Motor Crossover Info 


By Reynaldo Cortez


The manufacturing sector is changing fast, and every company is trying to keep up with new demands. One of the biggest challenges that the automation industry is currently facing is the negative impact that the 2020 pandemic has left on the supply chain. New parts and accessories are harder to come by. When you engineer the perfect system solution you now face long lead times for your components and you are left on a standstill, waiting to test out your concept. In order to combat long lead times manufacturers have resorted to diversifying. They have expanded their automation partner vendor list and are mix and matching components as they become available. In this blog, we will explore how to look at any servo motor specifications and then find equivalent performing motors from Rockwell to help diversify your motor inventory.

Step 1: Review the motor’s rotor inertia value.

Most servo motor manufacturers produce specification tables that give you important details about the characteristics of a motor. 

Fig. 1 shows what a typical specification table looks like for a servo motor. Let us say I was interested in crossing over Motor “A”, I now know that its rotor moment of inertia value is 43x10-4 kgm2. This value speaks to the capability to overcome the inertia of the mechanism connected to the motor.

You want to make sure that Rockwell’s motor rotor inertia value is equal or greater than the motor you are trying to replace.

Step 2: Review the motor’s Speed and Torque values.

Now that you know your Rockwell motor can overcome the inertia of the load, you want to make sure that the speed and torque values are the same or greater than the deposed motor.

When revisiting Motor “A”, I find that that the nominal motor speed is 2500 RPM, and the rated torque is 12Nm. These characteristics will help you narrow down your search, when looking at equivalent Rockwell motors.

fig 1
fig 2

Step 3: Review the speed-torque curves.

We have identified the rated speed and torque values for Motor “A”, but it is important to understand that these values are not constants. Speed and Torque are inversely affected by each other, the faster a motor is working the lower the torque capacity is, and vice versa. Therefore, it is important to look at the Torque vs Speed chart for your motor of interest.

Fig. 2 shows what you can expect from a Torque vs Speed chart, speed on the X-axis and Torque on the Y-axis.

The area highlighted in grey is the safe operation area, meaning that the torque and speed values that fall in this region are safe for the motor to perform in continuous use. Anything above this area is know as the intermittent operation area, torque or speed values that falls inside this area are possible to achieve but not for prolonged use. If you continue to use the motor within the intermittent operation area, you will start running into errors and it would decrease its life in operation.

When you start to look for Rockwell motors be sure to compare Rockwell’s Torque vs Speed charts with the motor’s charts you are trying to replace to see if they have similar or better continuous operating region.

After all these steps have been taken you can safely assume that you have found a Rockwell motor with equivalent performance to that of the motor you are trying to replace.


These steps focus on the performance of a motor but there are other factors to consider in motion application, such as precision, safety rating, form factor, etc. Gerrie Electric encourages you to perform the checks above and contact your local mechatronics specialist to verify your selection.

Useful links:

Rockwell Motion Webpage:

Motion Selection Guide: KNX-SG001I-EN-P

Motor Specification Information: KNX-TD001H-EN-P

reynaldo image

About The Author

Mechanical Engineer & Data Scientist for Gerrie Electric in Burlington ON, Canada

C# 289-861-7128,  [email protected] 


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