# What is the Inertia and Inertia Ratio of the Servo Motor?

What is inertia and inertia ratio?

Inertia refers to the measurement of the rotational inertia of the motor rotor itself, referred to as inertia, which is crucial to the acceleration and deceleration performance of the motor. Therefore, when selecting a servo motor, it is necessary to fully consider the size of the motor inertia, which directly affects the performance and operating stability of the motor. You can directly query its moment of inertia value in the motor's parameter table. The servo motors sold by STEPPERONLINE are all large inertia motors.

The load inertia ratio is the ratio of the total moment of inertia of the motor load to the motor's own moment of inertia. Load moment of inertia represents the inertia of the load. The greater the load moment of inertia, the worse the responsiveness of the system. If it is too large, it may cause unstable motion and produce vibration. The load inertia ratio is limited by the allowable load moment of inertia of the servo motor, and the load inertia ratio will vary depending on the driving conditions of the servo motor.

When the motor is used beyond the allowable load moment of inertia, an overvoltage alarm will occur during deceleration. In addition, when the servo drive has a built-in braking resistor, an "overload alarm" will occur. If such an alarm occurs, you can take any of the following actions:

● Reduce the torque limit value

● Reduce deceleration curvature

● Reduce the maximum speed

● If the alarm still cannot be cleared after taking the above measures, an external braking resistor is required

How to choose the appropriate inertia ratio?

There are two ways to set the load inertia ratio:

● Manual setting: Generally calculated by calculating the weight and composition of each part of the system, but this method is very cumbersome. For complex servo systems, it is difficult to calculate the correct load to inertia ratio.

● Automatic inertia identification: The most commonly used method currently is automatic identification through the inertia identification function of the servo driver. During the identification process, the driver will drive the servo motor in the forward or reverse direction several times to obtain the appropriate load inertia ratio. Currently, all servo drives sold by our company have this function.

It should be noted that the inertia identification conditions of different series of our company's servo drives are different. Before operation, please carefully read the detailed instructions on inertia identification in the driver manual.

Normally, the load inertia ratio of the servo motor is about 30 times (as shown in the figure). If your system load exceeds this upper limit, it is recommended that you add a reduction gearbox to reduce the load inertia of the system. The reduced load inertia is the square of the reduction ratio. At the same time, considering the impact of the reduction gearbox on the motor speed, it is recommended that you choose a reduction gearbox with a reduction ratio of 5 or 10, which can ensure a suitable inertia ratio without reducing the speed too much.

Updated on: 29/08/2024

Thank you!