will have more backlash and less rigidity than one larger diameter shaft. These points of course don't matter if you can't fit the larger motor in the machine.

Running 2 motors with one slightly behind the other in order to create preload may eliminate backlash in the system and allow the 2 motors to work better than one. I have heard of this being done on large rack and pinion systems to eliminate backlash. There will be some power lost to this preload, but it may offset the other issues.

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