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# Mass inertia has great influences on the drive train

What if there is a big difference between the load and motor inertia?
In this article, we use a video to discuss why it is important that both are in a good ratio. This can be easily solved by means of a gearbox and has the advantage that a lighter, more accurate drive train can be used.

One of the energy conversions used in almost all machines is the transition from electricity to motion. The tool to convert this is known as an “electric motor”. Broadly speaking, electric motor applications are divided into S1 (continuous motor) and S5 (intermittent motor, duty cycle less than 60% and less than 20 minutes continuous). Especially with S5 operation it is important to look closely at the mass inertia ratios.

## Video inertia difference

The sample video below show an S5 application.

Have you ever experienced anything like machine 1?
This problem arises because there is a big difference between the used IL (load inertia) and IM (Motor inertia). What is a good ratio of IL and IM to make the movement perfect like machine 2?
This is a bit confusing because there are a number of answers: Lighten the load, get a heavier motor, introduce a gear ratio, dampers, etc. etc.. We calculate the gear ratio as follows:

## Calculating the optimal inertia

For an optimal mass inertia ratio the empirical and dimensionless rule applies:

JL/ i2 <= 4 JM

JL = Mass inertia of the load, JM = Inertia of the motor and i = gear ratio. The most ideal situation for the system is when: JL/ i2 = JM.

From the above equation, many believe that making the motor or setting the JL to the servo motor parameters can solve the problem. That decision is only correct if the difference in inertia is not too much. But what if the difference between JL and JM is much greater? How large should we choose the electric motor?

With the equation JL/ i2 <= 4 JM we can achieve maximum results with a relatively small motor by using a gearbox with gear ratio i.

## Mass inertia practical example

The application from the video has a load of about 200 kg and has to rotate it 180 degrees in 1 second. Machine 1 is driven by a motor only, machine 2 with a motor and gearbox.
Of the latter, the motor specification is only 750 watts at 3,000 rpm. A light, precise and stable drive train. Is this interesting or not?

Contact Apex Dynamics for advice regarding issues with your application, we’ll be happy to help.