Good day. I have been looking for materials wherein the Moment of Inertia of the load is being considered in the design of motors. I have seen "inertia matching" in some books. It says that, in the motor-gearbox-load configuration, the Inertia of the load "as seen" by the motor should be the same as the inertia of the motor in order to obtain maximum acceleration. This inertia matching will be dependent on the gear-ratio of the gearbox. My question is this... If there is no gearbox, what will happen to the motor-load configuration if the load's moment of inertia is too high? Or, is it ok to put whatever amount of moment of inertia as a motor load? Including reference materials in your response will be very helpful. Thanks in advance.

This is my email just in case.
[email protected]
thank you very much.

 

The "inertia matching" that you referred to is only for guiding you in choosing a gear ratio. If there is no gearbox then you don't have that choice and that particular optimization is not open to you.

If the load inertia is "too high", then you won't get optimal acceleration, that's all. Of course the more you can reduce the load inertia by removing mass at the load, the more acceleration you will get. But that option is probably also not available to you. But forget about inertia matching unless you can affect the gear ratio.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

Matching of motor to load inertia is more of theroretical interest than for any practcal reason. The fastest acceleration will be obtained with the lowest load inertia. The load inertia can be much larger than the motor inertia in direct drive applications.

The most important reason for knowing the load to motor inertia ratio is when the motor is coupled to the load with a flexible coupling (as they amost always are). At inertia ratios above, say, 3 to 1, servo instability is a real possibility due to the resonant frequency of the load inertia/coupling spring constant.

 

No other reference materials are needed other than your mind:

What is Inertia? It is a resistance to change in velocity.

An analogy I use often with motion control is thinking of the load as a big round rock. The bigger and heavier the rock, the harder it is to
make it roll, and of course, to stop it from rolling.

That is what a motor is doing, making the load roll, and then being able to stop it.

If you are a little guy against a big rock your job is difficult, if not impossible, to make the rock stop and start like you want.

No difference with a motor. Without getting theoretical, you can assume a motor with a certain rotor inertia is designed so it can move itself
around well. The closer the load inertia is to the motor inertia the greater your chance of success.

The time you need to accelerate and decelerate the load is as important as the inertia ratio.

There are many factors to consider, but to distill things:

If you want to *position* your load with reasonable acceleration and deceleration parameters:

Keep: Jl / Jr < 10 (Jl = load inertia (as connected to motor), Jr = motor rotor inertia)

If you are just trying to adjust velocity you can get away with a much higher inertia ratio.

For Maximum Acceleration/Deceleration of a motor/drive combination Jl <= Jr.

Robert B. Trask, PE
[email protected]
Los Angeles, CA

 

If you have an inertia mismatch, the system wil be increasingly harder to tune and have lower performance as a result. The rule of thumb is 10:1 mismatch or less. 5:1 or less would be better for a "high performance" system. 1:1 is optimum, but is is overkill for most applications.

Bill Sturm

 

Well, tuning might not be a problem if the motor is not being used in a closed-loop servo. The original question did not say if this was closed-loop or not. If not, then there is nothing to tune.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting