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I would like to get some feedback on the topic of inertia ratio. I have recently been trying to gather information as to the importance of this specification. For those of you unsure of what I
mean, let me elaborate:
The inertia ratio is equal to the reflected load inertia divided by the motor's rotational inertia. The specifics of the calculation are not
important for this conversation. There are many recommended guidelines to address this topic. The most common is the 10:1 or 5:1 rule-of-thumb. Some manufacturers state that a stepper
system should have less than a 5:1 ratio, while a servo can have a 10:1 ratio.
The question is why does inertia ratio matter? If the motor/drive combination can handle the torque, speed, & regen shouldn't that be enough? I have an opinion on this topic but am curious as to what others think.
Let me throw out another idea. Let's assume that it is critial to maintain a certain inertia ratio. Every motor manufacturer spec's out the rotational inertia of its motors. If you'll look carefully, you'll see that the inertia spec increases if an absolute encoder &/or brake is added to the motor. Why is this? Are we to assume that by increasing the mass of the motor shaft we can effectively increase the motor's rotational inertia? If this is the case, is there
a limit to how much we can increase it or a set of guide-lines stipulating how the mass must be attached? If your answer states that the brake &/or absolute encoder are contained w/ in the motor housing & that's the reason the increase is relavant, you need to look at the construction of the motor. The brake &/or absolute encoder are not subjected to the magnetic field...therefore
they could be added outside the housing & the effect should be the same (w/ respect to increased rotor inertia). That being said, why
can't we simply put a mass on the shaft of a motor (outside the casing) to increase its inertia.
Some have said that it's a question of compliance & rigidity. Assume the load could be pressed on the shaft & that the bearing structure of the motor is sufficient to handle radial & axial loadings. If the drive/motor can handle the torque, speed, & regen is the inertia ratio important? If so, at what point does it matter?
This subject has long been a topic of conversation within our industry. I would really appreciate any feedback that is deemed relevant. I hope that I receive qualtity feedback, and if you do not feel comfortable with this topic, sit back and enjoy the responses.
Sincerely,
Guy H. Looney
Sales Engineer
Regan Controls, Inc.
475 Metroplex Dr.
Suite 212
Nashville, TN 37211
phone: (615) 333-1940, ext. 322
fax: (615) 333-1941
[email protected]
www.regancontrols.com
mean, let me elaborate:
The inertia ratio is equal to the reflected load inertia divided by the motor's rotational inertia. The specifics of the calculation are not
important for this conversation. There are many recommended guidelines to address this topic. The most common is the 10:1 or 5:1 rule-of-thumb. Some manufacturers state that a stepper
system should have less than a 5:1 ratio, while a servo can have a 10:1 ratio.
The question is why does inertia ratio matter? If the motor/drive combination can handle the torque, speed, & regen shouldn't that be enough? I have an opinion on this topic but am curious as to what others think.
Let me throw out another idea. Let's assume that it is critial to maintain a certain inertia ratio. Every motor manufacturer spec's out the rotational inertia of its motors. If you'll look carefully, you'll see that the inertia spec increases if an absolute encoder &/or brake is added to the motor. Why is this? Are we to assume that by increasing the mass of the motor shaft we can effectively increase the motor's rotational inertia? If this is the case, is there
a limit to how much we can increase it or a set of guide-lines stipulating how the mass must be attached? If your answer states that the brake &/or absolute encoder are contained w/ in the motor housing & that's the reason the increase is relavant, you need to look at the construction of the motor. The brake &/or absolute encoder are not subjected to the magnetic field...therefore
they could be added outside the housing & the effect should be the same (w/ respect to increased rotor inertia). That being said, why
can't we simply put a mass on the shaft of a motor (outside the casing) to increase its inertia.
Some have said that it's a question of compliance & rigidity. Assume the load could be pressed on the shaft & that the bearing structure of the motor is sufficient to handle radial & axial loadings. If the drive/motor can handle the torque, speed, & regen is the inertia ratio important? If so, at what point does it matter?
This subject has long been a topic of conversation within our industry. I would really appreciate any feedback that is deemed relevant. I hope that I receive qualtity feedback, and if you do not feel comfortable with this topic, sit back and enjoy the responses.
Sincerely,
Guy H. Looney
Sales Engineer
Regan Controls, Inc.
475 Metroplex Dr.
Suite 212
Nashville, TN 37211
phone: (615) 333-1940, ext. 322
fax: (615) 333-1941
[email protected]
www.regancontrols.com