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The Most Important Factors of Servo Motor Sizing

The Most Important Factors of Servo Motor Sizing

Do you know the best motor to use for your servo application? A lot more goes into choosing the right motor solution than simply calculating the torque requirements of your application, and you shouldn’t simply check the specs (horsepower, etc.) of your current motor. There may be something better than what you currently have installed, and optimization is always the name of the game.

Here are the four things you should consider when choosing a motor for your application.

Inertia Ratio

We’ll first remove a common misconception from the equation: it’s not essential to achieve a 1:1 inertia ratio, or any inertia ratio for that matter. It’s far more important to get the best motor for the job. If performance is critical to your application, aim for an inertia ratio of 2:1 or lower, but if high performance isn’t critical for your application, higher ratios (10:1 or even 100:1 and higher) can be managed with the higher bandwidth drives and higher feedback resolution motors available today. Lower inertia ratios can sometimes require excessively large (not to mention expensive) motors that may result in overkill.

Mechanical compliance from motor to load is an important factor when opting for higher inertia ratios; the higher the backlash or “springiness” the lower the inertia ration should be.  Know the performance requirements of your application before calculating your ideal inertia ratio. The choose a motor that will deliver that and nothing more.

The Speed-Torque Curve

Once you have your ideal inertia ratio determined, the next step is to find the smallest motor that can produce the speed and torque your application requires.

Speed and torque are inversely related at higher speeds; as speed increases beyond a certain point torque will decrease. That’s why it’s important to consult the servo motor speed-torque curve. The speed-torque curve of a motor will show you its rated torque, the maximum continuous torque available at the design speed (without the motor overheating), and the rated speed, the maximum speed that allows for rated torque. While motor can generally run faster than their rated speed, you will experience a drop in torque after its rated speed is exceeded.

The curve will also show you two regions: intermittent and continuous. If the motor’s speed and torque fall in the continuous region, it produce that torque indefinitely without overheating. If it falls in the intermittent region, it will only be able to run in short bursts to produce that torque.

Important to note that not all motor manufacturers plot their speed-torque curves with the same ambient temperature, motor heat rise, and (heatsink) mounting setup.  Be sure to look at the manufacturer specifications for this in marginal applications.

Max Torque

It’s important to ensure the motor is capable of producing the Max Torque required for your application’s speed. The motor must be able to overcome the static friction keeping the load at rest and maintain the speed required by the application. The ideal Max Torque of your chosen motor should fall in the intermittent region of the speed-torque curve. If the Max Torque falls in the continuous region, this could be a sign that motor is too large for your application.

Root Mean Square (RMS) Torque

We’re not done with torque yet. It’s also important to consider the RMS torque. RMS torque not only takes into account the different amounts of torque required throughout the application cycle, but also how long each amount of torque must be maintained. A motor that produces a certain amount of RMS torque will heat at a constant level when maintaining torque equal to its RMS torque. Since RMS torque deals with a constant value, a motor’s RMS torque should land within the continuous region of the speed-torque curve.


Need Some Help Selecting the Right Motor for Your Application? Let the Automation Experts at AMMC be Your Guide.


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